Bio

Academic Appointments


Administrative Appointments


  • Committee on Research, Stanford University (2012 - 2015)

Honors & Awards


  • Inaugural Fellow, Association for Research in Vision and Ophthalmology (2009)

Boards, Advisory Committees, Professional Organizations


  • Director, Vision Sciences Society (2012 - Present)
  • Director, Vision4Children, UK (2010 - Present)

Research & Scholarship

Current Research and Scholarly Interests


Vision, development, functional imaging, systems analysis

Teaching

2013-14 Courses


Graduate and Fellowship Programs


Publications

Journal Articles


  • The time course of shape discrimination in the human brain NEUROIMAGE Ales, J. M., Appelbaum, L. G., Cottereau, B. R., Norcia, A. M. 2013; 67: 77-88

    Abstract

    The lateral occipital cortex (LOC) activates selectively to images of intact objects versus scrambled controls, is selective for the figure-ground relationship of a scene, and exhibits at least some degree of invariance for size and position. Because of these attributes, it is considered to be a crucial part of the object recognition pathway. Here we show that human LOC is critically involved in perceptual decisions about object shape. High-density EEG was recorded while subjects performed a threshold-level shape discrimination task on texture-defined figures segmented by either phase or orientation cues. The appearance or disappearance of a figure region from a uniform background generated robust visual evoked potentials throughout retinotopic cortex as determined by inverse modeling of the scalp voltage distribution. Contrasting responses from trials containing shape changes that were correctly detected (hits) with trials in which no change occurred (correct rejects) revealed stimulus-locked, target-selective activity in the occipital visual areas LOC and V4 preceding the subject's response. Activity that was locked to the subjects' reaction time was present in the LOC. Response-locked activity in the LOC was determined to be related to shape discrimination for several reasons: shape-selective responses were silenced when subjects viewed identical stimuli but their attention was directed away from the shapes to a demanding letter discrimination task; shape-selectivity was present across four different stimulus configurations used to define the figure; LOC responses correlated with participants' reaction times. These results indicate that decision-related activity is present in the LOC when subjects are engaged in threshold-level shape discriminations.

    View details for DOI 10.1016/j.neuroimage.2012.10.044

    View details for Web of Science ID 000314144600008

    View details for PubMedID 23116814

  • An objective signature for visual binding of face parts in the human brain. Journal of vision Boremanse, A., Norcia, A. M., Rossion, B. 2013; 13 (11)

    Abstract

    Whether and how the parts of a visual object are grouped together to form an integrated ("holistic") representation is a central question in cognitive neuroscience. Although the face is considered to be the quintessential example of holistic representation, this issue has been the subject of much debate in face perception research. The implication of holistic processing is that the response to the whole cannot be predicted from the sum of responses to the parts. Here we apply techniques from nonlinear systems analysis to provide an objective measure of the nonlinear integration of parts into a whole, using the left and right halves of a face stimulus as the parts. High-density electroencephalogram (EEG) was recorded in 15 human participants presented with two halves of a face stimulus, flickering at different frequencies (5.88 vs. 7.14 Hz). Besides specific responses at these fundamental frequencies, reflecting part-based responses, we found intermodulation components (e.g., 7.14 - 5.88 = 1.26 Hz) over the right occipito-temporal hemisphere, reflecting nonlinear integration of the face halves. Part-based responses did not depend on the relative alignment of the two face halves, their spatial separation, or whether the face was presented upright or inverted. By contrast, intermodulations were virtually absent when the two halves were spatially misaligned and separated. Inversion of the whole face configuration also reduced specifically the intermodulation components over the right occipito-temporal cortex. These observations indicate that the intermodulation components constitute an objective, configuration-specific signature of an emergent neural representation of the whole face that is distinct from that generated by the parts themselves.

    View details for DOI 10.1167/13.11.6

    View details for PubMedID 24023273

  • On determining the intracranial sources of visual evoked potentials from scalp topography: A reply to Kelly et al. (this issue) NEUROIMAGE Ales, J. M., Yates, J. L., Norcia, A. M. 2013; 64: 703-711

    Abstract

    The cruciform model posits that if a Visual Evoked Potential component originates in cortical area V1, then stimuli placed in the upper versus lower visual field will generate responses with opposite polarity at the scalp. In our original paper (Ales et al., 2010b) we showed that the cruciform model provides an insufficient criterion for identifying V1 sources. This conclusion was reached on the basis of simulations that used realistic 3D models of early visual areas to simulate scalp topographies expected for stimuli of different sizes and shapes placed in different field locations. The simulations indicated that stimuli placed in the upper and lower visual field produce polarity inverting scalp topographies for activation of areas V2 and V3, but not for area V1. As a consequence of the non-uniqueness of the polarity inversion criterion, we suggested that past studies using the cruciform model had not adequately excluded contributions from sources outside V1. In their comment on our paper, Kelly et al. (this issue) raise several concerns with this suggestion. They claim that our initial results did not use the proper stimulus locations to constitute a valid test of the cruciform model. Kelly et al., also contend that the cortical source of the initial visually evoked component (C1) can be identified based on latency and polarity criteria derived from intracranial recordings in non-human primates. In our reply we show that simulations using the suggested critical stimulus locations are consistent with our original findings and thus do not change our conclusions regarding the use of the polarity inversion criterion. We further show that the anatomical assumptions underlying the putatively optimal locations are not consistent with available V1 anatomical data. We then address the non-human primate data, describing how differences in stimuli across studies and species confound an effective utilization of the non-human primate data for interpreting human evoked potential responses. We also show that, considered more broadly, the non-human primate literature shows that multiple visual areas onset simultaneously with V1. We suggest several directions for future research that will further clarify how to make the best use of scalp data for inferring cortical sources.

    View details for DOI 10.1016/j.neuroimage.2012.09.009

    View details for Web of Science ID 000312504200066

    View details for PubMedID 22982584

  • Linking perception to neural activity as measured by visual evoked potentials. Visual neuroscience Norcia, A. M. 2013: 1-5

    Abstract

    Linking propositions have played an important role in refining our understanding of the relationship between neural activity and perception. Over the last 40 years, visual evoked potentials (VEPs) have been used in many different ways to address questions of the relationship between neural activity and perception. This review organizes and discusses this research within the linking proposition framework developed by Davida Teller, and her colleagues. A series of examples from the VEP literature illustrates each of the five classes of linking propositions originally proposed by Davida Teller. The related concept of the bridge locus-the site at which neural activity can be said to first be proscriptive of perception-is discussed and a suggestion is made that the concept be expanded to include an evolution over time and cortical area.

    View details for PubMedID 23879990

  • Increasing the accuracy of electromagnetic inverses using functional area source correlation constraints HUMAN BRAIN MAPPING Cottereau, B. R., Ales, J. M., Norcia, A. M. 2012; 33 (11): 2694-2713

    Abstract

    Estimating cortical current distributions from electroencephalographic (EEG) or magnetoencephalographic data is a difficult inverse problem whose solution can be improved by the addition of priors on the associated neural responses. In the context of visual activation studies, we propose a new approach that uses a functional area constrained estimator (FACE) to increase the accuracy of the reconstructions. It derives the source correlation matrix from a segmentation of the cortex into areas defined by retinotopic maps of the visual field or by functional localizers obtained independently by fMRI. These areas are computed once for each individual subject and the associated estimators can therefore be reused for any new study on the same participant. The resulting FACE reconstructions emphasize the activity of sources within these areas or enforce their intercorrelations. We used realistic Monte-Carlo simulations to demonstrate that this approach improved our estimates of a diverse set of source configurations. Reconstructions obtained from a real EEG dataset demonstrate that our priors improve the localization of the cortical areas involved in horizontal disparity processing.

    View details for DOI 10.1002/hbm.21394

    View details for Web of Science ID 000310264500015

    View details for PubMedID 21938755

  • Spatial Contrast Sensitivity Vision Loss in Children with Cortical Visual Impairment INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Good, W. V., Hou, C., Norcia, A. M. 2012; 53 (12): 7730-7734

    Abstract

    Although cortical visual impairment (CVI) is the leading cause of bilateral vision impairment in children in Western countries, little is known about the effects of CVI on visual function. The aim of this study was to compare visual evoked potential measures of contrast sensitivity and grating acuity in children with CVI with those of age-matched typically developing controls.The swept parameter visual evoked potential (sVEP) was used to measure contrast sensitivity and grating acuity in 34 children with CVI at 5 months to 5 years of age and in 16 age-matched control children. Contrast thresholds and spatial frequency thresholds (grating acuities) were derived by extrapolating the tuning functions to zero amplitude. These thresholds and maximal suprathreshold response amplitudes were compared between groups.Among 34 children with CVI, 30 had measurable but reduced contrast sensitivity with a median threshold of 10.8% (range 5.0%-30.0% Michelson), and 32 had measurable but reduced grating acuity with median threshold 0.49 logMAR (9.8 c/deg, range 5-14 c/deg). These thresholds were significantly reduced, compared with age-matched control children. In addition, response amplitudes over the entire sweep range for both measures were significantly diminished in children with CVI compared with those of control children.Our results indicate that spatial contrast sensitivity and response amplitudes are strongly affected by CVI. The substantial degree of loss in contrast sensitivity suggests that contrast is a sensitive measure for evaluating vision deficits in patients with CVI.

    View details for DOI 10.1167/iovs.12-9775

    View details for Web of Science ID 000313053500043

    View details for PubMedID 23060143

  • Bridging the gap: global disparity processing in the human visual cortex JOURNAL OF NEUROPHYSIOLOGY Cottereau, B. R., McKee, S. P., Norcia, A. M. 2012; 107 (9): 2421-2429

    Abstract

    The human stereoscopic system is remarkable in its ability to utilize widely separated features as references to support fine depth discrimination. In a search for possible neural substrates of this ability, we recorded high-density EEG and used a distributed inverse technique to estimate population-level disparity responses in five regions of interest (ROIs): V1, V3A, hMT+, V4, and lateral occipital complex (LOC). The stimulus was a central modulating disk surrounded by a correlated "reference" annulus presented in the fixation plane. We varied a gap separating the disk from the annulus parametrically from 0 to 5.5° as a test of long-range disparity integration. In the V1, LOC, and hMT+ ROIs, the responses with gaps >0.5° were equal to those obtained in a control condition where the surround was composed of uncorrelated noise (no reference). By contrast, in the V4 and V3A ROIs, responses with gaps as large as 5.5° were still significantly higher than the control. As a test of the spatial distribution of the disparity reference information, we manipulated the properties of the stimulus by placing noise between the center and the surround or throughout the surround. The V3A ROI was particularly sensitive to disparity noise between the center and annulus regions, suggesting an important contribution of disparity edge detectors in this ROI.

    View details for DOI 10.1152/jn.01051.2011

    View details for Web of Science ID 000303600900011

    View details for PubMedID 22323636

  • The Time Course of Segmentation and Cue-Selectivity in the Human Visual Cortex PLOS ONE Appelbaum, L. G., Ales, J. M., Norcia, A. M. 2012; 7 (3)

    Abstract

    Texture discontinuities are a fundamental cue by which the visual system segments objects from their background. The neural mechanisms supporting texture-based segmentation are therefore critical to visual perception and cognition. In the present experiment we employ an EEG source-imaging approach in order to study the time course of texture-based segmentation in the human brain. Visual Evoked Potentials were recorded to four types of stimuli in which periodic temporal modulation of a central 3° figure region could either support figure-ground segmentation, or have identical local texture modulations but not produce changes in global image segmentation. The image discontinuities were defined either by orientation or phase differences across image regions. Evoked responses to these four stimuli were analyzed both at the scalp and on the cortical surface in retinotopic and functional regions-of-interest (ROIs) defined separately using fMRI on a subject-by-subject basis. Texture segmentation (tsVEP: segmenting versus non-segmenting) and cue-specific (csVEP: orientation versus phase) responses exhibited distinctive patterns of activity. Alternations between uniform and segmented images produced highly asymmetric responses that were larger after transitions from the uniform to the segmented state. Texture modulations that signaled the appearance of a figure evoked a pattern of increased activity starting at ?143 ms that was larger in V1 and LOC ROIs, relative to identical modulations that didn't signal figure-ground segmentation. This segmentation-related activity occurred after an initial response phase that did not depend on the global segmentation structure of the image. The two cue types evoked similar tsVEPs up to 230 ms when they differed in the V4 and LOC ROIs. The evolution of the response proceeded largely in the feed-forward direction, with only weak evidence for feedback-related activity.

    View details for DOI 10.1371/journal.pone.0034205

    View details for Web of Science ID 000303894900070

    View details for PubMedID 22479566

  • Dynamics of Normalization Underlying Masking in Human Visual Cortex JOURNAL OF NEUROSCIENCE Tsai, J. J., Wade, A. R., Norcia, A. M. 2012; 32 (8): 2783-2789

    Abstract

    Stimulus visibility can be reduced by other stimuli that overlap the same region of visual space, a process known as masking. Here we studied the neural mechanisms of masking in humans using source-imaged steady state visual evoked potentials and frequency-domain analysis over a wide range of relative stimulus strengths of test and mask stimuli. Test and mask stimuli were tagged with distinct temporal frequencies and we quantified spectral response components associated with the individual stimuli (self terms) and responses due to interaction between stimuli (intermodulation terms). In early visual cortex, masking alters the self terms in a manner consistent with a reduction of input contrast. We also identify a novel signature of masking: a robust intermodulation term that peaks when the test and mask stimuli have equal contrast and disappears when they are widely different. We fit all of our data simultaneously with family of a divisive gain control models that differed only in their dynamics. Models with either very short or very long temporal integration constants for the gain pool performed worse than a model with an integration time of ?30 ms. Finally, the absolute magnitudes of the response were controlled by the ratio of the stimulus contrasts, not their absolute values. This contrast-contrast invariance suggests that many neurons in early visual cortex code relative rather than absolute contrast. Together, these results provide a more complete description of masking within the normalization framework of contrast gain control and suggest that contrast normalization accomplishes multiple functional goals.

    View details for DOI 10.1523/JNEUROSCI.4485-11.2012

    View details for Web of Science ID 000300716600021

    View details for PubMedID 22357861

  • Disparity-Specific Spatial Interactions: Evidence from EEG Source Imaging JOURNAL OF NEUROSCIENCE Cottereau, B. R., McKee, S. P., Ales, J. M., Norcia, A. M. 2012; 32 (3): 826-840

    Abstract

    Using cortical source estimation techniques based on high-density EEG and fMRI measurements in humans, we measured how a disparity-defined surround influenced the responses to the changing disparity of a central disk within five visual ROIs: V1, V4, lateral occipital complex (LOC), hMT+, and V3A. The responses in the V1 ROI were not consistently affected either by changes in the characteristics of the surround (correlated or uncorrelated) or by its disparity value, consistent with V1 being responsive only to absolute, not relative, disparity. Correlation in the surround increased the responses in the V4, LOC, and hMT+ ROIs over those measured with the uncorrelated surround. Thus, these extrastriate areas contain neurons that are sensitive to disparity differences. However, their evoked responses did not vary systematically with the surround disparity. Responses in the V3A ROI, in contrast, were increased by correlation in the surround and varied with its disparity. We modeled these V3A responses as attributable to a gain modulation of the absolute disparity response, where the gain amplitude is proportional to the center-surround disparity difference. An additional experiment identified a nonlinear center-surround interaction in V3A that facilitates the responses when center and surround are misaligned but suppresses it when they share the same disparity plane.

    View details for DOI 10.1523/JNEUROSCI.2709-11.2012

    View details for Web of Science ID 000299324900008

    View details for PubMedID 22262881

  • Distinct effects of attention on the neural responses to form and motion processing: A SSVEP source-imaging study JOURNAL OF VISION Palomares, M., Ales, J. M., Wade, A. R., Cottereau, B. R., Norcia, A. M. 2012; 12 (10)

    Abstract

    We measured neural responses to local and global aspects of form and motion stimuli using frequency-tagged, steady-state visual evoked potentials (SSVEPs) combined with magnetic resonance imaging (MRI) data. Random dot stimuli were used to portray either dynamic Glass patterns (Glass, 1969) or coherent motion displays. SSVEPs were used to estimate neural activity in a set of fMRI-defined visual areas in each subject. To compare activity associated with local versus global processing, we analyzed two frequency components of the SSVEP in each visual area: the high temporal frequency at which the local dots were updated (30 Hz) and the much lower frequency corresponding to updates in the global structure (0.83 Hz). Local and global responses were evaluated in the context of two different behavioral tasks--subjects had to either direct their attention toward or away from the global coherence of the stimuli. The data show that the effect of attention on global and local responses is both stimulus and visual area dependent. When attention was directed away from stimulus coherence, both local and global responses were higher in the coherent motion than Glass pattern condition. Directing attention to coherence in Glass patterns enhanced global activity in areas LOC, hMT+, V4, V3a, and V1, while attention to global motion modulated responses by a smaller amount in a smaller set of areas: V4, hMT+, and LOC. In contrast, directing attention towards stimulus coherence weakly increased local responses to both coherent motion and Glass patterns. These results suggest that visual attention differentially modulates the activity of early visual areas at both local and global levels of structural encoding.

    View details for DOI 10.1167/12.10.15

    View details for Web of Science ID 000313886400015

    View details for PubMedID 23019120

  • Effect of Grade I and II Intraventricular Hemorrhage on Visuocortical Function in Very Low Birth Weight Infants SEEING AND PERCEIVING Madan, A., Norcia, A. M., Hou, C., Pettet, M. W., Good, W. V. 2012; 25 (2): 143-154

    Abstract

    The neurological outcome for infants with Grade I/II intraventricular hemorrhage (IVH) is debated. The aim of this study was to determine whether very low birth weight infants (VLBW, <1500 g) with Grade I/II (IVH) have altered visuocortical activity compared with infants with no IVH. We assessed the quantitative swept parameter visual evoked potential (sVEP) responses evoked by three different visual stimuli. Data from 52 VLBW infants were compared with data from 13 infants with Grade I or II IVH, enrolled at 5-7 months corrected age. Acuity thresholds and suprathreshold response amplitudes were compared. Grating acuity (GA), contrast sensitivity (CS) and vernier acuity (VA) were each worse in the Grade I/II IVH compared with the no IVH groups (8.24 cpd in IVH group vs. 13.07 cpd in no IVH group for GA; 1.44% vs. 1.18% for CS and 1.55 arcmin vs. 0.58 arcmin for VA). The slopes of the response amplitude for CS and VA were significantly lower in IVH infants. The spatial frequency tuning function was shifted downward on the spatial frequency axis, without a change in slope. These results indicate that Grade I/II IVH are associated with deleterious effects on cortical vision development and function.

    View details for DOI 10.1163/187847612X626381

    View details for Web of Science ID 000303371100002

    View details for PubMedID 22371027

  • An objective method for measuring face detection thresholds using the sweep steady-state visual evoked response JOURNAL OF VISION Ales, J. M., Farzin, F., Rossion, B., Norcia, A. M. 2012; 12 (10)

    Abstract

    We introduce a sensitive method for measuring face detection thresholds rapidly, objectively, and independently of low-level visual cues. The method is based on the swept parameter steady-state visual evoked potential (ssVEP), in which a stimulus is presented at a specific temporal frequency while parametrically varying ("sweeping") the detectability of the stimulus. Here, the visibility of a face image was increased by progressive derandomization of the phase spectra of the image in a series of equally spaced steps. Alternations between face and fully randomized images at a constant rate (3/s) elicit a robust first harmonic response at 3 Hz specific to the structure of the face. High-density EEG was recorded from 10 human adult participants, who were asked to respond with a button-press as soon as they detected a face. The majority of participants produced an evoked response at the first harmonic (3 Hz) that emerged abruptly between 30% and 35% phase-coherence of the face, which was most prominent on right occipito-temporal sites. Thresholds for face detection were estimated reliably in single participants from 15 trials, or on each of the 15 individual face trials. The ssVEP-derived thresholds correlated with the concurrently measured perceptual face detection thresholds. This first application of the sweep VEP approach to high-level vision provides a sensitive and objective method that could be used to measure and compare visual perception thresholds for various object shapes and levels of categorization in different human populations, including infants and individuals with developmental delay.

    View details for DOI 10.1167/12.10.18

    View details for Web of Science ID 000313886400018

    View details for PubMedID 23024355

  • Piecing it together: Infants' neural responses to face and object structure JOURNAL OF VISION Farzin, F., Hou, C., Norcia, A. M. 2012; 12 (13)

    Abstract

    Integration of local elements into a coherent global form is a fundamental aspect of visual object recognition. How the different hierarchically organized stages of visual analysis develop in order to support object representation in infants remains unknown. The aim of this study was to investigate structural encoding of natural images in 4- to 6-month-old infants and adults. We used the steady-state visual evoked potential (ssVEP) technique to measure cortical responses specific to the global structure present in object and face images, and assessed whether differential responses were present for these image categories. This study is the first to apply the ssVEP method to high-level vision in infants. Infants and adults responded to the structural relations present in both image categories, and topographies of the responses differed based on image category. However, while adult responses to face and object structure were localized over occipitotemporal scalp areas, only infant face responses were distributed over temporal regions. Therefore, both infants and adults show object category specificity in their neural responses. The topography of the infant response distributions indicates that between 4 and 6 months of age, structure encoding of faces occurs at a higher level of processing than that of objects.

    View details for DOI 10.1167/12.13.6

    View details for Web of Science ID 000313888000006

    View details for PubMedID 23220577

  • Visual Cortical Function in Very Low Birth Weight Infants without Retinal or Cerebral Pathology INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Hou, C., Norcia, A. M., Madan, A., Tith, S., Agarwal, R., Good, W. V. 2011; 52 (12): 9091-9098

    Abstract

    Preterm infants are at high risk of visual and neural developmental deficits. However, the development of visual cortical function in preterm infants with no retinal or neurologic morbidity has not been well defined. To determine whether premature birth itself alters visual cortical function, swept parameter visual evoked potential (sVEP) responses of healthy preterm infants were compared with those of term infants.Fifty-two term infants and 58 very low birth weight (VLBW) infants without significant retinopathy of prematurity or neurologic morbidities were enrolled. Recruited VLBW infants were between 26 and 33 weeks of gestational age, with birth weights of less than 1500 g. Spatial frequency, contrast, and vernier offset sweep VEP tuning functions were measured at 5 to 7 months' corrected age. Acuity and contrast thresholds were derived by extrapolating the tuning functions to 0 amplitude. These thresholds and suprathreshold response amplitudes were compared between groups.Preterm infants showed increased thresholds (indicating decreased sensitivity to visual stimuli) and reductions in amplitudes for all three measures. These changes in cortical responsiveness were larger in the <30 weeks ' gestational age subgroup than in the ?30 weeks' gestational age subgroup.Preterm infants with VLBW had measurable and significant changes in cortical responsiveness that were correlated with gestational age. These results suggest that premature birth in the absence of identifiable retinal or neurologic abnormalities has a significant effect on visual cortical sensitivity at 5 to 7 months' of corrected age and that gestational age is an important factor in visual development.

    View details for DOI 10.1167/iovs.11-7458

    View details for Web of Science ID 000297631400043

    View details for PubMedID 22025567

  • Distinct cortical responses to 2D figures defined by motion contrast VISION RESEARCH Fesi, J. D., Yannes, M. P., Brinckman, D. D., Norcia, A. M., Ales, J. M., Gilmore, R. O. 2011; 51 (19): 2110-2120

    Abstract

    Motion contrast contributes to the segregation of a two-dimensional figure from its background, yet many questions remain about its neural mechanisms. We measured steady-state visual evoked potential (SSVEP) responses to moving dot displays in which figure regions emerged from and disappeared into the background at a specific temporal frequency (1.2Hz, F1), based on regional differences of dot direction and global direction coherence. The goal was to measure the cortical response function across a range of motion contrast magnitudes. In two experiments using both a low channel count electrode array (Experiment 1) and a high density array (Experiment 2), we observed two distinct phase-locked evoked responses that were similar across motion contrast type. A response at 1.2Hz (1F1) increased in amplitude with increasing magnitudes of direction or coherence contrast. A response at 2.4Hz (2F1) increased in amplitude, but saturated at low levels of direction or coherence contrast. The two components showed different scalp distributions - the 1F1 was strongest along medial occipital channels, while the 2F1 was bilaterally distributed. Taken together, the studies suggest that figures defined by different types of motion contrast are processed by cortical systems with similar dynamics, and that there are separable neural systems devoted to (i) signaling the absolute magnitude of motion contrast and (ii) detecting when a figure defined by motion contrast appears and disappears from view.

    View details for DOI 10.1016/j.visres.2011.07.015

    View details for Web of Science ID 000295489200002

    View details for PubMedID 21820002

  • Contrast Gain Control Abnormalities in Idiopathic Generalized Epilepsy ANNALS OF NEUROLOGY Tsai, J. J., Norcia, A. M., Ales, J. M., Wade, A. R. 2011; 70 (4): 574-582

    Abstract

    The origin of neural hyperexcitability underlying idiopathic generalized epilepsy (IGE) is not known. The objective of this study is to identify evidence of hyperexcitability in precisely measured visual evoked responses and to understand the nature of changes in excitation and inhibition that lead to altered responses in human patients with IGE.Steady-state visual-evoked potentials (VEPs) to contrast reversing gratings were recorded over a wide range of stimulus contrast. VEPs were analyzed at the pattern reversal rate using spectral analysis. Ten patients with IGE and 13 healthy subjects participated. All subjects had normal visual acuity and had no history of photic-induced seizures or photoparoxysmal electroencephalograph (EEG) activity.At a group level, the amplitude of visual responses did not saturate at high stimulus contrast in patients, as it did in the control subjects. This reflects an abnormality in neuronal gain control. The VEPs did not have sufficient power to reliably distinguish patients from controls at an individual level. Parametric modeling using a standard gain control framework showed that the abnormality lay in reduced inhibition from neighboring neurons rather than increased excitatory response to the stimulus.Visual evoked responses reveal changes in a fundamental mechanism regulating neuronal sensitivity. These changes may give rise to hyperexcitability underlying generalized epilepsy.

    View details for DOI 10.1002/ana.22462

    View details for Web of Science ID 000296396700010

    View details for PubMedID 21710621

  • Orientation tuning in the visual cortex of 3-month-old human infants VISION RESEARCH Baker, T. J., Norcia, A. M., Candy, T. R. 2011; 51 (5): 470-478

    Abstract

    Sensitivity to orientation is critical for making a whole and complete picture of the world. We measured the orientation tuning of mechanisms in the visual cortex of typically developing 3-month-olds and adults using a nonlinear analysis of the two-input steady-state Visually Evoked Potential (VEP). Two gratings, one a fixed test and the other a variable orientation masker were tagged with distinct temporal frequencies and the corresponding evoked responses were measured at the harmonics of the test and masker frequencies and at a frequency equal to the sum of the two stimulus frequencies. The magnitude of the sum frequency component depended strongly on the relative orientation of the test and masker in both infants and adults. The VEP tuning bandwidths of the 3-month-olds measured at the sum frequency were similar to those of adults, suggesting that behavioral immaturities in functions such as orientation discrimination and contour integration may result from other immaturities in long-range lateral projections or feedback mechanisms.

    View details for DOI 10.1016/j.visres.2011.01.003

    View details for Web of Science ID 000288618900002

    View details for PubMedID 21236289

  • Disparity-Tuned Population Responses from Human Visual Cortex JOURNAL OF NEUROSCIENCE Cottereau, B. R., McKee, S. P., Ales, J. M., Norcia, A. M. 2011; 31 (3): 954-965

    Abstract

    We used source imaging of visual evoked potentials to measure neural population responses over a wide range of horizontal disparities (0.5-64 arcmin). The stimulus was a central disk that moved back and forth across the fixation plane at 2 Hz, surrounded either by binocularly uncorrelated dots (disparity noise) or by correlated dots presented in the fixation plane. Both disk and surround were composed of dynamic random dots to remove coherent monocular information. Disparity tuning was measured in five visual regions of interest (ROIs) [V1, human middle temporal area (hMT+), V4, lateral occipital complex (LOC), and V3A], defined in separate functional magnetic resonance imaging scans. The disparity tuning functions peaked between 2 and 16 arcmin for both types of surround in each ROI. Disparity tuning in the V1 ROI was unaffected by the type of surround, but surround correlation altered both the amplitude and phase of the disparity responses in the other ROIs. Response amplitude increased when the disk was in front of the surround in the V3A and LOC ROIs, indicating that these areas encode figure-ground relationships and object convexity. The correlated surround produced a consistent phase lag at the second harmonic in the hMT+ and V4 ROIs without a change in amplitude, while in the V3A ROI, both phase and amplitude effects were observed. Sensitivity to disparity context is thus widespread in visual cortex, but the dynamics of these contextual interactions differ across regions.

    View details for DOI 10.1523/JNEUROSCI.3795-10.2011

    View details for Web of Science ID 000286373700020

    View details for PubMedID 21248120

  • Impaired visual decision-making in individuals with amblyopia JOURNAL OF VISION Farzin, F., Norcia, A. M. 2011; 11 (14)

    Abstract

    This study examined the effects of amblyopia on perceptual decision-making processes to determine the consequences of visual deprivation on the development of higher level cortical networks outside of the visual cortex. A variant of the Eriksen flanker task was used to measure response time and accuracy for decisions made in the presence of response-selection conflict. Performance of adults with amblyopia was compared to that of neurotypical participants of the same age. Additionally, simple and choice reaction time tasks presented in the visual and the auditory modality were used to control for factors such as feature visibility, crowding, and motor execution speed. A selective deficit in response time for visual decisions was found when individuals with amblyopia used either the amblyopic or non-amblyopic (dominant) eye, and this deficit was independent of visual acuity, motor time, and performance accuracy. In trial conditions that provoked response-selection conflict, responses were significantly delayed in amblyopic relative to neurotypical participants and were not subject to standard trial sequence effects. Our results indicate that, beyond the known effects of abnormal visual experience on visual cortex, suboptimal binocular input during a developmental critical period may also impact cortical connections to downstream areas of the brain, including parietal and frontal cortices, that are believed to underlie decision and response-selection processes.

    View details for DOI 10.1167/11.14.6

    View details for Web of Science ID 000298635300006

    View details for PubMedID 22147222

  • V1 is not uniquely identified by polarity reversals of responses to upper and lower visual field stimuli NEUROIMAGE Ales, J. M., Yates, J. L., Norcia, A. M. 2010; 52 (4): 1401-1409

    Abstract

    The cruciform hypothesis states that if a visual evoked potential component originates in V1, then stimuli placed in the upper versus lower visual fields will generate responses with opposite polarity at the scalp. This diagnostic has been used by many studies as a definitive marker of V1 sources. To provide an empirical test of the validity of the cruciform hypothesis, we generated forward models of cortical areas V1, V2 and V3 that were based on realistic estimates of the 3-D shape of these areas and the shape and conductivity of the brain, skull and scalp. Functional MRI was used to identify the location of early visual areas and anatomical MRI data was used to construct detailed cortical surface reconstructions and to generate boundary element method forward models of the electrical conductivity of each participant's head. These two data sets for each subject were used to generate simulated scalp activity from the dorsal and ventral subdivisions of each visual area that correspond to the lower and upper visual field representations, respectively. The predicted topographies show that sources in V1 do not fully conform to the cruciform sign-reversal. Moreover, contrary to the model, retinotopic visual areas V2 and V3 show polarity reversals for upper and lower field stimuli. The presence of a response polarity inversion for upper versus lower field stimuli is therefore an insufficient criterion for identifying responses as having originated in V1.

    View details for DOI 10.1016/j.neuroimage.2010.05.016

    View details for Web of Science ID 000280695200028

    View details for PubMedID 20488247

  • Configural specificity of the lateral occipital cortex NEUROPSYCHOLOGIA Appelbaum, L. G., Ales, J. M., COTTEREAU, B., Norcia, A. M. 2010; 48 (11): 3323-3328

    Abstract

    While regions of the lateral occipital cortex (LOC) are known to be selective for objects relative to feature-matched controls, it is not known what set of cues or configurations are used to promote this selectivity. Many theories of perceptual organization have emphasized the figure-ground relationship as being especially important in object-level processing. In the present work we studied the role of perceptual organization in eliciting visual evoked potentials from the object selective LOC. To do this, we used two-region stimuli in which the regions were modulated at different temporal frequencies and were comprised of either symmetric or asymmetric arrangements. The asymmetric arrangement produced an unambiguous figure-ground relationship consistent with a smaller figure region surrounded by a larger background, while four different symmetric arrangements resulted in ambiguous figure-ground relationships but still possessed strong kinetic boundaries between the regions. The surrounded figure-ground arrangement evoked greater activity in the LOC relative to first-tier visual areas (V1-V3). Response selectivity in the LOC, however, was not present for the four different types of symmetric stimuli. These results suggest that kinetic texture boundaries alone are not sufficient to trigger selective processing in the LOC, but that the spatial configuration of a figure that is surrounded by a larger background is both necessary and sufficient to selectively activate the LOC.

    View details for DOI 10.1016/j.neuropsychologia.2010.07.016

    View details for Web of Science ID 000281890200018

    View details for PubMedID 20638395

  • Quantitative Fiber Tracking of the Optic Radiation Is Correlated with Visual-Evoked Potential Amplitude in Preterm Infants AMERICAN JOURNAL OF NEURORADIOLOGY Glass, H. C., Berman, J. I., Norcia, A. M., Rogers, E. E., Henry, R. G., Hou, C., Barkovich, A. J., Good, W. V. 2010; 31 (8): 1424-1429

    Abstract

    Children born preterm are at risk for adverse outcome, including visual impairment. We examined the relationship between neonatal DTI and sVEP in children born preterm to determine whether visual outcomes are related to early measurements of brain microstructure.Subjects were born at <34 weeks gestation and imaged before term-equivalent age. DTI fiber tracking was used to delineate the optic radiations and measure tract-specific average FA, D(av), and parallel and transverse diffusivity. Visual-evoked response amplitudes were measured as a function of spatial frequency, contrast, and vernier offset size with sVEP at 6-20 months after birth. The association between DTI and sVEP was assessed by using the Spearman correlation coefficient and linear regression for repeated measures.Nine children with 15 scans were included. The peak response amplitudes for spatial frequency sweeps were associated with increasing FA and decreasing D(av) and transverse diffusivity (P ? .006) but not with parallel diffusivity (P = 1). There was only modest association with the swept contrast condition and no detectable association with the vernier offset sweeps.Microstructure of the optic radiations measured shortly after birth is associated with quantitatively measured responses elicited by moderate-to-high contrast spatiotemporal gratings in infancy. These findings are in keeping with studies showing a relationship between brain microstructure and function. While the clinical impact is not known, quantitative neuroimaging of white matter may ultimately be important for predicting outcome in preterm neonates.

    View details for DOI 10.3174/ajnr.A2110

    View details for Web of Science ID 000282804100015

    View details for PubMedID 20448011

  • Connecting the Dots: How Local Structure Affects Global Integration in Infants JOURNAL OF COGNITIVE NEUROSCIENCE Palomares, M., Pettet, M., Vildavski, V., Hou, C., Norcia, A. 2010; 22 (7): 1557-1569

    Abstract

    Glass patterns are moirés created from a sparse random-dot field paired with its spatially shifted copy. Because discrimination of these patterns is not based on local features, they have been used extensively to study global integration processes. Here, we investigated whether 4- to 5.5-month-old infants are sensitive to the global structure of Glass patterns by measuring visual-evoked potentials. Although we found strong responses to the appearance of the constituent dots, we found sensitivity to the global structure of the Glass patterns in the infants only over a very limited range of spatial separation. In contrast, we observed robust responses in the infants when we connected the dot pairs of the Glass pattern with lines. Moreover, both infants and adults showed differential responses to exchanges between line patterns portraying different global structures. A control study varying luminance contrast in adults suggests that infant sensitivity to global structure is not primarily limited by reduced element visibility. Together our results suggest that the insensitivity to structure in conventional Glass patterns is due to inefficiencies in extracting the local orientation cues generated by the dot pairs. Once the local orientations are made unambiguous or when the interpolation span is small, infants can integrate these signals over the image.

    View details for Web of Science ID 000279057500015

    View details for PubMedID 19642888

  • Human neutrophil alloantigen-1a,-1b,-2,-3a and-4a frequencies in Brazilians TISSUE ANTIGENS Norcia, A. M., Sugano, E. Y., Chiba, A. K., Moritz, E., Guirao, F. P., Yamamoto, M., Bordin, J. O. 2009; 74 (5): 404-407

    Abstract

    Human neutrophil reactive antibodies may cause clinical disorders such as transfusion-related acute lung injury, febrile transfusion reactions, alloimmune neonatal neutropenia, immune neutropenia after stem cell transplantation, refractoriness to granulocyte transfusion, drug-induced neutropenia and autoimmune neutropenia. Using the granulocyte immunofluorescence test by flow cytometry, the phenotypic frequencies of the human neutrophil alloantigens (HNA)-1a, -1b, -2, -3a and -4a were determined in 100 healthy Brazilian persons. Neutrophils were separated from blood samples by sedimentation, centrifugated and incubated with HNA-specific alloantibody plus fluorescein isothiocyanate-labeled F(ab')(2) fragments of anti-human IgG. The results showed that the phenotype frequencies of HNA-1a, -1b, -2a, -3a and -4a were 65%, 83%, 97%, 95% and 94%, respectively. We detected that neutrophils from 17% of Brazilians typed positive only with anti-HNA-1a (HNA-1a/a), 35% only with anti-HNA-1b (HNA-1b/b) and 48% reacted with both antibodies (HNA-1a/b). The frequencies found for HNA-1a and -1b were quite similar to that reported among Africans and American-Africans, but different from those found in Japanese and Chinese. In addition, our data showed that the frequencies of HNA-2, -3a and -4a in Brazilians were comparable with those observed in Caucasians. The determination of HNAs frequencies among populations with distinct racial backgrounds is important not only for anthropological reasons, but also for neonatal typing in suspected cases of alloimmune neutropenia or when patients are severely neutropenic.

    View details for DOI 10.1111/j.1399-0039.2009.01357.x

    View details for Web of Science ID 000270833200005

    View details for PubMedID 19737365

  • Spatio-temporal tuning of coherent motion evoked responses in 4-6 month old infants and adults VISION RESEARCH Hou, C., Gilmore, R. O., Pettet, M. W., Norcia, A. M. 2009; 49 (20): 2509-2517

    Abstract

    Motion cues provide a rich source of information about translations of the observer through the environment as well as the movements of objects and surfaces. While the direction of motion can be extracted locally these local measurements are, in general, insufficient for determining object and surface motions. To study the development of local and global motion processing mechanisms, we recorded Visual Evoked Potentials (VEPs) in response to dynamic random dot displays that alternated between coherent rotational motion and random motion at 0.8 Hz. We compared the spatio-temporal tuning of the evoked response in 4-6 months old infants to that of adults by recording over a range of dot displacements and temporal update rates. Responses recorded at the frequency of the coherent motion modulation were tuned for displacement at the occipital midline in both adults in infants. Responses at lateral electrodes were tuned for speed in adults, but not in infants. Infant responses were maximal at a larger range of spatial displacement than that of adults. In contrast, responses recorded at the dot-update rate showed a more similar parametric displacement tuning and scalp topography in infants and adults. Taken together, our results suggest that while local motion processing is relatively mature at 4-6 months, global integration mechanisms exhibit significant immaturities at this age.

    View details for DOI 10.1016/j.visres.2009.08.007

    View details for Web of Science ID 000270861900008

    View details for PubMedID 19679146

  • Evidence for Visual Compromise in Preverbal Children with Orbital Vascular Birthmarks AMERICAN JOURNAL OF OPHTHALMOLOGY Good, W. V., Hou, C., Frieden, I. J., Norcia, A. M. 2009; 147 (4): 679-682

    Abstract

    To learn whether electrophysiological changes indicating amblyopia occur even in the absence of clinically recognizable amblyopia.Prospective study.Four consecutive infants between 7 and 19 months of age with unilateral periocular vascular lesions that intermittently obstructed vision in the affected eye and no clinical evidence of amblyopia were evaluated. No child had anisometropia greater than 0.50 diopter in the greatest meridian or strabismus. Sweep visual evoked potential vernier acuity was measured under monocular viewing conditions with the fellow eye tested as the control.Response amplitudes and acuity thresholds were significantly diminished in the affected eyes. A phase analysis showed slowing of the response in the affected eyes compared with the control eyes.An amblyopia-like effect on vernier acuity occurred in infants with unilateral periocular vascular birthmarks when the lesion caused intermittent occlusion of the eye. Whether long-term effects will occur is unknown, but children with no clinically apparent amblyopia in the setting of a vascular mark or other cause of intermittent occlusion of the visual axis should be followed, since these electrophysiology findings suggest amblyopia may be present.

    View details for DOI 10.1016/j.ajo.2008.11.007

    View details for Web of Science ID 000264566600018

    View details for PubMedID 19195640

  • Disparity Tuning of Binocular Facilitation and Suppression after Normal versus Abnormal Visual Development INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Norcia, A. M., Hale, J., Pettet, M. W., McKee, S. P., Harrad, R. A. 2009; 50 (3): 1168-1175

    Abstract

    To study the pattern of facilitatory and suppressive binocular interactions in stereodeficient patients with strabismus and in healthy controls.Visual evoked potentials were recorded in response to a Vernier onset/offset pattern presented to one eye, either monocularly or paired dichoptically with a straight vertical square-wave grating, which, when fused with the target in the other eye, gave rise to a percept of a series of bands appearing in depth from an otherwise uniform plane or with a grating that contained offsets that produced a standing disparity and the appearance of a constantly segmented image, portions of which moved in depth.Participants with normal stereopsis showed facilitative and suppressive binocular interactions that depended on which dichoptic target was presented. Patients with longstanding, constant strabismus lacked normal facilitative binocular interactions. The response to a normally facilitative stimulus was reduced below the monocular level when it was presented to the dominant eye of patients without anisometropia, consistent with classical strabismic suppression of the nondominant eye. The dominant eye of strabismic patients without anisometropia retained suppressive input from crossed but not uncrossed disparity stimuli presented to the nondominant eye.Abnormal disparity processing can be detected with the dichoptic VEP method we describe. Our results suggest that suppression in stereoblind, nonamblyopic observers is determined by a binocular mechanism responsive to disparity. In some cases, the sign of the disparity is important, and this suggests a mechanism that can explain diplopia in patients made exotropic after surgery for esotropia.

    View details for DOI 10.1167/iovs.08-2281

    View details for Web of Science ID 000263665000026

    View details for PubMedID 19098323

  • Assessing direction-specific adaptation using the steady-state visual evoked potential: Results from EEG source imaging JOURNAL OF VISION Ales, J. M., Norcia, A. M. 2009; 9 (7)

    Abstract

    Studying directional selectivity using neuroimaging in humans is difficult because the resolution is insufficient to directly access directionally selective activity. Here we used motion adaptation of the steady-state visual evoked potential (SSVEP) and source imaging in the frequency domain to detect brain areas that contain direction-selective cells. This study uses a definitive electrophysiological marker for direction-specific adaptation in the SSVEP to localize cortical areas that are direction selective. It has been shown previously that an oscillating stimulus produces an SSVEP response that is dominated by even harmonics of the stimulus frequency. This pattern of response is consistent with equal population responses to each direction of motion. Prolonged exposure to unidirectional motion induces an asymmetry in the population response that is consistent with adaptation of direction-selective cells. This asymmetry manifests itself in the presence of odd harmonic components after adaptation Critically, the feature that indicates the direction used for adaptation is the phase of the odd-harmonic responses. We recorded this signature of direction selectivity in a group of observers whose retinotopic visual areas had been defined from fMRI mapping. We find direction-specific responses throughout retinotopic cortex, with the largest effect in areas V1 (occipital pole) and V3/V3a (dorsal).

    View details for DOI 10.1167/9.7.8

    View details for Web of Science ID 000269824300008

    View details for PubMedID 19761323

  • Attentive and pre-attentive aspects of figural processing JOURNAL OF VISION Appelbaum, L. G., Norcia, A. M. 2009; 9 (11)

    Abstract

    Here we use the steady-state visual evoked potential (SSVEP) to study attentive versus non-attentive processing of simple texture-defined shapes. By "tagging" the figure and background regions with different temporal frequencies, the method isolates response components associated with the figure region, the background region, and with non-linear spatio-temporal interactions between regions. Each of these response classes has a distinct scalp topography that is preserved under differing attentional task demands. In one task, attention was directed to discrimination of shape changes in the figure region. In the other task, a difficult letter discrimination was used to divert attentive processing resources away from the texture-defined form. Larger task-dependent effects were observed for figure responses and for the figure/background interaction than for the background responses. The figure region responses were delayed in occipital areas in the shape versus letter task conditions, while the region interactions were enhanced, especially in frontal areas. While a basic differentiation of figure from background processing occurred independent of task, attentive processing of elementary shapes recruited later occipital activity for figure processing and sustained non-linear figure/background interaction in frontal areas. Collectively, these results indicate that basic aspects of scene segmentation proceed pre-attentively, but that directed attention to the object shape engages a widely distributed network of brain areas including frontal and occipital regions.

    View details for DOI 10.1167/9.11.18

    View details for Web of Science ID 000273280700018

    View details for PubMedID 20053081

  • Figure-ground interaction in the human visual cortex JOURNAL OF VISION Appelbaum, L. G., Wade, A. R., Pettet, M. W., Vildavski, V. Y., Norcia, A. M. 2008; 8 (9)

    Abstract

    Discontinuities in feature maps serve as important cues for the location of object boundaries. Here we used multi-input nonlinear analysis methods and EEG source imaging to assess the role of several different boundary cues in visual scene segmentation. Synthetic figure/ground displays portraying a circular figure region were defined solely by differences in the temporal frequency of the figure and background regions in the limiting case and by the addition of orientation or relative alignment cues in other cases. The use of distinct temporal frequencies made it possible to separately record responses arising from each region and to characterize the nature of nonlinear interactions between the two regions as measured in a set of retinotopically and functionally defined cortical areas. Figure/background interactions were prominent in retinotopic areas, and in an extra-striate region lying dorsal and anterior to area MT+. Figure/background interaction was greatly diminished by the elimination of orientation cues, the introduction of small gaps between the two regions, or by the presence of a constant second-order border between regions. Nonlinear figure/background interactions therefore carry spatially precise, time-locked information about the continuity/discontinuity of oriented texture fields. This information is widely distributed throughout occipital areas, including areas that do not display strong retinotopy.

    View details for DOI 10.1167/8.9.8

    View details for Web of Science ID 000258709300008

    View details for PubMedID 18831644

  • Neural correlates of transformational apparent motion PERCEPTION Mirabella, G., Norcia, A. M. 2008; 37 (9): 1368-1379

    Abstract

    Transformational apparent motion (TAM) arises when a shape that is abruptly flashed on and off next to a static shape of similar color or texture appears as a protrusion that extends and retracts smoothly from the static object. Here we report that the strength of the TAM percept can be predicted from the waveform of visual evoked potentials (VEPs) measured while observers rated their percepts. The VEPs at pattern onset and offset are maximally symmetric when the static inducer and the flashing patches of the display are of the same contrast. VEP symmetry is affected by how the two patches can be matched as a single surface and may reflect the relative contribution of different motion and object detection systems in visual cortex.

    View details for DOI 10.1068/p5972

    View details for Web of Science ID 000260949900005

    View details for PubMedID 18986063

  • Abnormalities of coherent motion processing in strabismic amblyopia: Visual-evoked potential measurements JOURNAL OF VISION Hou, C., Pettet, M. W., Norcia, A. M. 2008; 8 (4)

    Abstract

    Coherent motion responses of patients with mild to moderate strabismic amblyopia were compared to those of normals using visual-evoked potentials (VEPs). Responses were elicited by dynamic random-dot kinematograms that alternated at 0.83 Hz between globally coherent (left-right) and incoherent (random) motion states. Tuning curves were measured at the first harmonic of the global motion update rate (0.83 Hz) and at the first harmonic of the dot update rate (20 Hz) for spatial displacements 3.1 to 27.9 arcmin (1.6 to 9.3 deg/s). Responses locked to the changes in the global organization of the local direction vectors were an inverted U-shaped function of displacement/speed in the normal-vision observers and in the fellow eyes of the strabismus patients while the tuning function of the amblyopic eyes was shifted to larger displacements/higher speeds. Responses at the dot update rate were reduced in amplitude and altered in timing in both eyes of the patients. The results are consistent with both local and global deficits in motion processing in strabismic amblyopia.

    View details for DOI 10.1167/8.4.2

    View details for Web of Science ID 000255976000002

    View details for PubMedID 18484841

  • Development of cortical responses to optic flow VISUAL NEUROSCIENCE Gilmore, R. O., Hou, C., Pettet, M. W., Norcia, A. M. 2007; 24 (6): 845-856

    Abstract

    Humans discriminate approaching objects from receding ones shortly after birth, and optic flow associated with self-motion may activate distinctive brain networks, including the human MT+ complex. We sought evidence for evoked brain activity that distinguished radial motion from other optic flow patterns, such as translation or rotation by recording steady-state visual evoked potentials (ssVEPs), in both adults and 4-6 month-old infants to direction-reversing optic flow patterns. In adults, radial flow evoked distinctive brain responses in both the time and frequency domains. Differences between expansion/contraction and both translation and rotation were especially strong in lateral channels (PO7 and PO8), and there was an asymmetry between responses to expansion and contraction. In contrast, infants' evoked response waveforms to all flow types were equivalent, and showed no evidence of the expansion/contraction asymmetry. Infants' responses were largest and most reliable for the translation patterns in which all dots moved in the same direction. This pattern of response is consistent with an account in which motion processing systems detecting locally uniform motion develop earlier than do systems specializing in complex, globally non-uniform patterns of motion, and with evidence suggesting that motion processing undergoes prolonged postnatal development.

    View details for DOI 10.1017/S0952523807070769

    View details for Web of Science ID 000251977300008

    View details for PubMedID 18093371

  • Validation study of VEP vernier acuity in normal-vision and amblyopic adults INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Hou, C., Good, W. V., Norcia, A. M. 2007; 48 (9): 4070-4078

    Abstract

    Vernier displacement thresholds can be measured with swept-parameter visual evoked potentials (sVEPs) and may therefore be useful in pre- or nonverbal subjects. This study was conducted to test whether sVEP vernier thresholds are valid measures of the visibility of vernier offsets in two different settings.Vernier acuity thresholds were measured psychophysically and electrophysiologically using square-wave gratings containing vernier displacements modulated at 3.76 Hz. The detectability of the vernier alignment cue was degraded by introducing either gaps or standing offsets in the stimulus. These manipulations were performed in normal-vision observers. In a second experiment, psychophysical and sVEP vernier acuity were measured in amblyopic observers.sVEP thresholds and overall amplitudes in normal observers were strongly affected by the introduction of gaps or standing offsets, as were psychophysical thresholds. Psychophysical and sVEP vernier offset thresholds were significantly correlated in the amblyopic eyes, as were sVEP and optotype interocular threshold differences. sVEP amplitudes of patients with strabismus were lower than those of patients with anisometropic amblyopia, even though optotype acuities were the same in the two groups.Vernier acuity thresholds derived from the sVEP tap mechanisms that are specific for the relative position of stimulus elements, and they correlate with perceptual visibility in normal and amblyopic observers. Because of this correlation and because sVEP thresholds can be measured without the need for instruction or behavioral responses, they may be useful in assessing visual function in pre- and nonverbal patients.

    View details for DOI 10.1167/iovs.06-1368

    View details for Web of Science ID 000249061900024

    View details for PubMedID 17724189

  • Sensitivity and configuration-specificity of orientation-defined texture processing in infants and adults VISION RESEARCH Pei, F., Pettet, M. W., Norcia, A. M. 2007; 47 (3): 338-348

    Abstract

    Here we use textures made up of widely spaced Gabor patches to compare infant and adult sensitivity to the global organization of the elements comprising the textures. Visual Evoked Potentials (VEPs) were recorded to alterations between random images and images containing varying proportions of patches that were of the same orientation. The patches were placed on rectangular, hexagonal or random lattices. Texture-specific responses were robust in adults and their VEP threshold was reached when 1-17% of the patches had the same orientation in the structured image. Infant thresholds were approximately 20-60%. While infants are capable of detecting the global structure of our textures, their sensitivity is low. In adults we found, unexpectedly, that sensitivity and response gain were higher for horizontal compared to vertical global orientations. Infant sensitivity was the same for the two orientations. Comparable orientation anisotropies have not been previously reported for gratings, suggesting that the Gabor-defined textures are tapping different mechanisms. There were small, but measurable effects of the lattice type in adults, with the rectangular lattice producing the largest responses.

    View details for DOI 10.1016/j.visres.2006.10.005

    View details for Web of Science ID 000244016000006

    View details for PubMedID 17188321

  • Cue-invariant networks for figure and background processing in human visual cortex JOURNAL OF NEUROSCIENCE Appelbaum, L. G., Wade, A. R., Vildavski, V. Y., Pettet, M. W., Norcia, A. M. 2006; 26 (45): 11695-11708

    Abstract

    Lateral occipital cortical areas are involved in the perception of objects, but it is not clear how these areas interact with first tier visual areas. Using synthetic images portraying a simple texture-defined figure and an electrophysiological paradigm that allows us to monitor cortical responses to figure and background regions separately, we found distinct neuronal networks responsible for the processing of each region. The figure region of our displays was tagged with one temporal frequency (3.0 Hz) and the background region with another (3.6 Hz). Spectral analysis was used to separate the responses to the two regions during their simultaneous presentation. Distributed source reconstructions were made by using the minimum norm method, and cortical current density was measured in a set of visual areas defined on retinotopic and functional criteria with the use of functional magnetic resonance imaging. The results of the main experiments, combined with a set of control experiments, indicate that the figure region, but not the background, was routed preferentially to lateral cortex. A separate network extending from first tier through more dorsal areas responded preferentially to the background region. The figure-related responses were mostly invariant with respect to the texture types used to define the figure, did not depend on its spatial location or size, and mostly were unaffected by attentional instructions. Because of the emergent nature of a segmented figure in our displays, feedback from higher cortical areas is a likely candidate for the selection mechanism by which the figure region is routed to lateral occipital cortex.

    View details for DOI 10.1523/JNEUROSCI.2741-06.2006

    View details for Web of Science ID 000241892700021

    View details for PubMedID 17093091

  • Visual development in very low birth weight infants PEDIATRIC RESEARCH Mirabella, G., Kjaer, P. K., Norcia, A. M., Good, W. V., Madan, A. 2006; 60 (4): 435-439

    Abstract

    Extremely preterm infants are at risk for neurodevelopmental problems and the visual system is particularly vulnerable. However, development of visual function in preterm infants with little or no retinal or neurologic injury has not been well defined. This study compared development of visual function in preterm infants without severe retinopathy of prematurity (ROP), intraventricular hemorrhage (IVH) or periventricular leukomalacia (PVL) to that of term infants at 5-7 mo corrected age. Twenty-one very low birth weight (VLBW) preterm infants (24-32 wk gestational age, weighing < 1500 g), and 22 healthy term infants were tested at 5-7 mo corrected age. Infants with any IVH/PVL and > Stage II ROP or Plus disease were excluded. Contrast sensitivity, grating acuity, and vernier acuity were measured using swept-parameter visual evoked potentials. Thresholds and maximum amplitudes were compared between groups. VLBW and term infants showed no differences in sensitivity for contrast (67.5 versus 63.8), grating resolution (12.4 versus 12.5 cpd) or vernier acuity (1.2 versus 1.0 arcmin). However, the amplitudes for swept contrast (p < 0.03) and swept vernier offset (p < 0.04) stimuli were higher in VLBW infants. Visual thresholds in VLBW infants without serious retinal or neurologic abnormalities were not significantly different from those of term infants, suggesting that increased visual experience does not influence visual sensitivity. The higher amplitudes in VLBW infants, suggests that visual experience may affect responses to suprathreshold stimuli.

    View details for DOI 10.1203/01.pdr.0000238249.44088.2c

    View details for Web of Science ID 000240707500012

    View details for PubMedID 16940247

  • Neural correlates of shape-from-shading VISION RESEARCH Hou, C., Pettet, M. W., Vildavski, V. Y., Norcia, A. M. 2006; 46 (6-7): 1080-1090

    Abstract

    Visual evoked potentials were recorded during presentation of a single stimulus that generated bi-stable perceptual alternation between two different three-dimensional percepts. One interpretation (asymmetric) changed depth structure from flat to corrugated in depth and the other (symmetric) had the appearance of a flat surface translating laterally behind a set of apertures. Responses during perception of the asymmetric three-dimensional structure contained larger negative components than did responses during perception of the symmetric three-dimensional structure. Control experiments suggest that the interpretation of depth structure is selected after junction information caused by the interplay between shading and object shape is extracted.

    View details for DOI 10.1016/j.visres.2005.10.017

    View details for Web of Science ID 000236073400032

    View details for PubMedID 16321419

  • The repeatability of best corrected acuity in normal and amblyopic children 4 to 12 years of age INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Chen, S. I., Chandna, A., Norcia, A. M., Pettet, M., Stone, D. 2006; 47 (2): 614-619

    Abstract

    The main purpose of this work was to measure repeatability of line-by-line logMAR (logarithm of the minimum angle of resolution) acuity in normal and amblyopic children, while adequately controlling for optical defocus.The Lea Symbols Chart is a constant-crowding, equal-logMAR increment chart similar in design to the Early Treatment Diabetes Retinopathy Study [ETDRS] chart. LogMAR visual acuity was tested twice in each eye of 32 amblyopic and 11 normal children. Each test commenced with screening in which one of the three central symbols was chosen for identification starting with the 1.0- or 0.9-logMAR line, progressing to every second line until incorrect identification occurred. Symbol-by-symbol presentation then commenced at the logMAR line containing the last correctly identified symbol. The threshold was recorded as the last logMAR line where four of four or four of five correct responses occurred (i.e., line-by-line scoring). Retesting by the same examiner was identical and occurred within the same session.There was no significant difference in repeatability among normal, fellow, or amblyopic eyes. The difference between test and retest thresholds lay between +/-0.10 logMAR in 93% of eyes. The 95% limits of agreement for the difference was +/-0.18 logMAR. Repeatability in eyes tested first did not differ from that in those tested second in either the normal or amblyopic groups.In the age-group tested, the line-by-line method of threshold scoring compares favorably with previous reports of both line-by-line and interpolated threshold scoring. There was no clinically meaningful difference in repeatability between the normal and amblyopic children tested.

    View details for DOI 10.1167/iovs.05-0610

    View details for Web of Science ID 000234922100022

    View details for PubMedID 16431958

  • Measurement of position acuity in strabismus and amblyopia: Specificity of the vernier VEP paradigm INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Chen, S. I., Norcia, A. M., Pettet, M. W., Chandna, A. 2005; 46 (12): 4563-4570

    Abstract

    An objective measure of positional acuity is desirable in the nonverbal clinical population. This study was conducted to investigate the specificity of the vernier VEP as a measure of positional acuity, evaluating the potential confound of asymmetric motion responses that may be present in some groups of patients. These motion responses could masquerade as position-specific responses, since they occur at the same response frequency as the vernier-related response.Twelve observers with early-onset esotropia (EOE), 30 children with untreated amblyopia, and 15 control children underwent swept vernier VEP acuity testing accompanied by a swept motion control stimulus. The control condition was used to detect the presence of artifactual responses not related to position sensitivity. The patients with EOE were selected for high levels of motion asymmetry as documented with oscillating gratings presented monocularly. As a measure of motion confound (penetration), the proportion of first-harmonic responses recorded in the control condition was determined.The penetration rate in the vernier condition in each study group (EOE: 0.93%; amblyopes: 4.26%; normal subjects: 2.40%) and the entire group (2.85%) was acceptably low. The level of penetration was not significantly influenced by the presence of amblyopia.The vernier VEP paradigm, when applied in the manner described, can be interpreted as a measure of position sensitivity. The presence of motion asymmetry or untreated amblyopia does not affect the validity of vernier measurements made.

    View details for DOI 10.1167/iovs.05-0792

    View details for Web of Science ID 000233578600031

    View details for PubMedID 16303949

  • Event-related potentials show configural specificity of global form processing NEUROREPORT Pei, F., Pettet, M. W., Vildavski, V. Y., Norcia, A. M. 2005; 16 (13): 1427-1430

    Abstract

    Glass patterns are a type of moiré created when a random-dot field is overlaid with a rotated, translated or dilated copy. The overall form of the moiré cannot be detected using local processing mechanisms, and because of this, Glass patterns are useful probes of global form processing. Here, we use event-related potentials to show that certain global organizations (concentric structure created by rotation and radial structure produced by dilation) produce much larger brain responses than others (linear structure created by translation). The results are consistent with the existence of specialized form processing mechanisms in the extrastriate cortex.

    View details for Web of Science ID 000232456900004

    View details for PubMedID 16110264

  • A VEP measure of the binocular fusion of horizontal and vertical disparities INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Hale, J., Harrad, R. A., MCKEE, S. P., Pettet, M. W., Norcia, A. M. 2005; 46 (5): 1786-1790

    Abstract

    Because of the lateral separation of the orbits, the retinal images differ in the two eyes. These differences are reconciled into a single image through sensory and motor fusional mechanisms. This study demonstrates electrophysiologically the effects that normal horizontal and vertical fusional processes have on the processing of monocular position signals.VEPs were recorded in 16 healthy adults in response to a vernier onset-offset target presented to one eye. The vernier offsets appeared and disappeared at 2 Hz and were introduced into bar targets that were oriented either vertically (horizontal offsets) or horizontally (vertical offsets). The magnitude of the offsets was varied over the range of 0.5 to 10 arc min. VEP amplitude was measured as a function of the size of the dynamic offset under monocular viewing conditions and in the presence of two different static targets presented to the other eye. One of the static targets matched the dynamic test, except that it had no vernier offsets. The other static target, the static pedestal, matched the dynamic test, but contained a set of static vernier offsets in locations corresponding to the locations of the dynamic offsets presented to the other eye.VEP amplitude was a monotonically increasing function of vernier offset size under monocular viewing conditions. The addition of the static target without offsets in the other eye resulted in an increased amplitude VEP response. The addition of the static target with vernier offsets resulted in a decrease in VEP amplitude for both horizontal and vertical disparities.The normal process of fusion results in a single visual direction. To obtain a single visual direction, the visual system must synthesize a binocular visual direction that differs from the monocular components. One of the conditions (the static pedestal with offsets) produces binocular visual direction shifts that degrade the appearance of vernier onset-offset, and reduce VEP amplitude for both horizontal and vertical disparities. This characteristic evoked response marker is a promising tool for measuring binocular fusion objectively in patients with strabismus.

    View details for DOI 10.1167/iovs.04-0954

    View details for Web of Science ID 000228708000037

    View details for PubMedID 15851583

  • Development of sensitivity to texture and contour information in the human infant JOURNAL OF COGNITIVE NEUROSCIENCE Norcia, A. M., Pei, F., Bonneh, Y., Hou, C., Sampath, V., Pettet, M. W. 2005; 17 (4): 569-579

    Abstract

    Texture discrimination and bounding contour extraction are essential parts of the object segmentation and shape discrimination process. As such, successful texture and contour processing are key components underlying the development of the perception of both objects and surfaces. By recording visual-evoked potentials, we investigate whether young infants can detect orientation-defined textures and contours. We measured responses to an organized texture comprised of many Gabor patches of the same orientation, alternated with images containing the same number of patches, but all of random orientation. These responses were compared with a control condition consisting of the alternation between two independently random configurations. Significant difference potentials were found as early as 2-5 months, as were significant odd harmonics in the test conditions. Responses were also measured to Gabor patches organized either as circles (all patches tangent to an imaginary circular path) alternated with pinwheels (all patches having a fixed orientation offset from the path). Infants between 6 and 13 months also showed sensitivity to the global organization of the elements along contours. Differential responses to our texture and contour stimuli and their controls could only have been generated by mechanisms that are capable of comparing the relative orientation of 2 or more patches, as no local information at a single patch distinguished the random and organized textures or the circle and pinwheel configurations.

    View details for Web of Science ID 000228360600003

    View details for PubMedID 15829078

  • Suppression of monocular visual direction under fused binocular stimulation: Evoked potential measurements JOURNAL OF VISION Norcia, A. M., MCKEE, S. P., Bonneh, Y., Pettet, M. W. 2005; 5 (1): 34-44

    Abstract

    Visual evoked potentials (VEPs) were recorded in response to a vernier onset/offset target presented to one eye that was combined with matching static targets in the other eye. The monocular response was dominated by a negative peak at 160 ms that occurred after a set of offsets was introduced into a one-dimensional random bar pattern. The static targets produced no discernible VEP response by themselves, but when fused binocularly with the oscillating vernier target, they produced shifts in perceived visual direction that influenced the VEP response. A vernier target fused with static vertical bars was perceived to alternate in depth between a flat surface and one broken into two interleaved surfaces. The response to this "surface-breaking" was as large or larger than the response to the monocular vernier offset. This response was much reduced when the oscillating vernier was fused with a static offset vernier (5' offset) that produced a percept of segregated regions moving in depth. Apparently, the VEP is strongly driven by shifts in visual direction that alter surface, texture, or contour contiguity.

    View details for DOI 10.1167/5.1.4

    View details for Web of Science ID 000227037600004

    View details for PubMedID 15831065

  • Experience-expectant development of contour integration mechanisms in human visual cortex JOURNAL OF VISION Norcia, A. M., Sampath, V., Hou, C., Pettet, M. W. 2005; 5 (2): 116-130

    Abstract

    Extended contours are a common feature of natural images. Most previous studies have considered contour integration as a two-dimensional process of linking like-oriented elements along their common orientation axis. Yet contours exist in a three-dimensional world, and one might therefore ask about the relationship between contour integration and binocular vision. Using an event-related potential assay of contour integration, we demonstrate that patients with strabismic amblyopia show a relative insensitivity to Gabor-defined contours in their dominant eyes, all of which had normal acuity. These deficits were not seen in the dominant eyes of patients with anisometropic amblyopia without strabismus, but were present in the amblyopic eyes of patients with either strabismus or anisometropia. Deficits were also found in both eyes of strabismus patients who had normal visual acuity in each eye, but who had strongly reduced or absent stereopsis. These results suggest that the maturation of contour detection mechanisms depends at least in part on the presence of normal binocular interaction during a developmental critical period.

    View details for DOI 10.1167/5.2.3

    View details for Web of Science ID 000227249100003

    View details for PubMedID 15831072

  • Recovery of contour integration in relation to LogMAR visual acuity during treatment of amblyopia in children INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Chandna, A., Gonzalez-Martin, J. A., Norcia, A. M. 2004; 45 (11): 4016-4022

    Abstract

    In several studies, researchers have found that integration of orientation information along contours defined by Gabor patches is abnormal in patients with strabismus and in untreated patients with anisometropic amblyopia. In this study, the rate and degree of recovery of contour-integration deficits were compared with the recovery of logMAR (logarithm of the minimum angle of resolution) visual acuity deficits in patients newly diagnosed with amblyopia secondary to anisometropia, strabismus, or both.Contour-detection thresholds and optotype acuity were measured in 17 newly diagnosed anisometropic amblyopes, in 6 patients with strabismic amblyopia, and in 4 patients with combined anisometropic and strabismic amblyopia. Contour-detection thresholds were measured with a card-based procedure. Treatment comprised full refractive correction and full-time total occlusion therapy, when necessary. Visual function was measured at monthly visits during the course of treatment, with an average follow-up period of 16 weeks (12-24 weeks) for the entire group. Complete data were obtained from 23 patients through 8 weeks of follow-up.Significant interocular differences in contour-detection thresholds were present in 16 of the 27 patients at the first visit after initial refractive correction. Interocular differences in contour-detection thresholds declined to normal levels in most of the patients within 8 weeks of the initiation of treatment. Interocular acuity differences remained significant in many of the patients (19/23) at 8 weeks of follow-up and continued to decline, but did not fully normalize, over the remainder of the follow-up period.Refractive correction alone or in combination with occlusion therapy produces a normalization of contour-integration thresholds in amblyopia that is more rapid and complete than that achieved for visual acuity.

    View details for DOI 10.1167/iovs.03-0795

    View details for Web of Science ID 000224678200024

    View details for PubMedID 15505051

  • Development of the spatial organization and dynamics of lateral interactions in the human visual system JOURNAL OF NEUROSCIENCE Hou, C., Pettet, M. W., Sampath, V., Candy, T. R., Norcia, A. M. 2003; 23 (25): 8630-8640

    Abstract

    Psychophysical thresholds and neuronal responses for isolated stimuli are strongly modified by nearby stimuli in the visual field. We studied the orientation and position specificity of these contextual interactions using a dual-frequency visual-evoked potential technique in developing human infants and adults. One set of small, oriented stimulus elements (targets) was tagged with a temporal frequency f1 of 4.52 Hz. The addition of an abutting second set of similar patches (flankers) tagged at f2 = 2.58 Hz had three effects: (1) The flankers reduced the second and fourth harmonic responses to the targets. This reduction was independent of flanker orientation or position and age. (2) The response to the combination of targets and flankers also contained nonlinear interaction terms (1f1 +/- 1f2) that were tuned for flanker orientation and position in adults, but only for flanker orientation in infants 8-31 weeks of age. (3) Nonlinear interaction terms recorded at 2f1 +/- 2f2 were large and untuned for flanker orientation and position in adults but were nearly absent in the youngest infants. The three forms of nonlinear interaction, thus, have differences in sensitivity to flanker orientation and position and differential growth trends, indicating that they are generated by different mechanisms. These three forms of interaction could serve different functional roles. The first process provides a nonselective gain control that is fully functional in early infancy. The second process, which develops slowly, is selective for the specific form of the stimuli. The third process, which is also immature, pools across orientation.

    View details for Web of Science ID 000185534500002

    View details for PubMedID 14507962

  • Late maturation of visual hyperacuity PSYCHOLOGICAL SCIENCE Skoczenski, A. M., Norcia, A. M. 2002; 13 (6): 537-541

    Abstract

    We used a visual evoked-potential measure to study the development of two components of pattern vision, vernier acuity and grating acuity, in humans from early infancy through adolescence. These two visual functions develop at similar rates and have nearly the same absolute values between 1 month and 6 years of age. After age 6, grating acuity is constant at the adult level, but vernier acuity continues to improve, becoming a hyperacuity. Vernier acuity reaches asymptotic levels around age 14 years. These results suggest that adultlike vernier hyperacuity is not limited by spatial resolution or sensitivity of small receptive fields, but rather that the limitation is imposed by higher-level processing. Sensitivity, connections in visual cortical areas, or both therefore retain plasticity throughout childhood and into adolescence.

    View details for Web of Science ID 000178993800008

    View details for PubMedID 12430838

  • Temporal dynamics of the human response to symmetry JOURNAL OF VISION Norcia, A. M., Candy, T. R., Pettet, M. W., Vildavski, V. Y., Tyler, C. W. 2002; 2 (2): 132-139

    Abstract

    Symmetry is a highly salient feature of animals, plants, and the constructed environment. Although the perceptual phenomenology of symmetry processing is well understood, little is known about the underlying neural mechanisms. Here we use visual evoked potentials to measure the time course of neural events associated with the extraction of symmetry in random dot fields. We presented sparse random dot patterns that were symmetric about both the vertical and horizontal axes. Symmetric patterns were alternated with random patterns of the same density every 500 msec, using new exemplars of symmetric and random patterns on each image update. Random/random exchanges were used as a control. The response to updates of random patterns was multiphasic, consisting of P65, N90, P110, N140 and P220 peaks. The response to symmetric/random sequences was indistinguishable from that for random/random sequences up to about 220 msec, after which the response to symmetric patterns became relatively more negative. Symmetry in random dot patterns thus appears to be extracted after an initial response phase that is indifferent to configuration. These results are consistent with the hypothesis (Lee, Mumford, Romero, & Lamme, 1998; Tyler & Baseler, 1998) that the symmetry property is extracted by processing in extrastriate cortex.

    View details for DOI 10.1167/2.2.1

    View details for Web of Science ID 000206907600001

    View details for PubMedID 12678588

  • Neural correlates of object-based attention JOURNAL OF VISION Pei, F., Pettet, M. W., Norcia, A. M. 2002; 2 (9): 588-596

    Abstract

    Much research has been directed toward disentangling the "units" of attention: Is attention directed to locations in space, visual objects, or to individual features of an object? Moreover, there is considerable interest in whether attention increases the gain of neural mechanisms (signal enhancement) or acts by other means, such as reducing noise or narrowing channel tuning. To address these questions, we used a direct measure of signal strength: the amplitude of visual evoked potentials and a task in which selection could be based on a depth order cue but not on location. Attended and nonattended stimuli were presented at different temporal frequencies, and, thus, responses to the two stimuli could be analyzed separately even though they were presented simultaneously. Attention increased the amplitude of the second harmonic component of the response, but not the fourth harmonic. In addition, responses measured at the second harmonic, but not at the fourth harmonic, were larger for stimuli seen as behind. The results are consistent with the fourth harmonic being generated at a stage of processing that is not accessible to attention and where depth order has not been extracted. The second harmonic, on the other hand, is modifiable by attention and shows evidence for differential encoding of depth order.

    View details for DOI 10.1167/2.9.1

    View details for Web of Science ID 000206908200001

    View details for PubMedID 12678630

  • Normalization models applied to orientation masking in the human infant JOURNAL OF NEUROSCIENCE Candy, T. R., Skoczenski, A. M., Norcia, A. M. 2001; 21 (12): 4530-4541

    Abstract

    Human infants can discriminate the orientation of lines within the first week after birth (Atkinson et al., 1988; Slater et al., 1988) but have immature orientation-selective pattern masking until after 6 months of age (Morrone and Burr, 1986). Here the development of orientation processing is further examined using a visual-evoked potential paradigm and normalization models of pattern masking. Contrast response functions were measured for 1 cycle per degree (cpd) gratings, counterphase-reversed in contrast at either 3.3 or 5.5 Hz. A second 1 cpd, 20% contrast, 8.3 Hz grating of either the same or orthogonal orientation was added as a mask. Evoked responses associated with the test grating, the mask, and intermodulation between the two were individually extracted using spectral analysis of the scalp-recorded EEG. Adults exhibited orientation selectivity in the masking of their test component responses and in nonlinear intermodulation between the test and mask stimuli. Infants <5 months old, however, demonstrated nonselective masking or a reversed selectivity in their responses to the test component, with adult-like orientation selectivity in their intermodulation responses. Within the context of a normalization model of pattern masking, the results are consistent with the existence of oriented filters early in life the responses of which are normalized immaturely until approximately 5 months of age.

    View details for Web of Science ID 000169279900047

    View details for PubMedID 11404441

  • Colinear facilitation promotes reliability of single-cell responses in cat striate cortex EXPERIMENTAL BRAIN RESEARCH Kasamatsu, T., Polat, U., Pettet, M. W., Norcia, A. M. 2001; 138 (2): 163-172

    Abstract

    Behavior is controlled by neural activity in the brain. The final outcome of this neural control may critically depend on the firing reliability of individual neurons. A nearly constant, proportional relationship is usually found between the response mean and response variance. Here we asked whether lateral interactions within striate cortex that modulate response magnitude also proportionately modify the response variance of cortical neurons. In many cases, response variability depended on stimulus organization: discrete flankers colinearly placed well outside the neuron's receptive field increased response magnitude without a proportional increase in variance, thus improving the neuron's response reliability. Since colinear flanker facilitation is often seen near the neuron's firing threshold, increased response reliability for weak stimuli may contribute to enhancing perceptual saliency.

    View details for Web of Science ID 000168951000003

    View details for PubMedID 11417457

  • Contour integration deficits in anisometropic amblyopia INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Chandna, A., Pennefather, P. M., Kovacs, I., Norcia, A. M. 2001; 42 (3): 875-878

    Abstract

    Previous retrospective studies have found that integration of orientation information along contours defined by Gabor patches is abnormal in strabismic, but not in anisometropic, amblyopia. This study was conducted to reexamine the question of whether anisometropic amblyopes have contour integration deficits prospectively in an untreated sample, to isolate the effects of the disease from the effects of prior treatment-factors that may have confounded the results in previous retrospective studies.Contour detection thresholds, optotype acuity, and stereoacuity were measured in a group of 19 newly diagnosed anisometropic amblyopes before initiation of occlusion therapy. Contour detection thresholds were measured using a card-based procedure.Significant interocular differences in contour detection thresholds were present in 14 of the 19 patients with anisometropic amblyopia.Contour integration deficits are a common, but not universal, finding in untreated anisometropic amblyopia. Differences in the prevalence of contour integration deficits between the present study and that of another study may lie in differences in treatment history and/or in the sensitivity of the two different contour integration tasks.

    View details for Web of Science ID 000167206600047

    View details for PubMedID 11222553

  • Changes in cortical activity during suppression in stereoblindness NEUROREPORT Norcia, A. M., Harrad, R. A., Brown, R. J. 2000; 11 (5): 1007-1012

    Abstract

    Patients with strabismus or anisometropic amblyopia fixate and attend with one eye and suppress the image from the other eye. Here we use a visual evoked potential technique to show that patients who lack normal stereopsis retain suppressive binocular interactions but lack a characteristic form of non-linear binocular interaction that is present in normal observers. Oscillating grating targets presented at different temporal frequencies in the two eyes evoke a strong response in normal observers at a frequency equal to the sum of the two input frequencies for fusable targets but not for rivalrous ones. However increasing contrast in one eye reduces the response amplitude from the other eye under either fusable (dichoptic masking) or rivalrous conditions. Stereo-deficient observers lack the sum-frequency response, but retain dichoptic masking interactions. Dichoptic masking is stronger when the masker is presented to the patients' dominant rather than non-dominant eyes, suggesting that a subset of preserved binocular inhibitory interactions form the basis of clinical suppression.

    View details for Web of Science ID 000086385300026

    View details for PubMedID 10790873

  • A new test of contour integration deficits in patients with a history of disrupted binocular experience during visual development VISION RESEARCH Kovacs, I., Polat, U., Pennefather, P. M., Chandna, A., Norcia, A. M. 2000; 40 (13): 1775-1783

    Abstract

    Previous studies have suggested that the integration of orientation information across space is impaired in amblyopia. We developed a method for quantifying orientation-domain processing using a test format that is suitable for clinical application. The test comprises a graded series of cards where each card includes a closed path (contour) of high contrast Gabor signals embedded in a random background of Gabor signals. Contour visibility in both normals and patients with histories of abnormal binocular vision depends jointly on the spacing of elements on the contour as well as background element density. Strabismic amblyopes show significant degradation of performance compared to normals. Small but significant losses in sensitivity were also observed in a group of non-amblyopic strabismus patients. Threshold measurements made with contrast reducing diffusers indicated that the amblyopic loss is not due to the reduced contrast sensitivity of the amblyopic eye. An abnormal pattern of long-range connectivity between spatial filters or a loss of such connectivity appears to be the primary source of contour integration deficits in amblyopia and strabismus.

    View details for Web of Science ID 000087362500012

    View details for PubMedID 10814762

  • Electrophysiological correlates of vernier and relative motion mechanisms in human visual cortex VISUAL NEUROSCIENCE Norcia, A. M., Wesemann, W., Manny, R. E. 1999; 16 (6): 1123-1131

    Abstract

    Vernier onset/offset thresholds were measured both psychophysically and with the steady-state VEP by introducing a series of horizontal breaks in a vertical square-wave luminance grating. Several diagnostic tests indicated that the first harmonic component of the evoked response generated by periodic modulation of offset gratings taps mechanisms that encode the relative position of spatial features. In the first test, a first harmonic component was only found with targets that contained transitions between collinear and noncollinear states. VEP vernier onset/offset thresholds obtained with foveal viewing were in the range of 15-22 arc sec. Control experiments with transitions between symmetrical, noncollinear patterns (relative motion) did not produce first harmonic components, nor did full-field motion of a collinear grating. A second series of experiments showed that VEP thresholds based on the first harmonic component of the vernier onset/offset response had an eccentricity dependence that was very similar to that found in a psychophysical discrimination task that required a left/right position judgment (vernier acuity). Other recordings showed that the first harmonic of the vernier onset/offset VEP was degraded by the introduction of a gap between stimulus elements, as is the displacement threshold. The vernier onset/offset target also produced a second harmonic component that was virtually identical to the one produced by a relative motion stimulus. Displacement thresholds based on these second harmonic components showed a more gradual decline with retinal eccentricity than did the first harmonic component elicited by vernier offsets. The second harmonic of the vernier onset/offset VEP was relatively unaffected by the introduction of gaps between the stimulus elements. The first and second harmonic components of the vernier onset/offset VEP thus tap different mechanisms, both of which support displacement thresholds that are finer than the resolution limits set by the spacing of the photoreceptors (hyperacuity).

    View details for Web of Science ID 000084409800012

    View details for PubMedID 10614592

  • Development of VEP Vernier acuity and grating acuity in human infants INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Skoczenski, A. M., Norcia, A. M. 1999; 40 (10): 2411-2417

    Abstract

    To compare the developmental sequences of two basic measures of pattern vision, Vernier acuity and grating acuity, using steady state visual-evoked potentials (VEPs) and an analysis designed to isolate pattern-specific responses from those due to motion in the Vernier stimulus.The authors recorded VEPs from 57 healthy full-term infants and 4 adults. The grating acuity stimulus was a sinusoidal grating, temporally modulated (appearance-disappearance) at a rate of 3 Hz, with spatial frequency decreasing in linear steps during each 10-second trial. The Vernier acuity stimulus was a vertical square-wave grating with portions of each bar temporally modulated to make offsets appear and disappear at a rate of 3 Hz. Vernier offset size changed in log steps from small to large offsets. The authors recorded each observer's electroencephalogram (EEG) during multiple presentations of each stimulus type, and the EEG was digitized and filtered to obtain the amplitude and phase of the response at the first two harmonics of the stimulus temporal frequency. Thresholds were estimated with an extrapolation technique that took into account the signal-to-noise ratio and phase of the response.VEP Vernier acuity and grating acuity develop at different rates, with grating acuity approaching adult levels earlier than Vernier acuity. The within-subject relationship between VEP Vernier acuity and grating acuity follows the same developmental trajectory established by previous psychophysical studies of humans and monkeys.This VEP technique provides a rapid estimate of Vernier acuity in infants. VEP Vernier acuity remains strikingly immature throughout the first year of life, similar to behavioral Vernier acuity. Because Vernier acuity is a sensitive measure of amblyopia, this VEP test may be useful in the future to identify amblyopia and to follow its treatment progress in pediatric patients.

    View details for Web of Science ID 000082239800032

    View details for PubMedID 10476810

  • Asymmetric responses in cortical visually evoked potentials to motion are not derived from eye movements INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Wilson, J. R., Noyd, W. W., Aiyer, A. D., Norcia, A. M., Mustari, M. J., Boothe, R. G. 1999; 40 (10): 2435-2439

    Abstract

    Normal neonates and many adults after abnormal visual development have directional preferences for visual stimulus motions; i.e., they give better responses for optokinetic nystagmus (OKN) and visually evoked potentials (VEPs) in one direction than to those in the opposite direction. The authors tested whether the VEP responses were asymmetrical because of abnormal eye movements.VEPs were recorded from the visual cortices of five macaque monkeys: one normal, one neonate, and three reared with alternating monocular occlusion (AMO). They were lightly anesthetized, followed by paralysis to prevent eye movements. They then had "jittered" vertical grating patterns presented in their visual fields. The steady state VEPs were analyzed with discrete Fourier transforms to obtain the amplitudes and phases of the asymmetries.The normal, control monkey had small, insignificant amplitudes of its asymmetrical Fourier component and random phases that were not 180 degrees out of phase across the left and right eyes. The neonatal monkey and the AMO monkeys all had large, significant asymmetries that were approximately 180 degrees out of phase between the left and right eyes.The neonate and abnormally reared monkeys continued to have asymmetrical responses even after their eyes were paralyzed. Therefore, eye movements cannot be the source of the asymmetrical amplitudes of the VEPs, and the visual cortex is at least one source responsible for asymmetries observed in neonates and adults reared under abnormal visual inputs.

    View details for Web of Science ID 000082239800037

    View details for PubMedID 10476815

  • Oscillatory motion but not pattern reversal elicits monocular motion VEP biases in infantile esotropia VISION RESEARCH Shea, S. J., Chandna, A., Norcia, A. M. 1999; 39 (10): 1803-1811

    Abstract

    Patients with early disruptions of binocularity show cortical directional asymmetries in their steady state monocular VEP response to oscillatory motion. The VEP directional asymmetry is characterized by significant first harmonic components that show a 180 degrees difference in the response phase between the two eyes. By contrast, the normal response is dominated by even-order response harmonics, although some normal observers also have measurable responses at the first harmonic. Experiments and simulations were conducted to determine if the first harmonic in patients could reasonably be attributed to direction selective mechanisms. A secondary goal was to determine whether the first harmonic response of normals was also due to imbalances in direction selective mechanisms. Monocular steady state VEPs were elicited by oscillating 3 c/deg gratings presented at 6 and 10 Hz in normal observers and observers with infantile esotropia. Responses were also obtained to phase-reversing gratings of the same spatial and temporal frequencies. Phase reversal eliminated the majority of first harmonic responses which were recorded for normal observers to oscillatory motion. However, phase reversal did not elicit the cortical motion asymmetry in infantile esotropia. Modeling results suggest that the first harmonic response to oscillatory motion arises due to non-linearities in both direction selective and non-direction-selective mechanisms, with the latter being dominant in patients with early onset strabismus.

    View details for Web of Science ID 000079298900007

    View details for PubMedID 10343872

  • Contour detection threshold: repeatability and learning with 'contour cards' SPATIAL VISION Pennefather, P. M., Chandna, A., Kovacs, I., Polat, U., Norcia, A. M. 1999; 12 (3): 257-266

    Abstract

    Human observers are able to locate contours that are defined solely on the basis of long-range, orientation-domain correlations. The integrity of the mechanisms responsible for second-order contour detection is disrupted by amblyopia (Kovacs et al., 1996; Hess et al., 1997) and it is therefore of interest to develop methods for assessing pediatric patients undergoing treatment for amblyopia. In this study, we have determined the inter-observer and test-retest reliability of a card-based test of second-order contour integration. The magnitude of practice effects was also assessed in both adult and pediatric patient groups. Contour detection thresholds were measured for a closed contour, defined by Gabor patches, embedded in a randomly oriented Gabor-patch background. The visibility of the contour was controlled by varying the density of the background elements. Thresholds, defined in terms of the ratio of contour element spacing to average background spacing were measured with a clinical staircase procedure. Thresholds measured by two observers differed on average by 0.023 +/- 0.075 or about one half the increment between cards. Children and adults showed only small practice effects (0.022 +/- 0.051 vs 0.053 +/- 0.077, respectively) and average unsigned differences between repeated measures were equivalent to approximately 1 card across groups. A card-based test of second-order contour integration produces reliable estimates of contour integration performance in normal and amblyopic observers, including children.

    View details for Web of Science ID 000081631900001

    View details for PubMedID 10442513

  • Elongated physiological summation pools in the human visual cortex VISION RESEARCH Polat, U., Norcia, A. M. 1998; 38 (23): 3735-3741

    Abstract

    The visibility of gratings improves with increasing stimulus area. This effect is usually interpreted as being due to probability summation between the outputs of linear, independent spatial filters, although non-linear spatial summation can have similar effects [1]. In order to distinguish between probabilistic and physiological summation models, we measured contrast thresholds using the Visual Evoked Potential (VEP). Our previous work [2] suggests that spatial summation in the VEP is nonlinear and that it occurs preferentially for collinear configurations. Traditional probability summation models predict that areal summation will improve threshold independent of stimulus configuration. Contrast thresholds were derived from VEP contrast response functions for either circular or elongated Gabor patches with aspect ratios up to 6:1. The carrier orientation was either the same as the patch envelope orientation (collinear) or orthogonal to it. Response amplitudes were larger and contrast sensitivity was higher for collinear configurations. The results are consistent with nonlinear, configuration dependent summation that is more extensive along the axis of orientation.

    View details for Web of Science ID 000075693300007

    View details for PubMedID 9893803

  • OKN, perceptual and VEP direction biases in strabismus VISION RESEARCH Brosnahan, D., Norcia, A. M., Schor, C. M., Taylor, D. G. 1998; 38 (18): 2833-2840

    Abstract

    The present study quantified nasalward/temporalward biases in monocular optokinetic nystagmus (MOKN) and perceived velocity in patients with either early onset esotropia, late onset esotropia and in normals. MOKN was measured with low spatial frequency, small-field gratings drifting at 9.4 degrees/s. MOKN bias was quantified as the ratio of nasalward slow-phase velocity divided by the sum of temporalward and nasalward slow-phase velocities (N/(N + T)). Observers also rated the perceived velocity of gratings moving in nasalward and temporalward directions (3 or 9.4 degrees/s) using a two interval forced choice task. MOKN and perceived velocity biases were correlated negatively in both early onset and late onset groups in the perceptual task--nasalward moving targets were rated as slower than temporalward targets, but in the MOKN task, slow-phase gain was higher for nasalward than for temporalward targets. Oscillatory-motion, visual evoked potentials (VEPs), were recorded in response to 1 c/deg gratings undergoing apparent motion at 10 Hz in a subset of the observers. VEP direction biases were quantified by calculating the ratio of first harmonic response amplitudes to the sum of first and second harmonic amplitudes. Significant correlations were found between the direction biases obtained on all three measures. Perceived velocity and MOKN bias measures were also correlated negatively. Patients with early onset esotropia (infantile esotropia) had larger biases than late onset esotropes or normals on each measure and the biases were more frequently bilateral in the early onset patients. The pattern of result is consistent with early critical periods for the mechanism(s) underlying MOKN, perceived velocity and cortical responsiveness. A single site model for all three asymmetries is unlikely, at least in simple form, because of the negative correlation between MOKN and perceived velocity biases and because of the differences in relative magnitude between the perceptual and MOKN biases.

    View details for Web of Science ID 000074821900011

    View details for PubMedID 9775329

  • Development of directional motion symmetry in the monocular visually evoked potential of infant monkeys VISION RESEARCH Brown, R. J., Wilson, J. R., Norcia, A. M., Boothe, R. G. 1998; 38 (9): 1253-1263

    Abstract

    Motion processing in humans and monkeys exhibit a directional asymmetry during infancy which is not present in adults except following abnormal visual rearing conditions. To characterize the time course for maturation of a symmetric response, we measured the monocular visually evoked potential (MVEP) response to 0.26 c/deg gratings oscillating horizontally at 6 Hz in 13 infant rhesus monkeys between 1 and 52 weeks of age. An asymmetric (F1) and a symmetric (F2) frequency component were extracted from the MVEP using Fourier analysis. At early ages the asymmetric F1 component measured from the two eyes exhibited a 180 deg interocular phase shift, demonstrating that there was a directional bias in opposite directions between the left and right eyes. Although our methods could not determine whether the bias was in the nasal or temporal direction, our results would be consistent with a nasal bias, as has been observed in previous motion studies. Magnitude of the asymmetry was quantified in the form of an asymmetry index, F1/(F1 + F2). Based on developmental changes in the asymmetry index, and phase and amplitudes of F1 and F2, we conclude that the MVEP loses its directional asymmetry at 6 weeks of age. The development of directional motion symmetry observed in monkeys over the first 6 weeks is similar to that observed in humans over the first 5 months.

    View details for Web of Science ID 000073529000007

    View details for PubMedID 9666993

  • Neural noise limitations on infant visual sensitivity NATURE Skoczenski, A. M., Norcia, A. M. 1998; 391 (6668): 697-700

    Abstract

    Visual contrast sensitivity is poor in newborn human infants, but improves rapidly to approach adult levels by 8 months of age. During this period, infant sensitivity can be limited by physical factors affecting photon capture, such as eye size and photoreceptor density. Here we show that infant visual sensitivity is also limited by high levels of noise in the neural transduction process. Using a non-invasive electrophysiological measurement and a visual noise titration technique, we have found that intrinsic neural noise in neonates is approximately nine times higher than in adults. As intrinsic neural noise decreases during infancy, contrast sensitivity improves proportionally, suggesting that neural noise places critical limits on contrast sensitivity throughout development. Moreover, contrast gain control, an inhibitory process that adjusts visual responses to changing stimulation, is in place and operating in infants as young as 6 weeks of age, in spite of high levels of neural noise and significant immaturities in contrast sensitivity. The contrast gain control that we observed in human neonates may serve as a building block for more complex forms of visual inhibition, which develop later in infancy.

    View details for Web of Science ID 000071982500053

    View details for PubMedID 9490413

  • Collinear stimuli regulate visual responses depending on cell's contrast threshold NATURE Polat, U., Mizobe, K., Pettet, M. W., Kasamatsu, T., Norcia, A. M. 1998; 391 (6667): 580-584

    Abstract

    Neurons in the primary visual cortex are selective for the size, orientation and direction of motion of patterns falling within a restricted region of visual space known as the receptive field. The response to stimuli presented within the receptive field can be facilitated or suppressed by other stimuli falling outside the receptive field which, when presented in isolation, fail to activate the cell. Whether this interaction is facilitative or suppressive depends on the relative orientation of pattern elements inside and outside the receptive field. Here we show that neuronal facilitation preferentially occurs when a near-threshold stimulus inside the receptive field is flanked by higher-contrast, collinear elements located in surrounding regions of visual space. Collinear flanks and orthogonally oriented flanks, however, both act to reduce the response to high-contrast stimuli presented within the receptive field. The observed pattern of facilitation and suppression may be the cellular basis for the observation in humans that the detectability of an oriented pattern is enhanced by collinear flanking elements. Modulation of neuronal responses by stimuli falling outside their receptive fields may thus represent an early neural mechanism for encoding objects and enhancing their perceptual saliency.

    View details for Web of Science ID 000071842300050

    View details for PubMedID 9468134

  • Lack of lateral inhibitory interactions in visual cortex of monocularly deprived cats VISION RESEARCH Kasamatsu, T., Kitano, M., Sutter, E. E., Norcia, A. M. 1998; 38 (1): 1-12

    Abstract

    To gain new insight into the effects of monocular deprivation, we studied the visual cortex of adult cats deprived of vision in one eye. Local field potentials were recorded in response to contrast reversal of square-wave gratings modulated in time either by pseudorandom, m-sequences or periodically. We have found that: (1) stimulation of the retinotopic locus of the recording site elicits responses with abnormal waveforms and long latencies from the deprived eye; (2) stimulation of a remote, non-retinotopic locus elicits responses from the non-deprived eye but not from the deprived eye; (3) the monocularly deprived cortex lacks lateral inhibitory interactions which are characteristic of the normal cortex; and (4) steady-state responses showed little difference in spatial-frequency tuning and contrast sensitivity between the deprived and non-deprived eye, mostly conforming to earlier field-potential data in monocular deprivation. Functional lateral interactions appear to be greatly reduced in monocularly deprived cortex.

    View details for Web of Science ID A1998YH85300001

    View details for PubMedID 9474370

  • Spatial frequency masking with the sweep-VEP VISION RESEARCH Peterzell, D. H., Norcia, A. M. 1997; 37 (17): 2349-2359

    Abstract

    A suprathreshold sinewave grating can change the amplitude of the steady-state visual evoked potential (VEP) in response to a test grating if the two are close in spatial frequency (SF). The change in amplitude provides clues to underlying pattern analyzers. Masking was measured in 12 observers using the steady-state VEP. As a test grating reversed at 7 Hz, a masker of similar temporal frequency (9 Hz) but of variable SF was superimposed on it. Test gratings were 1, 3 and 8 c/deg (20% contrast). Within a 10 sec trial, the mask (20 or 40% contrast) was fixed at one of nine SFs or was swept across 19 SFs (5 octaves). The amplitude of the test response (at 14 Hz) was measured as a function of the SF of the masker. Group masking functions were broad (2-3 octaves) and revealed multiple minima. Functions for 1 and 3 c/deg tests each revealed minima near 1 and 3 c/deg. Functions for 8 c/deg tests revealed minima at 3 and 8 c/deg. Doubling the contrast of the mask from 20 to 40% increased masking but in a nonlinear fashion that enlarged the off-peak minima. Swept masks caused slightly more masking than fixed masks, and caused masked amplitudes to exceed unmasked amplitudes (i.e., enhancement) in one condition (3 c/deg.test, 20% contrast mask). The data suggest that each VEP masking function reflects the outputs of multiple spatial analyzers, that a discrete set of analyzers may underlie the data, and that the efficient sweep-VEP can measure SF tuning.

    View details for Web of Science ID A1997XP66800004

    View details for PubMedID 9381671

  • A method for investigating binocular rivalry in real-time with the steady-state VEP VISION RESEARCH Brown, R. J., Norcia, A. M. 1997; 37 (17): 2401-2408

    Abstract

    Under conditions in which the visual system cannot reconcile dissimilar images from the two eyes, perception typically alternates between the two half-images-a process known as binocular rivalry. We report a real-time, steady-state VEP method that is a sensitive detector of the continuous alternations in perceptual dominance across the eyes. This method works by labelling each half-image with a slightly different temporal frequency so that the record generated by each can be recovered from the EEG by spectrum analysis. In this way, one can track the "waxing" and "waning" of the VEP amplitudes for each eye simultaneously during spontaneous rivalry, permitting an analysis of the relative physiological dominance of each eye in real-time. Such alternations were clearly observed in the VEP amplitudes generated by each half-image during rivalry (the amplitudes for the two eyes correlated negatively). In contrast, VEP amplitudes for the two eyes varied either synchronously or randomly when the half-images were allowed to fuse. The instances of physiological dominance of each eye as evidenced by the VEP correlated well with the subjects' report of perceptual dominance. This purely electrophysiological method appears to be suitable for measuring rivalry in non-verbal human or animal subjects, as it does not require active participation from them.

    View details for Web of Science ID A1997XP66800008

    View details for PubMedID 9381675

  • Abnormal long-range spatial interactions in amblyopia VISION RESEARCH Polat, U., Sagi, D., Norcia, A. M. 1997; 37 (6): 737-744

    Abstract

    Neural interactions between widely separated stimuli were explored with psychophysical and visual evoked potential (VEP) measures in normal and amblyopic observers. Contrast detection thresholds were measured psychophysically for small foveally viewed Gabor patches presented in isolation and in the presence of similar, but laterally displaced flanks. The amplitude and phase of VEPs elicited by similar targets were also measured. The presence of neural interaction between the target and flank responses was assessed by comparing the unflanked threshold to the flanked threshold in the psychophysical experiments and by comparing the response predicted by the algebraic sum of test and flank responses to that measured when test and flanks were presented simultaneously. In normal observers simultaneous presentation of test and flank targets produces a VEP response that is up to a factor of two larger than the linear prediction (facilitation). Psychophysical threshold is also facilitated by a comparable factor. Facilitation was found mainly for configurations in which local (carrier) and global (patch) orientations resulted in collinearity, independent of global orientation (meridian). Amblyopic observers showed several deviations from the normal pattern. The facilitation for the collinear configurations was either markedly lower than normal or was replaced by inhibition. The normal pattern of spatial interaction may facilitate the grouping of collinear line segments into smooth curves. In contrast, abnormal long-range spatial interactions may underlie the grouping disorders and perceptual distortions found in amblyopia.

    View details for Web of Science ID A1997WM12200009

    View details for PubMedID 9156218

  • Neurophysiological evidence for contrast dependent long-range facilitation and suppression in the human visual cortex VISION RESEARCH Polat, U., Norcia, A. M. 1996; 36 (14): 2099-2109

    Abstract

    Long-range spatial interactions in human visual cortex were explored using a lateral masking paradigm. Visual evoked potentials (VEPs) elicited by a Gabor signal presented in isolation or in the presence of two flanking high-contrast Gabor signals (masks) were measured. Response amplitude and phase were recorded for a vertically oriented test, for horizontal and vertical masks and for combinations of vertical tests and vertical or horizontal masks. The amplitudes and phases of the test alone and mask alone responses were added coherently to predict the amplitude for collinear and orthogonal lateral masking conditions. Additivity failures were taken as evidence for neural interactions. At a target-to-mask distance of 2 deg, VEP amplitude exceeded the linear prediction for test contrasts in the range of 8-16% for the collinear, co-axial target/mask combination. Measured response phase also led predicted response phase over the same range of contrast. The VEP amplitudes were less than the linear prediction in the orthogonal target/mask combination and measured response phase lagged the predicted phase. Significant facilitation occurred with collinear test/mask combinations up to at least 3 deg of separation (nine wavelengths). Co-oriented, but non-collinear test/mask combinations (oblique test and mask, horizontal test and mask) did not produce facilitation. Contrast gain thus appears to be set over considerable distances in a configuration-specific fashion.

    View details for Web of Science ID A1996UY57500009

    View details for PubMedID 8776476

  • Development of grating acuity and contrast sensitivity in the central and peripheral visual field of the human infant VISION RESEARCH Allen, D., Tyler, C. W., Norcia, A. M. 1996; 36 (13): 1945-1953

    Abstract

    Central and peripheral visual functions were measured simultaneously in 39 infants from 10 to 39 weeks old using a dual-frequency VEP technique. Central acuity and contrast sensitivity over a 4 deg circular field were measured at 6 or 8 Hz. Peripheral acuity and contrast sensitivity were measured simultaneously at the other rate with a semi-circular stimulus extending from 8 to 16 deg. The EEG was analyzed at 12 and 16 Hz to determine the separate responses for the central and peripheral fields. Both central and peripheral VEP acuity developed over the age range tested. Central acuity improved by about a factor of 2.6 over the age range tested, while peripheral acuity improved by about a factor of 2.2. Central acuity was always higher by an average factor of about 2.3. Contrast sensitivity showed similar development for the central and peripheral fields with an early rapid rise in sensitivity.

    View details for Web of Science ID A1996UV00700010

    View details for PubMedID 8759434

  • Abnormal motion processing and binocularity: Infantile esotropia as a model system for effects of early interruptions of binocularity EYE Norcia, A. M. 1996; 10: 259-265

    Abstract

    Infantile esotropia, a common form of strabismus with onset prior to 6 months of age, occurs at a time of rapid visual development. While monocular visual acuity is relatively unaffected in these patients, the majority of them fail to achieve fully normal stereopsis. In addition, these patients show a spectrum of abnormalities in their ocular following responses, visual perception and visual evoked potentials (VEPs) that suggest a failure to develop a normal complement of motion processing mechanisms. While abnormalities of of stereopsis have been studied for many years, motion processing in strabismus is a rapidly evolving area of current research. Motion mechanisms are normally binocular and may form a distinct binocular sub-system. This review summarises which is known about sensory and motor abnormalities in infantile esotropia, with special emphasis on recent motion VEP recordings. The monocular motion VEP shows directional biases early in infancy that are consistent with a nasalward/temporalward response bias. Patients with infantile esotropia maintain their neonatal biases beyond the age at which they normally disappear. The motion VEP biases persist into visual maturity in patients whose strabismus is treated after about 2 years of age. Treatment prior to age 2 can lessen the magnitude of the motion VEP asymmetry and these improvements can be maintained into visual maturity. A recording from the striate cortex of a visually deprived macaque monkey indicates that the motion VEP asymmetry arises early in the visual pathway.

    View details for Web of Science ID A1996UK09600017

    View details for PubMedID 8776457

  • AN ADAPTIVE FILTER FOR STEADY-STATE EVOKED-RESPONSES EVOKED POTENTIALS-ELECTROENCEPHALOGRAPHY AND CLINICAL NEUROPHYSIOLOGY Tang, Y., Norcia, A. M. 1995; 96 (3): 268-277

    Abstract

    A 2-weight adaptive filter that determines the amplitude and phase of steady-state evoked potentials is presented. Reference signals are derived from the visual stimulator that are related to corresponding harmonics of the response and the filter weights are adjusted so as to minimize the squared estimation error between the reference and the recorded signal using the recursive least squares (RLS) method. The filter, which acts as an adaptive bandpass filter, is followed by a detector based on the T2circ statistic. The performance of the RLS adaptive filter was compared to that of the conventional Discrete Fourier Transform (DFT) and the filtered DFT of Tang and Norcia in a series of simulations with known sinusoids buried in Gaussian noise and in EEG noise. In the simulations, the RLS adaptive filter detected signals at about 3-4 times lower signal to noise ratios than did the DFT. The RLS filter also outperformed the filtered DFT. Qualitatively similar results were obtained with human visual evoked potential recordings. The adaptive RLS filter significantly outperforms both the DFT and filtered DFT and is much simpler to implement than the filtered DFT method of Tang and Norcia.

    View details for Web of Science ID A1995QZ16100008

    View details for PubMedID 7750452

  • SPATIALLY DISTRIBUTED RESPONSES INDUCED BY CONTRAST REVERSAL IN CAT VISUAL-CORTEX EXPERIMENTAL BRAIN RESEARCH Kitano, M., Kasamatsu, T., Norcia, A. M., Sutter, E. E. 1995; 104 (2): 297-309

    Abstract

    Field potentials were recorded from cat striate cortex, either between an epidural screw electrode and a cannula-electrode inserted deep in the gray matter (transcortical recording) or with a pair of metal microelectrodes. Electrodes were placed bilaterally near the cortical projection of the area centralis. The horizontal separation of the recording tips was approximately 2 mm and approximately 300 microns, respectively. The area of the visual field providing input to the recording site (receptive field) was determined by measuring the field potentials generated by contrast reversal of high-contrast, achromatic bar gratings. Five-degree-diameter grating patches were presented individually over a large area of the visual field. The gratings were contrast-reversed at 4, 6 or 10 Hz, while also being swept in spatial frequency between 0.56 and 5.24 c/deg. The receptive fields were approximately 20 deg across or more, substantially larger than expected on the basis of cortical retinotopy. Responses were also elicited by stimulation of the hemi-field contralateral to that contributing to the classical receptive field, implicating the presence of a callosal projection. The large, spatially distributed receptive fields consisted of patches of high and low sensitivity. Continuous cortical infusion of either 100 microM tetrodotoxin or 10 mM muscimol at the recording site totally suppressed the transcortically recorded field potentials, proving that the local field potentials were generated postsynaptically. The present findings suggest that a cluster of cortical cells near the projection site of the area centralis receives input from remote cortical regions to an extent that is comparable with that of anatomically demonstrated long-range lateral connections.

    View details for Web of Science ID A1995RC03200012

    View details for PubMedID 7672022

  • APPLICATION OF ADAPTIVE FILTERING TO STEADY-STATE EVOKED-RESPONSE MEDICAL & BIOLOGICAL ENGINEERING & COMPUTING Tang, Y., Norcia, A. M. 1995; 33 (3): 391-395

    Abstract

    A new method to detect steady-state evoked potentials (EPs) is presented. The technique is based on a two-weight recursive least squares (RLS) adaptive filter and the Tcirc2 statistic. Simulations with known sinusoids buried in Gaussian noise and in EEG noise indicate that the adaptive filter can detect signals at 3 or 4 times lower signal-to-noise ratios that the discrete Fourier transform (DFT). Qualitatively similar results were obtained with human visual evoked potential recordings.

    View details for Web of Science ID A1995RE99400005

    View details for PubMedID 7666685

  • RETINOTOPIC AND NONRETINOTOPIC FIELD POTENTIALS IN CAT VISUAL-CORTEX VISUAL NEUROSCIENCE Kitano, M., Niiyama, K., Kasamatsu, T., Sutter, E. E., Norcia, A. M. 1994; 11 (5): 953-977

    Abstract

    Two types of field potentials were identified in cat visual cortex using contrast reversal of oriented bar gratings: a short-latency fast-local component with a retinotopic organization similar to that seen with single-unit discharges at the same cortical site, and a slow, nonretinotopic component with a longer peak latency. The slow-distributed component had an extensive receptive field mapped by measuring the amplitude of binary kernels and showed strong inhibitory interactions within the receptive field. The peak latency of the slow-local component increased with distance from the retinotopic center, suggesting a possible conduction delay. Both components showed some orientation bias depending on the laminar location, but the bias could be independent of the orientation preferred by single units in the immediate vicinity. The present findings indicate that locally generated field potentials reflect cortical mechanisms for nonlinear integration over wide areas of the visual field.

    View details for Web of Science ID A1994PC70800011

    View details for PubMedID 7947408

  • THE DEVELOPMENT OF MOTION SENSITIVITY DURING THE FIRST YEAR OF LIFE VISION RESEARCH Hamer, R. D., Norcia, A. M. 1994; 34 (18): 2387-2402

    Abstract

    Using the sweep visual evoked potential (VEP), we have measured oscillatory displacement thresholds (OMTs) in 49 infants ranging in age from 7 to 54 weeks of age. The stimuli were high-contrast (80%), sine-wave gratings (1 c/deg) undergoing oscillatory displacements at 6 Hz. In addition to the motion thresholds, contrast thresholds for phase-reversing (6 Hz), 1 c/deg gratings were measured in the same session for 26 infants. In the main experiment, responses were recorded at the second harmonic (F2) of the stimulus frequency (12 Hz) under binocular viewing conditions. Our main finding is that, over the age range during which infants' peak contrast sensitivity (CS) first develops to within a factor of 2 of adult CS (9-12 weeks), infants' sensitivity to grating displacement is a factor of approximately 10 less than adults'. Moreover, infants' sensitivity to oscillatory motion undergoes relatively little development over the period between 2 and 15 months postnatal, gradually achieving a factor of 4.5 below adult values by 1 yr of age. Averaged over the entire age range tested, infants' OMTs were 167 sec arc, a factor of 6.4 times higher than the average OMT (26 sec arc) for 13 adults tested under identical conditions. In contrast, the infants' average CS for reversing gratings averaged only a factor of 2.5 less than the adults' average CS. In a second experiment, we took advantage of a developmental asymmetry in the monocular oscillatory motion VEP which allows for unambiguous identification of direction selective responses from very young infants. Monocular motion VEPs were measured in five infants (8-14 weeks) and their data analyzed at the fundamental frequency (F1). Responses at F1 were present in the monocular motion VEP from each infant and were 180 deg out of phase between the two eyes, identifying them as directional cortical responses with a nasalward/temporalward bias. These directional thresholds were equal to or lower than the symmetric (F2) thresholds. The presence of directional asymmetry in the motion VEP and the similarity of the monocular F1 and F2 OMTs support the notion that the OMTs measured in the main experiment were, in fact, derived from the responses of directionally selective cells in visual cortex. These data also imply that the OMTs are not derived from local contrast-reversal responses. Other models to explain infants' relative insensitivity to oscillatory motion are discussed.

    View details for Web of Science ID A1994PB29700005

    View details for PubMedID 7975278

  • PREOPERATIVE ALTERNATE OCCLUSION DECREASES MOTION PROCESSING ABNORMALITIES IN INFANTILE ESOTROPIA JOURNAL OF PEDIATRIC OPHTHALMOLOGY & STRABISMUS Jampolsky, A., Norcia, A. M., Hamer, R. D. 1994; 31 (1): 6-17

    Abstract

    We have examined the effects of preoperative, full-time alternate occlusion on the development of visual motion processing mechanisms. Motion visual evoked potentials (MVEPs) were recorded longitudinally in 14 infantile esotropia patients during the course of standard preoperative occlusion therapy. The MVEP in these patients was initially asymmetric in a fashion consistent with a nasalward/temporalward response bias, with a motion asymmetry significantly higher than that of age-matched normals. The magnitude of the developmental motion asymmetry declined significantly after an average of 24 weeks of alternate occlusion. This result implies that the binocular motion-sensitive cells underlying the MVEP retain some degree of plasticity up to at least 1 year of age. Our data suggest further that the persistence of motion asymmetries in untreated infantile esotropia patients is maintained by an active process that can be disrupted by alternate occlusion. Alternate occlusion apparently eliminates a form of abnormal binocular interaction that supports the persistence of the motion asymmetry. We propose that one of the necessary pre-conditions for symmetricization of motion processing in infantile esotropia is the absence of abnormal competitive binocular interactions.

    View details for Web of Science ID A1994NB81500001

    View details for PubMedID 8195968

  • DOES CHROMATIC SENSITIVITY DEVELOP MORE SLOWLY THAN LUMINANCE SENSITIVITY VISION RESEARCH Allen, D., Banks, M. S., Norcia, A. M. 1993; 33 (17): 2553-2562

    Abstract

    Chromatic sensitivity is very low in humans during the first few months of life. We examined whether low chromatic sensitivity reflects a deficiency among chromatic mechanisms or whether it is simply a manifestation of poor visual sensitivity in general. The sweep VEP was used to measure contrast sensitivity to gratings varying in the mixture of red and green components. For infants from 2 to 8 weeks of age, sensitivity to all mixtures was lower than color-normal adults' sensitivity, but infant and adult ratios of luminance/chromatic sensitivity were similar. This finding is consistent with the hypothesis that infants have functional MWS and LWS cones and the requisite post-receptor chromatic mechanisms to compare their signals.

    View details for Web of Science ID A1993LZ83700018

    View details for PubMedID 8249334

  • IMPROVED PROCESSING OF THE STEADY-STATE EVOKED-POTENTIAL ELECTROENCEPHALOGRAPHY AND CLINICAL NEUROPHYSIOLOGY Tang, Y., Norcia, A. M. 1993; 88 (4): 323-334

    Abstract

    Two related procedures for estimating the parameters of steady-state evoked potentials (SSEPs) are introduced. The first procedure involves an initial stage of digital bandpass filtering followed by a Discrete Fourier Transform analysis. In the second method, a high resolution method based on parametric modelling is applied to the filtered data. The digital pre-filter consists of a non-phase shifting Chebychev bandpass filter. The parametric modelling method considers the evoked-response-plus-noise distribution to consist of a set of exponentially damped sinusoids. The frequency, amplitude, phase and damping factors of these components are estimated by calculating the mean of the forward and backward prediction filters and linear regression. We compared the signal-to-noise ratio (SNR) of the new procedures to the conventional Discrete Fourier Transform method for Monte Carlo simulations utilizing known sinusoids buried in white noise, known sinusoids buried in human EEG noise and for a sample of visual evoked potential data. Both of the new methods produce substantially more accurate and less variable estimates of test sinusoid amplitude. For VEP recording, the EEG background noise level is reduced by 5-6 dB over that obtained with the DFT. The new methods also provide approximately 5 dB better SNR than the DFT for detection of sinusoids based on the Rayleigh statistic. The parametric modelling approach is particularly suited for the analysis of very short data records including cycle-by-cycle analysis of the SSEP.

    View details for Web of Science ID A1993LR33000009

    View details for PubMedID 7688287

  • MOTION VEPS IN LATE-ONSET ESOTROPIA CLINICAL VISION SCIENCES Hamer, R. D., Norcia, A. M., ORELBIXLER, D., Hoyt, C. S. 1993; 8 (1): 55-62
  • CONTRAST DEPENDENCE OF THE OSCILLATORY MOTION THRESHOLD ACROSS THE VISUAL-FIELD JOURNAL OF THE OPTICAL SOCIETY OF AMERICA A-OPTICS IMAGE SCIENCE AND VISION Wesemann, W., Norcia, A. M. 1992; 9 (10): 1663-1671

    Abstract

    Observer sensitivity to oscillatory step displacements of sine-wave gratings was investigated at various loci in the visual field (0-30 degrees) as a function of contrast. Detection thresholds at 10 Hz and high grating contrasts were approximately 11-15 arcsec in the fovea and 37-47 arcsec at 30 degrees eccentricity. At any given contrast, threshold displacement increases linearly with eccentricity. The data provide evidence against an interpretation based on cortical magnification, because the slope and the scale-free x intercept of the eccentricity function vary strongly with contrast. While foveal thresholds for high-contrast gratings are in the range of the hyperacuities, the oscillatory motion threshold falls off an order of magnitude more slowly than the traditional hyperacuities. Rather than conceiving of the oscillatory motion threshold as a spatial acuity limited by cortical magnification, we suggest an alternative approach that is based on a form of contrast discrimination. Oscillatory motion can be decomposed into the sum of a modulating counterphase grating and a static masking grating, both of which are in spatial quadrature (i.e., 90 degrees out of phase). At low grating contrast, oscillatory motion can be detected when the counterphase component exceeds a constant contrast value. Above a critical contrast value of the static component Cscrit, threshold rises as a power function of contrast with a slope near 1.0. The critical contrast value Cscrit increases linearly with eccentricity, indicating that oscillating gratings observed with the peripheral visual field are less easily masked compared with foveally fixated gratings.

    View details for Web of Science ID A1992JQ42200002

    View details for PubMedID 1403240

  • COMPARISON OF ON-AXIS AND OFF-AXIS PHOTOREFRACTION WITH CYCLOPLEGIC RETINOSCOPY IN INFANTS JOURNAL OF PEDIATRIC OPHTHALMOLOGY & STRABISMUS Hamer, R. D., Norcia, A. M., Day, S. H., HAEGERSTROMPORTNOY, G., Lewis, D., HSUWINGES, C. 1992; 29 (4): 232-239

    Abstract

    We have compared the performance of an off-axis (knife-edge) photorefractor with that of an on-axis (isotropic) system. Normal infants and children between the ages of 8 and 208 weeks were photographed with each camera both with and without cycloplegia. Refractive errors were estimated for each technique based on equations derived from ray-tracing. These refractions were compared to the results of retinoscopy under cycloplegia. Sensitivity and specificity of the two photorefraction systems were evaluated as a function of the magnitude of meridional hyperopia defined by retinoscopy. We also examined the effect of varying the photorefraction screening criterion. Thirteen percent of the infants in the screening sample presented with +3.50 diopters or more of meridional hyperopia. Using this level of ametropia as a referral criterion, the sensitivity and specificity of the off-axis system for infants without cycloplegia were 83% and 72%, respectively. For the on-axis system, sensitivity and specificity values were 85% and 53%. The use of cycloplegics did not significantly improve the performance of either system, but rather their use degraded the specificity of the on-axis system in the presence of moderate refractive errors. The results of the present study indicate that both on- and off-axis systems are effective in identifying highly ametropic infants, but that the off-axis system results in significantly fewer false positives. Moreover, the off-axis system has the advantages of an inherently greater dynamic range for a fixed camera design, and also more easily interpreted photographs.

    View details for Web of Science ID A1992JK32800009

    View details for PubMedID 1512665

  • THEORY OF ECCENTRIC PHOTOREFRACTION (PHOTORETINOSCOPY) - ASTIGMATIC EYES JOURNAL OF THE OPTICAL SOCIETY OF AMERICA A-OPTICS IMAGE SCIENCE AND VISION Wesemann, W., Norcia, A. M., Allen, D. 1991; 8 (12): 2038-2047

    Abstract

    An optical analysis of eccentric photorefraction (photoretinoscopy) of astigmatic eyes is presented. The size and the angular tilt of the dark crescent appearing in the subject's pupil are derived as a function of five variables: the ametropia of the eye (Dsph, Dcyl, axis), the eccentricity of the flash, e, and the distance of the camera from the subject's eye, dc. A simplified solution and a solution of the inverse problem, which enable one to calculate the degree of ametropia from the size and the tilt of the crescent, are also presented. If the crescent is smaller than the pupil, both the size and the tilt of the dark crescent are independent of the pupil size. The angular tilt of the crescent is also independent of the eccentricity. Characteristic changes of the crescent as a function of the cylinder axis are illustrated for compound and mixed astigmatisms. The validity of the theoretical predictions was experimentally verified on a model eye.

    View details for Web of Science ID A1991GX03200026

    View details for PubMedID 1783948

  • CORTICAL RECOVERY FROM EFFECTS OF MONOCULAR DEPRIVATION CAUSED BY DIFFUSION AND OCCLUSION BRAIN RESEARCH Ohashi, T., Norcia, A. M., Kasamatsu, T., Jampolsky, A. 1991; 548 (1-2): 63-73

    Abstract

    Two forms of visual deficit were induced in 12 pairs of kittens (4-6 weeks of age) by monocular lid suture combined with either an opaque soft contact lens ('occlusion' amblyopia) or a clear lens ('diffusion' amblyopia) which had been kept behind the sutured eyelids for 5-9 weeks. The kittens were then reverse-sutured at the age of 9-15 weeks, and the previously open eye was occluded with an opaque lens for the next 5-7 weeks. We compared across the two groups the proportion of binocularly driven cells (group 2-6) and cells predominantly activated by stimulation of the initially deprived eye (groups 1-3). Both values were significantly higher in the 'occlusion' kitten, though the difference was small, compared to the 'diffusion' kitten. In addition, we measured visual acuity and peak contrast sensitivity for the two eyes in 4 of the 12 pairs, using the sweep VEP method applied under anesthesia and paralysis. The 'occlusion' kitten showed consistently better acuity and higher peak contrast sensitivity than the 'diffusion' kitten, when the initially deprived eye was tested. Taken together, the present results suggest that there is a difference in the depth of amblyopia caused by monocular 'occlusion' and monocular 'diffusion'. The cortical effects of the latter are more difficult to reverse than those of the former.

    View details for Web of Science ID A1991FP17000010

    View details for PubMedID 1651147

  • ANOMALOUS MOTION VEPS IN INFANTS AND IN INFANTILE ESOTROPIA INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Norcia, A. M., Garcia, H., Humphry, R., Holmes, A., Hamer, R. D., ORELBIXLER, D. 1991; 32 (2): 436-439

    Abstract

    Visual evoked potentials (VEPs) were recorded monocularly in response to vertical gratings that underwent oscillatory apparent motion at a temporal frequency of 10 Hz. In normal infants 6 months or younger and in patients with a history of constant strabismus onset before 6 months of age, the oscillatory motion VEP contains a prominent first harmonic component that is temporally 180 degrees out of phase in the two eyes. This pattern is not seen in normal adults and is consistent with the presence of a nasalward/temporalward asymmetry of cortical responsiveness in infants and in patients with early onset strabismus.

    View details for Web of Science ID A1991EX76600027

    View details for PubMedID 1993597

  • DEVELOPMENT OF CONTRAST SENSITIVITY IN THE HUMAN INFANT VISION RESEARCH Norcia, A. M., Tyler, C. W., Hamer, R. D. 1990; 30 (10): 1475-1486

    Abstract

    Contrast sensitivity and grating acuity were measured using the sweep VEP method in a group of 48 infants from 2 to 40 weeks of age and in a group of 10 adults. Sinusoidal gratings were reversed in contrast at 12 alternations per sec at a space-average luminance of 220 cd/m2. During 10 sec trials, either the contrast or the spatial frequency was increased in a series of 19 steps. Thresholds were estimated by extrapolation of the VEP response functions to zero amplitude. The contrast threshold at low spatial frequencies developed rapidly from 7% contrast at 2-3 weeks to an asymptote of 0.5% at 9 weeks. For adults, maximum sensitivity at low spatial frequencies was 0.32-0.22%. The sweep VEP estimate of grating acuity showed a gradual increase in spatial frequency with age, starting at 5 c/deg during the first month and reaching 16.3 c/deg at 8 months. The mean adult acuity was 31.9 c/deg. There appeared to be two phases in the development of contrast sensitivity and acuity. Between 4 and 9 weeks overall contrast sensitivity increased by a factor of 4-5 at all spatial frequencies. Beyond 9 weeks, contrast sensitivity at low spatial frequencies remained constant, while sensitivity increased systematically at higher spatial frequencies.

    View details for Web of Science ID A1990DX33300009

    View details for PubMedID 2247957

  • Polaroid photorefractive screening of infants. Journal of pediatric ophthalmology & strabismus Hsu-Winges, C., Hamer, R. D., Norcia, A. M., Wesemann, H., Chan, C. 1989; 26 (5): 254-260

    Abstract

    We modified a Polaroid SE camera for use as a photoretinoscope. A total of 187 infants between 2 and 18 months of age were photographed using this device. About half of these infants (97) participated in a double blind study in which the results of photorefraction were compared with those of standard cycloplegic retinoscopy. Eighty-three infants were photographed without cycloplegia. Thirty-four infants were photographed while cyclopleged. Photographs were evaluated for significant refractive errors and other ocular abnormalities. The effectiveness of the camera system to screen for significant refractive errors without the use of cycloplegia was assessed. Infants were identified to be at risk by photorefraction if, in any photograph, a hyperopic bright crescent calculated to be greater than or equal to +1.25 D was present in the pupil. Clinically significant refractive errors were defined by the results of cycloplegic retinoscopy: "at-risk" infants had either 3.5 D or more hyperopia in either eye, or astigmatism in either eye greater than or equal to 2.5 D, or anisometropia greater than or equal to 1.5 D. With these clinical criteria and the above photographic screening criterion, the camera's sensitivity and specificity were 83% and 69%, respectively. The present system compares favorably with earlier, more sophisticated units in alerting practitioners to potentially significant refractive errors in infants. Additionally, as a screening tool, this device offers the benefits of being inexpensive and easy to use, and of providing immediate feedback.

    View details for PubMedID 2795416

  • THE DEVELOPMENT OF MONOCULAR AND BINOCULAR VEP ACUITY VISION RESEARCH Hamer, R. D., Norcia, A. M., Tyler, C. W., HSUWINGES, C. 1989; 29 (4): 397-408

    Abstract

    The development of monocular and binocular grating acuity was measured in 87 infants, 2-52 weeks of age, using the sweep VEP technique. Average monocular and binocular acuity growth functions were nearly identical, with a small (less than 0.2 octaves) binocular acuity superiority occurring only under 6 months. Interocular acuity differences were small (averaging less than 1/4 octave, unsigned, with a 95% confidence interval of less than +/- 0.6 octaves) and were not significant at any age. These characteristics make the sweep VEP technique a potentially sensitive tool for the detection of monocular visual losses in the early stages of amblyopia.

    View details for Web of Science ID A1989T971200004

    View details for PubMedID 2781730

  • MEASUREMENT OF SPATIAL CONTRAST SENSITIVITY WITH THE SWEPT CONTRAST VEP VISION RESEARCH Norcia, A. M., Tyler, C. W., Hamer, R. D., Wesemann, W. 1989; 29 (5): 627-637

    Abstract

    Contrast response functions (CRFs) for the VEP were obtained with a Discrete Fourier Transform (DFT) technique employing swept contrast gratings. VEP CRFs in infants were found to have a form similar to those observed in adults, being linear functions of log contrast over a range of near-threshold contrasts. CRFs with low and high contrast lobes were present in infants, as they are in adults. Contrast thresholds were estimated by extrapolation of the CRF to zero microvolts. The effects of additive EEG noise and of the DFT data window on the shape of the measured CRF are considered. For large signals, the measured CRF is nearly independent of the additive noise, but at small signal values additive noise introduces a small bias towards larger amplitudes. The VEP signal-plus-noise distribution was modeled as a family of Rice distributions in order to evaluate the effects of bias on the estimates of threshold. The amount of bias depends inversely upon the slope of the CRF. The amount of bias introduced by a smoothing window also depends upon slope of the CRF as well as the sweep rate. The combined effects of additive noise and window bias were such that the total bias was nearly independent of CRF slope. Sweep VEP contrast thresholds were shown empirically to be unaffected by changes in the range of contrast swept.

    View details for Web of Science ID A1989T971300012

    View details for PubMedID 2603399

  • ABNORMAL ACUITY DEVELOPMENT IN INFANTILE ESOTROPIA INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Day, S. H., ORELBIXLER, D. A., Norcia, A. M. 1988; 29 (2): 327-329

    Abstract

    Monocular and binocular grating acuities were measured using a swept spatial frequency visual evoked potential (VEP) technique in a group of fifteen infants with esotropia and alternating fixation. Both monocular and binocular acuity measures fell significantly below the mean for age-matched normals. Infants with esotropia and alternating fixation did not have significant interocular acuity differences.

    View details for Web of Science ID A1988M122000023

    View details for PubMedID 3338891

  • HIGH VISUAL CONTRAST SENSITIVITY IN THE YOUNG HUMAN INFANT INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE Norcia, A. M., Tyler, C. W., Hamer, R. D. 1988; 29 (1): 44-49

    Abstract

    The visual evoked potential (VEP) was used to estimate photopic contrast sensitivity of 10-week-old infants over a wide range of spatial frequencies including the acuity limit. Adult and infant VEP contrast sensitivity was compared for sinusoidal luminance gratings reversed in contrast at 6 Hz. Space-average luminance was 220 cd/m2. Grating contrast was swept from well below the measured thresholds to well above them in 10 sec trials. Contrast thresholds were defined as the zero voltage intercept of the initial rising portion of the VEP amplitude versus contrast function. The VEP contrast sensitivity of 10-week-old infants was close to that of the adults for spatial frequencies below about 1 cycle (c)/deg.

    View details for Web of Science ID A1988L896700006

    View details for PubMedID 3335433

  • Visual acuity development in normal and abnormal preterm human infants. Journal of pediatric ophthalmology & strabismus Norcia, A. M., Tyler, C. W., Piecuch, R., Clyman, R., GROBSTEIN, J. 1987; 24 (2): 70-74

    Abstract

    The grating acuity of preterm infants was determined by measurements of the visual evoked potential (VEP) produced by phase alternation of sinusoidal luminance gratings. The development of visual acuity in healthy preterm infants appears to be accelerated when compared with full term infants of the same post-conceptual age. Cortical insults and the more advanced stages of retinopathy of prematurity may adversely affect acuity development as indexed by the VEP. Preterm twins also appear to have lower acuity than preterm monoparous infants.

    View details for PubMedID 3585654

  • DIFFERENTIAL GROWTH OF ACUITY FOR STEADY-STATE PATTERN REVERSAL AND TRANSIENT PATTERN ONSET-OFFSET VEPS CLINICAL VISION SCIENCES ORELBIXLER, D. A., Norcia, A. M. 1987; 2 (1): 1-9
  • PHOTOREFRACTION WITH A CATADIOPTRIC LENS - IMPROVEMENT ON THE METHOD OF KAAKINEN ACTA OPHTHALMOLOGICA Norcia, A. M., Zadnik, K., Day, S. H. 1986; 64 (4): 379-385

    Abstract

    Kaakinen (1979) presented a method of measuring refractive error and ocular alignment by simultaneous photography of corneal and fundus reflexes. As presented, the technique was unable to detect refractive errors of less than 2-3 diopters (Howland 1980; Kaakinen 1979). We demonstrate that the use of a catadioptric lens and a long working distance can improve sensitivity to less than 1.0 D. Refractions of a model eye, an accommodating eye and ametropic eyes indicate that refractive errors greater than 0.75 D are readily detectable. The improvement in sensitivity achieved by the present system is attributable to a reduction of the angle between the flash source and the entrance pupil of the photorefractor.

    View details for Web of Science ID A1986E199000003

    View details for PubMedID 3776500

  • COMPARATIVE-STUDY OF ELECTROPHYSIOLOGICAL AND PSYCHOPHYSICAL MEASUREMENT OF THE CONTRAST SENSITIVITY FUNCTION IN HUMANS AMERICAN JOURNAL OF OPTOMETRY AND PHYSIOLOGICAL OPTICS Allen, D., Norcia, A. M., Tyler, C. W. 1986; 63 (6): 442-449

    Abstract

    We measured the human contrast sensitivity function (CSF) both electrophysiologically with the steady-state visual evoked potential (VEP) and psychophysically using a method of ascending limits. VEP contrast thresholds were determined using a rapid recording technique in which the contrast of a counterphase modulated sine wave grating was swept logarithmically from 0.5 to 40% over a period of 10 s. For this pattern reversal stimulus the amplitude and phase of the second harmonic response as a function of contrast were measured using a discrete Fourier transform (DFT). Psychophysical thresholds were determined on the same trials used to record the VEP. Near threshold the VEP amplitude vs. contrast function was approximately linear and VEP contrast thresholds were estimated by a linear extrapolation to zero amplitude. The contrast thresholds obtained by the two methods correlated at 0.914 for 5 observers, with a mean discrepancy of only 12%. At higher contrasts, the VEP amplitude vs. contrast function often became nonmonotonic, sometimes showing two amplitude peaks.

    View details for Web of Science ID A1986C735000008

    View details for PubMedID 3728639

  • METHODS FOR THE IDENTIFICATION OF EVOKED-RESPONSE COMPONENTS IN THE FREQUENCY AND COMBINED TIME FREQUENCY DOMAINS ELECTROENCEPHALOGRAPHY AND CLINICAL NEUROPHYSIOLOGY Norcia, A. M., Sato, T., Shinn, P., MERTUS, J. 1986; 65 (3): 212-226

    Abstract

    Two prominent frequency components designated f1 and f2 have been identified in the visual evoked response to the transient presentation of sinusoidal luminance gratings in the range of 0.5-8 c/deg. The components occur at temporal frequencies below the alpha band, with the f1 frequency being roughly half that of the f2 frequency. The f1 component is largest at low spatial frequencies with f2 becoming progressively dominant as spatial frequency is increased. The frequency and amplitude of f1 and f2 change substantially over the time course of the response. This has been studied by calculating the temporal frequency spectrum of the transient evoked potential over successive short-time epochs running through the response. Using this technique, the response is shown to consist of narrow-band frequency peaks or 'formants' emerging at different times after stimulus onset. These formants occur at frequencies other than those of the spontaneous EEG and undergo changes in frequency and amplitude over the time course of the response. Two spectrum analysis techniques were employed: the Discrete Fourier Transform and Linear Predictive Coding. Frequency components were successfully identified in single-trial responses using the LPC technique.

    View details for Web of Science ID A1986C198900007

    View details for PubMedID 2420574

  • ELECTROPHYSIOLOGICAL ASSESSMENT OF CONTRAST SENSITIVITY IN HUMAN INFANTS AMERICAN JOURNAL OF OPTOMETRY AND PHYSIOLOGICAL OPTICS Norcia, A. M., Tyler, C. W., Allen, D. 1986; 63 (1): 12-15

    Abstract

    Contrast sensitivity functions (CSF's) were measured for a group of 6-month-old infants using the visual evoked potential (VEP). Sine-wave luminance gratings were counterphase modulated at 12 contrast reversals per s and simultaneously swept in contrast. Each contrast sweep lasted 10 s, spanning a range of 0.5 to 40% contrast in 19 equal logarithmic steps. The amplitude and phase of the response at the second harmonic were determined by a discrete Fourier transform. Contrast thresholds were estimated from a linear extrapolation to zero-amplitude of the VEP amplitude vs. log-contrast function. Contrast sensitivity was found to be nearly adult-like at 1 c/deg, but was not yet mature at higher spatial frequencies.

    View details for Web of Science ID A1986AXY7900003

    View details for PubMedID 3942183

  • PHOTOGRAPHIC DETECTION OF AMBLYOGENIC FACTORS OPHTHALMOLOGY Day, S. H., Norcia, A. M. 1986; 93 (1): 25-28

    Abstract

    Refractive errors were determined photographically in a group of infants and children and were compared to conventional cycloplegic retinoscopy. The refractor consisted of a mirror telephoto lens and strobe flash designed to mimic the action of a retinoscope. Significant amblyogenic conditions such as anisometropia and high isoametropia were detectable. Strabismus and media opacities were also recordable. The technique provides a potential mass suited for infants.

    View details for Web of Science ID A1986AYV8900006

    View details for PubMedID 3951813

  • ELECTROPHYSIOLOGICAL EVIDENCE FOR THE EXISTENCE OF COARSE AND FINE DISPARITY MECHANISMS IN HUMAN VISION RESEARCH Norcia, A. M., Sutter, E. E., Tyler, C. W. 1985; 25 (11): 1603-1611

    Abstract

    Visual sensitivity to stereoscopic disparity changes was measured both psychophysically and by means of evoked potentials. The binocular disparity of a dynamic random-dot stereogram portraying a single flat plane alternated between two values symmetrical about the plane of fixation. The threshold for disparity alternation of the stereoscopic plane was determined at alternation rates between 4 and 12 depth reversals per second (rps). Evoked potential and forced-choice psychophysical estimates of stereoscopic threshold at each reversal frequency agreed, with a mean discrepancy of only +/- 0.1 log units. Evoked potential amplitude was a linear function of log disparity up to about 15 arc min peak to peak disparity. For larger disparities, the evoked potential amplitude versus log disparity function was found to be nonmonotonic with a dip occurring at approximately 26 arc min disparity. Responses to fine disparities of less than 20 arc min lay close to one temporal phase while those evoked by coarse disparities greater than 40 arc min lay near a different phase. The data suggest that disparity processing mechanisms either undergo dynamic changes as disparity increases or that processing shifts between at least two independent mechanisms.

    View details for Web of Science ID A1985AVZ8700008

    View details for PubMedID 3832583

  • INFANT VEP ACUITY MEASUREMENTS - ANALYSIS OF INDIVIDUAL-DIFFERENCES AND MEASUREMENT ERROR ELECTROENCEPHALOGRAPHY AND CLINICAL NEUROPHYSIOLOGY Norcia, A. M., Tyler, C. W. 1985; 61 (5): 359-369

    Abstract

    The reliability of visual evoked potential (VEP) measurements of acuity was determined by estimating acuity for sinewave luminance gratings which were counterphase modulated at either 12 or 20 reversals/sec (rps). Gratings were swept in spatial frequency beyond the acuity limit and acuity was estimated from an extrapolation based on the last peak in the VEP amplitude versus spatial frequency function. Twenty-five infants ranging in age from 17 to 25 weeks were studied. Individual 10 sec sweeps resulted in records with a criterion response in 65-75% of trials. The reliability of acuities obtained from individual 10 sec sweeps was +/- 0.54 octaves at 95% confidence across 12 and 20 rps recording conditions (RMS error of +/- 0.27 octaves). The best acuity attained by each infant on either a single sweep, or on their vector average, was reliable to +/- 0.38 octaves at 95% confidence (RMS error of +/- 0.19 octaves) compared to a range of individual acuities of about 2 octaves. Much of the variability of sweep VEP acuity in cross-sectional samples of infants is therefore attributable to reliable individual differences rather than to measurement error. In testing individual infants our analysis indicates that choice of temporal frequency accounts for only 14% of the variation in acuity estimates within subjects.

    View details for Web of Science ID A1985AUC7000004

    View details for PubMedID 2412787

  • SPATIAL-FREQUENCY SWEEP VEP - VISUAL-ACUITY DURING THE 1ST YEAR OF LIFE VISION RESEARCH Norcia, A. M., Tyler, C. W. 1985; 25 (10): 1399-1408

    Abstract

    The grating acuity of 197 infants from 1 week to 53 weeks of age was measured using the visual evoked potential (VEP) in response to counterphase grating stimulation. The gratings were presented as a 10 sec spatial frequency sweep which spanned the acuity limit. The amplitude and phase of the second harmonic response were extracted by discrete Fourier analysis. The VEP amplitude versus spatial frequency function showed narrow spatial frequency tuning with amplitude peaks at one or more spatial frequencies. The phase of the response at medium to high spatial frequencies was generally constant at a spatial frequency peak, followed by a progressive phase lag with increasing spatial frequency. Grating acuity was estimated by linear extrapolation to zero microvolts of the highest spatial frequency peak in the VEP amplitude versus spatial frequency function. This visual acuity estimate increased from a mean of 4.5 c/deg during the first month to about 20 c/deg at 8-13 months of age. The VEP acuities at 1 month are a factor of three to five higher than previously reported for pattern reversal or pattern appearance stimuli. By 8 months VEP grating resolution was not reliably different from adult levels in the same apparatus.

    View details for Web of Science ID A1985AUP2000005

    View details for PubMedID 4090273

  • DIGITAL FILTERING AND ROBUST REGRESSION TECHNIQUES FOR ESTIMATING SENSORY THRESHOLDS FROM THE EVOKED-POTENTIAL IEEE ENGINEERING IN MEDICINE AND BIOLOGY MAGAZINE Norcia, A. M., Clarke, M., Tyler, C. W. 1985; 4 (4): 26-32

    View details for Web of Science ID A1985AVV5700003

    View details for PubMedID 19493780

  • TEMPORAL FREQUENCY LIMITS FOR STEREOSCOPIC APPARENT MOTION PROCESSES VISION RESEARCH Norcia, A. M., Tyler, C. W. 1984; 24 (5): 395-401

    Abstract

    Temporal processing limits were determined for two types of stereoscopic percept associated with square wave disparity alternation: apparent depth motion and depth pulsation. The stimuli were dynamic random dot stereograms containing no monocular cues for either target motion or disparity change. The percept of a single noise plane undergoing apparent depth motion coincided with the extent of a large peak in the low frequency portion of the evoked potential amplitude spectrum. The limit for apparent depth motion was approximately 6 Hz. Above this frequency two pulsating depth planes were seen simultaneously. Depth pulsations wer visible up to 14 Hz and an evoked potential occurred in synchrony with each disparity change (up to 28 depth reversals/sec). Above 14 Hz two transparent planes were perceived without depth pulsation and no stereoscopic evoked potential could be recorded. The results indicate a higher temporal resolution for stereoscopic position change than has been reported in previous studies of apparent depth motion.

    View details for Web of Science ID A1984SQ13800001

    View details for PubMedID 6740960

  • EVENT-RELATED BRAIN POTENTIALS TO HUMAN FACES IN INFANTS CHILD DEVELOPMENT Courchesne, E., Ganz, L., Norcia, A. M. 1981; 52 (3): 804-811

    Abstract

    Event-related brain potentials (ERPs) in response to tachistoscopically presented photographs of 2 human faces were recorded for 4--7-month-old infants. For each infant 1 face was chosen to be presented frequently (p = .88, a low-information event) and the other infrequently (p = .12, a high-information event). Both types of events elicited in our infants a long-latency negative ERP wave (ca. 700 msec), termed Nc, and a long-latency positive wave (ca. 1,360 msec), termed Pc. We found that the discrepant, infrequently presented face elicited Nc waves which were higher in amplitude and longer in latency than those elicited by the frequent face. These differences suggest that our infants were able to remember the frequently presented face from trial to trial and to discriminate it from the discrepant face. The discrepant event elicited Pc waves which were insignificantly higher in amplitude than those elicited by frequent events. In adults and children, discrepant events have been found by numerous researchers to elicit positive P3 waves (latency ca. 300--800 msec). In our study, however, such waves could not be discerned. So, of all of the ERP waves which have been related to cognitive processes, the wave which is maturationally the earliest to appear is the Nc wave, which has been related to the perception of attention-getting events or events of interest to the subject. Our findings suggest that ERP responses could provide a sensitive means for investigating infant cognitive development since they do not depend upon an integrated motor-response system.

    View details for Web of Science ID A1981MH02200004

    View details for PubMedID 7285651

Conference Proceedings


  • Contrast response characteristics of long-range lateral interactions in cat striate cortex Chen, C. C., Kasamatsu, T., Polat, U., Norcia, A. M. LIPPINCOTT WILLIAMS & WILKINS. 2001: 655-661

    Abstract

    Single-cell responses in visual cortex to a target falling within their receptive field can be modified by collinear flanking stimuli concurrently presented outside the receptive field. Here, we report the presence of four types of contrast-dependent lateral effects: (1) facilitation at low target contrasts and suppression at high contrasts, (2) facilitation that increases with contrast, (3) suppression that increases with contrast, and (4) suppression at low contrasts with facilitation at high contrasts. We propose a sensitivity modulation model that accounts for all the four types of lateral effects by changes in two parameters. In this model, activation of neighboring neurons changes the sensitivities of the target neuron to both the direct feedforward input and inhibitory, divisive feedback from neighboring neurons.

    View details for Web of Science ID 000167504700007

    View details for PubMedID 11277558

  • Development of rivalry and dichoptic masking in human infants Brown, R. J., Candy, T. R., Norcia, A. M. ASSOC RESEARCH VISION OPHTHALMOLOGY INC. 1999: 3324-3333

    Abstract

    To examine the development of rivalry, dichoptic masking, and binocular interactions in infants more than 5 months of age using the visual evoked potential (VEP).VEPs were recorded in 35 infants between 5 and 15 months of age and 23 adults between 13 and 59 years of age. Counterphasing, sinusoidal, 1 cycle/deg gratings were presented dichoptically. Responses from each eye were isolated by "tagging" each half-image with a different temporal frequency (5 or 7.5 Hz). Observers were presented with fixed 80% contrast gratings in each eye in experiment 1. Rivalry was detected on the basis of a negative correlation between the simultaneously measured response amplitudes at the second harmonics of the two eye-tagging frequencies. In a second analysis of the same data, response amplitudes recorded under dichoptic viewing conditions were compared to those obtained in a monocular control condition (dichoptic masking). In experiment 2, a 40% fixed-contrast grating was presented to one eye, whereas the other eye viewed a grating that was swept in contrast from 1% to 67%. Dichoptic masking was measured as the reduction in the fixed-grating response caused by the variable contrast grating.Experiment 1: although adults showed evidence of VEP amplitude alternations between the eyes for cross-oriented half-images (physiological rivalry), infants did not. This immature response to rivalrous stimuli occurred despite the presence of responses at nonlinear combination frequencies recorded with gratings of the same orientation in each eye, a definitive indication of binocular interaction. In addition, both iso- and cross-oriented half-images produced less dichoptic masking in infants than in adults in this experiment. Experiment 2: dichoptic masking in the infants was equivalent to that seen in adults with parallel gratings in the two eyes; however, masking with cross-oriented configurations was approximately five times weaker in the infants relative to the adults.The authors have identified a set of stimulus conditions under which infants between 5 and 15 months of age fail to demonstrate physiological rivalry despite the presence of binocular interactions. The observed lack of binocular rivalry may be the result of a specific immaturity in dichoptic, cross-orientation suppression.

    View details for Web of Science ID 000083939500036

    View details for PubMedID 10586959

  • Measurement of vernier and motion sensitivity with the rapid-sweep-VEP Wesemann, W., Norcia, A. M., Manny, R. E. FERDINAND ENKE VERLAG. 1996: 11-17

    Abstract

    The visual abilities of infants and small children, who are unable to communicate verbally, can be determined with "objective" visual acuity tests as, e.g., preferential looking (PL) or visual evoked potentials (VEP). Both methods provide an estimate of grating visual acuity, but are unable to determine optotype visual acuity. Grating acuity, however, is not an optimum indicator for visual performance, as it is less affected than optotype acuity by pathological changes. The diagnostic relevance of objective methods may be improved by testing visual functions that are more strongly degraded by a malfunction of the fovea than is grating acuity. Visual functions such as vernier acuity and relative motion sensitivity are potential candidates.Characteristic properties of vernier- and motion-VEP have been determined on adult subjects with a rapid-sweep-technique at various eccentricities.Both vernier and motion stimuli elicited VEP-responses at the 2nd harmonic (F2) of the stimulus frequency. A VEP-response at the fundamental frequency (F1) could be recorded with the vernier stimulus only. Foveal VEP-thresholds were very similar to psychophysical thresholds lying in the range from 11 to 25 arcsecs. The eccentricity dependence of the VEP-threshold recorded with vernier- and motion stimuli shows different cortical magnifications and supports the notion that the F1 response is related to the detection of the vernier offset, whereas the F2 response is generated by motion components of the stimuli.Vernier and relative motion-related VEP thresholds can be recorded with the steady-state VEP. They may provide more sensitive tests of foveal vision loss than grating visual acuity tests.

    View details for Web of Science ID A1996TV31000003

    View details for PubMedID 8839339

  • PLASTICITY OF HUMAN MOTION PROCESSING MECHANISMS FOLLOWING SURGERY FOR INFANTILE ESOTROPIA Norcia, A. M., Hamer, R. D., Jampolsky, A., ORELBIXLER, D. PERGAMON-ELSEVIER SCIENCE LTD. 1995: 3279-3296

    Abstract

    Monocular oscillatory-motion visual evoked potentials (VEPs) were measured in prospective and retrospective groups of infantile esotropia patients who had been aligned surgically at different ages. A nasalward-temporal response bias that is present prior to surgery was reduced below pre-surgery levels in the prospective group. Patients in the retrospective group who had been aligned before 2 yr of age showed lower levels of response asymmetry than those who were aligned after age 2. The data imply that binocular motion processing mechanisms in infantile esotropia patients are capable of some degree of recovery, and that this plasticity is restricted to a critical period of visual development.

    View details for Web of Science ID A1995TE46000009

    View details for PubMedID 8560799

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