Interhemispheric inhibition induced by transcranial magnetic stimulation over primary sensory cortex

The present study investigated whether the long-interval interhemispheric inhibition (lihi) is induced by the transcranial magnetic stimulation over the primary sensory area (s1-tms) without activation of the conditioning side of the primary motor area (m1) contributing to the contralateral motor evoked potential (mep),whether the s1-tmsinduced lihi is dependent on the status of the s1 modulated by the tactile input,and whether the pathways mediating the lihi are different from those mediating the m1-tms-induced lihi. in order to give the tms over the s1 without eliciting the mep,the intensity of the s1-tms was adjusted to be the sub-motor-threshold level and the trials with the mep response elicited by the s1-tms were discarded online. the lihi was induced by the s1-tms given 40 ms before the test tms in the participants with the attenuation of the tactile perception of the digit stimulation (tpds) induced by the s1-tms,indicating that the lihi is induced by the s1-tms without activation of the conditioning side of the m1 contributing to the contralateral mep in the participants in which the pathways mediating the tpds is sensitive to the s1-tms. the s1-tmsinduced lihi was positively correlated with the attenuation of the tpds induced by the s1-tms,indicating that the s1-tms-induced lihi is dependent on the effect of the s1-tms on the pathways mediating the tpds at the s1. in another experiment,the effect of the digit stimulation given before the conditioning tms on the s1- or m1- tms-induced lihi was examined. the digit stimulation produces tactile input to the s1 causing change in the status of the s1. the s1-tms-induced lihi was enhanced when the s1-tms was given in the period in which the tactile afferent volley produced by the digit stimulation just arrived at the s1,while the lihi induced by above-motorthreshold tms over the contralateral m1 was not enhanced by the tactile input. thus,the s1-tms-induced lihi is dependent on the status of the s1 modulated by the tactile input,and the pathways mediating the sub-motor-threshold s1-tms-induced lihi are not the same as the pathways mediating the above-motor-threshold m1-tms-induced lihi. © 2016 iwata,jono,mizusawa,kinoshita and hiraoka.