This study explores how brain changes affect thinking in people with Lewy Body Dementia (LBD) and Parkinson's Disease Dementia (PDD). **Lewy Body Dementia** is a condition where abnormal protein deposits, called Lewy bodies, build up in the brain, affecting memory and thinking. **Cognitive Fluctuations (CF)** refer to changes in attention and thinking ability. The study uses brain scans and a test called **EEG** (Electroencephalogram) to measure brain waves. Participants include those with LBD or PDD, some of whom will try a medication called **cholinesterase inhibitor** (helps memory) for 8 weeks. The study lasts for 2 years, with yearly follow-ups to see how thinking changes over time.

• The research lasts 2 years, with annual follow-ups.
• Participants in the medication group will try the treatment for 8 weeks.
• Participants must be 50 to 89 years old and able to follow study procedures.