Impulsivity in spontaneously hypertensive rats: Within-subjects comparison of sensitivity to delay and to amount of reinforcement

Previous research has shown that spontaneously hypertensive rats (SHR) display higher levels of impulsive choice behavior, which is accompanied by a higher sensitivity to the delay of reinforcement, and by a normal sensitivity to the amount of reinforcement. Because those results were based on three different samples of subjects, in the present report we evaluated these three processes in the same individuals. SHR and WIS rats were exposed to concurrent-chains schedules in which the terminal links were manipulated to assess impulsivity, sensitivity to delay, and sensitivity to amount. For exploring impulsivity, a terminal link was associated with a small reinforcer (1 pellet) delivered after a short delay (2 s) while the other terminal link was associated with a larger reinforcer (4 pellets) delivered after a longer delay (28 s). For assessing sensitivity to delay, both alternatives delivered the same amount of reinforcement (1 pellet) and the only difference between them was in the delay before reinforcement delivery (2 s vs 28 s). For assessing sensitivity to amount, both alternatives were associated with the same delay (15 s), but the alternatives differed in the amount of reinforcement (1 vs 4 pellets). In addition to replicating previously observed effects within-subjects, we were interested in analyzing different aspects of the regularity of rats' actions in the choice task. The results confirmed that previous findings were not a consequence of between-group differences: SHR were more impulsive and more sensitive to delay, while their sensitivity to amount was normal. Analyses of response regularity indicated that SHR subjects were more periodic in their responses to levers and in their feeder entries, had a higher number of short-duration bouts of responding, and made a substantially higher number of switches between the alternatives. We discuss the potential implications of these findings for the possible behavioral mechanisms driving the increased sensitivity to delay in SHR.