INTRODUCTION: Motor learning is the process by which movements can be improved with practice. A large amount of research has demonstrated that this process strongly relies on reduction of movement errors based on sensory feedback (Shadmehr et al., 2010). In the recent years, studies have also shown that reinforcement feedback (providing knowledge of performance) and reward (in the form of extrinsic motivation), can also impact motor learning (Vassiliadis et al., 2021; 2022).

OBJECTIVE: Despite its potential clinical relevance for motor rehabilitation, the underlying neurophysiological mechanisms remain largely unexplored. Specifically, whether reward affects the excitability of motor structures during motor learning remains unclear.

METHODS: 65 healthy right-handed participants (44 Females; 23.85 ± 3.22 yr old) divided in 3 groups performed a right-hand pinch-grip motor skill learning task (6 training blocks of 40 trials) with sensory feedback only (GroupS), sensory and reinforcement feedback (GroupSR) or with both feedbacks combined with monetary reward (GroupSRR). Study was approved by University Ethical Committee.To probe the excitability of motor structures, we applied transcranial magnetic stimulation (TMS) on the left motor cortex (130% of resting motor threshold of Flexor Policis Brevis muscle) before, during and after training (pre, intra and post training blocks) in the three groups. This allowed us to evaluate the effects of learning on corticospinal (CS) excitability (measured as Motor Evoked Potentials- MEPs), GABA-ergic short-intracortical inhibition (SICI) and use-dependent plasticity (UDP assessed by considering directional changes in TMS-evoked movements). Here our main goal was to assess the impact of reward at the neurophysiological level by assessing and comparing the MEPs (mean amplitude and variability measured as coefficient of variance (CV)), SICI and UDP of the 3 groups (GroupS, GroupSR, GroupSRR) during the 2 training blocks (intra, post blocks {normalized and expressed in percentage of pre-training block}).

RESULTS: At the behavioral level, motor learning was clearly higher in the group who received reward (GroupSRR) compared to the two other groups who only received feedback (GroupS and GroupSR). Next, we found that motor training was associated with a general increase in corticospinal excitability as well as clear signs of UDP that were equivalent in all groups. Further, we did not find any significant effect of training or group on SICI measures. Yet interestingly for CV MEPs, we noted a group effect (no training or trainingXgroup effect) with the group who learned with reward (GroupSRR) displayed a lower variability of CS-excitability compared to the other groups- CV of MEPs (normalized to pre-training) was 93% for Group SRR, 101.8% for GroupSR and 118.69% for GroupS. Currently, we are performing further statistical and exploratory analyses to evaluate if there is an association between training-related changes in CS-excitability variability and performance error, as well as individual scores of Sensitivity to Punishment and Sensitivity to Reward (SPSRQ Questionnaire {Lardi et al., 2008; Torrubia, 2001} completed by each participant at the beginning of our experiment).

CONCLUSION: Our current results indicate that the observed facilitatory effect of reward on motor learning (reduction in errors) may be driven by a reduction in variability of cortico-motor excitability.