Rats were given tone-footshock pairings with a 0·, 10·, or 30-sec trace interval between tone offset and shock onset. Half the rats within each trace interval were tested for their conditioned fear of the tone through a lick suppression procedure; the remaining rats were evaluated for their fear of the background or contextual cues through their avoidance of the compartment in which conditioning had occurred. Less conditioning was observed to the tone with increasing trace intervals. However, conditioned fear of the context increased with increases in the trace duration. The ability of the more predictive stimulus, the tone, to overshadow the contextual cues was determined by the tone's temporal contiguity with the footshock. The need to incorporate temporal parameters within current theories of conditioning is discussed.The role of contextual cues in a learning situation and the associative strength that such contextual cues may acquire have recently received increased recognition. This trend is evident in studies of both animal learning

A series of experiments was performed to determine whether long-term habituation of the acoustic startle response in rats is mediated by conditioned associations between contextual cues and the test stimulus. Experiment 1 established parameters yielding demonstrable long-term habituation of the startle response. Experiment 2 attempted to overshadow the hypothesized associations to contextual cues by providing a more reliable predictor of the acoustic stimulus. Experiment 3 investigated the effect of changes in contextual cues on long-term habituation. Experiment 4 provided treatments designed to extinguish the hypothesized associations between the context and the habituated stimulus. Experiment 5 sought latent inhibition of the hypothesized association between the contextual cues and the acoustic stimulus. The results of these experiments uniformly failed to support an associative model of long-term habituation of the startle response, but they are consistent with a nonassociative model emphasizing habituation to the entire experimental situation rather than exclusively to the iterated stimulus.

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Trace procedures. with a gap between the conditioned stimulus (CS) and the unconditioned stimulus (US). often produce weaker conditioning than procedures with contiguous CS and US. Such deficits were found in this experiment using rat subjects and conditioned suppression techniques to assess the strength of fear conditioning. But the deficit was greatly reduced either by filling the CS-US gap with a second "filler" stimulus or by adding a brief "safety signal" at the start of the intertrial interval.It was Pavlov (I 927) who first noted that conditioning was possible with what he termed a trace procedure, in which the conditioned stimulus (CS) is presented only briefly and is not contiguous with the unconditioned stimulus (US). Pavlov observed that trace procedures were not particularly effective, and subsequent experimenters confirmed this observation. For example, Davitz, Mason, Mowrer, and Viek (I957) and Kamin (1961) reported that CS-US gaps of just a few seconds seriously impaired fear conditioning. Kamin (I 954) and Mowrer and Lamoreaux (I 95 1) reported severe decrements in avoidance learning with short gaps between the avoidance "cs" and shock. Although conditioning decrements are not always found (e.g., Brahlek, 1968), they do seem to occur under a wide variety of conditions.The present studies examine some procedures for eliminating or reducing the trace-conditioning deficit. These particular procedures were suggested by Mowrer and Lamoreaux's (l95 1) hypothesis that the deficit results from the animal's failure to discriminate the interstimulus interval from the inter trial interval. If the animal confuses the short, dangerous "silent" part of the interstimulus interval with the long, safe silence of the intertrial interval, then it should tend not to respond during the former and to respond too much during the latter (a result that Mowrer and Lamoreaux found). It also follows that the conditioning deficit should be ameliorated by marking or designating in some way one silent period or the other so that they are not so likely to be confused. We examined two ways of doing this ; first, we tried to mark the short interstimulus interval, then we attempted to mark the long intertrial interval. Both approaches were successful. EXPERIMENT 1Our first approach to the problem was to fill the gap in the interstimulus interval with a filler, a second CS, to make the CS-US gap discriminably different from the intertrial interval. MethodSubjects. The animals were 36 naive female rats, of Wistar descent, with initial body weight of approximately 300 g.Apparatus.The animals were trained and tested in Foringer Skinner boxes. Fear conditioning was carried out in wooden boxes of approximately the same dimensions, with the same type of grid floor, and illumination and masking noise (exhaust fan) conditions. Shock was scrambled, of .5-sec duration and I-rnA intensity . The CS was a white noise of approximately 75 dB in both apparatuses, and the additional stimulus was a 3,300-Hz tone, also about 75 dB.Procedure. The a...

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