The electroencephalogram was recorded throughout the task. For the detection of eyeblink and eye movement artifacts, a pair of electrodes was placed diagonally above and below the left eye. The 31 channels of the EEG and 1 channel of eye movement (EOG) activity were appropriately amplified (EEG gain = 20K, EOG gain = 2K) and filtered (bandpass = 0.01–12 Hz) using a SA Instrumentation Company amplification system. Along with markers indicating stimulus and response onsets, the EEG and EOG channels were routed to an A/D converter and sampled at a rate of 200 Hz for 50 ms preceding and 1950 ms following the onset of each stimulus. During off-line computations, single-trial data were sorted by electrode. Before averaging, trials containing an eye movement deviation greater than 50 μV were deleted. Trials with A/D converter overflow and omitted responses were also deleted.
