Some tricks for ameliorating the trace-conditioning deficit

Abstract

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...

Contextual associations in trace conditioning

“…The influence that contextual stimuli may have had during the trace interval in the serial compound procedure of Kehoe et al is unknown. However, it is clear from their data and from related studies (Bolles et al, 1978;Kaplan & Hearst, Note 1) that the use of a discrete CS2 is more successful in mediating trace conditioning to CSI than is the context.…”

“…Unlike typical investigations of overshadowing in which each element of the compound CS is a discrete stimulus of defmed duration, contextual or background cues are necessarily always present. Therefore, with extended training, the overshadowed element of the compound, the context, may actually become inhibitory in the O-sec group, while in the trace conditions there may be a discrimination as to whether a particular interval of exposure to the context alone was preceded by the CS (see Bolles et al, 1978). Support for such nonmonotonic conditioned responding with increased training is evident in the elegant series of experiments by Kehoe, Gibbs, Garcia, and Gormezano (1979).…”

“…Logue, 1979). Moreover, the temporal parameters for successful trace conditioning may be extended by various techniques (Bolles, Collier, Bouton, & Marlin, 1978;Kehoe, Gibbs, Garcia, & Gormezano, 1979;Kaplan & Hearst, Note 1). Nevertheless, given the appropriate temporal parameters, the greater the trace duration-the lSI between CS offset and US onset-the weaker is the conditioning to the nominal CS.…”

Hippocampal and cerebellar single-unit activity during delay and trace eyeblink conditioning in the rat

“…In support of this latter notion are findings that are not easily explained by a "gap filler" account of trace conditioning. For example, trace eyeblink conditioning with very long trace intervals (3-sec) can be produced by decreasing the number of trials per session (Kehoe, Cool, & Gormezano, 1991) and trace fear conditioning can be facilitated by a post-US stimulus that serves to distinguish the trace interval from early portions of the intertrial interval (Bolles et al, 1978).…”

“…Trace conditioning represents perhaps the simplest form of learning that requires the hippocampus. A number of proposals have been advanced regarding why the brief gap between the CS and the US should engage the hippocampus, including filling the trace period gap (Rodriguez & Levy, 2001;Sutton & Barto, 1981), timing the relation between the CS and US (McEchron & Disterhoft, 1999), configuring CS onset and offset into a single CS (Kehoe & Weidemann, 1999), discriminating the trace period from the intertrial interval (Bolles, Collier, Bouton, & Marlin, 1978;Kaplan & Hearst, 1982;Marchand, Luck, & DiScala, 2004), and subserving the awareness that the CS predicts the US (Clark & Squire, 1998Clark, Manns, & Squire, 2001;Manns, Clark, & Squire, 2000a,2000b. Comparison of hippocampal single-unit activity during trace conditioning with a procedure (delay conditioning) that does not require the hippocampus and that is identical to trace conditioning except for the lack of a trace period, would be helpful in beginning to discriminate among these alternatives.…”