Auditory evoked fields to illusory sound source movements.

Auditory motion can be simulated by presenting binaural sounds with time-varying interaural intensity differences. We studied the human cortical response to both the direction and the rate of illusory motion by recording the auditory evoked magnetic fields with a 122-channel whole-head neuromagnetometer. The illusion of motion from left to right, right to left, and towards and away from the subject was produced by varying a 6-dB intensity difference between the two ears in the middle of a 600-ms tone. Both the onset and the intensity transition within the stimulus elicited clear responses in auditory cortices of both hemispheres, with the strongest responses occurring about 100 ms after the stimulus and transition onsets. The transition responses were significantly earlier and larger for fast than slow shifts and larger in the hemisphere contralateral to the increase in stimulus intensity for azimuthal shifts. Transition response amplitude varied with the direction of the simulated motion, suggesting that these responses are mediated by directionally selective cells in auditory cortex.