Protocol for mapping of the supplementary motor area using repetitive navigated transcranial magnetic stimulation

Abstract

Background: Damage to the supplementary motor area (SMA) for example during surgery can lead to impairments of motor and language function. A detailed preoperative mapping of functional boarders of the SMA could therefore aid preoperative diagnostics in these patients. Objective: The aim of this study was the development of a repetitive nTMS protocol for non-invasive functional mapping of the SMA while assuring effects are indeed caused by SMA stimulation rather than activation of M1. Methods: To this purpose the SMA in the dominant hemisphere of twelve healthy subjects (28.2 +- 7.7 years, 6 females) was mapped using repetitive nTMS at 20 Hz (120% RMT), while subjects performed a finger tapping task. The location of induced errors was marked in each subjects individual MRI. To further validate the protocol, effects of SMA stimulation were directly compared to effects of M1 stimulation in four different tasks. Results: Mapping of the SMA was possible for all subjects, yet varying effect sizes were observed. Stimulation of the SMA led to a significant reduction of finger taps compared to baseline (BL: 45 taps, SMA: 35.5 taps; p < 0.01). Line tracing, writing and targeting of circles was less accurate during SMA compared to M1 stimulation. Conclusion: Mapping of the SMA using repetitive nTMS is feasible. While errors induced in the SMA are not entirely independent of M1 due to the proximity of both regions, disruption of the SMA induces functionally distinct errors. These error maps can aid preoperative diagnostics in patients with SMA related lesions.