F193. What remains as transcranial cortical response after accounting for the contribution of somatosensory and auditory evoked responses?

Abstract

Focal transcranial magnetic stimulation (TMS) induces somatosensory input due to excitation of peripheral trigeminal nerve branches as well as auditory input caused by the loud click produced during stimulus discharge. In a recent study, we have demonstrated that these peripheral sources of cortical activation make a strong contribution to the transcranial evoked potentials (TEPs) that can be recorded with EEG at the scalp. Here we investigated the components of the TEP after removal of multisensory contributions. In 10 healthy young volunteers, we applied single-pulse neuronavigated TMS using two coil orientations (perpendicular and parallel to the target gyrus) and two positions (superior frontal gyrus and superior parietal lobule) and recorded the TEPs. We also recorded the evoked EEG response in a sham condition during which the TMS click was given simultaneously with bipolar electrical skin stimulation at the site corresponding to the centre of the coil during real TMS. We applied a fitting procedure to the sham and real-TMS conditions to exclude the signal related to somatosensory and auditory processing from the transcranially evoked TMS response. Some of the standard peaks commonly reported in the TMS/EEG literature were reduced or modified, suggesting that they were also caused by multisensory peripheral stimulation. For example, this was the case for the negative deflection that appears at around 100 ms after single-pulse TMS. We also identified TEP components which were not changed after accounting for somatosensory and auditory EEG response. This uncovered TEP signal was sensitive to the direction of the coil orientation, showing statistical difference in time intervals that were not present in the original TEP. This study confirms the need to exclude the somatosensory and auditory confounds in order to have a deeper insight into the TMS/EEG results. The appropriate approach is to include a sham procedure that controls for the contribution of peripheral sensory stimulation to the TEPs. Our novel procedure renders it possible to study the brain response to direct magnetic stimulation of the cortex also in time intervals affected or covered by unwanted sensory inputs.