SEFs to Median Nerve and Finger Stimulation: Analysis of Interhemispheric Asymmetries in Healthy Subjects and Following Hemispheric Lesion

Abstract

A systematic investigation of the asymmetries between Equivalent Current Dipoles (ECDs) activated in the two hemispheres after left and right median nerve, thumb, and little finger electric stimulation has been performed on 20 healthy subjects and 10 patients with unilateral hemispheric lesion. We studied the location and strength of the ECDs activated in the primary somatosensory cortex contralateral to the stimulated side corresponding to the first cortical component of the Somatosensory Evoked magnetic Field (SEF), namely N20m. Aim of this work is twofold: i) define normal limits for the parameters of the ECD generating N20m component on normal population; ii) use these limits to look for sign of brain plasticity in patients following an hemispheric lesion. On this line a previous paper by ours was devoted to the analysis of interhemispheric asymmetries between ECDs elicited by median nerve stimulation [1]. In the present study we enlarged the sample of our previous control population and, in order to make the test more accurate, we added the stimualtion of single digits (thumb and little finger). In this way the “hand extension”, that is the dimension of the parietal region receiving the sensory input from the contralateral hand, was also calulated. In fact, we calculated the “hand extension” as the distance-expressed in millimeters — of the generators responsible for the N20m cortical responses to separate stimulation of thumb and little finger in the contralateral hand.