Motor overflow, typically described in the context of unimanual movements, refers to the natural tendency for a ‘resting’ limb to move during movement of the opposite limb and is thought to be influenced by inter-hemispheric interactions and intra-cortical networks within the ‘resting’ hemisphere. It is currently unknown, however, how motor overflow contributes to asymmetric force coordination task accuracy, referred to as bimanual interference, as there is need to generate unequal forces and corticospinal output for each limb. Here, we assessed motor overflow via motor evoked potentials (MEPs) and the regulation of motor overflow via inter-hemispheric inhibition (IHI) and short-intra-cortical inhibition (SICI) using transcranial magnetic stimulation in the presence of unimanual and bimanual isometric force production. All outcomes were measured in the left first dorsal interosseous (test hand) muscle, which maintained 30% maximal voluntary contraction (MVC), while the right hand (conditioning hand) was maintained at rest, 10, 30, or 70% of its MVC. We have found that as higher forces are generated with the conditioning hand, MEP amplitudes at the active test hand decreased and inter-hemispheric inhibition increased, suggesting reduced motor overflow in the presence of bimanual asymmetric forces. Furthermore, we found that subjects with less motor overflow (i.e., reduced MEP amplitudes in the test hemisphere) demonstrated poorer accuracy in maintaining 30% MVC across all conditions. These findings suggest that motor overflow may serve as an adaptive substrate to support bimanual asymmetric force coordination.
Bimanual interference Motor overflow Transcranial magnetic stimulation Inter-hemispheric inhibition Intra-cortical inhibition
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This work is supported by the Clinical and Translational Science Collaborative (RPC 2014-1067) to D.A.C as well as by the National Institutes of Health (1K01HD069504) and American Heart Association (13BGIA17120055) to E.P.
Addamo PK, Farrow M, Hoy KE et al (2007) The effects of age and attention on motor overflow production—a review. Brain Res Rev 54:189–204CrossRefPubMedGoogle Scholar
Gerloff C, Richard J, Hadley J et al (1998) Functional coupling and regional activation of human cortical motor areas during simple, internally paced and externally paced finger movements. Brain 121:1513–1531. doi:10.1093/brain/121.8.1513CrossRefPubMedGoogle Scholar
Hortobágyi T, Taylor JL, Petersen NT et al (2003) Changes in segmental and motor cortical output with contralateral muscle contractions and altered sensory inputs in humans. J Neurophysiol 90:2451–2459. doi:10.1152/jn.01001.2002CrossRefPubMedGoogle Scholar
Kenway LC, Bisset LM, Kavanagh JJ (2014a) Removing visual feedback for a single limb alters between-limb force tremor relationships during isometric bilateral contractions. Exp Brain Res 233:115–124. doi:10.1007/s00221-014-4098-3
Kenway LC, Bisset LM, Kavanagh JJ (2014b) The effect of isometric contraction on the regulation of force tremor in the contralateral limb. Neurosci Lett 558:126–131. doi:10.1016/j.neulet.2013.11.013
Meyer B-U, Roricht S, von Einsiedel HG et al (1995) Inhibitory and excitatory interhemispheric transfers between motor cortical areas in normal humans and patiens with abnormalities of the corpus callosum. Brain 118:429–440CrossRefPubMedGoogle Scholar