Elsevier

Biological Psychiatry

Volume 68, Issue 3, 1 August 2010, Pages 306-309
Biological Psychiatry

Brief Report
Phasic Nucleus Accumbens Dopamine Release Encodes Effort- and Delay-Related Costs

https://doi.org/10.1016/j.biopsych.2010.03.026Get rights and content

Background

Optimal decision-making requires that organisms correctly evaluate both the costs and benefits of potential choices. Dopamine transmission within the nucleus accumbens (NAc) has been heavily implicated in reward-learning and decision-making, but it is unclear how dopamine release might contribute to decisions that involve costs.

Methods

Cost-based decision-making was examined in rats trained to associate visual cues with either immediate or delayed rewards (delay manipulation) or low-effort or high-effort rewards (effort manipulation). After training, dopamine concentration within the NAc was monitored on a rapid time scale with fast-scan cyclic voltammetry.

Results

Animals exhibited a preference for immediate or low-effort rewards over delayed or high-effort rewards of equal magnitude. Reward-predictive cues but not response execution or reward delivery evoked increases in NAc dopamine concentration. When only one response option was available, cue-evoked dopamine release reflected the value of the future reward, with larger increases in dopamine signaling higher-value rewards. In contrast, when both options were presented simultaneously, dopamine signaled the better of two options, regardless of the future choice.

Conclusions

Phasic dopamine signals in the NAc reflect two different types of reward cost and encode potential rather than chosen value under choice situations.

Section snippets

Methods and Materials

The NAc dopamine concentration was monitored in rats (n = 10) trained in either effort-based or delay-based decision tasks (Figure S1 in Supplement 1 and Figure 1A;Supplementary Methods in Supplement 1). In these tasks, rewards of equal magnitude (45-mg sucrose pellets) were made available at either low or high value in pseudorandomly ordered trials, with 90 total trials/behavioral session. On forced-choice trials (60/session), distinct 5-sec cue lights signaled the available response option

Results

During behavioral sessions, animals readily overcame high-effort demands or long delays to obtain rewards (Figures 1B and 1E) and discriminated between reward-predictive cues to reduce errors on forced-choice trials (error rates significantly below chance levels; p < .0001 for all comparisons; Figures 1C and 1F). On free-choice trials, animals exhibited a marked preference for low-cost and immediate reward options over high-cost and delayed reward options (paired t test on choice allocation; p

Discussion

Dopamine neurons encode a reward prediction error signal in which cues that predict rewards evoke phasic increases in firing rate, whereas fully expected rewards do not alter dopamine activity (13). This signal is also sensitive to a number of features of the upcoming reward, because cues that predict larger, immediate, or more probable rewards evoke larger spikes in dopamine neuron activity than cues that predict smaller, delayed, or less probable rewards (9, 10, 11, 14). In this way, dopamine

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