gms | German Medical Science

Objective: Language mapping by transcranial magnetic stimulation (TMS) is commonly applied over the left language-dominant hemisphere to indicate language-related cortex at stimulation intensities close to the resting motor threshold (rMT). High-resolution mapping in immediate vicinity to the premotor cortex by use of higher stimulation intensities for more efficient language inhibition may raise concern about confounding unspecific effects. In this study, we performed inter-hemispheric comparisons in order to delineate unspecific premotor effects from true language inhibition by TMS. Likewise, we investigated the effects of stimulation intensities above the rMT on naming errors, latencies, and stimulation discomfort.

Methods: Fifteen healthy, right-handed German speakers underwent two language mapping sessions with a naming protocol adapted for high-resolution mapping purposes. In the first experiment, we applied navigated TMS over a left- and right-hemispheric target array covering the left inferior frontal junction (IFJ) area. Six mapping repetitions were conducted per hemisphere with pseudo-randomized sequence between participants. Stimulation intensity was determined according to the individual speech inhibition threshold. In the second experiment, TMS was applied over the same left-hemispheric target array with pseudo-randomized intensities of 110, 120, 130, and 140 % of rMT. Three mapping repetitions were conducted per intensity. In both experiments, participants rated the unspecific interference of TMS with speech due to facial twitching, discomfort, and pain on a 4-level Likert scale (1 = none, 4 = strong) after each stimulation/naming.

Results: In the first experiment, we found a minor, statistically significant preponderance of stimulation discomfort on the left side: Stimulation of left IFJ was elicited slight discomfort more frequently as compared to right IFJ, whereas no discomfort at all was reported more frequently after stimulation of right IFJ as compared to left IFJ. More severe discomfort ratings did not differ between hemispheres. Naming errors were observed significantly more frequently during stimulation of the left IFJ, although some participants showed comparable effects during right-hemispheric stimulation. In the second experiment, there was no increase in both naming latencies and discomfort ratings with increased stimulation intensities, except for a small number of subjects. It is important to note that the individual speech inhibition threshold, i.e. the stimulation strength which reliably elicits language errors during random test stimulations within the target area, was not reached in all subjects.

Conclusion: During high-resolution TMS language mapping over the IFJ area, only slight discomfort is marginally more frequent in the left hemisphere, whereas more severe interferences are equally distributed between hemispheres. A major contribution of not language-specific effects to TMS language mapping results in this area is therefore unlikely. Our findings also support the determination of the individual speech inhibition threshold even at higher stimulation intensities, prior to high-resolution TMS language mapping in a distinct cortical target area. Within the moderate range of intensities tested here, raised stimulation strength does not increase discomfort in most subjects.