We performed topographical mapping of somatosensory evoked potentials (SEPs) to the posterior tibial nerve using earlobe references both ipsilateral and contralateral to the stimulation side. The voltage of the frontal contralateral N37 and P50 components was enhanced, while the voltage of the parietal ipsilateral P37 and N50 components was reduced when the contralateral earlobe was substituted by the ipsilateral earlobe reference. Maps of the same data documented concomitant changes in negative and positive potential fields, showing an expansion of the pre-Rolandic N37 toward the centrotemporal contralateral regions, and a tendency of the parietal ipsilateral P37, N50 components to be more focally distributed at the vertex. SEPs recorded at each earlobe (Cv6 reference) provided an explanation of these results: The contralateral earlobe detected a negative potential corresponding to the N37 potential recorded over the scalp, followed by a P50 potential that attenuated the contralateral responses and enhanced the ipsilateral ones. The ipsilateral earlobe had no significant effects on scalp SEPs, since it detected only a large N33 negativity. Current source density (CSD) maps were, of course, not influenced by the ear used as reference. Our results suggest that the ipsilateral ear reference is better than the contralateral one for recording "genuine" cortical SEPs. Therefore, it can be recommended in the clinical domain for mapping studies of lower-limb cortical SEPs.
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