Accurate Coil Positioning is Important for Single and Paired Pulse TMS on the Subject Level

Abstract

Function-guided navigation is commonly used when assessing cortical excitability using transcranial magnetic stimulation (TMS). However, the required accuracy, stability and the effect of a change in coil positioning are not entirely known. This study investigates the accuracy of function-guided navigation for determining the hotspot. Furthermore, it evaluates the effect of a change in coil location on the single and paired pulse excitability measures: motor evoked potential (MEP) amplitude, TMS evoked potential (TEP) and long intracortical inhibition (LICI), and of a change in coil orientation on LICI. Eight healthy subjects participated in the single pulse study, and ten in the paired pulse study. A robot-guided navigation system was used to ensure accurate and stable coil positioning at the motor hotspot as determined using function-guided navigation. In addition, we targeted four locations at 2 mm and four at 5 mm distance around the initially defined hotspot, and we increased and decreased the coil orientation by 10°. In none of the subjects, the largest MEP amplitudes were evoked at the originally determined hotspot, resulting in a poor accuracy of function-guided navigation. At the group level, a change in coil location had no significant effect on the MEP amplitude, TEP, or LICI, and a change in coil orientation did not significantly affected LICI. However, at the subject level significant effects on MEP amplitude, TEP, and LICI were found for changes in coil location or orientation, although absolute differences were relatively small and did not show a consistent pattern. This study indicates that a high accuracy in coil positioning is especially required to measure cortical excitability reliably in individual subjects using single or paired pulse TMS.Electronic supplementary materialThe online version of this article (10.1007/s10548-018-0655-6) contains supplementary material, which is available to authorized users.

Local Differences in Cortical Excitability – A Systematic Mapping Study of the TMS-Evoked N100 Component

“…We predict that the topographic maximum of the N100 is located at the respective stimulation site, i.e., covaries with changes in coil position. We further predict that N100 amplitudes are similar in magnitude following stimulation at different positions given that cortical excitability could be equal across stimulation positions, consistent with the results of de Goede et al (2018) . Our second hypothesis concerns the cortical origin of the N100; we expect that primary motor cortex activation can be represented by a single equivalent dipole in the motor cortex.…”

“…Our results concerning the N100 amplitude and latency as well as GFP further support the conclusion that TMS stimulated regions beyond the primary motor cortex. Exclusive activations within the primary motor cortex should yield either both N100 and GFP amplitude reduction during stimulation away from the hot spot (similar to the MEP amplitude changes) or there should be no changes in TEP amplitudes, similar to the study from de Goede et al (2018) . These authors observed no differences in TEP amplitudes across nine stimulation positions 2 or 5 mm around the hot spot within a latency of 100 ms.…”

“…Considering that primary motor cortex extends medially (approximately 43.6 mm) and laterally (approximately 22.7 mm) from the motor hot spot ( Lotze et al, 2000 ) but to a lesser extent anteriorly and posteriorly (width of precentral gyrus is approx. 16 ± 3.6 mm) ( Ebeling et al, 1986 ), de Goede et al (2018) may have stimulated only the primary motor cortex and, therefore, measured similar TEP amplitudes. We hypothesize that, even with larger (5–10 mm) coil deviations around the hot spot, we also stimulated mainly the primary motor cortex during medial (E) and lateral (W) stimulation due to the medial and lateral primary motor cortex expansion.…”

“…If the N100 reflects local cortical excitability, amplitudes should covary with changes to the stimulation position around the motor cortex hot spot. One previous study compared TEP deflections across nine different stimulation targets, located 2 and 5 mm around the target muscle hot spot ( de Goede et al, 2018 ), and observed no group-level differences in TEP deflections at a latency of 100 ms. In our study, we aim to assess larger variation in coil positioning (5–10 mm) to test whether larger distances between coil positions reveal changes in local cortical excitability and N100 topography.…”

Sensory and auditory evoked responses mimic synchronization of cortical oscillations induced by Transcranial Magnetic Stimulation

“…The problem of low field strength is naturally even greater when TMS is combined with EEG because the electrodes mounted in the cap cause a distance of the coil from the scalp of at least approximately 2mm, rather more. The strength of the magnetic field is maximal close to the coil surface and decreases exponentially with the distance from there (Cohen et al, 1990;de Goede et al, 2018). An interesting way to attenuate this problem might be closed--1 2 -loop stimulation, through which TMS efficacy might be enhanced by better timing relative to the ongoing oscillatory cortical activity (Bergmann et al, 2012;Kraus et al, 2016;Raco et al, 2016).…”