Effects of intratrial stimulus change on fixed-interval performance: The roles of clock and memory processes

Abstract

In two experiments, pigeons were exposed to differentially cued training trials of fixed interval (FI) 30 and 60 sec. In addition, shift trials were presented in which the cue associated with one FI value was presented for a prearranged duration at trial onset, followed by offset of that cue and presentation of the other cue. Response-contingent reinforcement was scheduled during the second cue. During the first shift phase, the FI 30-sec cue was shifted to the FI 60-sec cue; in a second phase, the order of the cue shift was reversed. Inferences about accumulator and memory functions of the internal clock were based upon behavior during both training trials and shift trials. At the end of both shift phases, test-trial FI functions generally superimposed in a manner consistent with accumulator reset on cue shift. Individual differences in clustering of functions were accommodated by variation in reference-memory storage across subjects. This interpretation was tested in Experiment 2 by constraining reference-memory storage on shift trials. These conditions yielded a decrease in between-subject variability and provided data consistent with accumulator reset and control by a single reference-memory value on shift trials.