We are constantly making predictions about events that may happen in our environment and actively seeking information with predictive value. Predictability is so important that the signs used to make predictions also acquire affective value in their own right. Classical or Pavlovian conditioning is the mechanism that endows signals with predictive value and transforms them into events with affective significance. Consider the following example involving a change in preference for the ingestion of water. Six-month-old infants consume less water than formula, but this can be altered using a conditioning procedure. In a first stage, a scent is applied to the formula bottle so that the scent becomes a signal for the formula. In technical terms, the scent is called the conditioned stimulus (CS), the formula is called the unconditioned stimulus (US), and the trial is described as a CS-US pairing. In a second stage, the scent CS, now transformed into an appetitive stimulus, is applied to a water bottle and, as a result, infants consume more water than they had consumed before scent-formula pairings. Thus, the scent CS acquired signaling and affective value as a result of classical conditioning. In this example, the scent signals the impending presentation of the formula and also reinforces water consumption.
The distinctive feature of classical conditioning is its response-independent procedure for US administration. Although typically some form of behavior is measured at the time of CS presentation, the occurrence of a response is not required for US delivery. Despite this feature, behavioral changes may still be the result of instrumental learning (i.e., response-US contingency), rather than of the CS-US pairings. This can be clarified by introducing omission contingencies, in which the occurrence of a response cancels the presentation of an appetitive US, itself delivered only when the response does not occur. Unlike instrumental responses, Pavlovian responses are less modifiable by omission contingencies, occurring reflexively as a result of the predictive and affective value of the CS. For example, approach responses tend to be Pavlovian; thus, in an omission contingency experiment, approach to a CS signaling food or a receptive sexual partner causes the animal to lose the US—yet, these response may occur at relatively high rates.
A proper demonstration of conditioning requires a distinction between the effects of CS-US pairings and the individual effects of the CS and US. The original response to the CS may increase after exposure to the US (sensitization); moreover, the repeated elicitation of a response by the US may cause other stimuli to elicit the same response (pseudoconditioning). Sensitization and pseudoconditioning mimic classical conditioning without being associative phenomena (hence, they are called nonassociative). Controls that eliminate nonassociative effects must be included in demonstrations of classical conditioning. For example, the CS and US may be presented an equal number of times in a control and in an experimental group, but explicitly unpaired from each other in the control group. Most control conditions introduce their own problems. The choice of an appropriate control often depends on the nonassociative effects that need to be eliminated.
Classical conditioning has been implicated in situations involving behavioral changes, changes in the response to drugs, neural-immune interactions, causal judgments, protective reflexes, fear conditioning, the development of attachment in young organisms, and endocrine responses, among others. Although it can be isolated under laboratory conditions, classical conditioning is often embedded into complex contingencies, including instrumental learning and behaviors mediated by language, where it can provide predictive value and regulate the acquired affective significance of stimuli. The elucidation of classical conditioning mechanisms is providing a detailed picture of how the brain acquires, stores, and retrieves information.
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