Two cortical areas mediate multisensory integration in superior colliculusneurons

The majority of multisensory neurons in the cat superior colliculus (SC) ar e able to synthesize cross-modal cues (e.g., visual and auditory) and there by produce responses greater than those elicited by the most effective sing le modality stimulus and, sometimes, greater than those predicted by the ar ithmetic sum of their modality-specific responses. The present study examin ed the role of corticotectal inputs from two cortical areas, the anterior e ctosylvian sulcus (AES) and the rostral aspect of the lateral suprasylvian sulcus (rLS), in producing these response enhancements. This was accomplish ed by evaluating the multisensory properties of individual SC neurons durin g reversible deactivation of these cortices individually and in combination using cryogenic deactivation techniques. Cortical deactivation eliminated the characteristic multisensory response enhancement of nearly all SC neuro ns but generally had little or no effect on a neuron's modality-specific re sponses. Thus, the responses of SC neurons to combinations of cross-modal s timuli were now no different from those evoked by one or the other of these stimuli individually. Of the two cortical areas, AES had a much greater im pact on SC multisensory integrative processes, with nearly half the SC neur ons sampled dependent on it alone. In contrast, only a small number of SC n eurons depended solely on rLS. However, most SC neurons exhibited dual depe ndencies, and their multisensory enhancement was mediated by either synergi stic or redundant influences from AES and rLS. Corticotectal synergy was ev ident when deactivating either cortical area compromised the multisensory e nhancement of an SC neuron, whereas corticotectal redundancy was evident wh en deactivation of both cortical areas was required to produce this effect. The results suggest that, although multisensory SC neurons can be created as a consequence of a variety of converging tectopetal afferents that are d erived from a host of subcortical and cortical structures, the ability to s ynthesize cross-modal inputs, and thereby produce an enhanced multisensory response, requires functional inputs from the AES, the rLS, or both.