Three channels of ERP data (FZ, CZ, PZ, and referenced to linked ear lobes with a forehead ground, international 10–20 system) were obtained by using gold-plated electrodes with impedance held below 5K ohms. An electrode placed left lateral infraorbitally and referenced to the left earlobe was used to monitor both horizontal and vertical eye movement. ERP signals were recorded on a Nihon-Kohden polygraph amplified (sensitivity 7 microvolts/mm, time constant 0.1 s, 35 Hz low pass). Signals were transferred to an Apple computer and digitized at a rate of 256 Hz. The EEG amplifier input range corresponding to the full range of the 12-bit analog-to-digital converter was about +/- 250 microvolts. Periodic calibration results were used to scale the digitized EEG to microvolts. Auditory stimuli and ERPs were elicited using an oddball plus “noise” paradigm described previously (Ehlers et al., 1996, 1998a). Each subject was instructed to depress a counter each time he detected a rare tone. Individual trials containing excessive eye movement artifact as well as trials where the EEG exceeded ±250 microvolts (< 5% of the trials) were eliminated.
