Automatic visual bias of perceived auditory location

Studies of reactions to audiovisual spatial conflict (alias "ventriloquism") are generally presented as informing on the processes of intermodal coordination. However, most of the literature has failed to isolate genuine perceptual effects from voluntary postperceptual adjustments. A new approach, based on psychophysical staircases, is applied to the case of the immediate visual bias of auditory localization. Subjects have to judge the apparent origin of stereophonically controlled sound bursts as left or right of a median reference line. Successive trials belong to one of two staircases, starting respectively at extreme left. and right locations, and are moved progressively toward the median on the basis of the subjects' responses. Response reversals occur for locations farther away from center when a central lamp is flashed in synchrony with the bursts than without flashes (Experiment 1), revealing an attraction of the sounds toward the flashes. The effect cannot originate in voluntary postperceptual decision, since the occurrence of response reversal implies that the subject is uncertain concerning the direction of the target sound. The attraction is contingent on sound-flash synchronization, for early response reversals did no longer occur when the inputs from the two modalities were desynchronized (Experiment 2). Taken together, the results show that the visual bias of auditory localization observed repeatedly in less controlled conditions is due partly at least to an automatic attraction of the apparent location of sound by spatially discordant but temporally correlated visual inputs.