A Study of LY3154207 in Participants With Dementia Due to Lewy Body Dementia (LBD) Associated With Idiopathic Parkinson's Disease (PD) or Dementia With Lewy Bodies (DLB)

Inclusion Criteria:

   - Have dementia as defined by a decline in cognitive function, which in the opinion of
   the investigator has resulted in functional impairment.

   - Meet diagnostic criteria for PD per MDS criteria or DLB per 4th Consensus Report of
   the DLB Consortium.

   - Have a score on the MoCA of 10 - 23.

   - Are Modified Hoehn and Yahr Stages 0 - 4.

   - Have a blood pressure (BP) or pulse rate at screening and randomization, as determined
   by three sequential BP/pulse rate measurements in a seated position:

      - Participants <60 years old:

         1. A mean systolic BP less than or equal to 140 millimeters of mercury (mmHg),
         a mean diastolic BP less than or equal to 90 mmHg and a mean pulse rate less
         than or equal 90 beats/minute in a seated position.

         2. Each of the 3 systolic BP measurement must be less than 180 mmHg

      - Participants ≥60 years old:

         1. A mean systolic BP less than or equal to 150 mmHg, a mean diastolic BP less
         than or equal to 90 mmHg and a mean pulse rate less than or equal to 90
         beats/min in a seated position.

         2. Each of the 3 systolic BP measurement must be less than 180 mmHg

   - If on anti-parkinsonian agents, participants must be on stable dosage for at least 3
   weeks prior to screening, and should remain on stable doses during the course of the
   study.

   - If on medications affecting cognition (rivastigmine, galantamine, donepezil,
   memantine), participants must be on stable dosage for at least 3 weeks prior to
   screening and should remain at a stable dosage during the course of the study.

   - If on antidepressant medications, participants must be on stable dosage for at least 3
   weeks prior to screening and should remain at a stable dosage during the course of the
   study.

   - If on clozapine, quetiapine, and pimavanserin to address drug induced or disease
   related psychosis, participants must be on stable dosage for 3 weeks prior to
   screening and should remain at a stable dosage during the course of the study.

   - If on antihypertensive medications, participants must be on stable dosage for at least
   3 weeks prior to screening.

   - Men should use appropriate contraception.

   - All participants must have a reliable caregiver who is in frequent contact with the
   participant (defined as at least 10 hours per week) and will accompany the participant
   to screening, baseline, day 7, day 42, day 84 and follow-up.

Exclusion Criteria:

   - Are women of childbearing potential.

   - Have significant central nervous system or psychiatric disease, other than PD or DLB,
   that in the investigator's opinion may affect cognition or the ability to complete the
   study.

   - Have a history in the last 6 months of transient ischemic attacks or ischemic stroke.

   - Have a history of intra cerebral hemorrhage due to hypertension.

   - Have a history of hypertensive encephalopathy.

   - Have atypical or secondary parkinsonism due to drugs (e.g., antipsychotics) or disease
   (such as progressive supranuclear palsy, essential tremor, multiple system atrophy
   (e.g. striatonigral degeneration, olivopontocerebellar atrophy), or postencephalitic
   parkinsonism).

   - Have a current implantable intracranial stimulator or history of intracranial ablation
   surgery (e.g., subthalamic, globus pallidus-internal segment [GPi]).

   - Have a history of substance abuse within the past 1 year (drug categories defined by
   the Diagnostic and Statistical Manual of Mental Disorder, 5th Edition [DSM-5], and/or
   substance dependence within the past 1 year, not including caffeine and nicotine.

   - Have a serious or unstable medical illness, other than idiopathic LBD (PDD or DLB),
   including cardiovascular, hepatic, respiratory, hematologic, endocrinologic,
   neurologic, or renal disease, or clinically significant laboratory or
   electrocardiogram (ECG) abnormality as determined by the investigator.

      - Have a history in the last 6 months of exertional angina, unstable angina,
      myocardial infarction, and acute coronary syndrome.

      - Have a history of heart failure of either New York Heart Association Class III or
      IV.

      - A history of additional risk factors for Torsades de Pointes (TdP; [e.g., chronic
      hypokalemia, family history of Long QT Syndrome]).

   - Participants with acute liver disease (e.g. acute viral hepatitis, alcoholic
   hepatitis); participants with a known chronic liver disease (e.g. hepatitis B, C,
   alcoholic liver disease, cirrhosis); alanine aminotransferase (ALT) or aspartate
   aminotransferase (AST) equal to or higher than 2X upper limit of normal (ULN); total
   bilirubin (TBL) equal to or higher than 1.5X ULN; (except for participants with
   Gilbert's syndrome); or alkaline phosphatase (ALP) equal to or higher than 2X ULN.

   - Participants have answered 'yes' to either Question 4 (Active Suicidal Ideation with
   Some Intent to Act, Without Specific Plan) or Question 5 (Active Suicidal Ideation
   with Specific Plan and Intent) on the "Suicidal Ideation" portion of the Columbia
   Suicide Severity Rating Scale (C-SSRS)- Children's version, or answer "yes" to any of
   the suicide-related behaviors (actual attempt, interrupted attempt, aborted attempt).

   - Have used antipsychotic medications, with the exception of clozapine, quetiapine,
   pimavanserin in the 6 months prior to screening and at any time during the course of
   the study.

   - Have used anticholinergics trihexyphenidyl and benztropine in the 4 weeks prior to
   screening and at any time during the course of the study.

   - Have motor conditions for which the antiparkinsonian treatment is expected to change
   during the course of the study, as well as unpredictable motor fluctuations that in
   the investigator's opinion would interfere with administering assessments.

   - Are taking any medications or food, herbal or dietary supplements that are inhibitors
   (e.g., ketoconazole, grapefruit juice), or strong/moderate inducers of cytochrome P450
   3A4 (CYP3A4) (e.g., rifampicin) or are unable or unwilling to discontinue usage of
   them 4 weeks prior to first dose of study drug.