Muscular reflex gains reflect changes of mind in reaching.

Abstract

Decisions for action are accompanied with a continual processing of sensory information, sometimes resulting in a revision of the initial choice, called a change of mind (CoM). While the motor system is tuned during the formation of a reach decision, it is unclear whether its preparatory state differs between CoM and non-CoM decisions. To test this, participants (n=14) viewed a random dot motion (RDM) stimulus of various coherence levels for a random viewing duration. At the onset of a mechanical perturbation, which rapidly stretched the pectoralis muscle, they indicated the perceived motion direction by making a reaching movement to one of two targets. Using electromyography (EMG), we quantified the reflex gains of the pectoralis and posterior deltoid muscle. Results show that reflex gains scaled with both the coherence level and viewing duration of the stimulus. We fit a drift diffusion model (DDM) to the behavioral choices. The decision variable (DV), derived from the DDM, correlated well with the measured reflex gain at the single trial level. However, when matched on DV magnitude, reflex gains were significantly lower in CoM than non-CoM trials. We conclude that the internal state of the motor system, as measured by the spinal reflexes, reflects the continual deliberation on sensory evidence for action selection, including the post-decisional evidence that can lead to a change of mind.