UC Irvine

AIMS OF THE STUDY:The time course of motor excitability during a task-related unilateral right thumb movement was studied using sub-threshold transcranial magnetic stimulation (TMS) to the contralateral left motor cortex. The level of stimulation evoked a motor evoked potential (MEP) in the thumb when the subject was at rest in approximately 10% of the trials. METHODS:Subjects made a brief right thumb movement to the predictable omission of regularly presented tone bursts allowing experimental definition of TMS relative to the cue to move. Motor cortical excitability was characterized by amplitude and/or probability of eliciting MEPs. RESULTS:There were four periods of altered motor excitability during task performance compared to a control resting state: a first period of weak facilitation before movement between -500 to -200 ms, a second period without increased excitability approximately 150 ms before movement onset when MEPs amplitude was below that seen in rest, a third period of strong facilitation between -100 ms before movement and +200 ms after facilitation and a fourth period of weak facilitation between +200 to +500 ms. CONCLUSION:These results show that during performance of a task requiring a motor response, motor cortical excitability is increased above resting for hundreds of millisecond before and after the response, except for a transient period between 75 and 150 ms prior to movement onset. The temporal pattern of these excitability changes is compatible with multiple excitatory and inhibitory inputs interacting on motor cortex.