Temporal maps and informativeness in associative learning

Abstract

Neurobiological research on learning assumes that temporal contiguity is essential for association formation, but what constitutes temporal contiguity has never been specified. We review evidence that learning depends, instead, on learning a temporal map. Temporal relations between events are encoded even from single experiences. The speed with which an anticipatory response emerges is proportional to the informativeness of the encoded relation between a predictive stimulus or event and the event it predicts. This principle yields a quantitative account of the heretofore undefined, but theoretically crucial, concept of temporal pairing, an account in quantitative accord with surprising experimental findings. The same principle explains the basic results in the cue competition literature, which motivated the Rescorla-Wagner model and most other contemporary models of associative learning. The essential feature of a memory mechanism in this account is its ability to encode quantitative information.

Figure I

Learning protocols and the behavioral results they produce (a CR to the CS, or not). Basic : the US occurs only during the CS but at unpredictable times within it: leads to a CR. Delay: the US occurs only at the termination of a CS of fixed duration, I_CS: leads to a CR. Inhibitory: the US occurs only when CS is absent (ergo, CS never paired with US): leads to a CR (such as CS avoidance if the US is appetitive or CS approach if the US is aversive). The remaining protocols are cue-competition protocols. Truly random: the times of US occurrence are completely independent; hence, they sometimes occur during CS ’by coincidence’: a CR to the CS does not develop but a strong CR does develop to the experimental chamber (the spatial context), which is a competing cue. Blocking: trained during Phase 1 with CS₁ and then during Phase 2 with CS₂ and CS₁, presented ’in compound’ (together): a CR to CS₂ does not develop. Overshadowing: CS₁ and CS₂ presented in compound from the outset; a CR develops to one or the other CS but not both (Box 4). Relative validity: (1) CS₂ is always presented in compound; half the time it is compounded with CS₁ and half the time with CS₃. The US occurs only when it is compounded with CS₁. A CR develops only to CS₁, despite fact that US is paired with CS₂ on half of all CS₂ occurrences. (2) Same as in RV–1 except that now pairing of US with CS₂ occurs on the half of the trials when it appears in compound with CS₁ and half the trials when it appears in compound with CS₃. CR develops only to CS₂, despite fact that the US is paired with the other two CSs just as frequently (on half of all their occurrences) and with CS₂ no more frequently than in RV–1.

Figure I

(a) Trials to acquisition (a measure of associability) plotted as a function of the CS–US interval, either when the average interval between USs, Ī_B, is fixed (solid line) or when the ratio of this interval to CS duration, Ī_B/I_CS, is fixed (dashed line). (b) Trials to acquisition versus Ī_B/I_CS on double logarithmic coordinates. The solid line is the best fitting linear regression; the dashed curves are the 95% confidence interval. Both plots adapted, with permission, from Ref. [5].

Figure 1

CR timing in goldfish as Pavlovian training progresses. A 5 s visual CS terminated in mild shock (US). Training trials were intermixed with probe trials, during which the CS remained on for 45 s with no US. Movement as a function of time in the CS is shown for blocks of 5 sessions (50 CS–US pairings) during the long unreinforced trials. The amount of anticipatory activity increased as training progressed, but the timing of the CR was appropriate even in the earliest part of training. Modified, with permission, from Ref. [18].