Cutaneomotor integration in human hand motor areas: somatotopic effect and interaction of afferents

The aim of this study was to elucidate sensorimotor integration in human hand motor areas, its time course, somatotopy and the interaction of sensory fields arising from two different fingers. We studied the influence of different intensities of electrical digital stimulation of two different fingers on motor-evoked potentials elicited in hand muscles by transcranial magnetic stimulation (TMS). Single conditioning electrical stimuli were applied to the right second (D2) and fifth fingers (D5) individually and also to both fingers (D2+D5) simultaneously in six normal volunteers. Magnetic tests, adjusted to produce a response in the abductor digiti minimi muscle of the right hand, were delivered using a circular and a focal coil. The digital stimuli were delivered to the finger at the sensory threshold (ST), at 3 and 5 times the ST, and over the pain threshold at interstimulus intervals (ISIs) ranging from 10 to 100 ms. In order to define the anatomical level of the sensorimotor interactions, the effect of the digital stimulation on TMS was compared to the effect on transcranial electrical stimulation. When the peripheral stimulation was delivered at the ST a small inhibitory effect was found only when stimulating both fingers. At 3 times the ST we detected a topographic distribution of motor-evoked potential inhibition, which partially disappeared at higher intensity (5 times the ST); two types of convergence effects took place at different ISIs. When conditioning stimuli were painful, somatotopy and convergence were lost. Sensorimotor integration shows somatotopy and interaction of afferents at different sites. The intensity of the conditioning stimulus plays an important role in topography and sensory convergence. The importance of these mechanisms in physiology and physiopathology is discussed.