Effects of lesions of the orbitofrontal cortex on sensitivity to delayed and probabilistic reinforcement
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Rationale: Lesions of the orbital prefrontal cortex (OPFC) can cause pathologically impulsive behaviour in humans. Inter-temporal choice behaviour (choice between reinforcers differing in size, delay and/or probability) has been proposed as a model of "impulsive choice" in animals. Objective: The effect of lesions of the OPFC on rats' inter-temporal choice behaviour was examined in two experiments: (1) rats chose between a smaller immediate reinforcer and a larger delayed reinforcer; (2) rats chose between a smaller certain reinforcer and a larger probabilistic reinforcer. Methods: Under halothane anaesthesia, rats received injections of the excitotoxin quinolinate into the OPFC (0.1 M, 0.5 µl, two injections in each hemisphere), or sham lesions (injections of vehicle). They were trained to press two levers (A and B) for food-pellet reinforcers in discrete-trials schedules. In free-choice trials, a press on A resulted in immediate delivery of one food pellet; a press on B resulted in delivery of two pellets, either following a delay (d) (experiment 1), or with a probability (p) <1 (experiment 2). The values of d and p were manipulated across phases of the experiments. The locations of the lesions were verified histologically at the end of the experiment. Results: In experiment 1, both groups showed declining choice of lever B as a function of d. The lesioned rats showed significantly shorter indifference delays (D50: the value of d corresponding to 50% choice of lever B) than the sham-lesioned rats. In experiment 2, both groups showed declining choice of lever B as a function of the odds against delivery of the two-pellet reinforcer, θ (θ=[1/p]–1). The lesioned rats showed lower indifference odds (θ50: the value of θ corresponding to 50% choice of lever B) than the sham-lesioned rats. In both experiments, the lesioned rats showed extensive atrophy of the OPFC, with sparing of the dorsolateral prefrontal cortex. Conclusions: The results show that lesions of the OPFC can promote preference for the smaller and more immediate, and the smaller and more certain of two reinforcers. The results are consistent with two interpretations: the lesion may have altered (i) the rates of delay and odds discounting, and/or (ii) sensitivity to the ratio of the sizes of the two reinforcers.
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