TOPOGRAPHIC ANALYSES OF SOMATOSENSORY-EVOKED POTENTIALS FOLLOWING STIMULATION OF TIBIAL, SURAL AND LATERAL FEMORAL CUTANEOUS NERVES

Using topographic maps, we studied the scalp field distribution of som atosensory evoked potentials (SEPs) in response to the stimulation of the tibial (TN), sural (SN) and lateral femoral cutaneous (LFCN) nerve s in 24 normal volunteers. Cortical peaks, i.e., N35, P40, N50 and P60 were generally dominant in the contralateral hemisphere for the LFCN- SEP, whereas all peaks except N35 had dominance in the ipsilateral hem isphere for TN- and SN-SEPs. The findings imply that ipsilateral or co ntralateral peak dominance for the lower extremity SEP is determined b y where the cortical leg representation occurs. As a result, mesial he misphere representation results in peak dominance projected to the hem isphere ipsilateral to stimulation. Representations at the superior li p of the interhemispheric fissure or lateral convexity lead to midline or contralateral peak dominance. These findings also suggest that the paradoxically lateralized P40 is not the result of a positive field d ipole shadow generated by the primary negative wave in the mesial hemi sphere, but is the primary positive wave, analogous to P26 of the medi an nerve SEP. Accordingly, contralaterally dominant N35 is likely equi valent to the first cortical potential of N20 in the median nerve SEP. The difference in vector directions of potential fields between N35 a nd P40 may account for the opposite hemispheric dominance for these pe aks in TN- and SN-SEPs.