Choice “under pressure”: Impulsive behavior and preference reversal of rats exposed to response-independent shocks

Abstract

Some research with human participants has shown that the choice of a smaller-sooner reinforcer (SS) over a larger-later reinforcer (LL) is more frequent in aversive than non-aversive contexts (e.g., presence versus absence of response-independent intense noise). Although there is evidence that this effect could be reproduced in rats (e.g., response-independent noise disrupts rats’ performance on schedules of reinforcement), no explicit attempt has been reported to date. The present study tested the generality of the disruptive effect of aversive contexts in rats’ impulsive choice. To emulate the procedures implemented with humans, response-independent (Random-Time 60 s) mild-intensity footshocks (.25 mA) were incorporated into a systematic replication of Green and Estle’s (2003) task designed to study preference reversal phenomena (i.e., SS preference changes to LL with the addition of delays before both the SS and LL alternatives). In doing so, we explored the effects of an aversive context on preference reversal, which also have not been reported to date. Male Wistar rats (16) were exposed to three different conditions; eight of them experienced shocks throughout the study. During an initial delay configuration condition (SS = 2 pellets after 0.5-s; LL = 4 pellets after 6 s), responding of non-shocked rats showed an increase from indifference (∼ 50 % LL) towards preference for the LL option (∼ 75 % or higher LL responses), whereas responding of shocked rats did not deviate from indifference. Next, delays were added to the LL reinforcer until SS preference was individually established (+6 s, +9 s, +15 s, etc.). The behavior of non-shocked rats seemed more affected by the added delays, e.g., they reached SS preference with less added delays. Preference-reversal conditions consisted of adding 5-s, 15-s, and 25-s delays to both SS and LL alternatives. Shocked rats showed a more robust and consistent preference reversal effect than non-shocked rats. Research on manipulations that reduce impulsive choice suggests that similar processes could explain the disruptive effects of aversive contexts and the effects of interventions; namely, aversiveness of delays and discrimination of contingent relations between temporally distant events. The results of the present study are discussed in that framework, focusing on covariations between rats’ choice patterns across the different delay configurations and the distribution of shocks pre- and post- reinforcement delivery.