Nature and sources of urgency during human motor behaviour
The aim of this project is to gain a better understanding of how the brain controls our decisions and movements in a coordinated manner. Indeed, all our motor behaviors rely on the ability to make decisions among several options, and to implement these choices by means of appropriate movements. Over the past decade or so, a new model of decision making has emerged supporting the existence of an evidence-independent urgency signal which allows to speed up decisions (and movements) to reduce the time before receiving an expected reward. In several past studies, we found that this urgency signal not only affects the speed of decisions but also speeds up ensuing movements, although some flexibility exists between both levels of control to maintain an optimal reward rate. In another study, by taking advantage of the high spatial resolution of motor evoked potentials to TMS over M1 in a variant of the Tokens task, we were able to show that urgency materializes into two main adjustments of motor neural activity. We are currently testing the hypothesis that these effects reflect two main sources of urgency: motivation by reward and the degree of caution implemented during our decisions. To do so, we apply M1 TMS during the Tokens task with manipulations of both motivation by reward (on a single-trial basis) and the level of caution (different block types).
Causal role of arousal in shaping speed-accuracy trade-off
This project aims at understanding the causal role of arousal in the regulation of speed-accuracy trade-off during decision making and movement execution. Arousal depends on various neuromodulators, including norepinephrine whose primary source is the locus coeruleus. Interestingly, several lines of evidence suggest that the locus coeruleus norepinephrine system can be modulated in humans by means of tVNS. A first pilot study in our lab aimed at assessing the effectiveness and reliability of several tVNS protocols by considering the impact of the procedure on pupil dilation, a marker of arousal. Then, we conducted decision-making experiments with active sham or tVNS, with behavior and pupillometry as endpoint measures. Ongoing analyses suggest that a higher level of arousal, as induced by active compared to sham tVNS, enhances decision accuracy, consistent with the view that the locus coeruleus norepinephrine system optimizes information processing. Planned experiments also involve testing the impact of tVNS on EEG/TMS markers of decision making and on reaching tasks.
Mechanisms underlying skill generalization & retention of a newly acquired motor skill
This project aims at understanding the neural mechanisms underlying interlimb generalization of a novel motor skill in healthy human subjects. In a first study, we aim to uncover how skill memory consolidation by brief reactivation, which has been widely studied in animal models, can influence retention and generalization of the skill memory. Our goal here is to specifically understand the effects of a brief memory reactivation session on subsequent motor skill performances (by testing for retention and generalization). We hypothesize that skill memory reactivation will strengthen the newly consolidated skill memory, and thereby enhance both retention and generalization test performances. Our preliminary findings indicate that skill memory reactivation may strengthen the memory in an effector-dependent manner which leads to better intralimb retention test performance. On the other hand, this form of memory reactivation appears to be detrimental for interlimb generalization of a newly learned skill, which may possibly rely more on effector-independent memory representations. Next, in order to uncover the neural basis of such human skill behavior, a second ongoing study is aimed at understanding the causal role of contralateral and ipsilateral primary motor cortices (M1) in skill generalization and retention using repetitive TMS intervention. Our hypothesis is that the contralateral M1 is causally involved in retention as well as interlimb generalization of a newly learned skill memory.
Impact of extrinsic motivation and intrinsic individual traits on motor skill learning
This project aims at understanding how extrinsic motivation (such as monetary reward), on top of reinforcement learning, can enhance learning of a new motor skill. A first study highlighted the behavioural benefits of reward on motor skill learning both at the level of consolidation and retention. Then, we focused on changes in motor excitability by applying TMS over M1 at different stages of learning. Our results indicate an overall increase in corticospinal excitability at the end of learning. In the context of reward, we noted a reduction in variability of corticospinal output excitability in the group that learned the skill with reward/motivation (as compared to the groups that learned the skill in the absence of reward). However, we did not observe the effects of reward on short-intracortical inhibition or use-dependent plasticity, suggesting that the effects of reward on motor learning may rely on layer-specific cortical plasticity, or on more complex subcortico-cortical interactions during learning. Furthermore, we are also in the process of assessing intrinsic individual trait characteristics of subjects (sensitivity to reward and punishment, anxiety, apathy and sleep quality assessments) to explore and quantify additional factors that may influence and predict skill learning in young healthy humans.
Impact of the phase of sensorimotor oscillations on corticospinal excitability and motor learning
This research project utilizes cutting-edge real-time closed-loop EEG-TMS technology to investigate the influence of sensorimotor mu-alpha (8-12 Hz) phases on corticospinal (CS) circuits during both rest and motor behavior in a cohort of healthy young adults. Comprising several key experiments, this project aims to uncover the specific impact of mu-alpha trough and peak phases on CS output during rest, the dynamic relationship between mu-alpha phases and CS excitability during movement preparation, as well as phase-dependent plastic changes associated with reinforcement motor learning. The overarching goals include shedding light on phase-dependent modulation of CS excitability, elucidating the functional relevance of sensorimotor phases in shaping motor behavior, and identifying pivotal neural phases, particularly troughs, that mediate sensorimotor output and behavior in humans. This project represents a significant step towards understanding the intricate interplay between mu-alpha phases and CS circuits, with potential implications for our comprehension of motor behavior and neurological processes.
Characterization of preparatory suppression in Parkinson’s disease
This project aims at advancing our understanding of the role and neural sources of preparatory suppression, a phenomenon consisting of the systematic suppression of corticospinal excitability during action preparation, evidenced using single-pulse TMS over primary motor cortex. In a first study, we found that Parkinson’s disease patients display a lack of preparatory suppression, which may be responsible for the motor slowness (bradykinesia) found in this clinical population. Surprisingly, we did not find any effect of treatment, whether it consisted of dopamine replacement therapy or deep brain stimulation, which calls into questions the proposed role of basal ganglia in generating preparatory suppression. Ongoing analyses also suggest an effect of gender on the deficit in preparatory suppression in Parkinson’s disease. A case to follow…
