People with epilepsy often experience problems with their memories and other thinking skills that get worse over time. The investigators hope to learn more about whether a drug called memantine can help improve or stabilize (keep the same) memory and other thought processes in people with epilepsy by blocking a chemical that is released in the brain during seizures. The investigators also want to see if memantine changes the frequency (how often) people with epilepsy have seizures. Memantine is currently approved by the United States Food and Drug Administration (FDA) for treatment of patients with Alzheimer's disease.