Additional Study Information: The purpose of this study is to investigate how well a drug called RVT-101 works to improve cognitiveand overall function as well as to investigate the safety of RVT-101. RVT-101 is an investigational drug.Investigational means that the drug tested in this study has not been approved by the U.S. Food and Drug Administration (FDA) or any other Health Authority including European or Canadian Health Authorities. It is believed that patients with dementia with Lewy bodies have an imbalance of acetylcholine, which is a chemical in the brain thought to be responsible for cognition. RVT-101 promotes the release of this chemical. Therefore, RVT-101 is being tested in this Phase 2b study to evaluate whether it can help cognition and overall function in patients with dementia with Lewy bodies.