ABSTRACT
Reaction time tasks are characterised by two features: preparatory suppression of corticospinal excitability that precedes facilitation and movement onset; and intersensory facilitation, in which receipt of a second stimulus around the time of the imperative signal shortens reaction times. These are usually explained using a model of subthreshold accumulation of motor commands. Preparatory suppression prevents premature release of “subthreshold” commands; intersensory facilitation allows preparatory activity to reach threshold faster.
Here we question that interpretation by studying movements that are self-paced or timed with predictable external events. In all cases, corticospinal excitability evolves over the same time course regardless of movement type. Thus, inhibition is not a brake on release, it is an integral part of movement preparation. Similarly, intersensory facilitation-like effects occur in self-paced movements, suggesting that they require a trigger event before movement is initiated. Intersensory facilitation is not seen in predictive movements, suggesting they use different mechanisms.
SIGNIFICANCE Intuitive reasoning suggests that planning for a forthcoming movement should involve subthreshold preparation of motor commands. In reaction movements release of these commands is triggered by an external input whereas in self-paced tasks, movement could start as soon as preparation is complete. Here we provide evidence in humans using TMS of motor cortex that this is incorrect. Preparation for movement appears to involve the motor cortex entering a novel state characterised by a small reduction in overall excitability. This is then triggered into execution by either an external event, or, on the case of self-paced tasks, by an equivalent internal event.
ACKNOWLEDGEMENTS
JI was supported in part by Grant No. #H2020-MSCA-IF-2015-700512 from the European Commission. RH was supported by the Biotechnology and Biological Sciences Research Council (BBSRC) (Grant No. BB/N016793/1).
Footnotes
CONFLICT OF INTEREST:
The authors declare no competing financial interests.
