Subjects were presented binaurally with two tones of different frequencies. One stimulus was a low tone (600 Hz) and the other a high tone (1600 Hz) produced by a tone generator. Each stimulus had a 60 ms. duration (10 ms. rise and fall times and 40 ms. plateau) and an intensity level of 60 dB. A computer initiated the stimulus. The probabilities of the rare and the frequent tones were 0.125 and 0.875 respectively. The use of the low or high tone as the rare (target) tone was alternated across the subjects. The auditory stimuli were presented through headphones (model ER-3A Tubephone Insert Earphones, 50 W impedance; Etymotic Research, Elk Grove Village, IL); the earpiece and a short length of the Tubephone were fitted under the electrode cap, and the individual left and right transducer cases were situated on either side of the neck. Subjects received a maximum of 400 trials with a uniform interstimulus interval of 1500 ms.
