EEG activity was amplified by a factor of 10 K (Sensorium EPA-2 Electrophysiology Amplifiers), filtered to pass frequencies between 0.02 and 50 Hz, and sampled at a rate of 256 Hz. EEG epochs from 187 ms preceding to 750 ms following the onset of each stimulus were retained by the computer and processed offline through a 32-Hz low-pass digital filter. Epochs with eye movement or EEG voltage deviations > 73.3 microvolts (peak-to-peak) were excluded from analysis. Remaining epochs were formed into time-point averages sorted by stimulus type.
