Extent and Location of the Excitatory and Inhibitory Cortical Hand Representation Maps: A Navigated Transcranial Magnetic Stimulation Study
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Voluntary muscle action and control are modulated by the primary motor cortex, which is characterized by a well-defined somatotopy. Muscle action and control depend on a sensitive balance between excitatory and inhibitory mechanisms in the cortex and in the corticospinal tract. The cortical locations evoking excitatory and inhibitory responses in brain stimulation can be mapped, for example, as a pre-surgical procedure. The purpose of this study was to find the differences between excitatory and inhibitory motor representations mapped using navigated transcranial magnetic stimulation (nTMS). The representations of small hand muscles were mapped to determine the areas and the center of gravities (CoGs) in both hemispheres of healthy right-handed volunteers. The excitatory representations were obtained via resting motor evoked potential (MEP) mapping, with and without a stimulation grid. The inhibitory representations were mapped using the grid and measuring corticospinal silent periods (SPs) during voluntary muscle contraction. The excitatory representations were larger on the dominant hemisphere compared with the non-dominant (p < 0.05). The excitatory CoGs were more medial (p < 0.001) and anterior (p < 0.001) than the inhibitory CoGs. The use of the grid did not influence the areas or the CoGs. The results support the common hypothesis that the MEP and SP representations are located at adjacent sites. Furthermore, the dominant hemisphere seems to be better organized for controlling excitatory motor functions with respect to TMS. In addition, the inhibitory representations could provide further information about motor reorganization and aid in surgery planning when the functional cortical representations are located in abnormal cortical regions.
KeywordsTranscranial magnetic stimulation Neuronavigation Motor cortex Motor evoked potential Silent period Center of gravity
The study was funded by the State Research Funding (Project 5041730, Kuopio, Finland). In addition, Minna Pitkänen was supported by the Research Foundation of Helsinki University of Technology, Espoo, Finland, and Elisa Kallioniemi was supported by the Kaute Foundation, Helsinki, Finland, The Finnish Brain Research and Rehabilitation Center Neuron, Kuopio, Finland, The Finnish Concordia Fund, Helsinki, Finland, The Paulo Foundation, Helsinki, Finland. The funding sources had no involvement in the study design, in the collection, analysis and interpretation of data; in the writing of the report, and in the decision to submit the article for publication.
Compliance with Ethical Standards
Conflicts of interest
Petro Julkunen has received unrelated consulting pay from Nexstim Plc, manufacturer of the nTMS devices. The rest of the authors declared that they have no conflict of interest.
All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee (ethical permission 1/2014) and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
Informed consent was obtained from all individual participants included in the study.
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