Paradoxical Lateralization of Cortical Potentials Evoked by Stimulation of Posterior Tibial Nerve

To study the distribution of the early (first 80 ms) human cortical potentials evoked by stimulation of the posterior tibial nerve at the ankle, scalp electrodes were placed within a 12-cm radius from the vertex and were separated by approximately 3 cm. With unilateral stimulation the response at the hemisphere ipsilateral to the stimulus was consistently of substantially higher amplitude and at times opposite in polarity to the contralateral response. An explanation of this paradoxical lateralization is that the cortical generators of the evoked potentials to posterior tibial nerve stimulation are located in the mesial surface of the cortex, adjacent to the interhemispheric fissure, and therefore project transversely or parallel (not perpendicular) to the scalp surface. A similar paradoxical lateralization with similar cause has been reported concerning occipital evoked potentials in response to half-field pattern stimulation.