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 (M1hand) 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 M1hand (8-shaped coils, monophasic current waveform, stimulus intensity 120% above motor threshold, TMS of right M1hand preceding TMS of left M1hand by 0-1000 micros), or by unilateral TMS of left M1hand 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 micros compared to unilateral TMS, whereas both measures increased at the interval of 150 micros. 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 M1hand 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 M1hand is relatively non-focal, even if a 'focal' stimulating coil and low stimulus intensity are used.


CONCLUSIONS
Potentially marked physical interaction between induced electrical fields must be taken into account when testing or disrupting distant brain sites with simultaneous focal TMS.
Cincotta
M
M
Unità Operativa di Neurologia, Azienda Sanitaria di Firenze, Ospedale S. Maria Nuova, Piazza S. Maria Nuova, 1, 50122 Firenze, Italy. cincotta@unifi.it
Borgheresi
A
A
Jung
P
P
Balestrieri
F
F
Giovannelli
F
F
Zaccara
G
G
Ziemann
U
U
D000328:Adult; D016477:Artifacts; D002540:Cerebral Cortex / Q000033:anatomy & histology / Q000502:physiology / Q000528:radiation effects*; D004558:Electric Stimulation / Q000295:instrumentation / Q000379:methods; D004574:Electromagnetic Fields; D019054:Evoked Potentials, Motor / Q000502:physiology / Q000528:radiation effects*; D006801:Humans; D008280:Magnetics / Q000295:instrumentation*; D008875:Middle Aged; D009433:Neural Inhibition / Q000502:physiology / Q000528:radiation effects; D009474:Neurons / Q000502:physiology / Q000528:radiation effects*; D013997:Time Factors