Pigeons may not remember the stimuli that reinforced their recent behavior

In two experiments the conditioned reinforcing and delayed discriminative stimulus functions of stimuli that signal delays to reinforcement were studied. Pigeons' pecks to a center key produced delayed-matching-to-sample trials according to a variable-interval 60-s (or 30-s in 1 pigeon) schedule (Experiment 1) or a multiple variable-interval 20-s variable-interval 120-s schedule (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 comparison stimuli on the side keys. Pecks to the key color that matched the sample were reinforced with 4-s access to grain. Under some conditions of Experiment 1, pecks to nonmatching comparison stimuli produced a 4-s blackout 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 effect and trials ended only when pigeons pecked the other, matching stimulus and received food. The functions relating pretrial response rates to delays differed markedly from those relating matching-to-sample accuracy to delays. Specifically, response rates remained relatively high until the longest delays (15.0 to 27.0 s) were arranged, at which point they fell to low levels. Matching accuracy was high at short delays, but fell to chance at delays between 3.0 and 9.0 s. In Experiment 2, both matching accuracy and response rates remained high over a wider range of delays in the variable-interval 120-s component relative to the variable-interval 20-s component. The difference in matching accuracy between the components was not due to an increased tendency in the variable-interval 20-s component toward proactive interference following short intervals. Thus, under these experimental conditions the conditioned reinforcing and the delayed discriminative functions of the sample stimulus depended on the same variables (delay and variable-interval value), but were nevertheless dissociated.     

Resurgence of time allocation

The resurgence of time allocation with pigeons was studied in three experiments. In Phase 1 of each experiment, response‐independent food occurred with different probabilities in the presence of two different keylights. Each peck on the key changed its color and the food probability in effect. In Phase 2, the food probabilities associated with each keylight were reversed and, in Phase 3, food was discontinued in the presence of either keylight. The food probabilities were .25 and .75, in Experiment 1, and 0.0 and 1.0 in Experiment 2. More time was allocated to the keylight correlated with more probable food in Phases 1 and 2, and in Phase 3 resurgence of time allocation occurred for two of three pigeons in Experiment 1, and for each of four pigeons in Experiment 2. Because time had to be allocated to either of the two alternatives in Experiments 1 and 2, however, it was difficult to characterize the time allocation patterns in Phase 3 as resurgence when changeover responding approached zero. In Experiment 3 this issue was addressed by providing a third alternative uncorrelated with food such that in each phase, after 30 s in the presence of either keylight correlated with food, the third alternative always was reinstated, requiring a response to access either of the two keylights correlated with food. In this experiment, the food probabilities were similar to those in Experiment 1. Resurgence of time allocation occurred for each of three pigeons under this procedure. The results of these experiments suggest that patterns of time allocation resurge similarly to discrete responses and to spatial and temporal patterns of responding.