Frontal distribution of early cortical somatosensory evoked potentials to median nerve stimulation

The topography of early frontal SEPs (P20 and N26) to left median nerve stimulation was studied in 30 normal subjects and 3 patients with the left frontal bone defect. The amplitudes of P20 and N26 were maximum at the frontal electrode (F4) contralateral to the stimulation and markedly decreased at frontal electrodes ipsilateral to the site of stimulation. There was, however, no latency difference of P20 and N26 between ipsilateral and contralateral frontal electrodes. These results suggest that the origin of the ipsilateral and contralateral P20 and N26 is the same. The wide distribution of P20 and N26 over both frontal areas could be explained by assuming a smearing effect from generators actually located in the rolandic fissure and motor cortex.