The effects of US habituation on conditioned and unconditioned responding: a direct comparison with CS extinction

Just as fear can be learned, it can also be inhibited. The most common way of reducing learned fear is through CS extinction, where the conditioned stimulus (CS) previously paired with an aversive unconditioned stimulus (US) is repeatedly presented on its own. Another, much less commonly studied, way to inhibit learned fear is by habituating, or devaluing, the US. In this procedure, fear responding to a CS is reduced by repeatedly presenting the US in the absence of the CS. The purpose of the present project was to directly compare the effects of US habituation and CS extinction on a learned fear response (freezing). Experiment 1 demonstrated that US habituation given either after (Experiment 1A) or before (Experiment 1B) fear conditioning reduced freezing to the CS at test. It was then showed that the reduction in freezing resulting from either US habituation or CS extinction was context specific (i.e., a change in context led to a renewal of the conditioned fear response; Experiment 2A) and, furthermore, was attenuated when a pre-test shock was given (i.e., reinstatement of conditioned fear was observed in both cases; Experiment 3A). Follow-up experiments tracing the unconditioned response revealed that such context- and shock-dependent recovery of responding did not extend to this response system (Experiments 2B and 3B). Finally, Experiment 4 demonstrated that an injection of the NMDA antagonist MK-801 prior to US habituation impaired long-term retention of the learning that takes place during this procedure. Together, these results suggest that the decrement in conditioned fear responses produced by US habituation and CS extinction could rely on overlapping processes, and, furthermore, that there is a dissociation in US habituation effects across the conditioned response and the unconditioned response under identical experimental conditions