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.
SIGNIFICANCE Brain responses to transcranial magnetic stimulation provide insight into how the brain prepares planned movements. However, most experiments involve temporally-controlled stimulus-driven actions. We provide evidence that cortical excitability evolves over the same time course in self-paced and reaction time movements. This is paralleled by previously unreported similarities in the way movement onset times are altered (speeded) when stimuli are delivered some hundreds of milliseconds before movements are to be started. This contrasts starkly with movements timed according to predictable countdowns. Here we see equivalent temporal evolution of pre-movement excitability, but the timing of these movements is unaffected by the external stimuli, thus suggesting a higher control of the time of action initiation.
ACKNOWLEDGEMENTS
JI was supported in part by Grant No. #H2020-MSCA-IF-2015-700512 from the European Commission (JI). RH was supported by a Biotechnology and Biological Sciences Research Council (BBSRC) (Grant No. BB/N016793/1).
Footnotes
CONFLICT OF INTEREST: The authors declare no competing financial interests.