three square wave tones (rise/fall times,1 ms): a frequent tone (50 ms, 2 KHz, 70 dB SPL) presented on 83% of the trials (n=200), an infrequent tone (50 ms, 2 KHz, 80 dB SPL) presented on 10.4% of the trials (n=25), and a noise burst (50 ms, noise, 80 dB SPL) presented on 6.3% of the trials (n=15). Each subject was hearing tested prior to the recording to ensure that he had no trouble easily identifying the three different tones. Infrequent tones were interspersed with frequent tones so that no two rare tones occurred consecutively. A noise burst was substituted for an infrequent tone every 12 trials to avoid habituation to the infrequent tone. The digitizing epoch was 1 s and a 0.5- to 1.0-s inter-trial interval was used. The ERP trials were digitized at a rate of 256 Hz. The total number of trials in each recording session was 240. These stimulus characteristics have been used extensively in our laboratory and have been described previously (see [42, 46, 53]). Each subject was instructed to depress a counter each time he detected an infrequent (target) tone. Individual trials containing excessive eye movement artifact as well as trials where the EEG
