Effects on the right motor hand-area excitability produced by low-frequency rTMS over human contralateral homologous cortex

Repetitive transcranial magnetic stimulation (rTMS) has long lasting effects on cortical excitability at the site of stimulation, on interconnected sites at a distance and on the connections between them. In the present experiments we have used the technique of transcallosal inhibition between the motor cortices to examine all three effects in the same protocol. Ten healthy subjects received 900 rTMS stimuli at 1 Hz from a figure of eight coil over the left motor hand area. The intensity of rTMS was above the threshold for inducing short latency interhemispherical inhibition with a single stimulus (equivalent to 115-120 % resting motor threshold). Before and after the rTMS we evaluated: (1) in the left hemisphere, the amplitude of motor-evoked potentials (MEPs), and contralateral and ipsilateral cortical silent periods (CSP, ISP); (2) in the right hemisphere, MEP, CSP, ISP and short-interval intracortical inhibition and intracortical facilitation (SICI/ICF), and (3) interhemispherical inhibition (IHI) from the left-to-right hemisphere using a paired-pulse method. There were two main effects after rTMS to the left hemisphere: first, the amplitude of MEPs from the right hemisphere increased; second, there was a reduction in the IHI from the left-to-right hemisphere at interstimulus intervals of 7 and 10 ms but not at longer intervals (15-75 ms). Control experiments showed that these effects were not due to afferent inputs produced by the muscle twitches induced during the rTMS. The data are compatible with the notion that rTMS to the left hemisphere leads to reduced interhemispherical inhibition of the right hemisphere and a consequent increase in corticospinal excitability in that hemisphere.