There Is a Time and a Place for Everything: Bidirectional Modulations of Latent Inhibition by Time-Induced Context Differentiation

Article excerpt

Latent inhibition (LI) is defined as poorer evidence of learning with a stimulus that previously was presented without consequence, as compared with a novel or previously attended stimulus. The present article reviews the evidence, mostly from three-stage conditioned taste aversion studies (preexposure, conditioning, and test), that LI can be either attenuated or enhanced depending on the length of the retention interval between conditioning and test and where that interval was spent. Time-induced reduction in LI is observed when the interval context is the same as that of the preexposure, conditioning, and test stages. Super-LI is obtained when a long retention interval is spent in a context that is different from that of the other stages. The differential modulations of LI appear to be the result of the strengthening of primacy effects (i.e., first training disproportionately stronger than subsequent training) by long-interval different contexts, thereby producing super-LI, and the reversal of this effect by long-interval same contexts, thereby producing attenuated LI. The bidirectional effects of time/ context modulations on LI, unaccounted for by current learning theories, are explained, in part, by a time-induced context differentiation process. Implications for theories of LI, learning, and memory are discussed.

Time is a sort of river of passing events, and strong is its current; no sooner is a thing brought to sight than it is swept by and another takes its place, and this too will be swept away. (Marcus Aurelius, Meditations [trans. 1964], iv. 43)

That memory is a function of time is a truism. Indeed, what is memory but some present mark of a past event. Most commonly, as time between study and test increases, memory performance decreases, a situation that we call forgetting. Sometimes, however, memory improves over time, as with reminiscence (an increase with repeated testing), hypermnesia (increased recall), and incubation.

Context, too, has profound effects on memory. In humans, for example, local context can disambiguate the meaning of a word (see, e.g., Kintsch & Mross, 1985), and more global or tonic contexts, when changed from learning to subsequent test sessions, can result in relatively poor memory performance (e.g., Smith, Glenberg, & Bjork, 1978). The effects of context on memory have been extensively studied also with animals, as in habituation (e.g., Hall & Channell, 1985; Honey, Pye, Lightbrown, Rey, & Hall, 1992), spontaneous recovery (e.g., Bouton & Peck, 1989; Harris, Jones, Bailey, & Westbrook, 2000), latent inhibition (e.g., Lubow, Rifkin, & Alek, 1976; McLaren, Bennett, Plaisted, Aitken, & Mackintosh, 1994), and simple conditioning (e.g., Bouton & King, 1983; for a review of these literatures, see Bouton, 1993; Bouton, Nelson, & Rosas, 1999). Irrespective of the particular research domain, the usual explanation of context-based effects incorporates some version of the hypothesis that contextual stimuli serve as retrieval cues for previously learned associations or information.

Recent experiments have demonstrated that context and time appear to be interchangeable (e.g., Bouton, 1993; Bouton et al., 1999), such that time and physical context manipulations have additive effects (e.g., Rosas, Vila, Lugo, & Lopez, 2001 ; Westbrook, Jones, Bailey, & Harris, 2000). To explain this, it has been proposed that with increased time between acquisition and test, there is a gradual change in the internal representation of context. As a consequence, with long intervals, identical acquisition and test contexts become subjectively differentiated (see Estes, 1997; McGeoch, 1942; Mensink & Raaijmakers, 1988; Spear, 1973, 1978).

Although it has long been recognized that memory is affected by both the passage of time and the relation between acquisition and test contexts, the role of the retention interval context has been largely ignored. …