Assessing Corticospinal Excitability During Goal-Directed Reaching Behavior.

Reaching is a widely studied behavior in motor physiology and neuroscience research. While reaching has been examined using a variety of behavioral manipulations, there remain significant gaps in the understanding of the neural processes involved in reach planning, execution, and control. The novel approach described here combines a two-dimensional reaching task with transcranial magnetic stimulation (TMS) and concurrent electromyography (EMG) recording from multiple muscles. This method allows for the noninvasive detection of corticospinal activity at precise time points during the unfolding of reaching movements. The example task code includes a delayed response reaching task with two possible targets displayed ± 45° off the midline. Single pulse TMS is delivered on the majority of task trials, either at the onset of the preparatory cue (baseline) or 100 ms prior to the imperative cue (delay). This sample design is suitable for investigating changes in corticospinal excitability during reach preparation. The sample code also includes a visuomotor perturbation (i.e., cursor rotation of ± 20°) to investigate the effects of adaptation on corticospinal excitability during reach preparation. The task parameters and TMS delivery can be adjusted to address specific hypotheses about the state of the motor system during reaching behavior. In the initial implementation, motor evoked potentials (MEPs) were successfully elicited on 83% of TMS trials, and reach trajectories were recorded on all trials.

Response preparation involves a release of intracortical inhibition in task-irrelevant muscles.

Action preparation involves widespread modulation of motor system excitability, but the precise mechanisms are unknown. In this study, we investigated whether intracortical inhibition changes in task-irrelevant muscle representations during action preparation. We used transcranial magnetic stimulation (TMS) combined with electromyography in healthy human adults to measure motor-evoked potentials (MEPs) and cortical silent periods (CSPs) in task-irrelevant muscles during the preparatory period of simple delayed response tasks. In experiment 1, participants responded with the left index finger in one task condition and the right index finger in another task condition, whereas MEPs and CSPs were measured from the contralateral nonresponding and tonically contracted index finger. During experiment 2, participants responded with the right pinky finger whereas MEPs and CSPs were measured from the tonically contracted left index finger. In both experiments, MEPs and CSPs were compared between the task preparatory period and a resting intertrial baseline. The CSP duration during response preparation decreased from baseline in every case. A laterality difference was also observed in experiment 1, with a greater CSP reduction during the preparation of left finger responses compared to right finger responses. Despite reductions in CSP duration, consistent with a release of intracortical inhibition, MEP amplitudes were smaller during action preparation when accounting for background levels of muscle activity, consistent with earlier studies that reported decreased corticospinal excitability. These findings indicate that intracortical inhibition associated with task-irrelevant muscles is transiently released during action preparation and implicate a novel mechanism for the controlled and coordinated release of motor cortex inhibition.NEW & NOTEWORTHY In this study, we observed the first evidence of a release of intracortical inhibition in task-irrelevant muscle representations during response preparation. We applied transcranial magnetic stimulation to elicit cortical silent periods in task-irrelevant muscles during response preparation, and observed a consistent decrease in the silent period duration relative to a resting baseline. These findings address the question of whether cortical mechanisms underlie widespread modulation in motor excitability during response preparation.