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The term "classical conditioning" is used here to describe a type of associative learning in which there is no contingency between response and reinforcer. This situation resembles most closely the original experiment of Pavlov (1927), who trained dogs to associate a tone with a food-reward. In such experiments, the subject shows a weak or no response to a conditioned stimulus (CS, e.g. a tone), but a measurable unconditioned response (UR, e.g. saliva production) to an unconditioned stimulus (US, e.g. food) at t1. In the course of the training, the CS is repeatedly presented together with the US; eventually the subject forms an association between the US and the CS. In a subsequent test-phase (t2), the subject will show the conditioned response (CR, e.g. saliva production) to the CS alone, if such an association has been established and memorized. The subject is said to have learned about salient contingencies in the world. Control subjects usually receive unpaired CS and US presentations or CS and US alone. Such "Pavlovian" conditioning is opposed to instrumental or "operant conditioning", as described below (1.3), where producing a CR controls the US presentations.
Findings from Kandel and coworkers (Kandel et al. 1983; Hawkins et al. 1983; Carew et al. 1983; Carew and Sahley, 1986 and references therein) investigating the cellular and molecular processes underlying classical conditioning in Aplysia suggest that the US is 'replaced' by the CS during training: simultaneous stimulation of the sensory neuron receiving the CS (SN1) from the sensory neuron receiving the US (SN2), facilitates synaptic efficacy of the SN1 presynaptically (Fig. 1). After a few conditioning trials, stimulation of the SN1 alone elicits the reflexive behavior - the UR eliciting properties of the reinforcer have been transferred to SN1.
Fig. 1: Schematic diagram of the classical conditioning model inferred from evidence in Aplysia. According to the model, activity dependent presynaptic facilitation (ADPF) is induced by the conjoint firing of the sensory neurons of the CS+ pathway and the facilitatory interneurons mediating the US. After conditioning, firing of SN1 alone will activate the motor neuron. CS+ - conditioned stimulus contiguous with the US; CS- - conditioned stimulus unpaired with the US; SN1 - sensory neuron receiving the CS+, SN2 - sensory neuron receiving the US; SN3 - sensory neuron receiving the unpaired control stimulus CS-. (Redrawn from Glanzman ,1995)
For more information on classical conditioning see the documents discribing various properties of classical conditioning.
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