Several recent studies of fear conditioning are based on trace-conditioning, which allows manipulations of the balance between conditioning to explicit and background contextual cues (55). However, because in trace conditioning a temporal gap (the trace interval) separates the conditioned stimulus (CS) from the unconditioned stimulus (US), trace conditioning remains a challenge to most association theories which require contiguity between the events to be associated (21). While already puzzling when limited to trace intervals of a few hundred milliseconds, as in conditioning of the nictitating membrane (31), this question becomes unavoidable when dealing with extremely long trace intervals (hours) as in taste aversion learning (18). We will focus on the intermediate case of fear conditioned responses (CRs) which can be obtained over trace intervals of one minute or more (28, 46). Through a brief survey of the literature relevant to trace fear conditioning, this article explores the idea that background contextual cues might provide the missing link between the temporally separate CS and US.
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