US Exposure Research Articles

Contextual conditioning and the US preexposure effect in conditioned fear.

A series of five experiments was carried out in which fear of context caused by exposure to shocks was manipulated by signaling the shocks with a discrete stimulus, signaling the days during which shocks occurred with a session-long stimulus, or switching the context between exposure and the subsequent test. All these manipulations influenced fear of the context in the manner predicted by the Rescorla-Wagner associative model. Following this, all the rats were given conditioning trials with shock and a different discrete stimulus. All preexposure treatments produced consistent and reliable interference with conditioning with the exception of signaling the shocks with a discrete stimulus, which greatly reduced interference. These results are interpreted as being consistent both with a cognitive explanation of the US exposure effect, which claims that animals learn that shocks are unpredictable during conditioning and this knowledge retards future conditioning when they are predictable, and with an adaptation explanation, which claims that unpredictable shocks produce chronic fear and this fear through either a change in adaptation level or through emotional exhaustion renders the shocks less reinforcing during the conditioning test.

Bait-shyness acquisition and resistance to extinction as functions of US exposure prior to conditioning

Cyclophosphamide-induced aversions to saccharin-flavored tap water were observed in normal control rats, in rats subjected to varying numbers of cyclophosphamide injections prior to conditioning, and in rats similarly subjected to preconditioning saline injections. Both initial magnitude and resistance to extinction of a conditioned flavor aversion were found to be inverse functions of preconditioning familiarity with drug-induced illness. Six preconditioning cyclophosphamide injections markedly reduced both initial aversion magnitude and resistance to extinction. In contrast, three such injections failed to affect initial aversion magnitude and resulted in a small acceleration of extinction rate, while one preconditioning cyclophosphamide injection produced no observable effects. These findings depict preconditioning familiarity with illness as one important variable modulating the strength of conditioned flavor aversions and emphasize the importance of viewing resistance to extinction as one indicator of aversion strength.