Hemispheric asymmetry of the auditory evoked N100m response in relation to the crossing point between the central sulcus and Sylvian fissure

The positions of the bilateral N100m sources of the auditory evoked magnetic fields (AEFs) were measured in relation to the central sulcus (CS) using an MRI-linked whole head magnetoencephalography system in 20 right-handed normal male subjects. The location of the N20m source of the median nerve-stimulated somatosensory evoked magnetic fields (SEFs), in the left hemisphere was 3.9±5.4 mm (mean±SD) posterior to that in the right hemisphere (P<0.005). The crossing point (CP) between the CS and Sylvian fissure in the left hemisphere was 4.3±4.8 mm posterior to that in the right hemisphere (P<0.001). The N100m sources were posterior to the CP in both hemispheres. The left hemispheric N100m source was 9.4±6.4 mm posterior to that on the right (P<0.0001) in absolute position. The relative distance between CP and the N100m source was 22.7±8.5 mm in the left hemisphere and 17.7±5.3 mm in the right hemisphere (P<0.01). Comparison of positions of the AEF sources and the CS as defined by the SEF demonstrated functional asymmetry of the human temporal lobe and possible source extension of the AEF-N100m beyond the Heschl gyrus over the planum temporale.