Unimanual, low-force instability control facilitates the corticospinal excitability in the ipsilateral M1 with no evidence of ipsilateral silent periods

Abstract

Precision pinch against unstable objects elicits dynamic control of fingertip forces. Unilateral dynamic precision grip engages both hemispheres as evidenced by several imaging studies. However, our understanding of underlying mechanisms of functional communication between hemispheres is still lacking. Here, we tested corticospinal excitability to assess changes in interhemispheric communication with increasing instability of objects during unimanual-pinch tasks. Ten right-dominant healthy adults (29.5±3.5yrs, 4M/6F) performed three force-matched pinch tasks at low forces (<3N) with right thumb and index finger while their left hand was resting. The pinch tasks included compression of (i) a rigid dowel, (ii) short (stable) spring, and (iii) long (unstable) spring while single-pulse TMS was delivered over the right M1. Motor-evoked-potentials (MEPs) and mirror EMG activity from the left first dorsal interosseous (left_FDI) and ipsilateral silent periods (ISPs) from the right_FDI were recorded and analyzed. We found that the average MEPs in the resting left_FDI were highest for the unstable spring (p<0.001). The stable spring and dowel elicited similar MEP amplitudes (p=0.79), but greater than a control, rest condition (p<0.05). Mirror EMG activity in the left_FDI did not differ across tasks, nor correlate with MEP amplitudes. Importantly, ISPs in the right_FDI did not differ among conditions. These demonstrate that instability control demands required for unimanual tasks can modulate the net excitability of the unengaged corticospinal tract without eliciting mirror EMG activity in the resting hand for healthy individuals. Furthermore, inhibitory mechanisms from the ipsilateral M1 of the manipulating hand may not play an important role during low-force, dynamic instability control. These modulations of ipsilateral corticospinal excitability with instability control demands may provide alternative neurorehabilitation strategies for individuals with hemiparesis. Moreover, measuring the ipsilateral corticospinal-excitability and mirror EMG activity in the paralyzed hand may be a practical means to quantify disruption or recovery of brain function after injuries.