Choice with delayed or uncertain reinforcers in rats: influence of timeout duration and session length

Interest is rising for animal modeling of impaired behavioral inhibition. Impulsivity and risk proneness, key symptoms of impulse-control disorders, are classically measured by Intolerance to Delay (ID) and Probabilistic Delivery (PD) tasks, requiring choice between a "Small & Soon" or "Sure" (SS) versus a "Large & Late" or "Luck-Linked" (LL or LLL, respectively) reinforcer. Several temporal parameters shall be set, which are not always explicit. Here, we focused on duration of timeout (TO; three groups: 15, 30, or 45 s; Exp. 1) and on session length (SL; three groups: 60, 90, or 120 min; Exp. 2) to determine whether these parameters may affect rats' performance in ID and PD tasks, respectively. In Exp. 1, rats' reaction to increasing experimental delays (absolute values 0-90 s, delay-equivalent odds 0 to 1.94 ± 0.11) was critically affected by TO duration: a steeper impulsivity curve was found in subjects tested with the longest TO, while random performance was elicited with too short TO. In Exp. 2, a specific "gambling" part was presented (LLL probability lower than 20%). Subjects tested with the shortest session length (60 min), who had a low number of gambling opportunities (performed trials = 84.33 ± 1.91), exhibited a profile of risk proneness, with sustained LLL preference despite high uncertainty and low payoff. Present data demonstrate that TO and SL crucially influence rats' performance in these operant tasks. Their methodological refinement is highly relevant to validate preclinical models for inhibitory-control impairments.