Titration During the Titration Phase of each Drug Treatment Period, doses of levetiracetam (or placebo) will be started at 250 mg twice a day (500 mg total daily dose) and increased over a period of two weeks to a target maintenance dose of 1000mg/day in divided dose, according to the dosing schedule outlined in the table that follows (Appendix 2). Upon achieving the target maintenance dose, subjects will receive this dose for one week during the Maintenance Phase. Following the Maintenance Phase, study drug will be tapered off over a period of one week and maintained off drug for one week (weeks 6 through 7) (washout period). The second treatment period will then begin (6 weeks).