R (salivation)

FIGURE 3 Classical conditioning. In the procedure introduced by Pavlov, the production of saliva is monitored continuously. Presentation of meat powder reliably leads to salivation, whereas some neutral stimulus such as a bell initially does not. With repeated pairings of the bell and meat powder, the animal learns that the bell predicts the food and salivates in response to the bell alone. (Modified from Rachlin, 1970.)

In Pavlovian conditioning, contingency is generally established by the close temporal pairing (contiguity) of the CS and US. (For a more detailed discussion see Mackintosh, 1974,1983; Rescorla, 1967.) Indeed, there is an optimum time interval for conditioning. Shorter or longer intervals between the two stimuli result in less effective conditioning. The interval between CS onset and US onset is called the CS-US interval or the interstimulus interval (ISI). Most conditioning procedures involve repeated pairings of the CS and US. The interval between these pairings is called the intertrial interval.

Specificity of behavioral change to pairing can be most clearly shown by using a differential conditioning procedure. In this procedure, two different conditioned stimuli are used in the same animal; one is specifically paired with the US and is therefore called the CS+, whereas the other, the CS-, is specifically unpaired. Learned changes in behavior can be assessed by comparing the response to the CS+ with that to the CS -.

In Pavlov's experiments, the bell (the CS) did not produce salivation when initially presented alone. In some examples of conditioning procedures, the CS initially produces a small response similar to that evoked by the US. After pairing, the response to the CS is enhanced. This type of conditioning is known as «-conditioning. Both classical conditioning and «-conditioning are similar in that they require a close temporal relationship between the CS and the US; however, in classical conditioning, the CS is initially neutral, whereas in «-conditioning the CS initially produces a weak response that is subsequently enhanced. Some have argued that the distinction between «-conditioning and classical conditioning is somewhat descriptive, because in principle the two could be mediated by identical cellular mechanisms. «-Conditioning and sensitization also resemble one another in that both involve modification of a previously existing response to a stimulus. They differ, however, in their temporal requirements; «-conditioning requires a close temporal association between the CS and US, whereas sensitization does not.

Operant Conditioning

Operant conditioning, or instrumental conditioning, is also a form of associative learning but it differs from classical conditioning in that during this training procedure the reinforcing stimulus is contingent on the performance of a behavior produced by the animal rather than on a CS delivered by the experimenter. The animal therefore learns the consequences of its own behavior and alters that behavior as a result of training. An example of this type of conditioning has been described by Skinner (1938). Before training, a pigeon confined in a small compartment pecks randomly at the walls. During training, delivery of food reinforcement is made contingent on the animal pecking a single location in the compartment (such as a small disk). Because food and the peck were paired, the pigeon continues to peck at the disk after training, even in the absence of food reinforcement. As in classical conditioning, the learned behavior is extinguished when repeated pecks are no longer followed by food reinforcement. Both operant conditioning and classical conditioning are similar in that both require a close temporal association between two stimuli. Operant conditioning differs from classical conditioning in that the reinforcement is contingent on the animal's response, rather than on the presentation of the CS.

In this brief introduction to conditioning procedures, various simple behavioral modifications have been described. These definitions, however, are operational, and at a mechanistic level (e.g., neural) some of the distinctions between various examples of conditioning may not hold. At the cellular level, basic mechanisms underlying these different examples of learning may be similar.

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