The effect of effort on reward prediction error signals in midbrain dopamine neurons

Disentangling effort from probability of success: Temporal dynamics of frontal midline theta in effort-based reward processing

The ability to weigh a reward against the effort required to acquire it is critical for decision-making. However, extant experimental paradigms oftentimes confound increased effort demand with decreased reward probability, thereby obscuring neural correlates underlying these cognitive processes. To resolve this issue, we designed novel tasks that disentangled probability of success - and therefore reward probability - from effort demand. In Experiment 1, reward magnitude and effort demand were varied while reward probability was kept constant. In Experiment 2, effort demand and reward probability were varied while reward magnitude remained fixed. Electroencephalogram (EEG) data was recorded to explore how frontal midline theta (FMT; an electrophysiological index of mPFC function) and component P3 (an index of incentive salience) respond to effort demand, and reward magnitude and probability. We found no evidence that FMT tracked effort demands or net value during cue evaluation. At feedback, however, FMT power was enhanced for high compared to low effort trials, but not modulated by reward magnitude or probability. Conversely, P3 was sensitive to reward magnitude and probability at both cue and feedback phases and only integrated expended effort costs at feedback, such that P3 amplitudes continued to scale with reward magnitude and probability but were also increased for high compared to low effort reward feedback. These findings suggest that, when likelihood of success is equal, FMT power does not track net value of prospective effort-based rewards. Instead, expended cognitive effort potentiates FMT power and enhances the saliency of rewards at feedback. SIGNIFICANCE STATEMENT: The way the brain weighs rewards against the effort required to achieve them is critical for understanding motivational disorders. Current paradigms confound increased effort demand with decreased reward probability, making it difficult to disentangle neural activity associated with effort costs from those associated with reward likelihood. Here, we explored the temporal dynamics of effort-based reward (via frontal midline theta (FMT) and component P3) while participants underwent a novel paradigm that kept probability of reward constant between mental effort demand conditions. Our findings suggest that the FMT does not track net value and that expended effort enhances, instead of attenuates, the saliency of rewards.

Expending effort may share neural responses with reward and evokes high subjective satisfaction

Throughout our daily lives, the levels of effort we invest in various tasks are influenced by reward processing. The subjective expectation after expending effort is a primary factor affecting reward processing. However, recent studies indicate that individual differences in reward anticipation influence this subjective valuation. To better understand the relationship between effort expenditure and the subjective valuation of rewards, in this study, we perform an experiment in which we manipulate effort, control reward expectation implicitly, and measure the subjective valuation of rewards using event-related potentials (ERPs) and physical effort through behavioral measures (number of keystrokes). In the reward-task paradigm, 30 subjects performed effort and control trials, with the reward probability comparable across the effort and control conditions. We also examined the ERPs associated with the valuation of subjective rewards, including reward positivity (RewP) and set reward expectation controlled as the baseline. The results showed that the ERP amplitudes, the number of keystrokes, and explicit satisfaction ratings were all significantly greater in the effort condition than in the control condition. The participants maintained high levels of effort throughout the sessions associated with the experiment. The results of this study suggest that when reward expectations are controlled, effort expenditure evokes neural responses similar to reward feedback being given, which is linked with increased subjective satisfaction.

Dopaminergic medication increases motivation to exert cognitive control by reducing subjective effort costs in Parkinson's patients

Engaging in demanding mental activities requires the allocation of cognitive control, which can be effortful and aversive. Individuals thus tend to avoid exerting cognitive effort if less demanding behavioral options are available. Recent accounts propose a key role for dopamine in motivating behavior by increasing the sensitivity to rewards associated with effort exertion. Whether dopamine additionally plays a specific role in modulating the sensitivity to the costs of cognitive effort, even in the absence of any incentives, is much less clear. To address this question, we assessed cognitive effort avoidance in patients (n = 38) with Parkinson's disease, a condition characterized by loss of midbrain dopaminergic neurons, both ON and OFF dopaminergic medication and compared them to healthy controls (n = 24). Effort avoidance was assessed using the Demand Selection Task (DST), in which participants could freely choose between performing a high-demand or a low-demand version of a task-switching paradigm. Critically, participants were not offered any incentives to choose the more effortful option, nor for good performance. While healthy controls and patients OFF their dopaminergic medications consistently preferred the low-demand option, effort avoidance in patients ON dopaminergic medications was reduced compared to patients OFF, a difference which seems to lessen over trials. These differences in preference could not be explained by altered task-switching performance. Although patients ON were less accurate at detecting the different effort levels, as measured during instructed forced-choice blocks, their detection ability was not associated with effort avoidance, unlike in the healthy controls and the patients OFF. Our findings provide evidence that dopamine replacement in Parkinson's patients increases the willingness to engage in cognitively demanding behavior, and that this cannot be explained by possible effects of dopamine replacement on performance nor on the ability to detect effort demands. These results suggest that dopamine plays a role in reducing the sensitivity to effort costs that is independent of its role in enhancing the sensitivity to the benefits of effort exertion.