Method and theory in the study of avoidance.

Two-factor theories of avoidance were conceived to explain responding in avoidance procedures that closely resemble the Pavlovian paradigm in superficial features, although differing in the fundamental contingency of reinforcement. Both typically involve an arbitrary conditioned stimulus and a trial-by-trial sequence of pairings between the conditioned and unconditioned stimuli. According to two-factor theory, the instrumental reinforcement of avoidance is based on the Pavlovian reinforcement of a drive state in the presence of the conditioned stimulus. It has been shown, however, that the presence of the conditioned stimulus is not necessary for the occurrence of avoidance responding. A procedure in which the sole effect of the avoidance response was a reduction in the average frequency of occurrence of an aversive electric shock proved to be fully adequate to maintain lever pressing in rats, thereby suggesting that not all avoidance requires two factors. Further experiments with various new procedures suggested that the conditioned stimulus may function as a discriminative stimulus for the avoidance response, rather than as a stimulus whose removal is inherently reinforcing, as two-factor theory requires.