Involuntary oscillations of the eyes (nystagmus) impairs vision so that affected patients, who have neurological disorders such as Multiple Sclerosis (MS) , cannot read or watch TV. Two medicines have been reported to suppress nystagmus and improve vision in such patients: gabapentin and memantine. The investigators set out to test which of these two drug was more effective by carrying out a double-blind cross-over study. In this way, we could determine which drug worked best in each patient.