Induction of receptive field plasticity in the auditory cortex of the guinea pig during instrumental avoidance conditioning.

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Abstract
Classical tone conditioning shifts frequency tuning in the auditory cortex to favor processing of the conditioned stimulus (CS) frequency versus other frequencies. This receptive field (RF) plasticity is associative, highly specific, rapidly acquired, and indefinitely retained--all important characteris- tics of memory. The investigators determined whether RF plasticity also develops during instnunental learning. RFs were obtained before and up to 24 hr after 1 session of successful 1-tone avoidance conditioning in guinea pigs. Long-term RF plasticity developed in all subjects (N = 6). Two-tone discrimination training also produced RF plasticity, like classical conditioning. Because avoidance responses prevent full elicitation of fear by the CS, long-term RF plasticity does not require the continual evocation of fear, suggesting that neural substrates of fear expression are not essential to RF plasticity. Classical conditioning involves the rapid acquisition of an association between the conditioned stimulus (CS) and the unconditioned stimulus (US), usually indexed by an autonomic conditioned response (CR), followed by a more slowly develop- ing association between the CS and a specific somatic CR (e.g., eyeblink; reviewed by Lennartz & Weinberger, 1992a). During the initial phase of CS-US association, neuronal responses to the CS are facilitated in the primary sensory cortex of the CS modality. This has been best documented for the auditory cortex (Diamond & Weinberger, 1986; reviewed in Wein- berger & Diamond, 1987). In recent years, it has been discovered that conditioning with tonal CSs produces a systematic change in the frequency receptive fields (RFs) of neurons in the primary auditory cortex. Responses to the frequency of the CS are increased, whereas responses to the preconditioning best frequency (BF; frequency eliciting the greatest discharges) and many other frequencies are decreased simultaneously. These opposite changes are often large enough to shift RF tuning toward or even to the frequency of the CS, so that it becomes the new best frequency (Bakin & Weinberger, 1990). Thus, classical conditioning enhances the processing of the CS frequency compared with other frequencies. This CS-specific RF plasticity constitutes "physiological memory" because it has the same major characteristics as behaviorally defined "fact" memory. It (a) is induced by normal learning experiences (i.e., classical conditioning; Bakin

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