Interhemispheric connectivity during bimanual isometric force generation.

Jinyi Long,Monica A Perez,Toshiki Tazoe,Demetris S Soteropoulos

doi:10.1152/jn.00876.2015

Jinyi Long, Monica A Perez + Show 2 more

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https://doi.org/10.1152/jn.00876.2015

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Abstract

Interhemispheric interactions through the corpus callosum play an important role in the control of bimanual forces. However, the extent to which physiological connections between primary motor cortices are modulated during increasing levels of bimanual force generation in intact humans remains poorly understood. Here we studied coherence between electroencephalographic (EEG) signals and the ipsilateral cortical silent period (iSP), two well-known measures of interhemispheric connectivity between motor cortices, during unilateral and bilateral 10%, 40%, and 70% of maximal isometric voluntary contraction (MVC) into index finger abduction. We found that EEG-EEG coherence in the alpha frequency band decreased while the iSP area increased during bilateral compared with unilateral 40% and 70% but not 10% of MVC. Decreases in coherence in the alpha frequency band correlated with increases in the iSP area, and subjects who showed this inverse relation were able to maintain more steady bilateral muscle contractions. To further examine the relationship between the iSP and coherence we electrically stimulated the ulnar nerve at the wrist at the alpha frequency. Electrical stimulation increased coherence in the alpha frequency band and decreased the iSP area during bilateral 70% of MVC. Altogether, our findings demonstrate an inverse relation between alpha oscillations and the iSP during strong levels of bimanual force generation. We suggest that interactions between neural pathways mediating alpha oscillatory activity and transcallosal inhibition between motor cortices might contribute to the steadiness of strong bilateral isometric muscle contractions in intact humans.

Highlights

ANIMAL STUDIES SHOWED that static bimanual force generation involves activity-dependent adaptations in both primary motor cortices (Murthy and Fetz 1996; Soteropoulos et al 2011)
Post hoc testing showed that mean rectified EMG activity increased during 40% and 70% compared with 10% of maximal isometric voluntary contraction (MVC) (10% ϭ 13.9 Ϯ 6.7, 40% ϭ 34.7 Ϯ 11.1, 70% ϭ 53.5 Ϯ 11.3; P Ͻ 0.001)
We found an effect of FORCE [F(2,15) ϭ 146.2, P Ͻ 0.001] but not HAND [F(1,15) ϭ 0.3, P ϭ 0.6] or their interaction [F(2,30) ϭ 0.9, P ϭ 0.4] on mean rectified first dorsal interosseous (FDI) EMG activity during bilateral contractions [nondominant hand: 10% ϭ 12.9 Ϯ 7.3, 40% ϭ 33.9 Ϯ 11.4, 70% ϭ 54.5 Ϯ 16.7% of MVC, P Ͻ 0.001 (Fig. 2C); dominant hand: 10% ϭ 15.9 Ϯ 7.9, 40% ϭ 36.8 Ϯ 11.6, 70% ϭ 55.8 Ϯ

Summary

Introduction

ANIMAL STUDIES SHOWED that static bimanual force generation involves activity-dependent adaptations in both primary motor cortices (Murthy and Fetz 1996; Soteropoulos et al 2011). 2009; Perez et al 2014; Tazoe et al 2013), the extent to which physiological connections between motor cortices are modulated during increasing levels of bimanual force generation remains poorly understood. Studies showed that oscillations between sensorimotor cortices in the alpha frequency band (Abdul-latif et al 2004; Svoboda et al 2002) and the iSP (Fling and Seidler 2012; Soteropoulos and Perez 2011) are sensitive to detect changes during strong levels of force generation. To further examine the relationship between coherence and the iSP during increasing levels of bimanual forces we used electrical stimulation of a peripheral nerve because alpha oscillations (Budini et al 2014) and interhemispheric inhibition measured by TMS (Tsutsumi et al 2012) can be modulated by similar afferent inputs

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