Primary motor cortex and movement prevention: Where Stop meets Go

Processes that engage frontal cortex and the basal ganglia are responsible for the prevention of planned movements. Here, we review the role of primary motor cortex (M1) in this function. M1 receives and integrates input from a range of cortical and subcortical sites. It is also the final cortical processing site for voluntary motor commands, before they descend to the spinal cord. Inhibitory networks within M1 may be an important mechanism for the prevention or suppression of movement. Transcranial magnetic stimulation (TMS) has been used to evaluate corticospinal excitability and intracortical inhibition in humans, during the performance of a range of movement selection and prevention tasks. This review explores how M1 intracortical inhibition is selectively reduced to initiate desired voluntary movements, while movement prevention is associated with rapid, non-selective recruitment of inhibition within M1. The relationship between deficient intracortical inhibition and behavioural inhibition is also explored. Examples of neuropathology are reviewed, including focal dystonia, attention deficit hyperactivity disorder and Tourette syndrome. The strengths and limitations of TMS in the study of movement prevention are also discussed. While the precise functional links between M1 neuronal populations and the fronto-basal-ganglia network activated by movement prevention have yet to be elucidated, it is clear that M1 plays a critical role in the final processing stage of response inhibition.