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.

No abstract