Serial overshadowing in long-delay taste aversion learning

Abstract

Taste aversion learning (conditioned taste aversions; CTA) can occur even when there is delay of some hours between experience of the taste and the subsequent onset of illness. This property of CTA is quite distinct from other forms of associative learning, where typically no association between two events is acquired if they are separated by more than a minute. Through a series of experiments based on the assumption that long-delay CTA is possible only when no potentially overshadowing – or ‘concurrently interfering’ (Revusky, 1971) – events occur during the delay, this thesis investigates the mechanisms and parameters involved in serial overshadowing to gain a better understanding of long-delay CTA. The general method was a conditioning session in which rats were first given 8% sucrose and later injected with lithium chloride at a dose that produces mild and transient malaise. What varied across experiments was the potentially interfering events that occurred during the delay period. When the interfering event was a second, and quite different taste, namely sour-tasting hydrochloric acid solution (HCl), one-trial overshadowing of the sucrose aversion was observed, to a degree that was greater when HCl was given late in the delay period and greater when HCl had not been pre-exposed. Other intervening events were also found to overshadow sucrose aversion learning. These included placement into a novel context, as long as this occurred immediately before injection, and even stimuli that evoked memories of food-related experiences. Events that occurred outside the delay period can also proactively overshadow the acquisition of a sucrose aversion, though this was only found following two conditioning sessions, but not after a single conditioning session. No evidence was found for release from overshadowing following extinction of the aversion to the overshadowing taste. Whether the findings observed in CTA would be relevant and applicable to general features of learning observed in other conditioning preparations was tested. Two experiments involving fear conditioning assessed whether the amount of overshadowing depended on the temporal location of the overshadowing cue and whether overshadowing produced inverse fear of the context after a single trial. Critically, the results were similar to those for taste aversion. Overall, the results of these various experiments suggest that Revusky’s account provides a simple and effective framework for understanding why taste aversion learning can tolerate long delays between the taste and subsequent illness.