Taste aversion learning despite long delays: How best explained?
Taste aversion learning (aka conditioned taste aversions or 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. This paper provides an overview of a series of recent 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. The general method is one in which in a single conditioning session the rats are first given 8% sucrose, providing the sweet target taste, and 65 min later are injected with lithium chloride. What vary across experiments are the potentially interfering events occurring during the 65-min delay period. When the interfering event is a second, and quite different, taste, namely sour-tasting hydrochloric acid solution (HCl), this produces 1-trial overshadowing of the sucrose aversion, to a degree that is greater when HCl is given late in the delay period, greater when HCl is given in the same context as sucrose and greater when HCl has not been pre-exposed. Other intervening events can also overshadow sucrose aversion learning. These include placement into a novel context, as long as this occurs immediately before injection, and even stimuli that evoke memories of food-related experiences. These results can be accounted for by adding to the Rescorla-Wagner model (Rescorla & Wagner, 1972) the assumption that sickness is comprised of a succession of mini-bouts and the assumption that context-event associations (Wagner, 1981) are important in long-delay CTA.