The effect of auditory cortex deactivation on stimulus-specific adaptation in the inferior colliculus of the rat.

Many neurons in the central auditory pathway, from the inferior colliculus (IC) to the auditory cortex (AC), respond less strongly to a commonly occurring stimulus than one that rarely occurs. The origin of this phenomenon, called stimulus-specific adaptation (SSA), remains uncertain. The AC sends descending projections to the IC that terminate most densely upon the dorsal, lateral and rostral IC cortices - areas where strong SSA has been reported. To investigate whether SSA in the IC is dependent upon the AC for its generation, we recorded the response from single IC neurons to stimuli presented in an oddball paradigm before, during and after reversibly deactivating the ipsilateral AC with a cryoloop. While changes in the basic response properties of the IC neurons were widespread (89%), changes in SSA sensitivity were less common; approximately half of the neurons recorded showed a significant change in SSA, while the other half remained unchanged. Changes in SSA could be in either direction: 18% enhanced their SSA sensitivity, while 34% showed reduced SSA sensitivity. For the majority of this latter group, cortical deactivation reduced, but did not eliminate, significant SSA levels. Only eight neurons seemed to inherit SSA from the AC, as their pre-existing significant level of SSA became non-significant during cortical deactivation. Thus, the presence of SSA in the IC is generally not dependent upon the corticocollicular projection, suggesting the AC is not essential for the generation of subcortical SSA; however, the AC may play a role in the modulation of subcortical SSA.