Minimum-Norm Estimation of Motor Representations in Navigated TMS Mappings
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Navigated transcranial magnetic stimulation (nTMS) can be applied to locate and outline cortical motor representations. This may be important, e.g., when planning neurosurgery or focused nTMS therapy, or when assessing plastic changes during neurorehabilitation. Conventionally, a cortical location is considered to belong to the motor cortex if the maximum electric field (E-field) targeted there evokes a motor-evoked potential in a muscle. However, the cortex is affected by a broad E-field distribution, which tends to broaden estimates of representation areas by stimulating also the neighboring areas in addition to the maximum E-field location. Our aim was to improve the estimation of nTMS-based motor maps by taking into account the E-field distribution of the stimulation pulse. The effect of the E-field distribution was considered by calculating the minimum-norm estimate (MNE) of the motor representation area. We tested the method on simulated data and then applied it to recordings from six healthy volunteers and one stroke patient. We compared the motor representation areas obtained with the MNE method and a previously introduced interpolation method. The MNE hotspots and centers of gravity were close to those obtained with the interpolation method. The areas of the maps, however, depend on the thresholds used for outlining the areas. The MNE method may improve the definition of cortical motor areas, but its accuracy should be validated by comparing the results with maps obtained with direct cortical stimulation of the cortex where the E-field distribution can be better focused.
KeywordsNavigated transcranial magnetic stimulation Minimum-norm estimate Motor cortex Motor-evoked potential Motor representation Electric field
The study was funded by the State Research Funding (Research Committee of the Kuopio University Hospital Catchment Area, projects 5041730 and 5041747, Kuopio, Finland); Finnish Cultural Foundation, Helsinki, Finland; Cancer Society of Finland, Helsinki, Finland; Päivikki and Sakari Sohlberg Foundation, Helsinki, Finland; Academy of Finland (Decisions Nos. 255347, 265680, and 294625), Helsinki, Finland; Russian Academic Excellence Project ‘5-100’, RFBR Grant (No. 16-04-01883) and Skolkovo personal grant “Umnik”. The funding sources had no involvement in the study design, in the collection, analysis and interpretation of data, in the writing of the report, or in the decision to submit the article for publication.
Compliance with Ethical Standards
Conflict of interest
Petro Julkunen and Jaakko O. Nieminen have received unrelated consulting fees from Nexstim Plc, Elisa Kallioniemi has received unrelated travel support from Nexstim Plc, and Risto J. Ilmoniemi is an advisor and a minority shareholder of the company. The other authors declare 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 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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