Physical interactions between induced electrical fields can have substantial effects on neuronal excitation during simultaneous TMS of two brain areas

Objective: To analyse the interactions between simultaneous or nearly simultaneous focal transcranial magnetic stimulation (TMS) of the motor cortex hand area (M1(hand)) of both hemispheres. Methods: In 7 healthy subjects, motor evoked potential (MEP) amplitude and cortical silent period (CSP) duration were elicited in the right hand by bihemispheric focal TMS of M1(hand) (8-shaped coils, monophasic current waveform, stimulus intensity 120% above motor threshold, TMS of right M1(hand) preceding TMS of left M1(hand) by 0-1000 mu s), or by unilateral TMS of left M1(hand) alone. A dipole probe was used to measure the physical interactions between the two stimulating coils. Results: Bihemispheric TMS markedly decreased MEP and CSP at intervals of 0 and 50 mu s compared to unilateral TMS, whereas both measures increased at the interval of 150 mu s. The dipole probe experiments showed that the physical interactions between the electrical fields of the two coils entirely explained the MEP and CSP findings, but only under the assumption that excitation of M1(hand) is not point-focal but extends over several centimetres. Conclusions: First, simultaneous focal TMS of distant brain sites may result in marked 'distortion' of brain excitation through physical interaction between the induced electrical fields. Second, these findings support the notion that excitation of human M1(hand) is relatively non-focal, even if a 'focal' stimulating coil and low stimulus intensity are used. Significance: Potentially marked physical interaction between induced electrical fields must be taken into account when testing or disrupting distant brain sites with simultaneous focal TMS. (c) 2005 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.