4-Week Double Blind Treatment Phase: The study is a 4-week, placebo controlled, double blind study. Subjects will be randomized to either: MK-0777 GEM 3mg BID; MK-0777 GEM 8mg BID; or placebo. The unblinded site pharmacist will be notified of the treatment assignment, and will dispense study medication. Subjects will receive biweekly symptom assessments and weekly side effect and vital sign assessments. At week 4, subjects will undergo repeat administration of the neuropsychological test battery and the functional capacity and patient self-report of cognitive function measures. These assessments will be done over a two-day period. At week 4, subjects will also undergo a repeat slit lamp eye examination. We will also attempt to contact and schedule subjects who dropped out of the study prior to week 4 for the week 4 slit lamp eye examination. Finally, subjects will have blood samples collected for antipsychotic and MK-0777 levels at week 4.