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Brain Topogr. 2017 Nov;30(6):711-722. doi: 10.1007/s10548-017-0577-8. Epub 2017 Jul 18.

Minimum-Norm Estimation of Motor Representations in Navigated TMS Mappings.

Author information

1
Department of Neuroscience and Biomedical Engineering, Aalto University School of Science, P. O. Box 12200, FI-00076, Aalto, Espoo, Finland. minna.pitk@gmail.com.
2
Department of Clinical Neurophysiology, Kuopio University Hospital, P. O. Box 100, FI-70029, Kuopio, Finland. minna.pitk@gmail.com.
3
Department of Clinical Neurophysiology, Kuopio University Hospital, P. O. Box 100, FI-70029, Kuopio, Finland.
4
Department of Clinical Radiology, Kuopio University Hospital, P. O. Box 100, FI-70029, Kuopio, Finland.
5
Department of Applied Physics, University of Eastern Finland, P. O. Box 1627, FI-70211, Kuopio, Finland.
6
Centre for Cognition and Decision Making, National Research University Higher School of Economics, 46B Volgogradskiy Prospekt, Moscow, Russian Federation, 109316.
7
Department of Neuroscience and Biomedical Engineering, Aalto University School of Science, P. O. Box 12200, FI-00076, Aalto, Espoo, Finland.

Abstract

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.

KEYWORDS:

Electric field; Minimum-norm estimate; Motor cortex; Motor representation; Motor-evoked potential; Navigated transcranial magnetic stimulation

PMID:
28721533
DOI:
10.1007/s10548-017-0577-8
[Indexed for MEDLINE]

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