Transcranial or non-transcranial stimulation? Results of a combined TMS-EEG feasibility study

BackgroundEntrainment of cortical oscillations by repetitive Transcranial Magnetic Stimulation (rTMS) is an attractive approach to modulate brain function non-invasively in humans. Here, we applied rTMS in order to modulate oscillatory activity in ventral premotor cortex (PMv), primary motor cortex (M1), and anterior intraparietal sulcus (aIPS). These areas are thought to contribute to recovery after motor stroke and our overarching goal is to enhance their impact by rTMS. To this end, we established a setup with bifocal, neuronavigated rTMS combined with EEG and tested its technical feasibility. MethodsBifocal zero-phase lag synchronized rTMS at 11Hz was applied in seven young healthy volunteers to the target pairs (i) PMv and M1 and (ii) aIPS and M1. Adapting to the close vicinity between target areas, we used two small, commercially available coils and applied subthreshold stimuli in order to avoid motor evoked potentials (MEPs). Besides a parieto-occipital sham stimulation, we also included auditory and sensory stimulation in a further control experiment. ResultsFirst, subthreshold TMS led to a phase synchronization and evoked time-averaged potentials in the EEG. However, the same findings could be elicited by peripheral, somatosensory stimulation combined with auditory stimulation. Second, despite the small coils neuronavigation analysis showed that in most participants aIPS and M1 or PMv and M1 could not precisely be targeted due to their vicinity and restriction in coil positioning. Third, bifocal subthreshold rTMS tended to sum up where the induced fields showed the greatest overlap resulting in overt MEPs and thus raising potential safety issues. ConclusionsThe presented data show refinements for bifocal rTMS studies regarding (i) spurious entrainment or resetting effects on brain oscillations, (ii) precise anatomical targeting of areas in close vicinity, and (iii) summing up of overlapping induced electrical fields.