Auditory Response Enhancement During Differential Conditioning in Behaving Rats

Summary

Neural unit activity has been recorded from the subcortical portions of the auditory system and surrounding structures during differential appetitive conditioning and reversal in rats. The goal has been to identify and characterize the circuitry involved in learned changes in response to auditory stimuli which are made behaviorally significant by pairing with food pellet presentation. During a single conditioning session, probes throughout the medial division of medial geniculate, the external nucleus of inferior colliculus, an anterior medial portion of inferior colliculus, and the deep portion of superior colliculus showed response enhancements which were selective for the tone paired with pellet presentation. These enhancements started as early as the first 30 ms after stimulus onset and persisted for at least 200 ms. With repeated reversal sessions the locus of points where units continued to show selective enhancement became much more restricted. Within the region of the medial geniculate, only probes in the most caudal portion showed selective enhancement throughout differential conditioning and reversal sessions. No units were found in inferior colliculus which did so. Within the deep portion of superior colliculus and subadjacent tegmentum 73% of auditory responsive units within the most posteriorregion showed selective enhancement throughout differential conditioning and reversal. No units in the most anterior region did so.

The known topographically organized sensory and motor relationships of neurons in some of the above regions suggest the possibility that these enhancements of response to auditory stimuli paired with food presentation may be functioning to connect the auditory stimuli with particular behavioral responses. With successive reversal sessions, as the behavioral response becomes more efficient, the number of neurons involved might become more restricted. The response enhancements are, however, much more closely time locked to stimulus onset than to behavior onset. The units are therefore not simply behaving in a pre-motor fashion. The fact that neurons in some of these regions have topographically organized relationships to both sensory stimuli and to behavior also suggests that the task of describing the sequence of changes which leads to a new behavioral response to a previously neutral stimulus may be more amenable to analysis than might have been thought.