Physiological studies have demonstrated that inputs from different sensory modalities converge on, and are integrated by, individual superior colliculus neurons and that this integration is governed by specific spatial rules. The present experiments were an attempt to relate these neural processes to overt behavior by determining if behaviors believed to involve the circuitry of the superior colliculus would show similar multisensory dependencies and be subject to the same rules of integration.
The neurophysiological-behavioral parallels proved to be striking. The effectiveness of a stimulus of one modality in eliciting attentive and orientation behaviors was dramatically affected by the presence of a stimulus from another modality in each of the three behavioral paradigms used here. Animals trained to approach a low intensity visual cue had their performance significantly enhanced when a brief, low intensity auditory stimulus was presented at the same location as the visual cue, but their performance was significantly depressed when the auditory stimulus was disparate to it. These effects were independent of the animals' experience with the modifying (i.e. auditory) stimulus and exceeded what might have been predicted statistically based on the animals' performance with each single-modality cue. The multiplicative nature of these multisensory interactions and their dependence on the relative positions and intensities of the two stimuli were all very similar to those observed physiologically for single cells. The few differences that were observed appeared to reflect the fact that understanding integration at the level of the single cell requires reference to the individual cell's multisensory receptive field properties, while at the behavioral level populations of receptive fields must be evaluated. These data illustrate that the rules governing multisensory integration at the level of the single cell also predict responses to these stimuli in the intact behaving organism.