HUMAN AUDITORY CORTICAL MECHANISMS OF SOUND LATERALIZATION .1. INTERAURAL TIME DIFFERENCES WITHIN SOUND

Neuromagnetic responses to 600-ms binaural click trains, presented once every 1.1 s, were recorded with a 24-channel gradiometer from 6 healthy humans. During the first 300 ms, the left-ear stimulus led the right by 0.7 ms and the sound was lateralized to the left ear. At 300 ms, the interaural time difference (ITD) changed and the lateralization moved to one of 5 different locations between the ears. An N100m response peaked about 110 ms after the sound onset and an N130m(c) response (c to stress a response to the change) about 135 ms after the ITD change. The source locations of N100m and N130m(c) agreed with activity in the supratemporal auditory cortex; this was confirmed in one subject by superimposing MEG results on MR images. The sources of N100m and N130m(c) did not differ statistically significantly from each other, nor were there differences in N130m(c) sources to various lateralization changes. N130m(c) grew larger when the ITD change increased, in parallel with the increase in the change of the perceived location. We suggest that N130m(c) is analogous to N100m, but is delayed due to postmasking induced by the early part of the sound.