Data analysis: Subjects will be recruited according to a two-part Gehan design. A "responder" is defined as a 30% reduction in the tremor score. To assess whether memantine has a potential responder rate of 30 percent, 9 subjects will be recruited in the first phase. If at least one subject is a responder, another 16 subjects will be recruited to estimate the actual responder rate with a standard error of 10%.