Dopamine modulates reward-related vigor

Abstract

Subjects routinely control the vigor with which they emit motoric responses. However, the bulk of formal treatments of decision-making ignores this dimension of choice. A recent theoretical study suggested that action vigor should be influenced by experienced average reward rate and that this rate is encoded by tonic dopamine in the brain. We previously examined how average reward rate modulates vigor as exemplified by response times and found a measure of agreement with the first suggestion. In the current study, we examined the second suggestion, namely the potential influence of dopamine signaling on vigor. Ninety healthy subjects participated in a double-blind experiment in which they received one of the following: placebo, L-DOPA (which increases dopamine levels in the brain), or citalopram (which has a selective, if complex, effect on serotonin levels). Subjects performed multiple trials of a rewarded odd-ball discrimination task in which we varied the potential reward over time in order to exercise the putative link between vigor and average reward rate. Replicating our previous findings, we found that a significant fraction of the variance in subjects' responses could be explained by our experimentally manipulated changes in average reward rate. Crucially, this relationship was significantly stronger under L-Dopa than under Placebo, suggesting that the impact of average reward levels on action vigor is indeed subject to a dopaminergic influence.

Figure 1

(a) Structure of one trial of the behavioral task. Subjects are shown their potential reward, followed by an odd-one-out task to be completed within 500 ms (400 ms for 20% of trials). After a further 500 ms, they were shown their received reward. (b) The induced fluctuation in available reward (blue) and averaged reward (for learning rate α=0.012 in red, the fixed value used in Guitart-Masip et al, 2011, and α=0.113 in green, the mean value across subjects found in the current analysis) varying over time. The color reproduction of this figure is available on the Neuropsychopharmacology journal online.

Figure 2

Mean β Values for data sets D_orig (blue; Guitart-Masip et al, 2011) and D_tired (red), estimated through the expectation maximization algorithm. Error bars are SEs and asterisks indicates significant difference in means at p<0.05 based on a two-sample t-test. The color reproduction of this figure is available on the Neuropsychopharmacology journal online.

Figure 3

Mean β Values for subjects given placebo (blue), L-Dopa (red), and citalopram (green) estimated through expectation maximization algorithm. Error bars are SEs and asterisks indicate significant difference in means at p<0.05 based on a two-sample t-test. The color reproduction of this figure is available on the Neuropsychopharmacology journal online.

Figure 4

β Values for average reward rate regressor across conditions and experiments. Error bars are SEs and asterisk indicates significant difference in means at p<0.05 based on a two-sample t-test.