Abstract
The conditioned response (CR) that develops during classical conditioning of the autonomic nervous system has been characterized as either a discrete response or a nonspecific response to the conditioned stimulus (CS). A discrete CR is a learned response that has been elaborated from an unconditioned reflexive response to a highly specific unconditioned stimulus (CS), whereas a nonspecific autonomic CR is one of a cluster of concurrent responses elicited by a CS. The unconditioned autonomic response (UR) is viewed as a component of a reactive homeostatic mechanism that services to return a controlled variable to its preset reference level in response to a regulatory challenge—the unconditioned stimulus (US). In contrast, the autonomic CR is seen as an element of a predictive homeostatic mechanism that is engaged prior to the onset of a noxious US. Viewed from this perspective, a discrete CR is a preemptive response that mitigates or totally nullifies the impact of the regulatory challenge (the US). Similarly, the development of nonspecific autonomic CRs may be thought of as the initial stage of a predictive homeostatic mechanism that is followed by the development of somatic CRs (e.g., eye blink) or an integrated somatic motor response pattern, learned through instrumental conditioning, that mitigates (or totally nullifies) a noxious US; the autonomic CRs are thought to reflect a change in the CNS, referred to as conditioned fear. Alternatively, the autonomic CR may be interpreted solely in terms of autonomic responses involved in predictive homeostasis. For example, a bradycardia CR develops in an aversive conditioning experiment in which the animal is restrained, thereby attenuating the blood pressure increase elicited by the US. Although there is ample evidence for instrumental conditioning of autonomic responses, the level of specificity of operant control seems limited. It appears that the CNS programs for somatomotor and cardiorespiratory responses are coupled and these functionally related systems are influenced in parallel by the same process.