Group‐level variations in motor representation areas of thenar and anterior tibial muscles: Navigated Transcranial Magnetic Stimulation Study

Abstract

Navigated transcranial magnetic stimulation (TMS) can be used to stimulate functional cortical areas at precise anatomical location to induce measurable responses. The stimulation has commonly been focused on anatomically predefined motor areas: TMS of that area elicits a measurable muscle response, the motor evoked potential. In clinical pathologies, however, the well-known homunculus somatotopy theory may not be straightforward, and the representation area of the muscle is not fixed. Traditionally, the anatomical locations of TMS stimulations have not been reported at the group level in standard space. This study describes a methodology for group-level analysis by investigating the normal representation areas of thenar and anterior tibial muscle in the primary motor cortex. The optimal representation area for these muscles was mapped in 59 healthy right-handed subjects using navigated TMS. The coordinates of the optimal stimulation sites were then normalized into standard space to determine the representation areas of these muscles at the group-level in healthy subjects. Furthermore, 95% confidence interval ellipsoids were fitted into the optimal stimulation site clusters to define the variation between subjects in optimal stimulation sites. The variation was found to be highest in the anteroposterior direction along the superior margin of the precentral gyrus. These results provide important normative information for clinical studies assessing changes in the functional cortical areas because of plasticity of the brain. Furthermore, it is proposed that the presented methodology to study TMS locations at the group level on standard space will be a suitable tool for research purposes in population studies.

Spatial localization and distribution of the TMS-related ‘hotspot’ of the tibialis anterior muscle representation in the healthy and post-stroke motor cortex

“…A few studies have estimated the approximate locations of the TA hotspot to be within 1–3 cm lateral to the vertex [2,17]. In a previous study using image guided TMS, Niskanen et al [22] reported that optimal stimulation site for the TA muscle was near the longitudinal fissure for healthy individuals, and that the observed variance in the location did not correlate with age or head circumference. In our study, we confirmed the optimal stimulation location using standard TMS, and furthermore, we reported the precise location in the anteroposterior and mediolateral coordinates on the skull.…”

Exploring the impact of visual and movement based priming on a motor intervention in the acute phase post-stroke in persons with severe hemiparesis of the upper extremity

“…[36] TMS pulses were delivered within the bounds with special attention paid to regions surrounding the hotspot territory. [37] For each stimulation point MEPs were calculated as the peak-to-peak amplitude of the filtered (2nd order Butterworth filter, 5–250 Hz band-pass) EMG signal 20–50 ms after the TMS pulse. MEP amplitudes were interpolated to a 10 × 10 cm mesh of 5 mm resolution centered on the M1 hotspot, using cubic surface interpolation.…”

Results on the spatial resolution of repetitive transcranial magnetic stimulation for cortical language mapping during object naming in healthy subjects

“…Furthermore, the reference electrode was placed at the elbow. Then, a coarse round of single-pulse stimulations was conducted to localize the spot with the highest MEP amplitude, the motor hotspot, which is usually found within the area of the hand knob [14]. During pulse application, the induced electrical field was oriented perpendicular to the precentral gyrus, and the RMT was then determined at the motor hotspot.…”