Visual-auditory integration in cat superior colliculus: implications for neuronal control of the orienting response.

Previous physiological studies have demonstrated that inputs from different sensory modalities converge on individual neurons in the superior colliculus. Moreover, in anesthetized, paralyzed animals, those tectal neurons which are most directly connected to brain stem circuits mediating orienting eye and head movements are highly likely to exhibit significant integration of sensory inputs from multiple modalities. The purpose of the present study was to examine the responses of tectal neurons in the alert cat when visual and auditory stimuli were presented as targets for ocular fixation and orienting responses. For comparison to previous work in anesthetized, paralyzed animals, we also examined the responses of tectal neurons to the presentation of these stimuli during periods when the cats voluntarily maintained their eyes near primary position in the absence of a fixation target. Under these conditions, there were significant differences between the strength of the response to the simultaneous presentation of visual and auditory targets and the strength of response to the most effective unimodal stimulus in about 40% of the cells tested. Many tectal neurons also responded tonically during fixation of visual, auditory and bimodal targets, and some of these also exhibited significant bimodal interactions. However, among individual neurons which responded phasically to stimulus onset or offset and tonically during fixation, there was only a weak correlation between the extent of bimodal interaction under the two conditions. Finally, among saccade-related neurons, the magnitude of saccade-related activity was only slightly affected when a biomodal target was used to elicit a saccade, and the extent of bimodal interactions was generally less than was found for the onset and offset of sensory targets. Such multisensory interactions can be significant for behavior. Indeed, simply using a multisensory target has been shown to influence the probability and latency of overt orienting responses, although the extent of such effects will probably vary across both tasks and stimulus conditions. Strong multi-sensory interactions are most likely to occur when low intensity stimuli are used. Our use of moderately intense sensory stimuli probably accounts for our finding of a relatively small percentage of cells in which bimodal responses were greater than the sum of their unimodal responses.