IS 13. Mean corticospinal excitability and its variability during decisions for actions

Abstract

The neural underpinnings that link decision making and action selection remain largely undetermined. It is thought that action selection in motor regions originates from a competitive process that is gradually biased by evidence signals originating in other regions, such as those specialized in value computations. Biases reflecting the evaluation of choice options should thus emerge in the motor system before the decision process is complete. We here review recent evidence that this is indeed the case. Mean changes in cortico-spinal excitability (CSE) for chosen versus unchosen actions distinguish forthcoming choices before completion of the decision process. Additionally, the variability of CSE prior to movement declines contralateral to the responding hand, and more strongly so on fast compared to slow response trials; this is consistent with data in non-human primates. For the non-responding hand, CSE variability also decreased, but only on choice trials and earlier compared to the responding hand, possibly reflecting choice-specific suppression of unselected actions. Internally generated (value-based) decisions thus influence the competition between action representations in motor cortex before the decision process is complete. In addition, the variability of human CSE elicited by TMS over M1 tracks the state of motor preparation, and may reflect the optimization of preparatory population activity. Collectively these results show how the mean changes in CSE and their variability reveal distinct influences of decision processes on regions specified in action preparation and selection.