Excitability changes in the motor cortex in preparation for selfpaced and cue-guided movements: a transcranial magnetic stimulation study

ABSTRACT

A simple of model of movement preparation, involving subthreshold accumulation of motor commands readily explains two features of a simple reaction time task. One is preparatory inhibition of corticospinal excitability, which is viewed as necessary to prevent premature release of movement. The other is intersensory facilitation in which receipt of a second stimulus at the same time as the imperative signal shortens the time taken to trigger a reduction in inhibition and increase in excitation. This allows preparatory activity to reach threshold more quickly and release action.

Here we question that interpretation by asking what happens in movements that are self-paced or timed to be coincident with an external event (predictive movements). Surprisingly we find that both types of movement show preparatory inhibition at approximately the same time prior to movement initiation as in a reaction time movement.

This suggests that rather than preventing premature release, premovement inhibition is an integral part of preparation to move, and may reflect preparatory neural population dynamics shared between actions initiated in different contexts. We also observed intersensory facilitation in self-paced movements when a sensory event preceded predicted movement onset by about 200-300ms. We suggest that this is consistent with the idea that the preparatory state requires a separate trigger event before movement is initiated. Intersensory facilitation was not seen in predictive movements, perhaps because they filter out external signals that could interfere with internal timing events.