Elsevier

Behavioural Brain Research

Volume 245, 15 May 2013, Pages 22-28
Behavioural Brain Research

Research report
Effect of orbitofrontal cortex lesions on temporal discounting in rats

https://doi.org/10.1016/j.bbr.2013.02.014Get rights and content

Abstract

Although choices of both humans and animals are more strongly influenced by immediate than delayed rewards, methodological limitations have made it difficult to estimate the precise form of temporal discounting in animals. In the present study, we sought to characterize temporal discounting in rats and to test the role of the orbitofrontal cortex (OFC) in this process. Rats were trained in a novel intertemporal choice task in which the sequence of delay durations was randomized across trials. The animals tended to choose a small immediate reward more frequently as the delay for a large reward increased, and, consistent with previous findings in other species, their choice behavior was better accounted for by hyperbolic than exponential discount functions. In addition, model comparisons showed that the animal's choice behavior was better accounted for by more complex discount functions with an additional parameter than a hyperbolic discount function. Following bilateral OFC lesions, rats extensively trained in this task showed no significant change in their intertemporal choice behavior. Our results suggest that the rodent OFC may not always play a role in temporal discounting when delays are randomized and/or after extensive training.

Highlights

► Temporal discounting of the rat was studied using a novel inter-temporal choice task. ► Rat's choices were better explained by hyperbolic than exponential discount functions. ► Two-parameter discount functions were superior to single-parameter discount functions. ► Orbitofrontal cortex lesions did not alter rat's inter-temporal choice behavior.

Introduction

Humans and other animal species preferentially choose an immediate reward over a similar, but more delayed reward, suggesting that the subjective value of a reward is discounted as a function of its delay. Temporal discounting in humans has initially been proposed to be exponential, such that the subjective value is discounted at the same rate during the entire delay [33]. However, subsequent empirical studies have repeatedly shown that temporal discounting in humans is better described by a hyperbolic discount function than by an exponential function [7], [27]. Similar results have been reported for rats [20], [21] and pigeons [21], [29], suggesting that hyperbolic temporal discounting might be a universal form of value discounting across different animal species. However, previous studies in rats and pigeons have employed the adjusting delay procedure [18] or block-wise variation of delay durations (e.g., [11], [16]). Such procedures often lead to choices that are non-stationary or serially correlated across trials, which can potentially lead to biases in estimating temporal discount functions [5]. By contrast, recent behavioral studies in monkeys have employed an intertemporal choice task with symbolic cues and randomized reward delays, and have shown that their choice behavior is in general more consistent with hyperbolic than exponential discounting of subjective values [3], [9], [12].

The first goal of this study was to estimate the precise form of temporal discount function in rodents, using a novel intertemporal choice task in which the sequence of delay durations was randomized across trials. The results showed that a hyperbolic discount function accounted for the rat's temporal discount function better than an exponential function. The second goal was to examine the role of orbitofrontal cortex (OFC) in temporal discounting during intertemporal choice of rats. OFC lesions are known to induce impulsive choice behavior in humans [1], [2]. Neural activity in the monkey OFC is also correlated with temporally discounted values [30]. However, previous lesion studies in rats have yielded conflicting results on this matter. Following OFC lesions, the rate of temporal discounting increased [11], [24], [32], decreased [37], did not change [6], [17], or varied depending on the extent of lesions [16] and the presence or absence of a distinct cue bridging the delay between choice and reward delivery [38]. In the present study, we tested the effect of OFC lesions on the rat's temporal discount functions that were estimated more accurately than in the previous studies. In particular, to distinguish the OFC role in learning response–outcome contingency [36] from its contribution to temporal discounting, we extensively trained rats before the lesions. We found that OFC lesions did not make a significant change in temporal discounting of rats.

Section snippets

Subjects

Seven young male Sprague-Dawley rats (approximately 9–11 weeks old, 390–400 g) were used. The animals were individually housed in a colony room and initially allowed free access to food and water with extensive handling for 1 week. Their body weight was gradually reduced to 80–85% of their free-feeding weight by water deprivation and maintained at this level throughout the study. Experiments were performed in the dark phase of a 12-h light/dark cycle. The experimental protocol was approved by

Extent of lesions

Fig. 2 shows schematic representations of the largest (light gray), medium (dark gray), and the smallest (black) OFC lesions (n = 7 animals). As shown, quinolinic acid injections consistently produced extensive damages to the medial, lateral, ventral and dorsolateral OFC [26].

Choice behavior

The animals performed 2263 ± 711 and 2343 ± 440 trials in S0 and S3 sessions, respectively, before the OFC lesions, and 1371 ± 451 and 1474 ± 206 trials in S0 and S3 sessions, respectively, after the OFC lesions (mean ± SD). When the

Discussion

The first goal of the present study was to determine the precise form of temporal discounting in rodents while avoiding methodological complications associated with the adjusting delay procedure. To accomplish this, all reward delays were explicitly signaled to the animals by the arm locations (rather than requiring the animals to estimate them through experience) and their sequence was randomized across trials in our task. Hence, our task was free from the problem of serial correlation or

Acknowledgments

This work was supported by the Research Center Program of Institute for Basic Science, the National Research Foundation grant (2011-0015618) and the Original Technology Research Program for Brain Science (2011-0019209) funded by the Ministry of Education, Science and Technology, Korea (M.W.J.) as well as a grant (R01 DA029330) from the National Institute of Health, USA (D.L.).

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