Auditory-visual interaction in the generation of saccades in man.

Four normal human subjects were investigated for evidence of auditory-visual interaction in the generation of horizontal saccades. In a first experiment it was shown that the mean amplitudes of initial saccades from primary position to auditory targets were significantly affected by the simultaneous occurrence of a distracting visual stimulus. If both auditory and visual stimuli were in the same hemifield, the mean amplitude of initial saccades to a fixed buzzer position was consistently increased or decreased depending on the position of the visual stimulus. The phenomenon is felt to be analogous to the "centre-of-gravity" effect previously described for two simultaneous visual stimuli. It did not occur if visual and auditory stimuli were in opposite hemifields when a simultaneous visual stimulus caused a slight reduction of mean initial saccadic amplitude compared to the mean amplitude to buzzer alone. In this case the reduction was independent of visual stimulus position. Similar effects were seen for mean final eye positions. In a second experiment, a similar procedure was carried out, but the eyes started by looking at a point at 15 degrees eccentricity. The same pattern of auditory-visual interaction was obtained in both experiments, consistent with the concept of eye-movement related movement of modality-specific sensory "maps" which has recently been shown to occur in the superior colliculus of primates.