CONDITIONING

in physiology, a behavioral process whereby a response becomes more frequent or more predictable in a given environment as a result of reinforcement (providing a natural stimulus or reward for the desired response). The history of this concept shows its dependence upon experimental techniques for the study of reflexes. Physiologists in Russia, England, and the United States contributed the original procedures, observations, and definitions. After the 1920s psychologists carried on the larger share of experimental work on the nature and prerequisites of conditioning. At the heart of the principles of conditioning are the stimulus-response theories, various theoretical positions based on the assumption that human behaviour is learned. One of the early contributors to the field, American psychologist Edward L. Thorndike (18741949), postulated the Law of Effect which stated that those behavioral responses (R) that were most closely followed by a satisfactory result were most likely to become established patterns and to reoccur in response to the same stimulus (S). This basic stimulus-response (S-R) scheme is referred to as unmediated. When an individual organism (O) affects the stimuli in any way, for example, by thinking about a response, the response is considered mediated. The S-O-R theories of behaviour are often drawn to explain social interaction between individuals or groups. Conditioning is a form of learning in which either (1) a given stimulus (or signal) becomes increasingly effective in evoking a response or (2) a response occurs with increasing regularity in a well-specified and stable environment. The type of reinforcement used determines which of these consequences obtains. When two stimuli are presented in an appropriate time and intensity relationship, one of them will come to induce a response resembling that of the other. The process can be described as one of stimulus substitution. This procedure is called classical (or respondent) conditioning. In the traditional technique, based on the work of the Russian physiologist Ivan P. Pavlov, a dog is placed in a harness within a sound-shielded room. On each conditioning trial the sound of a bell or a metronome is promptly followed by food powder blown by an air puff into the dog's mouth. Here the tone of the bell is known as the conditioned (or sometimes conditional) stimulus, abbreviated as CS. The dog's salivation upon hearing this sound is the conditioned response (CR). The strength of conditioning is measured in terms of the number of drops of saliva the dog secretes during test trials in which food powder is omitted after the bell has rung. The dog's original response of salivation upon the introduction of food into its mouth is called the unconditioned response (UR) to food, which is the unconditioned stimulus (US). Using a variety of stimuli and responses, psychologists in the United States have shown that in humans and other mammals there is a very consistent optimum interval between the CS and the US which the experimenter may utilize. This interval is approximately half a second. For cold-blooded animals the optimum interval for conditioning is longer. In fish, for example, the CS may best precede the US by 1.5 seconds. When the CS is appropriately followed by the US, reinforcement is said to occur. If the US is omitted on any trial or series of trials, the procedure is referred to as extinction, since without reinforcement the frequency of the CR gradually decreases. Instrumental, or operant, conditioning differs from classical conditioning in that reinforcement occurs only after the organism executes a predesignated behavioral act. When no US is used to initiate the specific act to be conditioned, the required behaviour is known as an operant or, once it occurs with regularity, also as a conditioned response to correspond to its counterpart in classical conditioning. Operant conditioning was studied in great detail by the American psychologist B.F. Skinner and his students. A spontaneous (or operant) behaviour is either rewarded, that is, reinforced, or punished. With rewarded behaviour, the frequency of that behaviour increases; with punishment it decreases. For example, a hungry animal will respond to a situation in a way that is most natural for that animal. If one of these responses leads to the reward of securing food, it often turns out that the specific response which led to the food reward is repeated and thus learned. The behaviour that has been instrumental in obtaining the reward becomes especially important to the organism. The same type of conditioning can also be applied to an action that allows the organism to escape from or avoid painful or noxious stimuli. Psychologists generally assume that most learning occurs as a result of instrumental rather than of classical conditioning. Central to all forms of behavioral interaction, however, is the concept that a change occurs in an organism's behaviour and that the change, brought about by some form of conditioning, results in learning.