at 10,000 times on Sensorium EPA-2 Electrophysiology amplifiers (Charlotte, VT), with a high pass filter of 0.02 Hz and low pass filter of 50 Hz and digitized on a Concurrent 5550 computer (Concurrent Computer Corp. Atlanta, GA). The sampling rate was 256 Hz with sampling beginning 187 msec prior to and continuing for 1413 msec after the stimulus onset. Online artifact rejection was used whenever the voltages exceeded ± 73.3 µV. Offline analysis was performed at SUNY Downstate Medical Center, New York. Data were further processed using an 8 Hz low pass digital filter and epochs including 200 ms prior to the stimulus and 800 ms after the stimulus onset were extracted. The average waveforms of each subject were inspected and the records of subjects who had anomalous and artifact contaminated wave forms were removed from further analysis. The trials in which the response time exceeded 1000 msec were excluded from all analyses. Further, only the correct response trials were included. In order to avoid the variation in the number of correct trials, the minimum and maximum numbers of correct trials were taken as 20 and 35 respectively. The mean number of accepted trials for the final analysis was 27
