gms | German Medical Science

Objective: Task-locked navigated transcranial magnetic stimulation (nTMS) allows for direct investigation of cortical functions in the stimulated area by analyzing behavioural changes during the period of train duration. Using short stimulation trains like paired-pulse (PP) nTMS instead of the commonly applied 10-Hz repetitive TMS (rTMS), enable to assess the time-course of cortical processing on the ms-level. We, therefore, compared the behavioural results (i.e., language errors) of PP-nTMS using different stimulation onset times.

Methods: Twelve healthy, right-handed subjects were investigated during a picture naming task, testing PP-nTMS (3 ms inter-stimulus-interval) starting between 0 and 500 ms (100 ms steps, pseudorandomized sequence) after picture presentation. Stimulation intensity was 82±12% of the resting motor threshold. PP-nTMS was performed over selected speech-related sites (fMRI-based) in random order and four times per target area.

Results: Performance errors (i.e., dysarthria, anomia, phonematic paraphasia, speech motor disturbances) were most frequent (5% cumulative error frequency). Most errors occurred after PP-nTMS at 0 ms (7.6%) and 400 ms (6.9%) whereas least errors were induced after stimulation at 300 ms (4.0%). In particular, PP-TMS simultaneous to picture presentation (0 ms offset), led to more phonematic errors (3.1%) than any other condition (p<0.1; FDR-corrected). In contrast, syntax errors seemed specific for a stimulation onset after 200 ms or later, most frequent after 400 ms (1.6%).

Conclusion: The results confirm that PP-TMS is a feasible alternative to conventional rTMS for language mapping. Overall, simultaneous stimulation and picture presentation seems most appropriate to induce short-lasting interference with speech performance. However, the heterogeneous error distribution depending on the timing suggests the need for an adaptive design of stimulation parameters, to address the clinical or research question optimally.