Hemispheric asymmetry and somatotopy of afferent inhibition in healthy humans

There are hemispheric asymmetries of the primary motor cortices (M1) in humans thought to be related to handedness. Whether this is also true for interactions between the somatosensory and the motor cortex is unclear. We thus investigated sensory-motor interactions over both hemispheres in right hander using conditioning electrical pulses of 3 times of the sensory threshold applied to the right or left index finger and transcranial magnetic stimulation (TMS) test pulses given over the corresponding M1 (afferent inhibition; AI). Typically, conditioning electrical pulses suppress motor evoked potentials (MEP) following TMS pulses at different interstimulus intervals (ISI). In experiment 1, we compared the amount of AI between the two hemispheres. In addition, in experiment 2 and 3 we explored the somatotopy of AI in the left hemisphere.

Methods: Experiment 1: Conditioning electrical pulses were given to the right and left index finger, respectively. MEPs were recorded from first dorsal interosseus (FDI) and abductor digiti minimi (ADM) muscles bilaterally. Experiment 2 and 3: Electrical pulses were applied to the right index finger only. MEP were recorded from right FDI, ADM, extensor digitorum communis (EDC) and extensor digiti minimi (EDM) muscles in experiment 2 and from right FDI, ADM and biceps brachii (BB) muscles in experiment 3.

Results: AI in FDI and ADM was significantly larger in the right hand compared to the left (experiment 1). The amount of AI did not differ between right FDI, ADM, EDC and EDM (experiment 2). However, AI in the right BB was significantly smaller than in FDI and ADM (experiment 3).

Conclusion: These data suggest that hemispheric asymmetry of AI is another facet of the motor dominance of the left hemisphere in right handed humans. They also imply that AI is not specific for adjacent muscles but rather body site specific.