This study will measure brain excitability in patients with succinic semialdehyde dehydrogenase (SSADH) deficiency, and in their parents. SSADH is a rare inherited disease in which changes in certain brain chemicals affect brain cell activity. Symptoms vary greatly among patients, and may include mental retardation, impaired ability to coordinate movements, and delays in language and speech development. Other symptoms may include poor muscle tone, uncontrolled seizures and other neurological or behavioral abnormalities. Test findings in patients and their parents will be compared with those of healthy normal volunteers.