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

Abstract

Abstract Background The combination of Transcranial Magnetic Stimulation (TMS) and Electroencephalography (EEG) provides the possibility to record neurophysiological responses to non-invasive brain stimulation. Previous studies found reproducible time-averaged potentials evoked by TMS (TEPs). Brain electric responses to TMS, including some components of TEPs, might, however, reflect somatosensory and auditory stimulation, rather than direct cortical neuronal responses to the transcranial stimuli. This is of great relevance for interpreting experimental data geared at modulating neuronal networks by TMS. Here we report observations from a feasibility study suggesting that TMS-induced effects on cortical oscillations need to be interpreted with caution. Methods Subthreshold, monofocal TMS to M1 and PO3 (sham) and bifocal single-pulse TMS was applied to the targets (i) ventral premotor cortex (PMv) and primary motor cortex (M1) and (ii) anterior intraparietal sulcus (aIPS) and M1 using small coils (25mm inner diameter). Besides sham stimulation, we conducted control experiments with a pure auditory and sensory co-stimulation. Results TMS led to EEG phase synchronization and a pattern of evoked potentials which were comparable with those patterns evoked by sham-stimulation. Moreover, somatosensory stimulation when combined with auditory stimulation mimicked TMS-induced EEG responses. Despite small coils (inner radius 25mm), 3D-neuronavigation analysis showed that aIPS and M1 or PMv and M1 could not precisely be targeted due to their vicinity. Conclusions TEPs and phase-synchronization of EEG can be affected by TMS-induced non-transcranial somatosensory and auditory components. Simultaneous stimulation of nearby targets like PMv and M1 or M1 and aIPS may be hampered by limited possibilities to position even small TMS coils next to each other. Significance Statement The present data provide transparency on relevant limitations of a promising neuromodulation approach. While subthreshold TMS can induce resetting and entrainment effects on cortical oscillations, the specificity of these effects needs to be interpreted with caution. Non-transcranial stimulation effects and limitations of coil positioning directly next to each other need to be considered and controlled for.