Hemispheric Asymmetry and Somatotopy of Short-Latency Cutaneomotor Inhibition in Healthy Humans

A conditioning electrical stimulus to a digital nerve can inhibit the motor response in adjacent hand muscles elicited by a supra-motor threshold transcranial magnetic stimulation (TMS) test stimulus to the contralateral primary motor cortex (M1) hand area when given 25 to 50 ms before the TMS pulse. Such sensorimotor inhibition is referred to as short-latency afferent inhibition (SAI). There is good evidence that SAI is mediated through sensory-motor interaction at a cortical level. In right-handed healthy volunteers, we studied inter-hemispheric differences and within-limb somatotopy of SAI in two experiments. In experiment 1, conditioning electrical pulses were applied to the right or left index finger and motor evoked potentials (MEPs) were recorded from relaxed first dorsal interosseus (FDI) and abductor digiti minimi (ADM) muscles ipsilateral to the conditioning stimulus. Conditioning electrical stimulation of the right index finger was more effective in producing SAI in ipsilateral intrinsic hand muscles than stimulation of the left index finger. In experiment 2, electrical stimulation was applied to the right index finger only and MEPs were recorded from ipsilateral FDI, ADM, extensor digitorum communis (EDC) and extensor digiti minimi (EDM) muscles (experiment 2a) or from right FDI, ADM and biceps brachii (BB) muscles (experiment 2b). Experiment 2a revealed that the amount of SAI did not differ between right FDI, ADM, EDC and EDM muscles. In contrast, in experiment 2b SAI was also present in right BB muscle but significantly smaller as compared to FDI and ADM muscles. These data demonstrate inter-hemispheric differences in cortical processing of cutaneous input from the hand with stronger SAI in the dominant left hemisphere. In addition, these results show that apart from intrinsic hand muscles adjacent to electrical digital stimulation SAI occurs to the same extent in distant hand and forearm muscles and is also present in proximal arm muscles, albeit less pronounced. This can be taken as evidence that SAI is not a focal and somatotopically specific phenomenon.