The effect of stimulus distribution form on the acquisition and rate of conditioned responding: Implications for theory.

Abstract

This is the accepted version of the paper.This version of the publication may differ from the final published version. In four experiments rats were conditioned to an auditory conditioned stimulus (CS) that was paired with food, and learning about the CS was compared across two conditions in which the mean duration of the CS was equated. In one the CS was of a single, fixed duration on every trial, while in the other the CS duration was drawn from an exponential distribution, and hence changed from trial to trial. Higher rates of conditioned responding to the fixed than to the variable stimulus were observed, in both between-(Experiment 1) and within-subject designs (Experiments 2 and 3). Moreover, this difference was maintained when stimuli trained with fixed or variable durations were tested under identical conditions (i.e. with equal numbers of fixed and variable duration trials) -suggesting that the difference could not be attributed to performance effects (Experiment 3). In order to estimate the speed of acquisition of conditioned responding, the scaled cumulative distribution of a Weibull function was fitted to the trial-by-trial response rates for each rat. In the within-subject experiments specific differences in the pattern of acquisition to fixed and variable CS were shown; a somewhat different pattern was found when ITI was manipulated (Experiment 4). The implications of these findings for theories of conditioning and timing are discussed. Permanent repository link

Overshadowing by fixed- and variable-duration stimuli

“…Nonetheless, the difference in the overshadowing ratios is consistent with the suggestion that, although of the same mean duration, fixed duration CSs condition better than cues whose duration varies from trial to trial (Jennings et al, 2013).…”

“…In summary, there is evidence that a fixed duration CS acquires more associative strength than its variable counterpart (Jennings et al, 2013). This result is consistent with a trial-based approach, but cannot be explained by time-accumulation theories.…”

“…Thus acquisition of the CR to CSs that are either of a fixed duration, or vary in duration from trial to trial but have the same overall mean duration as the fixed CS, should be the same. However, Jennings et al (2013) reported a higher level of CR to a fixed than to a variable CS -a difference which persisted even when animals were tested under identical conditions. 3 Jennings et al (2013) interpreted these findings as evidence that the fixed duration CS had acquired more associative strength than the variable CS.…”

“…Third, our account would predict higher response rates to the fixed than to the variable noise (cf. Jennings et al, 2013), but Experiment 1 did not examine responding to this stimulus. Thus in Experiment 2 responding to the noise alone was examined, both in the probe trials just described, and also by adding two sessions of test trials with the noise CS after the test of the light.…”

“…This result is consistent with a trial-based approach, but cannot be explained by time-accumulation theories. The present experiments were designed to provide further evidence for this proposal: Jennings et al (2013) only demonstrated differences in performance of the CR to fixed and variable duration CSs; however, it has long been argued that conditioned responding to a CS may not always be the best indicator of its associative strength (e.g. Hull, 1943;cf.…”