EEG responses to combined somatosensory and transcranial magnetic stimulation

Objectives: To investigate a possible interaction between sensory processing and transcranial magnetic stimulation (TMS), an experimental set-up permitting multichannel EEG measurements was used. Methods: A somatosensory stimulus was delivered to the right wrist, while single-pulse TMS was applied to the contralateral somatosensory cortex, either concurrent with the somatosensory stimulus or 10 ms after it. A control condition served to mimic the sound of TMS without actually resulting in brain stimulation. Results: An enhancement of the P25 component of the somatosensory-evoked potential (SEP) was consistently observed for TMS concurrent with somatosensory stimulus. The effect was topographically specific to the EEG recording sites below the TMS coil, i.e. above the somatosensory cortex contralateral to the stimulated peripheral nerve. Conclusions: The results can be interpreted (1) as an indication of local interaction between the somatosensory-evoked cortical activity and TMS-evoked activity or (2) as support of a relationship between the background EEG and the evoked potential (EP), this relationship being ‘disrupted’ by TMS.