Brain mechanisms for changes in processing of conditioned stimuli in Pavlovian conditioning: Implications for behavior theory

This article is a review of the results of a series of experiments designed to identify brain systems involved in appetitive conditioning of rats. It discusses some of their implications for behavioral theories of learning, especially those that concern changes in processing of conditioned stimuli (CSs). Evidence is presented which suggests that separable brain circuits are involved in (1) the production of CS-dependent conditioned orienting responses, (2) the enhancement of CS associability produced when expectancies about upcoming events are violated, (3) the reduction of CS associability produced when stimuli are consistent predictors of other events or are presented without consequence, and (4) the abilities of CSs to serve as reinforcers for second-order conditioning and to be sensitive to postconditioning changes in the value of the unconditioned stimulus (US). Finally, none of these circuits seems critical for normal acquisition of the most common indicator of Pavlovian conditioning, US-dependent conditioned responses (CRs). Although the independence of brain pathways does not demand independence of behavioral function, clustering of behavioral phenomena on anatomical grounds may provide useful guides for constructing behavior theories.