Evoked responses of human auditory cortex may be enhanced by preceding stimuli.

We report enhancement of the 100 msec deflection N100m of the auditory evoked magnetic field in paired-stimulus paradigms. Noise bursts of 50 msec duration were delivered in pairs to the left ear at interpair intervals of 1.2-1.4 sec. Stimulus onset asynchrony (SOA) within the pair was either 70, 150, 230, 300, 370 or 500 msec, all intervals being presented randomly within the same block. Magnetic responses were recorded over the right hemisphere with a 7-channel first-order SQUID gradiometer. The mean amplitude of N100m to the second stimulus was maximal at an SOA of about 150 msec, decreasing at longer SOAs to an amplitude about equal to that of the N100m evoked by the first stimulus. Similar enhancement effects were elicited by noise bursts, square-wave tones and sinusoidal tones, by pauses in a continuous noise, and when the two stimuli of a pair were led to different ears.