and amplified 10,000 times using a set of amplifiers (Sensorium, Charlotte, VT). The continuous EEG was digitally low-pass filtered at 32 Hz and then segmented into epochs of 100 ms pre-stimulus to 750 ms post-stimulus. The mean EEG activity for 100 ms prior to stimulus onset served as the pre-stimulus baseline. All segments exceeding ±75 µV threshold were automatically excluded from further processing. After correcting eye-movement artifacts, the averaged segments for each individual were screened visually for further artifact rejection. The artifact detection was done on all channels including the electroocculogram (EOG) channels. Artifact-free data obtained from 9 channels located in the central scalp were included for genetic analyses.
