Association for Behavior Analysis International

Differential responding to long intervals—i.e., long-interval timing—requires a functional hippocampus (HPC). However, reduced timing capacity in HPC-lesioned animals performing timing tasks is complicated by ostensible motivational effects which arise from the delay-to-reward imposed by interval timing tasks, as well as overlap between timed and non-timed responses. To address these concerns, two adjustments to long-interval timing tasks are proposed. First, subjects should be afforded with reinforced non-timing behaviors concurrent with timing. Second, subjects should initiate the onset of timed stimuli. Under these conditions, interference by extraneous behavior would be detected in the rate of concurrent non-timing behaviors, and changes in motivation would be detected in the rate at which timed stimuli are initiated. In a task with these characteristics, rats initiated a concurrent fixed-interval (FI) random-ratio (RR) schedule of reinforcement. Changes in schedule requirements affected the rate of initiating responses and the timing of changeovers, but reduced reinforcer efficacy, via pre-session feeding, only affected the former. A similar task using concurrent FI FI schedules revealed the effects of chronic variable stress—which reliably impacts hippocampal function—on motivation and interval timing.