Asymmetries in motor cortex inhibition during bimanual isometric muscle activation.

Verstynen, Stinear, Konkle, Ivry & Byblow

During unimanual movements, excitability is modulated in the ipsilateral motor cortex. This modulation is usually observed as an increase in the amplitude of motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS). We examined whether this modulation is also reflected in measures of cortical inhibition. MEPs and silent period (SP) durations were examined in the abductor pollicis brevis (APB) muscles of the nondominant and dominant hand in nine right-handed participants. For all trials, participants were instructed to maintain APB contraction in the target hand at 15% of its maximum voluntary contraction (MVC). Participants performed one of five conditions with the opposite, non-target hand: remain at rest (REST), abduct the contralateral shoulder while the hand remained at rest (ABDUCT), or maintain APB activity at 15, 35 or 55% of MVC. Participants received online visual feedback of EMG activity of the APB for both hands. During the APB contraction conditions, increasing the activity of the dominant hand resulted in increased SP durations in the homologous muscle of the nondominant hand (p < 0.05). No such effects were observed with increasing activity in the nondominant hand. While there was a tendency for SP duration of the nondominant APB to decrease in the ABDUCT condition (p = 0.06), no differences in MEP amplitude or SP duration between hemipsheres were observed in the REST condition (p > 0.25). These findings suggest that increasing activity in the dominant motor cortex results in greater inhibition of the nondominant motor cortex.

Verstynen, T., Stinear, C., Konkle, T., Ivry, R. B., & Byblow, W. Asymmetries in motor cortex inhibition during bimanual isometric muscle activation. Program No. 992.3. 2004 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2004. Online.