Renewal and reinstatement of the conditioned but not the unconditioned response following habituation of the unconditioned stimulus.

Research on the inhibition of learned fear currently relies almost exclusively on one specific procedure, namely extinction of the conditioned stimulus (CS). Importantly, however, learned fear responses can be reduced by a number of other procedures, including habituation of the unconditioned stimulus (US). We recently demonstrated that reductions in learned fear following US habituation, like CS extinction, were subject to both renewal and reinstatement (Storsve et al., 2010). The present study further investigates the associative and non-associative processes shared between habituation and extinction. Given that habituation is typically context-independent (Mackintosh, 1987), in the present study we directly compared renewal and reinstatement of both a conditioned response (CR; freezing) and an unconditioned response (UR; startle) following habituation. It was found that the reduction in conditioned freezing resulting from habituation was context specific (i.e., a change in context led to a renewal of the conditioned fear response; Experiment 1) and was attenuated when a pre-test shock was given (i.e., reinstatement of conditioned fear was observed; Experiment 2). In contrast, habituation of an unconditioned response elicited by the US (i.e., a startle response) was unaffected by either a change in test context or administration of a pre-test shock. This dissociation in the effects of habituation on learned and unlearned responses is discussed in relation to theories of fear extinction.