The perceived effort level of an action shapes everyday decisions. Despite the importance of these perceptions for decision-making, the behavioral and neural representations of the subjective cost of effort are not well understood. While a number of studies have implicated anterior cingulate cortex (ACC) in decisions about effort/reward trade-offs, none have experimentally isolated effort valuation from reward and choice difficulty, a function that is commonly ascribed to this region. We used functional magnetic resonance imaging to monitor brain activity while human participants engaged in uncertain choices for prospective physical effort. Our task was designed to examine effort-based decision-making in the absence of reward and separated from choice difficulty—allowing us to investigate the brain’s role in effort valuation, independent of these other factors. Participants exhibited subjectivity in their decision-making, displaying increased sensitivity to changes in subjective effort as objective effort levels increased. Analysis of blood-oxygenation-level dependent activity revealed that the ventromedial prefrontal cortex (vmPFC) encoded the subjective valuation of prospective effort, and ACC activity was best described by choice difficulty. These results provide insight into the processes responsible for decision-making regarding effort, partly dissociating the roles of vmPFC and ACC in prospective valuation of effort and choice difficulty.

Performance-based incentives tend to increase an individual's motivation, resulting in enhancements in behavioral output. While much work has focused on understanding how the brain's reward circuitry influences incentive-motivated performance, fewer studies have investigated how such reward representations act on the motor system. Here we measured motor cortical excitability with transcranial magnetic stimulation while female and male human participants performed a motoric incentive motivation task for prospective monetary gains and losses. We found that individuals' performance increased for increasing prospective gains and losses. While motor cortical excitability appeared insensitive to prospective loss, temporal features of motor cortical excitability for prospective gains were modulated by an independent measure of an individual's subjective preferences for incentive (i.e., loss aversion). Those individuals that were more loss averse had a greater motor cortical sensitivity to prospective gain, closer to movement onset. Critically, behavioral sensitivity to incentive and motor cortical sensitivity to prospective gains were both predicted by loss aversion. Furthermore, causal modeling indicated that motor cortical sensitivity to incentive mediated the relationship between subjective preferences for incentive and behavioral sensitivity to incentive. Together, our findings suggest that motor cortical activity integrates information about the subjective value of reward to invigorate incentive-motivated performance.Increasing incentives tend to increase motivation and effort. Using a motoric incentive motivation task and transcranial magnetic stimulation, we studied the motor cortical mechanisms responsible for incentive-motivated motor performance. We provide experimental evidence that motor cortical sensitivity to incentive mediates the relationship between subjective preferences for incentive and incentive-motivated performance. These results indicate that, rather than simply being a reflection of motor output, motor cortical physiology integrates information about reward value to motivate performance.

1Performance-based incentives tend to increase an individual's motivation, 2 resulting in enhancements in behavioral output. While much work has focused on 3 understanding how the brain's reward circuitry influences incentive motivated 4 performance, fewer studies have investigated how such reward representations act on 5 the motor system. Here we measured motor cortical excitability with transcranial magnetic 6 stimulation (TMS) while female and male human participants performed a motoric 7 incentive motivation task for prospective monetary gains and losses. We found that 8 individuals' performance increased for increasing prospective gains and losses. While 9 motor cortical excitability appeared insensitive to prospective loss, temporal features of 10 motor cortical excitability for prospective gains were modulated by an independent 11 measure of an individual's subjective preferences for incentive (i.e., loss aversion). Those 12 individuals that were more loss averse had a greater motor cortical sensitivity to 13 prospective gain, closer to movement onset. Critically, behavioral sensitivity to incentive 14 and motor cortical sensitivity to prospective gains were both predicted by loss aversion. 15 Furthermore, causal modeling indicated that motor cortical sensitivity to incentive 16 mediated the relationship between subjective preferences for incentive and behavioral 17 sensitivity to incentive. Together our findings suggest that motor cortical activity integrates 18 information about the subjective value of reward to invigorate incentive motivated 19 performance. 20 SIGNIFICANCE STATEMENT 21Increasing incentives tend to increase motivation and effort. Using a motoric 22 incentive motivation task and transcranial magnetic stimulation, we studied the motor 23 cortical mechanisms responsible for incentive motivated motor performance. We provide 24 experimental evidence that motor cortical sensitivity to incentive mediates the relationship 25 between subjective preferences for incentive and incentive motivated performance. 26These results indicate that, rather than simply being a reflection of motor output, motor 27 cortical physiology integrates information about reward value to motivate performance. 28

Experiences of physical exertion guide our assessments of effort. While these assessments critically influence our decisions to engage in daily activities, little is known about how they are generated. We had female and male human participants exert grip force and assess how effortful these exertions felt; and used magnetic resonance spectroscopy to measure their brain GABA concentration. We found that variability in exertion (i.e., the coefficient of variation in their force exertion profile) was associated with increases in assessments of effort, making participants judge efforts as more costly. GABA levels in the sensorimotor cortex (SM1) moderated the influence of exertion variability on overassessments of effort. In individuals with higher sensorimotor GABA, exertion variability had a diminished influence on overassessments of effort. Essentially, sensorimotor GABA had a protective effect on the influence of exertion variability on inflations of effort assessment. Our findings provide a neurobiological account of how the brain's GABAergic system integrates features of physical exertion into judgments of effort, and how basic sensorimotor properties may influence higher-order judgments of effort.

The perceived effort level of an action shapes everyday decisions. Despite the importance of these perceptions for decision-making, the behavioral and neural representations of the subjective cost of effort are not well understood. While a number of studies have implicated anterior cingulate cortex (ACC) in decisions about effort/reward trade-offs, none have experimentally isolated effort valuation from reward and choice difficulty, a function that is commonly ascribed to this region. We used functional magnetic resonance imaging (fMRI) to monitor brain activity while human participants engaged in uncertain choices for prospective physical effort. Our task was designed to examine effort-based decision making in the absence of reward and separated from choice difficulty -allowing us to investigate the brain's role in effort valuation, independent of these other factors. Participants exhibited subjectivity in their decision-making, displaying increased sensitivity to changes in subjective effort as objective effort levels increased. Analysis of blood-oxygenation level dependent (BOLD) activity revealed that the ventromedial prefrontal cortex (vmPFC) encoded the subjective valuation of prospective effort and ACC encoded choice difficulty. These results provide insight into the processes responsible for decision-making regarding effort, dissociating the roles of vmPFC and ACC in prospective valuation of effort and choice difficulty.