Nucleus accumbens lesions decrease sensitivity to rapid changes in the delay to reinforcement

Abstract

Both humans and non-humans discount the value of rewards that are delayed or uncertain, and individuals that discount delayed rewards at a relatively high rate are considered impulsive. To investigate the neural mechanisms that mediate delay discounting, the present study examined the effects of excitotoxic lesions of the nucleus accumbens (NAC) on discounting of reward value by delay and probability. Rats were trained on delay (n = 24) or probability discounting (n = 24) tasks. Following training, excitotoxic lesions of the NAC were made by intracranial injections of 0.5 µl 0.15 M quinolinic acid (n = 12) or vehicle (n = 12) aimed at the NAC (AP +1.6, ML ±1.5, DV −7.1). NAC lesions did not alter performance in animals tested with a constant delay (4 s) or probability (0.4) of reinforcement. However, when tested with between session changes in the delay (0, 1, 2, 4, and 8 s) of reinforcement, the lesioned rats had flatter discount curves than the sham group, indicating that they were less sensitive to frequent changes in the delay to reward. In contrast, the NAC lesions did not affect discounting of probabilistic rewards. NAC lesions impaired the ability to adapt to frequent between session changes in the delay to reward but did not increase or decrease discounting when the delay was held constant across sessions. NAC lesions may disrupt the ability of the animals to predict the timing of delayed rewards when the delay to reward is changed frequently.

Effects of quinolinic acid-induced lesions of the nucleus accumbens core on inter-temporal choice: a quantitative analysis

“…A recent study by Acheson et al (2006) reported findings that are in apparent contrast with the findings of Cardinal et al (2001Cardinal et al ( , 2003. Acheson et al (2006) used an adjusting amount schedule (Richards et al 1999) in which the delay to a liquid reinforcer of fixed volume was held constant while the volume of an immediate reinforcer was varied in response to the animals' choices.…”

“…Acheson et al (2006) found that while the lesion had no effect on performance when the delay was held constant, lesioned rats showed flatter preference functions when the delay was changed from day to day. Acheson et al (2006) proposed that the lesion may have disrupted the ability of the rats to adapt to changes in delay, without altering the rate of delay discounting. The present finding of flatter slopes of the psychometric function and larger Weber fractions is consistent with the suggestion that discrimination of within-session changes in d B was less precise in the AcbC-lesioned rats than in the intact rats.…”

“…Acheson et al (2006) used an adjusting amount schedule (Richards et al 1999) in which the delay to a liquid reinforcer of fixed volume was held constant while the volume of an immediate reinforcer was varied in response to the animals' choices. Acheson et al (2006) found that while the lesion had no effect on performance when the delay was held constant, lesioned rats showed flatter preference functions when the delay was changed from day to day. Acheson et al (2006) proposed that the lesion may have disrupted the ability of the rats to adapt to changes in delay, without altering the rate of delay discounting.…”

“…Bowman and Brown 1998;Cardinal et al 2001;Pothuizen et al 2005;Acheson et al 2006). With the exception of three rats whose data were excluded from the analysis, the lesions were mainly restricted to the AcbC.…”

“…Among the structures that have been implicated in inter-temporal choice is the nucleus accumbens core (AcbC) (Cardinal et al 2001(Cardinal et al , 2003Pothuizen et al 2005;Acheson et al 2006). Cardinal et al (2001) found that selective lesions of the AcbC in the rat induced a decrease in preference for a larger/delayed reinforcer over a smaller/immediate reinforcer.…”