Conditioning Intensity Dependent Interaction between Short-latency Interhemispheric Inhibition and Short-latency Afferent Inhibition Authors 4 Revised the Manuscript Critically for Important Intellectual Content and Approved The

32 Relation between sensory and transcallosal inputs into the motor cortex may be 33 important in motor performance, but it has not been well studied, especially in humans. 34 The aim of this paper is to reveal this relation by investigating the interaction between 35 short-latency interhemispheric inhibition (SIHI) and short-latency afferent inhibition 36 (SAI) in humans using transcranial magnetic stimulation. SIHI is the inhibition of the 37 primary motor cortex (M1) elicited by the contralateral M1 stimulation given at around 38 10 ms before, and it reflects the transcallosal inhibition. SAI is the inhibition of M1 39 elicited by the contralateral median nerve stimulation preceding M1 stimulation by 40 around 20 ms. In this investigation, we studied the intensity dependency of SIHI and 41 SAI, and the interaction between SIHI and SAI in various conditioning intensities. 42 Subjects were eleven normal volunteers. The degree of effects was evaluated by 43 comparing motor evoked potential sizes recorded from the first dorsal interosseous 44 muscle between a certain condition and control condition. Both the SIHI and SAI were 45 potentiated by increment of the conditioning stimulus intensity, and saturated at 1.4 46 times resting motor threshold for SIHI and 3 times sensory threshold for SAI. No 47 significant interaction was observed when either of their intensities was subthreshold 48 for the inhibition on its own. Only when both intensities were strong enough for their 49 inhibition, the presence of one inhibition lessened the other one. Based on these, we 50 conclude that interneurons mediating SIHI and SAI have mutual, direct and inhibitory 51