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1 Centre for Brain and Mind, University of Western Ontario, London, Canada
2 Cell and Developmental Biology, University of Pennsylvania, Philadelphia, Pennsylvania, United States
* To whom correspondence should be addressed. E-mail: steve.lomber{at}uwo.ca.
Removal of all visual cortical areas of one hemisphere results in a contralateral hemianopia. Subsequent deactivation of the contralesional superior colliculus (SC) nullifies the effects of the visual cortex ablation and restores visual orienting responses into the cortically blind hemifield. This deficit nullification has become known as the Sprague Effect. Similarly, in the auditory system, unilateral ablation of auditory cortex results in sound localization deficits to stimuli in the contralateral hemifield. The purpose of this study was to examine if auditory orienting responses can be restored into the impaired hemifield during deactivation of the contralesional SC. Cats were trained to orient towards and approach an acoustic stimulus (noise burst) that was presented centrally, or at one of twelve peripheral loci. Following training, a cryoloop was implanted on the right SC. During cooling to deactivate the superficial and intermediate layers, auditory orienting responses were eliminated into the left (contracooled) hemifield. Next, auditory cortex was removed from the left hemisphere. As expected, the auditory cortex ablation resulted in a profound deficit in orienting to acoustic stimuli presented in the right (contralesional) hemifield. Additional deactivation of the superficial and intermediate layers of the contralesional SC nullified the deficit caused by the auditory cortex ablation. Therefore, deactivation of the right superior colliculus following ablation of left auditory cortex yields a fundamentally different result to that identified during deactivation of the right superior colliculus prior to the removal of left auditory cortex. Hence, the Sprague Effect is not unique to the visual system.
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