Abstract
The purpose of this study was to further develop methods to assess corticospinal divergence and muscle coupling using transcranial magnetic stimulation (TMS). Ten healthy right-handed adults participated (7 females, age 34.0 ± 12.9 years). Monophasic single pulses were delivered to 14 sites over the right primary motor cortex at 40, 60, 80 and 100% of maximum stimulator output (MSO), using MRI-based neuronavigation. Motor evoked potentials (MEPs) were recorded simultaneously from 9 muscles of the contralateral hand, wrist and arm. For each intensity, corticospinal divergence was quantified by the average number of muscles that responded to TMS per cortical site, coactivation across muscle pairs as reflected by overlap of cortical representations, and correlation of MEP amplitudes across muscle pairs. TMS to each muscle’s most responsive site elicited submaximal MEPs in most other muscles. The number of responsive muscles per cortical site and the extent of coactivation increased with increasing intensity (ANOVA, p < 0.001). In contrast, correlations of MEP amplitudes did not differ across the 60, 80 and 100% MSO intensities (ANOVA, p = 0.34), but did differ across muscle pairs (ANOVA, p < 0.001). Post hoc analysis identified 4 sets of muscle pairs (Tukey homogenous subsets, p < 0.05). Correlations were highest for pairs involving two hand muscles and lowest for pairs that included an upper arm muscle. Correlation of MEP amplitudes may quantify varying levels of muscle coupling. In future studies, this approach may be a biomarker to reveal altered coupling induced by neural injury, neural repair and/or motor learning.
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Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.
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Acknowledgements
This work was funded by the Comprehensive Opportunities for Rehabilitation Research Training (CORRT), a Multicenter Career Development Program for Physical and Occupational Therapists, National Institutes of Health Grant Number K12 HD055931, the University of Iowa Carver College of Medicine and the Department of Physical Therapy and Rehabilitation Science. MR scans were conducted on an MRI instrument funded by NIH 1S10OD025025-01. The authors acknowledge Lauren Werner for assistance with data collection.
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DeJong, S.L., Bisson, J.A., Darling, W.G. et al. Simultaneous Recording of Motor Evoked Potentials in Hand, Wrist and Arm Muscles to Assess Corticospinal Divergence. Brain Topogr 34, 415–429 (2021). https://doi.org/10.1007/s10548-021-00845-1
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DOI: https://doi.org/10.1007/s10548-021-00845-1