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Time-dependent impact of urgency on the activity of the human M1 during motor decisions
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Vanhemelrijck_41361300_2017.pdf
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- Recent studies suggest that while animals decide between action opportunities, urgency plays a major role in the process. Different urgency levels lead to different decision policies, which impacts the speed-accuracy trade-off. The aim of this study was to assess the impact of urgency on CSE modulations in humans. Hence, subjects performed a tokens task in which a penalty-ratio varied dynamically to produce different time-dependent changes in urgency in three block conditions. Single-pulse TMS was exploited to provide a muscle-specific readout of CSE modulations. The different levels of urgency induced specific changes in CSE depending on the time at which it was probed during action selection. As such, an initial high level of urgency led to an increase in task-relevant right-hand CSE in both right-hand selected and non-selected trials (i.e., compared to lower initial levels of urgency); conversely, a late high level of urgency led to a decrease in CSE specifically in right-hand non-selected trials. These results suggest that the urgency signal could be implemented differently, putatively by the basal ganglia, depending on how close to the movement onset it is. As such, it may initially boost motor neural activity, and then rather inhibit it, to enhance the signal-to-noise ratio. Finally, no significant impact of urgency was found for a task-irrelevant motor representation. A mechanism that could be implicated is the center-surround inhibition, in which the indirect basal ganglia pathway could act as a selective inhibitor of the competing motor representations.