gms | German Medical Science

Objective: Navigated transcranial magnetic stimulation (nTMS) is increasingly applied to map human language functions. However, despite its promising role especially for preoperative assessment of patients with intracranial lesions, studies on protocol comparisons are mostly lacking. Thus, the present study systematically investigates different stimulation adjustments with the aim of optimizing current procedures.

Methods: Twenty healthy, right-handed volunteers (25.7 ± 3.8 years, 12 females and 8 males) underwent left-hemispheric language mapping by nTMS, combined with an object-naming task, over a cortical spot with reproducible naming errors within the triangular or opercular part of the inferior frontal gyrus (trIFG, opIFG: anterior stimulation) and the posterior part of the superior temporal gyrus (pSTG: posterior stimulation), respectively. Various stimulation intensities (80%, 100%, and 120% of the resting motor threshold [rMT]), stimulation frequencies (2 Hz, 5 Hz, 10 Hz, and 20 Hz), and coil orientations (in steps of 45°) were evaluated in terms of their error rates (ERs; number of errors / number of stimulations) for different error categories.

Results: Regarding anterior stimulation, 100% rMT, 5 Hz, and anterior-posterior (a-p) coil orientation resulted in optimal results, meaning the highest ERs when comparing the different adjustments. For posterior stimulation, 100% rMT and 5 Hz were optimal (without a clearly optimal coil orientation). Errors due to facial muscle stimulation, regarded as relevant confounders of language mapping, only played a significant role during analyses of high-intensity (120% rMT) and high-frequency stimulation (20 Hz).

Conclusion: This is one of the first studies to systematically investigate different stimulation protocols of nTMS language mapping. Due to the partially contradictory findings in relation to the sparse amount of previous literature on the matter, further studies have to clarify whether the optimal adjustments revealed in the present study can serve as universal parameters enabling optimized mapping, or whether they should rather serve as reliable starting parameters for subsequent individualization of adjustments. Optimized protocols can improve the correlation between preoperative nTMS and intraoperative direct cortical stimulation as the current gold-standard method for mapping of human language functions.