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Minimum-Norm Estimation of Motor Representations in Navigated TMS Mappings

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An Erratum to this article was published on 06 September 2017

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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.

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  • 06 September 2017

    An erratum to this article has been published.

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Acknowledgements

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.

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Authors and Affiliations

  1. Department of Neuroscience and Biomedical Engineering, Aalto University School of Science, P. O. Box 12200, FI-00076, Aalto, Espoo, Finland

    Minna Pitkänen, Jaakko O. Nieminen & Risto J. Ilmoniemi

  2. Department of Clinical Neurophysiology, Kuopio University Hospital, P. O. Box 100, FI-70029, Kuopio, Finland

    Minna Pitkänen, Elisa Kallioniemi & Petro Julkunen

  3. Department of Clinical Radiology, Kuopio University Hospital, P. O. Box 100, FI-70029, Kuopio, Finland

    Elisa Kallioniemi

  4. Department of Applied Physics, University of Eastern Finland, P. O. Box 1627, FI-70211, Kuopio, Finland

    Petro Julkunen

  5. Centre for Cognition and Decision Making, National Research University Higher School of Economics, 46B Volgogradskiy Prospekt, Moscow, Russian Federation, 109316

    Maria Nazarova

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  1. Minna Pitkänen
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  2. Elisa Kallioniemi
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  3. Petro Julkunen
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  4. Maria Nazarova
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  5. Jaakko O. Nieminen
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  6. Risto J. Ilmoniemi
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Correspondence to Minna Pitkänen.

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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.

Ethical Approval

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

Informed consent was obtained from all individual participants included in the study.

Additional information

An erratum to this article is available at https://doi.org/10.1007/s10548-017-0587-6.

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Pitkänen, M., Kallioniemi, E., Julkunen, P. et al. Minimum-Norm Estimation of Motor Representations in Navigated TMS Mappings. Brain Topogr 30, 711–722 (2017). https://doi.org/10.1007/s10548-017-0577-8

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  • DOI: https://doi.org/10.1007/s10548-017-0577-8

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