gms | German Medical Science

Objective: Using navigated repetitive transcranial magnetic stimulation (rTMS) to disrupt language function during an online-task (e.g., picture-naming) is a novel mapping technique, which has recently been introduced into pre-surgical diagnostics in brain tumor patients. However, results are variable. Not only the task itself but also the distinct error categories may strongly influence mapping results. Therefore, we studied the test-retest-reliability and the spatial localization of rTMS-induced naming errors related to motor-speech function in contrast to semantic language errors.

Methods: 13 right-handed, healthy volunteers were investigated three times each in consecutive sessions (short-term: 2-5 days, long-term: 21-40 days) by 10Hz-rTMS. After determination of the individual threshold for motor cortex excitability, rTMS was applied “online” during a picture-naming task: black-and-white drawings of everyday objects were presented simultaneously (triggered, delay = 0 ms) with rTMS. The rTMS bursts lasted for 1.5s (15 pulses) and were applied over the frontolateral, parietolateral and superior/middle temporal area of the left hemisphere, using a grid for guidance. Errors were rated by two independent examiners by a post-hoc video analysis. The error frequency (number of errors per 100 rTMS pulses) as well as spatial representation and reliability of distinct motor-speech-related errors (i.e., speech arrest and dysarthria) were compared to language errors associated with semantic processing (i.e., anomia and semantic paraphasia). Test-retest-reliability was assessed by the average intraclass correlation coefficients for a fixed rater (ICC).

Results: Overall, speech/language errors occurred only rarely: Amongst the four analyzed error categories, dysarthria was most frequent (4.0 ± 2.5 %) while only very few semantic paraphasias were induced (0.5 ± 0.4 %). Motor-speech errors were found significantly more often than semantic language errors (p<0.01). In comparison to semantic errors, the test-retest-reliability of the error rates was lower for motor-speech errors (motor-speech: ICC=0.45 vs. semantic: ICC=0.64). By contrast, the spatial reliability was higher for motor speech errors and showed a clear accumulation over M1 and the premotor cortex, especially for the category “arrest”.

Conclusion: The novel approach of using rTMS for language mapping is still challenging due to the high variability of induced errors and possible interpretations. Most rTMS-induced picture-naming errors seem to be at least partially due to motor network disruption rather than to interference with non-motor language processing. Further studies, combining different neuroimaging techniques and online-EEG, are mandatory to elucidate the neurophysiological mechanisms and particular network effects of rTMS during language tasks.