Behavioral Studies

Whatever the reason that two cortical areas have assumed shared control over multisensory integration in SC neurons, it is interesting to note that these "association" cortices achieve at least some of their associative functions via a very distant subcortical structure. This allows them to influence, quite directly, SC-mediated behaviors. One might expect, then, that the same perturbations that compromise multisensory integration in single SC neurons would also compromise SC-mediated multisensory behaviors.

This possibility was tested experimentally using a paradigm similar to that described earlier. Although cats were trained to respond as described previously, their performance was assessed when association cortex was intact or temporarily deactivated. Cortical deactivation was accomplished either by locally anesthetizing an area via lidocaine injections through chronically implanted cannulas or by cryogenic blockade of the activity in that area via implanted cooling probes. Deactivating association cortex had no effect on orientation to an individual modality-specific cue, but the spatially coincident combination of visual and auditory stimuli no longer could enhance performance and spatially disparate cues could no longer degrade performance. These effects were not seen when other visual or auditory cortical regions were deactivated. That these effects were dependent on disrupting SC multisensory integration by depriving its neurons of critical cortical inputs, and were not due to the loss of association cortex per se, was evident from the results of experiments in which the functional integrity of the

Figure 6 Deactivation of association cortex disrupts multisensory integration in the SC. The shadowed image shows the electrophysiological paradigm. While recording from a multisensory neuron in the SC, the anterior ectosylvian sulcus (AES) was reversibly deactivated by cooling. Representative data are shown for a single SC visual-somatosensory neuron. Prior to AES deactivation (precool), this SC neuron showed a modest response to the visual and somatosensory stimuli when presented individually and a large response enhancement when they were paired. When the AES was deactivated, the neuron's modality-specific visual and somatosensory responses remained intact, but its multisensory response enhancement was abolished. Multisensory response enhancement was reinstated after rewarming the AES.

Figure 6 Deactivation of association cortex disrupts multisensory integration in the SC. The shadowed image shows the electrophysiological paradigm. While recording from a multisensory neuron in the SC, the anterior ectosylvian sulcus (AES) was reversibly deactivated by cooling. Representative data are shown for a single SC visual-somatosensory neuron. Prior to AES deactivation (precool), this SC neuron showed a modest response to the visual and somatosensory stimuli when presented individually and a large response enhancement when they were paired. When the AES was deactivated, the neuron's modality-specific visual and somatosensory responses remained intact, but its multisensory response enhancement was abolished. Multisensory response enhancement was reinstated after rewarming the AES.

SC was destroyed with a neurotoxin. In these cases, the enhanced multisensory behaviors were lost in the absence of the SC despite the functional integrity of association cortex. However, modality-specific behaviors were unaffected. The results of behavioral and electrophysiological experiments underscore the dependence of SC multisensory integration on influences from association cortex.

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