Effect of auditory cortex deactivation on stimulus-specific adaptation in the medial geniculate body.

An animal's survival may depend on detecting new events or objects in its environment, and it is likely that the brain has evolved specific mechanisms to detect such changes. In sensory systems, neurons often exhibit stimulus-specific adaptation (SSA) whereby they adapt to frequently occurring stimuli, but resume firing when "surprised" by rare or new ones. In the auditory system, SSA has been identified in the midbrain, thalamus, and auditory cortex (AC). It has been proposed that the SSA observed subcortically originates in the AC as a higher-order property that is transmitted to the subcortical nuclei via corticofugal pathways. Here we report that SSA in the auditory thalamus of the rat remains intact when the AC is deactivated by cooling, thus demonstrating that the AC is not necessary for the generation of SSA in the thalamus. The AC does, however, modulate the responses of thalamic neurons in a way that strongly indicates a gain modulation mechanism. The changes imposed by the AC in thalamic neurons depend on the level of SSA that they exhibit.