What Is the Relationship between Dopamine and Effort?

Abstract

The trade-off between reward and effort is at the heart of most behavioral theories, from ecology to economics. Compared to reward, however, effort remains poorly understood, both at the behavioral and neurophysiological levels. This is important because unwillingness to overcome effort to gain reward is a common feature of many neuropsychiatric and neurological disorders. A recent surge in interest in the neurobiological basis of effort has led to seemingly conflicting results regarding the role of dopamine. We argue here that, upon closer examination, there is actually striking consensus across studies: dopamine primarily codes for future reward but is less sensitive to anticipated effort cost. This strong association between dopamine and the incentive effects of rewards places dopamine in a key position to promote reward-directed action.

Keywords: cost–benefit decision making; electrophysiology; midbrain; motivation; striatum; voltammetry.

Figure 1

Relative Sensitivity of Dopamine (DA) Neurons to reward Benefits and Effort Costs. Sensitivity of substantia nigra pars compacta dopamine neurons in monkeys to information about upcoming reward benefits (blue) and effort costs (red) in two recent neurophysiological studies (A,B) in behaving monkeys. In both studies, monkeys were required to perform a given action to obtain a given reward. Reward sizes and physical difficulty (effort cost) were manipulated independently across trials, and each trial started with a visual cue indicating the upcoming effort and reward. Regression coefficients were calculated using a sliding-window procedure to evaluate the difference in firing across reward (blue) and effort (red conditions) at each time-point around stimulus onset. The firing of dopamine neurons shows reliable positive encoding of reward size (firing rates are greater for cues indicating large versus small rewards) within 200 ms after cue onset. At the same time, dopamine neurons also display negative modulation by effort level (firing rates are smaller for larger effort levels). Crucially, the magnitude of the reward modulation is greater than the effort modulation in both studies, even though they clearly differ in the way animals needed to cope with the expected difficulty. The difference in sensitivity cannot be simply due to a difference in subjective sensitivity to reward, as compared to effort, because these two variables had an equivalent weight on the willingness to work of the animal, at least in . Panel (A) reproduced, with permission, from ; panel (B) adapted, with permission, from .

Figure 2

Cue-Elicited Dopamine (DA) Levels Are Primarily Modulated by Expected Future Benefits and Not by Anticipated Costs. (A) Behavior (choice performance and response latencies, upper panels) and cue-elicited dopamine levels (lower panels), recorded with fast-scan cyclic voltammetry in rat nucleus accumbens core, in conditions where rats were presented with options signaling availability of a future reward (food pellets) after paying an effort cost (repeated lever presses). In each condition there was one reference option (16 presses/1 pellet, blue bar/line) and one alternative that was associated with a higher benefit (16 presses/4 pellets, purple bar/line, left panels), lower cost (2 presses/1 pellet, red bar/line, mid panels), or higher cost (32 presses/1 pellet, burgundy bar/line, right panels). Filled lines correspond to the preferred option of each pair. (B) Difference in peak cue-elicited dopamine on higher benefit/lower cost trials ([DA]_HR/LC) compared to reference trials ([DA]_REF) in individual rats as a function of the amount of experience they had with those particular cost–benefit contingencies (standard training, <10 sessions of experience, lighter colored dots; extended training, ≥10 sessions of experience, darker colored dots). Although a greater peak dopamine was consistently recorded on higher-benefit trials regardless of training experience (left panel), the difference in dopamine between lower-cost and reference trials reduced with increasing experience of this condition (right panel). Specifically, after extended training with the lower-cost contingencies, there was no reliable difference in cue-elicited dopamine on lower cost and reference trials, even though rats still exhibited a strong preference for the lower-cost option and responded faster on lower-cost trials. Note that, unlike in some studies (e.g., [60]), the cost/benefit contingencies in the experiments depicted here reversed each day, and that these data come only from trials after animals had achieved a stable preference for the higher-benefit or lower-cost option in each session. Therefore, these results neither reflect learning (in early sessions) nor habitual responding (in later sessions). Panels adapted, with permission, from . (C) (Upper panel, left) dopamine responses to cues signaling availability of either a low-reward/low-cost option (LR/LC: 1 reward/4 presses) or a high-reward/mid-cost option (HR/MC, 8 presses/4 rewards; left panel). (Upper panel, right) As in the left panel, but comparing responses to the LR/LC option to a HR/high-cost option (HR/HC, ≥32 presses/4 rewards). The cost–benefit contingencies were again reversed every few sessions, and dopamine data were collected from trials after animals had achieved a stable preference for the HR/MC (versus LR/LC) or LR/LC (versus HR/HC). (Lower panel) A ‘dopamine discriminability index’ plotted against average preference across a session, quantified using a ‘choice index’ (HR−LR choices). The dopamine discriminability index was based on the area under the receiver operating characteristic (auROC) classifying dopamine release as discriminable on HR from LR trials in each session. As can be observed, although the choice index spans the full range of values, the dopamine index is strongly positively skewed, showing that it was more common to classify dopamine release as greater on HR than LR trials irrespective of preference (blue dots: LR/LC versus HR/MC; red dots: LR/LC versus HR/HC). Panels adapted, with permission, from .

Figure 3

Key Figure: Influence of Expected Reward and Effort on Behavior Information about upcoming benefits and costs are separately integrated into incentives and effort, respectively. Incentives have a dual effect on behavior: first, they have a positive influence on action selection (animals select the most beneficial options) and, second, incentive processes stimulate action execution. Effort, defined as the amount of anticipated resources necessary for action, negatively affects decisions: animals tend to select actions that minimize energy expenditure (Box 2). Nonetheless, the influence of this information on action execution might be dualistic: although anticipated greater demand can retard action initiation, animals once committed may need to boost their motivation to overcome effort costs. However, to date, the processes involved in surmounting effortful challenges remain little explored. Abbreviation: DA, dopamine.