Effects of lesions of the nucleus accumbens core on inter-temporal choice: further observations with an adjusting-delay procedure.

Previous experiments using progressive-delay schedules showed that destruction of the nucleus accumbens core (AcbC) altered rats' choice between food reinforcers differing in size and delay. Application of a quantitative model of inter-temporal choice suggested that lesions of the AcbC increase the delay-dependent degradation of reinforcer value (delay discounting) without altering instantaneous reinforcer value. This experiment examined the effect of lesions of the AcbC on inter-temporal choice using an adjusting-delay schedule. Rats received excitotoxin-induced lesions of the AcbC or sham lesions. They were trained to press levers A and B for food-pellet reinforcers in an adjusting-delay schedule in which the delay to the larger reinforcer, d(B), varied in accordance with the rats' choices between the two levers. In two experimental conditions, the reinforcers associated with levers A and B were 1 vs. 4 and 2 vs. 4 pellets. The AcbC-lesioned group showed shorter indifference delays to reinforcer B (d(B(50))) than the sham-lesioned group under both conditions. In confirmation of a prediction derived from the model of inter-temporal choice, the ratio of the indifference delays from the two conditions did not differ between the groups. Analysis of the cyclical changes in d(B) by Fourier transform showed that the period of oscillation and power within the dominant frequency band did not differ between the groups, suggesting that the lesion did not disrupt the rats' ability to detect short-term changes in delay of reinforcement. The results are consistent with previous findings that indicate a role for the AcbC in delay discounting.