Those learning processes where the presentation of two stimuli in TEMPORAL CONTIGUITY leads to the development of an ASSOCIATION between them, such that one stimulus, typically a neutral event, becomes imbued with some of the properties of the other, typically a motivationally salient event (see PAVLOVIAN CONDITIONING). Both the presentation of the two stimuli and the formation of the association between them are entirely independent of the actions of the subject, distinguishing such processes from INSTRUMENTAL CONDITIONING. Classical conditioning typically involves the pairing of an UNCONDITIONED STIMULUS, or US (for example, the delivery of sucrose solution) with a CONDITIONED STIMULUS, or CS (for example, the presentation of a light). Following a number of exposures to the two paired stimuli, presentation of the previously neutral light stimulus (CS) on its own becomes capable of eliciting a range of responses from the subject that were previously elicited only by the sucrose solution (US). These responses could include ORIENTATION and approach to the stimulus, contact and salivation. Such learning has been shaped by evolutionary processes to permit the detection and storage of information about predictive relationships in the environment.
Depending on the organization of stimuli in an experiment, a stimulus acting as a CS in one situation may act as a US in another. Thus, the definition of a stimulus as a CS or as a US is relative and is determined by which stimulus elicits the specified response under study. The US will elicit the specified response when presented alone. The CS will only elicit the specified response following a period of training. The pace with which learning (see CONDITIONING) proceeds and the ultimate strength of the association is dependent upon the intrinsic properties of the stimuli involved, the subject’s prior experience of the stimuli and the temporal relationship between them. Classical conditioning can occur with both appetitive (see APPETITIVE CONDITIONING) and aversive (see AVERSIVE CONDITIONING) unconditioned stimuli. Conditioning by classical (or Pavlovian) processes plays an important role in the regulation of AUTONOMIC NERVOUS SYSTEM activity and also in the expression of complex behaviours.
