Commonalities Among Structures Superior Colliculus Versus Multisensory Cortex

Using the information obtained from the SC as a guide, experiments were conducted with multisensory neurons in AES to determine if they are governed by the same principles of multisensory integration. This cortex consists of three largely modality-specific subregions (visual, auditory, and somatosensory), with multisensory neurons clustered most heavily at the borders between these subregions. AES multi-sensory neurons proved to have overlapping receptive fields, much like their counterparts in the SC. This was particularly surprising given that no overall topographic organization appears to be present in the visual and auditory subregions of AES. Only the somatosensory representation is topographic. This suggested that cross-modal receptive field register would also be a key feature of multisensory integration in these neurons.

Indeed, just as in SC neurons, the spatial correspondence of cross-modal cues was a critical determinant of the nature of an integrated multisensory response: spatially coincident stimuli produced multisensory response enhancement in AES neurons, whereas combinations of spatially disparate cross-modal cues either failed to produce an interaction or resulted in response depression (albeit overall the response depression appeared to be less severe than in SC neurons). Furthermore, the greatest proportionate response enhancements were produced by combining weakly effective modality-specific cues, just as in the SC, and the temporal window within which multi-sensory integration was possible was similar to that found in the SC. Unfortunately, it is not known if it is possible to selectively depress multisensory integration in AES neurons by deactivating a specific input without eliminating their modality-specific responses.

Similar principles of multisensory integration have also been found in polysensory cortical regions in rodents, and the overlapping nature of cross-modal receptive fields has proved to be present in nearly every polysensory area of primate cortex, suggesting that here, too, the spatial principles of multisensory integration will be operative. Such a constancy in the principles of multisensory integration would be a parsimonious way of uniformly increasing (or decreasing) the salience of the same stimulus complex throughout the brain and would also be an effective way of appropriately matching the intensity of immediate behaviors with the higher order perceptual, cognitive, and emotive processes that provide conscious dimension to sensory experience.

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