Conditioning intensity-dependent interaction between short-latency interhemispheric inhibition and short-latency afferent inhibition.

The relationship between sensory and transcallosal inputs into the motor cortex may be important in motor performance, but it has not been well studied, especially in humans. The aim of this study was to reveal this relationship by investigating the interaction between short-latency interhemispheric inhibition (SIHI) and short-latency afferent inhibition (SAI) in humans with transcranial magnetic stimulation. SIHI is the inhibition of the primary motor cortex (M1) elicited by contralateral M1 stimulation given ∼10 ms before, and it reflects transcallosal inhibition. SAI is the inhibition of M1 elicited by contralateral median nerve stimulation preceding M1 stimulation by ∼20 ms. In this investigation, we studied the intensity dependence of SIHI and SAI and the interaction between SIHI and SAI in various conditioning intensities. Subjects were 11 normal volunteers. The degree of effects was evaluated by comparing motor evoked potential sizes recorded from the first dorsal interosseous muscle between a certain condition and control condition. Both SIHI and SAI were potentiated by increment of the conditioning stimulus intensity and saturated at 1.4 times resting motor threshold for SIHI and 3 times sensory threshold for SAI. No significant interaction was observed when either of their intensities was subthreshold for the inhibition on its own. Only when both intensities were strong enough for their inhibition did the presence of one inhibition lessen the other one. On the basis of these findings, we conclude that interneurons mediating SIHI and SAI have mutual, direct, and inhibitory interaction in a conditioning intensity-dependent manner.