Neural Substrates of Classical Conditioning

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[Classical or Pavlovian conditioning, first described by Ivan Pavlov (see PAVLOV, IVAN ), is a procedure where a neutral stimulus such as a light or sound (conditioned stimulus, CS) is presented together with an unconditioned stimulus (US) that elicits a behavioral response (UR). As a result of pairing, the CS comes to elicit a conditioned response (CR). In Pavlov's original experiments with dogs, a bell (CS) was paired with meat powder in the mouth (US). The salivation UR elicited by the meat powder came to be elicited by the bell, the CR. The CS must precede the US for learning to occur; in the delay procedure the CS and US co-terminate; in the trace procedure the CS offset occurs prior to the onset of the US. Although contiguity of the CS and US is necessary for learning, the contingency between them, the probability that the CS will predict the occurrence of the US, is critically important.

The second entry in this section, on conditioning ofD iscreteB ehavioralR esponses, uses eyeblink conditioning as the prototypic example (tone CS, corneal airpuff US). The cerebellum and its associated brainstem circuitry is the necessary and sufficient circuit for this form of learning; however, the hippocampus also becomes important in the trace procedure. Elsewhere in this book, NEURAL SUBSTRATES OF EMOTIONAL MEMORY provides an overview of fear learning, where a neutral CS is paired with an aversive, emotionally arousing US such as shock. The amygdala plays a key role in all aspects of fear conditioning. A major component of fear learning is classical conditioning ofC ardiovascularR esponses. Here, the amygdala, prefrontal cortex, and cerebellum are all involved. The most widely used behavioral index ofF earC onditioningis freezing. Here it appears that critical components of the fear memory are stored in a region of the amygdala. However the hippocampus also becomes critically important in learned freezing to context. Another productive method to study learned fear isF ear-P otentiatedS tartle. Here a light CS is paired with shock to establish conditioned fear to the light. This CS is then given together with a loud acoustic stimulus that elicits behavioral startle response. Presentation of the light CS enhances the startle response to the acoustic stimulus. Much of the circuitry for the startle response and fear potentiation of the response has been identified. As in freezing, the amygdala plays a critical role.

The article on NEURAL SUBSTRATES OF AVOIDANCE LEARNING is included in this book in part for contrast. It describes an example of instrumental learning, where the animal can influence the outcome, unlike classical conditioning. The focus in this entry is on active avoidance, where the animal can make a response (e.g., locomotion) when a CS occurs to avoid a shock US. But until the animal first moves to avoid the shock, the training is Pavlovian. Critical neural structures for this form of learning include the amygdala, certain nuclei of the thalamus, and the cingulated area of the cerebral cortex.]