Bimodal interactions in the superior colliculus of the behaving cat.

Bimodal enhancement, a form of nonlinear summation of physiological responses from two sensory modalities, has been demonstrated in the intermediate layers of the superior colliculus (SCi) and is thought to be a manifestation of a neural mechanism underlying behavioral facilitation to such stimuli. Most physiological studies, however, have been performed in anesthetized animals. We tested for bimodal enhancement in the SCi of behaving cats trained to orient to acoustic, visual, and bimodal stimuli. Surprisingly, we never observed the large enhanced responses reported in anesthetized animals, even when we varied the time between presentation of the visual and acoustic stimuli and/or decreased the level of the stimuli. Using three different behavioral paradigms, we found no support for enhanced interactions between auditory and visual modalities. Prominent depressive effects were seen, however, particularly when the cats were required to fixate a visual target during presentation of an acoustic stimulus.