The present study is a proof-of-concept clinical trial to test the efficacy of low doses of a repurposed anti-epileptic drug (levetiracetam) in treating memory problems in Parkinson's disease (PD). Neuroimaging techniques will be used to determine the effect of the drug on specific brain regions (hippocampal subfields). Finally, baseline brain activity of PD patients with memory problems will be compared to PD patients without memory problems and healthy older adults to determine if activity in specific brain regions (hippocampal subfields) can be used to predict memory problems in PD. This information will be useful for future clinical trials to target drugs to these brain regions.