Auditory evoked fields covary with perceptual grouping.

Abrupt acoustic events evoke a transient magnetic response (N100m) at the supratemporal plane. Such responses decrease in amplitude as the interval between successive stimuli decreases to about 1 s. However, when pairs of stimuli are separated by still smaller intervals the second stimulus evokes a larger response than the first. This enhancement depends on the duration of the pause between the offset of the first stimulus and onset of the second. The range over which enhancement of N100m is observed agrees quite well with the range over which subjects experience perceptual grouping of the two stimuli with loudness enhancement of the second. Recordings from multi-channel SQUID gradiometers show that the effect involves not only a change in source strength but also a change in source location within supratemporal cortex. The results suggest that inhibition induced by onset of the first stimulus may be disinhibited by its offset. Physiological and psychological implications are discussed.