MECHANISMS OF WITHIN-MODALITY AND CROSS-MODALITY SUPPRESSION IN THE SUPERIOR COLLICULUS

The present studies were initiated to explore the basis for the respon se suppression that occurs in cat superior colliculus (SC) neurons whe n two spatially disparate stimuli are presented simultaneously or in c lose temporal proximity to one another. Of specific interest was exami ning the possibility that suppressive regions border the receptive fie lds (RFs) of unimodal and multisensory SC neurons and, when activated, degrade the neuron's responses to excitatory stimuli. Both within-and cross-modality effects were examined. An example of the former is whe n a response to a visual stimulus within its RF is suppressed by a sec ond visual stimulus outside the RF. An example of the latter is when t he response to a visual stimulus within the visual RF is suppressed wh en a stimulus from a different modality (e.g., auditory) is presented outside its (i.e., auditory) RF. Suppressive regions were found border ing visual, auditory, and somatosensory RFs. Despite significant modal ity-specific differences in the incidence and effectiveness of these r egions, they were generally quite potent regardless of the modality. I n the vast majority (85%) of cases, responses to the excitatory stimul us were degraded by greater than or equal to 50% by simultaneously sti mulating the suppressive region. Contrary to expectations and previous speculations, the effects of activating these suppressive regions oft en were quite specific. Thus powerful within-modality suppression coul d be demonstrated in many multisensory neurons in which cross-modality suppression could not be generated. However, the converse was not tru e. If an extra-RF stimulus inhibited center responses to stimuli of a different modality, it also would suppress center responses to stimuli of its own modality. Thus when cross-modality suppression was demonst rated, it was always accompanied by within-modality suppression. These observations suggest that separate mechanisms underlie within-and cro ss-modality suppression in the SC. Because some modality-specific tect opetal structures contain neurons with suppressive regions bordering t heir RFs, the within-modality suppression observed in the SC simply ma y reflect interactions taking place at the level of one input channel. However, the presence of modality-specific suppression at the level o f one input channel would have no effect on the excitation initiated v ia another input channel. Given the modality-specificity of tectopetal inputs, it appears that cross-modality interactions require the conve rgence of two or more modality-specific inputs onto the same SC neuron and that the expression of these interactions depends on the internal circuitry of the SC. This allows a cross-modality suppressive signal to be nonspecific and to degrade any and all of the neuron's excitator y inputs.