P301 Contribution of somatosensory and auditory processing to the TMS evoked cortical response: A sham TMS-EEG study

Abstract

Objectives Transcranial Magnetic Stimulation (TMS) is capable to non-invasively stimulate the human cortex. Electroencephalography (EEG) can record the cortical response evoked by TMS (TEPs), which are a summation of the brain responses to the TMS-induced electric field in the cortex, and to the multisensory peripheral stimulation derived from the TMS coil discharge. This multisensory stimulation is composed by a somatosensory component relative to the activation of trigeminal nerve fibers and by an auditory component due to the loud click of the coil. Here, we aim to describe the contribution of somatosensory and auditory processing to the TEPs by comparing a somatosensory-auditory sham condition with real TMS. Methods In 18 healthy individuals, TMS was delivered with a figure-of-eight coil over two target sites (parietal and frontal) using two coil orientations. On the same target sites, a sham condition was applied by delivering a cutaneous electrical stimulation concurrently with the TMS click. The EEG was recorded with a 61-channel TMS-compatible system. Results The potentials evoked by real TMS and by somatosensory-auditory sham TMS were highly similar in terms of shape and spatial distribution, particularly at later latencies, 70–200 ms, after stimulus administration. Conclusion The present results challenge a straight-forward interpretation of TEPs as an index of cortical connectivity of the focally stimulated brain region, because of the strong contribution of TMS-induced somatosensory and auditory stimulation. This work has been funded by the Novo Nordisk Foundation Interdisciplinary Synergy Program 2014 [“Biophysically adjusted state-informed cortex stimulation (BASICS); Grant No. NNF14OC0011413 ].