The goal of this research project is to determine the involvement of the primary auditory cortex (Al) in auditory associative memory (AAM) and to identify the mechanisms of AAM. In contrast to traditional views that Al is a static acoustic processor, research during the past decade has established that learning modifies information processing to specifically emphasize the frequency of a behaviorally acquired important stimulus. Notably, neuronal tuning shifts toward/to the frequency of a tone that signals reinforcement. This plasticity, like memory, is associative, highly specific, rapidly acquired, retained indefinitely and consolidates over time. These tuning shifts increase the cortical area that represents behaviorally important frequencies. Thus, the auditory cortex may use a memory code that connotes behavioral importance by increasing the number of cells tuned to that stimulus. We will evaluate this hypothesis by differentially training groups of rats in simple and complex acoustic tasks, so that a tone gains different levels of behavioral importance, as indexed by levels of correct performance. Subsequent mapping of Al will determine the relationship between the area of frequency representation, other cellular response properties, and learned stimulus importance. The nucleus basalis (NB) and its cholinergic (ACh) projections to the auditory cortex have been deeply implicated as mechanisms of learning-induced plasticity because pairing a tone with NB stimulation produces the same types of plasticity as does behavioral training and atropine applied to the cortex blocks the plasticity. Recent work from our laboratory has revealed that pairing a tone with stimulation of the NB actually induces predicted behavioral AAM. Therefore, we will fully characterize NB-induced AAM in simple and complex tasks, including its ability to facilitate new discrimination learning, and determine its correlated Al plasticity. We will also compare Al plasticity from these two approaches to seek general rules relating behavioral auditory associative memory to representational plasticity in primary auditory cortex. The findings will elucidate cortical mnemonic function and provide a foundation for therapeutic treatments, including recovery of higher auditory function following insult and learning to perceive speech using cochlear implants.
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