Subjects were seated on a comfortable chair in a sound-attenuated, temperature-regulated, and dimly lit booth (Industrial Acoustics Company; Bronx, NY). An Electro-Cap (Electro-Cap International Inc.; Eaton, OH) with 32 leads based on the International 10-20 system (Jasper, 1958) was placed on the scalp of each subject. A forehead electrode served as the ground and the nose electrode served as the common reference. Electrode impedance was maintained below 5 kΩ. Vertical and horizontal electro-ocular (EOG) activities were recorded with the electrodes placed supraorbitally and at the outer canthus of the left eye. Amplifier gain was set 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 ms prior to and continuing for 1413 ms after the stimulus onset. Artifact rejection thresholds were set at 73.3 μV. Waveforms were computed using epochs filtered with 32 Hz low pass digital filter. The epochs were 1000 milliseconds
