Spatio-temporal constraints for auditory-visual integration

The perceptual coherence of auditory and visual information is achieved by integrative brain processes. Specialized single neurons with spatial and te mporal interactions of auditory and visual stimuli have been demonstrated b y several neurophysiological studies. The present, psychophysical, study in vestigates possible perceptual correlates of these neuronal features. Subje cts had to indicate the point of subjective spatial alignment (PSSA) for a horizontally moving visual stimulus that crossed the position of a stationa ry sound source. Auditory and visual stimuli consisted of periodic pulses t hat were systematically varied in their phase relationship or repetition ra te. PSSAs obtained for continuous visual stimuli served as a reference. Whe n sound and light pulses were coincident in phase at a repetition rate of 2 Hz, PSSAs were shifted by similar to 3 degrees in a direction opposite to the movement of the visual stimulus (with respect to the reference conditio n). This shift markedly decreased when the temporal disparity exceeded simi lar to 100 ms and disappeared near phase opposition (250 ms disparity). Wit h 4 Hz repetition rate (temporal disparity less than or equal to 125 ms), t here was no significant effect of phase relationship on PSSAs, but still an approximately constant shift with respect to the reference value. Variatio n of the repetition rate resulted in almost constant shifts in PSSA of simi lar to 3 degrees between 1 and 4 Hz and a linear decrease (slope 0.27 degre es /Hz) with higher repetition rates. These results suggest a spatio-tempor al 'window' for auditory-visual integration, that extends over similar to 1 00 ms and similar to 3 degrees: when auditory and visual stimuli are within this window, they are always perceived as spatially coincident. These psyc hophysical findings may be related to properties of bimodal neurons such as have been demonstrated by neurophysiological recordings in midbrain and co rtex. (C) 2001 Elsevier Science B.V. All rights reserved.