gms | German Medical Science

Objective: Recently, repetitive transcranial magnetic stimulation (rTMS) has gained a lot of interest in preoperative planning of brain tumor resections for detection of cortical language-related areas. However, the right rTMS protocol is still a matter of debate since rTMS-evoked speech-errors appear relatively widespread over the brain and are rather poorly reliable, depending on the type of speech-errors. We, here, compared locations of rTMS-evoked speech-errors of distinct categories to language-related fMRI clusters (widely used standard for language mapping).

Methods: Thirteen right-handed, healthy volunteers were investigated by fMRI and navigated rTMS using the same picture naming-task. In fMRI a sparse-sampling design was used with the following parameters: TR = 11 000 ms, delay in TR = 9000 ms, TE = 30 ms, flip angle = 90°, voxel size 3x3x3 mm3, FoV = 192 mm2, 79 images. After preprocessing, the clusters of the superior temporal gyrus (STG) and the inferior frontal gyrus (IFG) were extracted. In the rTMS sessions, 10, 30 and 50 Hz rTMS were applied in a randomized order over the left hemisphere, continuously covering facial (pre-) motor and language-related cortical areas. Bursts were triggered to picture presentation without delay. Errors were rated by two independent examiners using a post-hoc video analysis and categorized as follows: arrest, anomia, delayed term, complete delay, dysarthria, morphosyntactic errors, speech-motor disturbance, semantic and phonematic paraphasia. The coordinates of the sites corresponding to the speech-errors were extracted. Then the amount of rTMS-speech errors lying within the STG/IFG-clusters was calculated (relative to the total number of errors, “hit rate”).

Results: Overall, 17 % of the rTMS-evoked speech-errors were located within the STG (10 Hz: 0.15 ± 0.05, 30 Hz: 0.19 ± 0.07, 50 Hz: 0.18 ± 0.07). Only 6% were located within the IFG (10 Hz: 0.06 ± 0.05, 30 Hz: 0.05 ± 0.02, 50 Hz: 0.07 ± 0.04). However, when corrected for the different cluster sizes (STG>IFG) this difference was not significant. Within the STG, a significantly higher rate of speech-errors was observed for 30 Hz as compared to 10 Hz (p<0.05). The rate of motor-speech associated errors increased along with higher rTMS-frequencies (30 & 50 Hz, p<0.05).

Conclusion: The rate of rTMS-induced language errors within the STG seems to increase with higher frequencies, especially for errors associated with motor-speech function. This finding suggests that higher rTMS frequencies may be more powerful to induce virtual lesions, i.e., to interfere with language processing via short-lasting cortico-cortical inhibition in the area of the STG.