New Perspectives on Classical Conditioning: a Synthesis of Hebbian and Non-Hebbian Mechanisms

conditioning in the isolated nervous system is attenuated in presence of the NMDA receptor antagonist APV. This finding is consistent with the results of Lin and Glanzman (1994), which indicated that the influx of calcium through NMDA receptors in the postsynaptic neu-ron is necessary for facilitation, and suggests that a Houston, Texas 77030 Hebbian mechanism is important for classical conditioning in Aplysia. Bao et al. (1998) confirmed a postsynaptic Since the pioneering studies of Pavlov, a major goal in contribution to the induction of associative plasticity neurobiology has been the elucidation of the cellular by showing that postsynaptic injection of the calcium mechanisms underlying classical conditioning, a form of chelator BAPTA attenuated associative facilitation of the associative learning. An important step in understanding sensorimotor connection. However, they also found that classical conditioning was taken in 1983, when Walters associative facilitation was attenuated by manipulations and Byrne (1983) and Hawkins et al. (1983) indepen-of signaling cascades in the presynaptic sensory neu-dently described a possible mechanism for classical ron, consistent with the presynaptic induction mechanism identified previously (Ocorr et al., 1985; Yovell and conditioning in Aplysia that involved the associative en-Abrams, 1992). A new model for associative facilitation hancement of synaptic strength (associative facilitation) might therefore involve a combination of non-Hebbian at identified synapses. This mechanism pointed to a and Hebbian induction mechanisms. To put the new presynaptic site for the induction of changes in synaptic results into perspective, it is useful to review the features strength, which did not appear to critically involve activ-of classical conditioning and some of the previous be-ity in the postsynaptic motor neuron. Moreover, these havioral and cellular findings in Aplysia. and other studies indicated that a mechanism for asso-Typically, in classical conditioning, the behavioral re-ciative facilitation was an elaboration of mechanisms sponse of an animal to a conditioned stimulus (CS) is for nonassociative facilitation (i.e., sensitization). The altered by an unconditioned stimulus (US) when the CS resulting model of a purely presynaptic induction mech-is followed immediately by the US. The US has two anism for associative facilitation contrasted with a effects. It reflexively produces a response (the uncondi-widely recognized proposal made by Donald Hebb in tioned response [UR]), and it modifies the animal's naive 1949, in which postsynaptic activity is required for the response to the CS, so that after paired presentation of induction of neural plasticity. Therefore, the mechanism CS and US the animal shows …