Pigeons may not remember the stimuli that reinforced their recent behavior

In two experiments the conditioned reinforcing and delayed discriminative s timulus functions of stimuli that signal delays to reinforcement were studi ed. Pigeons' pecks to a center key produced delayed-matching-to-sample tria ls according to a variable-interval 60-s (or 30-s in 1 pigeon) schedule (Ex periment 1) or a multiple variable-interval 20-s variable-interval 120-s sc hedule (Experiment 2). The trials consisted of a 2-s illumination of one of two sample key colors followed by delays ranging across phases from 0.1 to 27.0 s followed in turn by the presentation of matching and nonmatching co mparison stimuli on the side keys. Perks to the key color that matched the sample were reinforced with 4-s access to grain. Under some conditions of E xperiment 1, pecks to nonmatching comparison stimuli produced a 4-s blackou t and the start of the next interval. Under other conditions of Experiment 1 and each condition of Experiment 2, pecks to nonmatching stimuli had no e ffect and trials ended only when pigeons pecked the other, matching stimulu s and received food. The functions relating pretrial response rates to dela ys differed markedly from those relating matching-to-sample accuracy to del ays. Specifically, response rates remained relatively high until the longes t delays (15.0 to 27.0 s) were arranged, at which point they fell to low le vels. Matching accuracy was high at short delays, but fell to chance at del ays between 3.0 and 9.0 s. In Experiment 2, both matching accuracy and resp onse rates remained high over a wider range of delays in the variable-inter val 120-s component relative to the variable-interval 20-s component. The d ifference in matching accuracy between the components was not due to an inc reased tendency in the variable-interval 20-s component toward proactive in terference following short intervals. Thus, under these experimental condit ions the conditioned reinforcing and the delayed discriminative functions o f the sample stimulus depended on the same variables (delay and variable-in terval value), but were nevertheless dissociated.