Efficient Mapping of the Motor Cortex with Navigated Biphasic Paired-Pulse Transcranial Magnetic Stimulation
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Navigated transcranial magnetic stimulation (nTMS) can be applied to locate cortical muscle representations. Usually, single TMS pulses are targeted to the motor cortex with the help of neuronavigation and by measuring motor evoked potential (MEP) amplitudes from the peripheral muscles. The efficacy of single-pulse TMS to induce MEPs has been shown to increase by applying facilitatory paired-pulse TMS (ppTMS). Therefore, the aim was to study whether the facilitatory ppTMS could enable more efficient motor mapping. Biphasic single-pulse TMS and ppTMS with inter-stimulus intervals (ISIs) of 1.4 and 2.8 ms were applied to measure resting motor thresholds (rMTs) as a percentage of the maximal stimulator output and to determine the cortical representation areas of the right first dorsal interosseous muscle in healthy volunteers. The areas, shapes, hotspots, and center of gravities (CoGs) of the representations were calculated. Biphasic ppTMS with ISI of 1.4 ms resulted in lower rMTs than those obtained with the other protocols (p = 0.001). With ISI of 2.8 ms, rMT was lower than with single-pulse TMS (p = 0.032). The ppTMS mapping was thus performed with lower intensity than when using single-pulse TMS. The areas, shapes, hotspots, and CoGs of the muscle representations were in agreement. Hence, biphasic ppTMS has potential in the mapping of cortical hand representations in healthy individuals as an alternative for single-pulses, but with lower stimulation intensity by utilizing cortical facilitatory mechanism. This could improve application of nTMS in subjects with low motor tract excitability.
KeywordsNavigated transcranial magnetic stimulation Short-interval intracortical facilitation Motor mapping Motor evoked potential Motor cortex
The study was funded by the Finnish Cultural Foundation, Helsinki, Finland; Päivikki and Sakari Sohlberg Foundation, Helsinki, Finland; State Research Funding (Research Committee of the Kuopio University Hospital Catchment Area, Projects 5041730 and 5041747, Kuopio, Finland) and Cancer Society of Finland, 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, or in the decision to submit the article for publication.
Compliance with Ethical Standards
Conflict of interest
Gustaf Järnefelt and Jari Karhu are employed by the TMS-device manufacturing company Nexstim Plc, Helsinki, Finland. Petro Julkunen has received unrelated consulting pay and Elisa Kallioniemi unrelated traveling support from Nexstim Plc. Minna Pitkänen has 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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