EEG responses to combined somatosensory and transcranial magnetic stimulation

Objectives: To investigate a possible interaction between sensory processin g and transcranial magnetic stimulation (TMS), an experimental set-up permi tting multichannel EEG measurements was used. Methods: A somatosensory stimulus was delivered to the right wrist, while s ingle-pulse TMS was applied to the contralateral somatosensory cortex, eith er concurrent with the somatosensory stimulus or 10 ms after it. A control condition served to mimic the sound of TMS without actually resulting in br ain stimulation. Results: An enhancement of the P25 component of the somatosensory-evoked po tential (SEP) was consistently observed for TMS concurrent with somatosenso ry stimulus. The effect was topographically specific to the EEG recordings sires 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 i nteraction between the somatosensory-evoked cortical activity and TMS-evoke d activity or (2) as support of a relationship between the background EEG a nd the evoked potential (EP), this relationship being 'disrupted' by TMS. ( C) 2001 Elsevier Science Ireland Ltd. All rights reserved.