Taste Aversion Learning Despite Long Delays: How Best Explained?

Abstract

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.

Mediated overshadowing and potentiation of long-delay taste aversion learning: Two versus six cue-taste pairings.

“…As noted earlier, the context of this study is a series of experiments (Kwok & Boakes, 2015b) aimed at understanding long-delay CTAs and inspired by Revusky’s (1971) proposal that taste–nausea associations can be acquired despite intervals of several hours between the two events, because under the normal conditions under which such learning is studied few other events occur that overshadow (‘are relevant to’) the taste–nausea association. Nevertheless, the fact that taste aversion learning displays a delay gradient, namely, that the longer the interval between taste and a nausea-inducing event, the weaker the resultant taste aversion, is explained in terms of increasing interference from low associability events as the interval is lengthened (Revusky, 1971).…”

“…The present focus on mediated overshadowing arose in the context of our interest in long delay learning (Kwok, Livesey, & Boakes, 2012; Kwok & Boakes, 2015b). As commonly observed, rats can develop aversions to tastes despite long delays between the consumption of a target taste and the occurrence of some kind of internal malaise (Garcia, Kimeldorf, & Koelling, 1955; Freeman & Riley, 2009).…”

“…These experiments found that overshadowing is weaker when rats have been pre-exposed to HCl a day before the conditioning session, but not when preexposed a week earlier (Kwok & Boakes, 2015a), and also weaker when HCl is presented early in the target–LiCl interval rather than shortly before the LiCl injection (Kwok, Harris, & Boakes, 2015). Furthermore, this example of overshadowing appears to reflect an acquisition deficit rather than modulation of responding to the overshadowed cue: In an experiment briefly described in Kwok and Boakes (2015b), in which HCl again produced serial overshadowing of a sucrose aversion, there was no indication that sucrose intakes at test were affected by prior extinction of the HCl aversion, that is, no evidence for recovery from overshadowing (cf. Kaufman & Bolles, 1981).…”