Variability of initial cortical sensory evoked potentials to posterior tibial nerve stimulation

Scalp topographies and distributions of initial cortical positive-negative components of somatosensory evoked potentials (SEPs) to posterior tibial nerve stimulation were studied in 12 normal controls in order to investigate the generator sources of the cortical SEPs. There are three variations in topographies and distributions of early cortical SEPs in normal subjects. In 5 of 12 cases initial cortical positive-negative components were distributed over the centro-parietal areas ipsilateral to the stimulation site or midline parasagittal areas. The origins of these components are speculated to be vertical dipolar generators located at the contralateral interhemispheric fissure. In 5 of 12 cases P37/N36, and N45/P43 showed a phase reversal between the left and right hemispheres and may be generated from horizontal dipoles located at the contralateral interhemispheric fissure. In 2 of 12 cases N36 and P43 were distributed predominantly over the contralateral hemisphere. Oblique dipolar generators located at the contralateral interhemispheric fissure may be oriented prominently to the contralateral hemisphere. Normal variations of initial cortical positive-negative components of SEPs to posterior tibial nerve stimulation should be considered in their clinical applications.