Analysis of interhemispheric asymmetries of somatosensory evoked magnetic fields to right and left median nerve stimulation

This paper represents the first neuromagnetic systematic investigation of the asymmetries between the sources activated in the right and left hemispheres after electric median nerve stimulation. We focused our attention on the location and strength of the equivalent sources activated in the primary somatosensory cortex contralateral to the stimulated nerve in the 50 msec post-stimulus epoch. The spatial coordinates of the equivalent sources did not differ statistically significantly in the two hemispheres. Minor individual asymmetries are shown to be related to the interhemispheric differences in the position of the central sulcus as revealed by MRI investigation. The equivalent sources were significantly stronger in the left hemisphere. When comparing the location of the generators across individuals, we show that interhemispheric differences fluctuate less than absolute values. A quantitative evaluation of these findings is also given. Based on these results, a normative data set has been established, to be used as a baseline in following up changes of interhemispheric asymmetries due to hemispheric lesions and subsequent cortical reorganization.