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1 Division of Neurology, University of Toronto, Toronto, Ontario, Canada; Krembil Neuroscience Centre, Toronto Western Research Institute, Toronto, Ontario, Canada
2 Krembil Neuroscience Centre, Toronto Western Research Institute, Toronto, Ontario, Canada
* To whom correspondence should be addressed. E-mail: robert.chen{at}uhn.on.ca.
We studied the effects of 1Hz repetitive transcranial magnetic stimulation (rTMS) on the excitability of interhemispheric connections in 13 right-handed healthy volunteers. TMS was performed using figure-of-eight coils and surface EMG was recorded from both first dorsal interosseous (FDI) muscles. A paired pulse method with a conditioning stimulus (CS) to the motor cortex (M1) followed by a test stimulus to the opposite M1 was used to study the interhemispheric inhibition (ppIHI). Both CS and TS were adjusted to produce motor-evoked potentials of about 1mV in the contralateral FDI muscles. After baseline measurement of right-to-left IHI (pre-RIHI) and left-to-right IHI (pre-LIHI), rTMS was applied over left M1 at 1Hz with 900 stimuli at 115% of resting motor threshold. Following rTMS, ppIHI was studied using both the pre-rTMS CS (post-RIHI and post-LIHI) and an adjusted post-rTMS CS set to produce 1 mV MEPs (post-RIHIadj and post-LIHIadj). The TS was set to produce 1 mV MEPs. There was a significant reduction in post-LIHI (p=0.0049) and post-LIHIadj (p=0.0169) compared to pre-LIHI at both interstimulus intervals of 10 and 40 ms. Post-RIHI was significantly reduced compared to pre-RIHI (p=0.0015) but pre-RIHI and post-RIHIadj were not significantly different. We conclude that 1 Hz rTMS reduces IHI in both directions, but is predominantly from the stimulated to the unstimulated hemisphere. Low frequency rTMS may be used to modulate the excitability of IHI circuits. Treatment protocols using low frequency rTMS to reduce cortical excitability in neurological and psychiatric conditions need to take into account their effects on IHI.
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