Motor Cortex Excitability Reflects the Subjective Value of Reward and Mediates Its Effects on Incentive-Motivated Performance

Abstract

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.

Reward-based improvements in motor control are driven by multiple error-reducing mechanisms

“…While spinal stretch reflex is extremely fast, it is difficult to assume an effect of reward or motivation occurring at the spinal level. On the other hand, transcortical feedback includes primary motor cortex processing (Pruszynski et al, 2011), a structure that shows sensitivity to reward (Bundt et al, 2016; Galaro et al, 2019; Thabit et al, 2011). Consequently, an exciting possibility for future research is that transcortical feedback gain is directly enhanced by the presence of reward.…”

“…Similarly, in a saccade task, reward reduced participant’s reaction time whilst making them less sensitive to distractors (Manohar et al, 2015). It has also been shown that reward invigorates movement execution by increasing peak velocity and accuracy during saccades (Manohar et al, 2015; Takikawa et al, 2002) and reaching movements (Carroll et al, 2019; Galaro et al, 2019; Summerside et al, 2018). Therefore, this body of work suggests that reward can consistently shift the speed-accuracy function, at least in isolation, of both selection and execution.…”

Modulation of neural activity in frontopolar cortex drives reward-based motor learning

“…Additional TMS work demonstrated an increase in motor cortex excitability during motor preparation in anticipation of reward delivery, thus providing compelling evidence for an involvement of M1 in reward-guided motor processing [21,22]. This effect could be mediated by ventral striatal reward circuitry, given its established role in instrumental learning for reward [45,46].…”

“…Nineteen right-handed participants (10 females, mean = 27.7 yrs, std = 3.3 yrs, range [21][22][23][24][25][26][27][28][29][30][31][32][33] with no musical training were recruited. Laterality quotient was assessed by the Oldfield handedness inventory ( [49]; mean = 90, SEM = 3.2; values available in 17/19 participants).…”

“…More consistent evidence for a role of M1 in processing reward signals during motor learning comes from TMS studies. Specifically, using M1-TMS to assess motor-evoked potentials, different authors have demonstrated that motor cortex excitability is enhanced in anticipation of rewarding movements [21,22], and more generally, upcoming appetitive stimuli [23,24]. However, it is still unclear whether M1-tDCS consistently affects reward-based motor learning and, thus, motor decision-making.…”