Article
Protocol for mapping of the supplementary motor area using repetitive navigated transcranial magnetic stimulation (rnTMS)
Protokoll zur Kartierung des supplementär motorischen Areals mittels repetitiver navigierter transkranieller Magnetstimulation (rnTMS)
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Authors
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Melina Engelhardt
- Charité Universitätsmedizin, Klinik für Neurochirurgie, Berlin, Deutschland; Charité Universitätsmedizin, Einstein Center für Neurowissenschaften, Berlin, Deutschland
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Jari Karhu
- University of Eastern Finland, Department of Physiology, Kuopio, Finnland; Nexstim, Helsinki, Finnland
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Thomas Picht
- Charité Universitätsmedizin, Klinik für Neurochirurgie, Berlin, Deutschland; Charité Universitätsmedizin, Einstein Center für Neurowissenschaften, Berlin, Deutschland; Humboldt Universität Berlin, Cluster of Excellence “Matters of Activity. Image Space Material”, Berlin, Deutschland
| Published: | June 4, 2021 |
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Outline
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Objective: The supplementary motor area (SMA) has been suggested to mediate movement planning, execution and coordination. Navigated transcranial magnetic stimulation (nTMS) offers the possibility to induce transient lesions of the SMA to study the specific role on this region in performance of motor tasks. Further, detailed mapping of functionally relevant areas within the SMA could potentially aid preoperative diagnostics in patients. The aim of this study was the development of a repetitive nTMS protocol for non-invasive functional mapping of the SMA.
Methods: The SMA was mapped in the dominant hemisphere of nine healthy subjects (28.3 ± 8.1 years, 5 females) using repetitive nTMS at 20 Hz (120% RMT), while subjects performed a finger tapping task. The location of induced errors was marked in each subject’s individual MRI. Additionally, a SMA hotspot was defined as the point consistently eliciting the largest disruptions of task performance upon stimulation. To exclude effects due to indirect stimulation of M1, effects of SMA stimulation were directly compared to effects of M1 stimulation in four different tasks (finger tapping, writing, line tracing, pointing at small circles with a pencil). M1 was targeted with the intensity of the residual electric field reaching the motor hotspot during SMA stimulation.
Results: Mapping of the SMA was possible in 7 of 9 subjects. Stimulation of the SMA led to a significant reduction of finger taps compared to baseline (BL: 45.8 taps, SMA: 35.2 taps; p = 0.017), as well as a non-significant reduction compared to M1 stimulation (M1: 40.0 taps; p = 0.104). Line tracing, writing and targeting of circles was less accurate during SMA stimulation. Further, effects of SMA disruption increased with stimulation time, while effects of M1 stimulation were present from the beginning on. Noteworthy, a significant variation of effect size was observed between subjects.
Conclusion: Mapping of the SMA using repetitive nTMS is feasible in the majority of subjects. While errors induced in the SMA are not entirely independent of M1 due to the proximity of both regions, disruption of the SMA seems to induce functionally distinct errors. Thus, error maps of the SMA assessed with nTMS can give valuable insights into the functional organization of this region, potentially aiding preoperative diagnostics in patients with SMA related brain lesions.
