The electroencephalogram (EEG) was recorded continuously throughout the experimental task from 32 tin electrodes fixed in a stretch-lycra cap (ElectroCap, Eaton, OH) placed on the scalp in standard locations (Electrode Position Nomenclature Committee, 1994) and referenced to the right mastoid; an average mastoid reference was derived offline. The EEG signal was amplified with a Synamps2 amplifier (Compumedics Neuroscan, Charlotte, NC), filtered on-line at 0.05 to 40 Hz and sampled at 1000 Hz. Electrode locations were cleaned until the measured impedance of the skin was below 5 kΩ. Ocular artifacts (blinks) were removed from the EEG off-line using a regression-based procedure (Semlitsch, Anderer, Schuster, & Presslich, 1986). EEG data were segmented into epochs of −100 to 1200 ms of post-stimulus activity to derive stimulus-locked ERPs, and −400 to 600 ms of post-response activity to derive response-locked ERPs. Epochs containing artifacts (e.g., muscle movement) were rejected (on the basis of visual inspection of each participant’s single-trial waveforms) prior to averaging according to participant and stimulus conditions.
