Changes in the retrievability of associations to elements of the compound CS determine the expression of overshadowing

Abstract

In two experiments, rats were presented with a taste conditioned stimulus (CS) alone, an odor CS alone, or an odor-taste compound followed by lithium chloride injection. When tested 1 day following conditioning, there was evidence that the odor cue overshadowed conditioning to the taste; however, there was no indication of following a longer (2I-day) retention interval, despite undiminished strength of the aversion in animals conditioned with only the single element (taste). The observed at the I-day retention interval was not reciprocal. Rats conditioned with the odor CS alone or with the compound CS expressed odor aversions of comparable strength-that is, no overshadowing. However, in contrast to the taste aversion, over­ shadowing of conditioning to the odor by taste was evident following a 2I-day retention interval. Rather than reflecting a failure of the overshadowed stimulus to acquire associative strength, these data suggest that may be expressed, or not expressed, as a result of changes in the relative retrievability of learned associations over time. When a compound conditioned stimulus (CS) made up of two separable elements (AB) is paired with an uncon­ ditioned stimulus (US), the conditioned responding elicited by the less salient element (B) is weaker than if it alone had been paired with the US. This overshadowing of B by A has been interpreted by major theories of Pav­ lovian conditioning as a failure of the overshadowed ele­ ment to be associated with the US (Mackintosh, 1975, 1976; Rescorla & Wagner, 1972). Recent evidence indi­ cates, however, that the overshadowed stimulus may in fact become associated with the US during conditioning and that represents a failure to express this learned association behaviorally. A variety of studies have reported that nonreinforced presentations of the stimulus following conditioning with a compound CS result in an enhanced level of conditioned responding to the overshadowed stimulus that may be comparable to the level observed to the more salient stimulus. This recovery from over­ shadowing, via extinction of the stimu­ lus, has been reported with either general activity (Kauf­ man & Bolles, 1981) or suppression of drinking (Matzel, Schachtman, & Miller, 1985; Matzel, Shuster, & Miller, 1987) as the index of conditioning. Recovery from has also been reported as a function of an increase in the retention interval be