Interhemispheric transfer of paired associative stimulation-induced plasticity in the human motor cortex

To evaluate the interhemispheric interaction of paired associative stimulation (PAS)-induced plasticity, we performed a transcranial magnetic stimulation study on nine healthy volunteers after PAS, motor evoked potentials were significantly enhanced in both the nonstimulated and stimulated primary motor cortex (M1). Short-interval intracortical inhibition and intracortical facilitation were not changed in the nonstimulated M1, but interhemispheric inhibition was significantly reduced after PAS. Motor evoked potential enhancement in the nonstimulated M1 was significantly correlated to that in the stimulated M1 and tended to correlate with the degree of pre-PAS interhemispheric inhibition. These results show that PAS-induced plasticity in the dominant M1 can transfer to contralateral M1 depending on the amount of plastic change induced in the stimulated M1 and, also probably, on the amount of transcallosal connection.