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Theoretical Background Analyses of learning experiments demonstrated very early that conditioning promotes the association of certain relationships between particular external events, eg, stimuli and/or responses. Psychological learning theory distinguishes between classical conditioning and instrumental aversive (or appetitive) conditioning based on the different ways (contingencies) by which the aversive event (the unconditioned stimulus; US) is related to the conditioned stimulus (CS) and the unconditioned response (UCR). Instrumental (aversive) conditioning refers to contingencies in which the behavior of the subject determines whether or not the unconditioned stimulus (US) will occur (Ogren 1985). In active avoidance paradigms, the animal has to perform a discrete response of a low probability, eg, running from one side of a two-compartment box to the other when a discrete stimulus, eg, the conditioned stimulus (CS), is presented in order to escape or avoid the US. In the passive avoidance task, the animal learns to suppress a motor response to avoid exposure to the test area (context) associated with or predictive of the aversive event, such as a dark compartment of the passive avoidance system that is normally preferred over the brightly illuminated compartment. A conflict situation is created in which the behavioral responses are analyzed. Analyses of avoidance learning indicate that it involves different processes. Initially, presentation of the US results in a learned emotional state (eg, conditioned fear) involving Pavlovian classical conditioning followed by the acquisition of the discrete, adaptive behavioral responses, the escape or …