Giant somatosensory evoked potentials : scalp topography and dipole source analysis

Scalp topography and equivalent dipoles (EDs) of giant SEPs to median nerve stimulation were investigated in 10 patients: 4 with essential cortical myoclonus; 1 with progressive myoclonic epilepsy; 1 with hyperekplexia; 2 with unilateral porencephalic lesions; 2 with tactile-evoked spikes in their EEGs. Spherical spline maps of amplitudes and current source densities were computed for different SEP peaks recorded from 20 scalp electrodes. Dipolar sources were modelled by means of a spatio-temporal analysis.Five sets of early cortical components contralateral to the stimulated limb were identified in topographic maps of normal subjects: a parieto-frontal N20-P20 complex; a central positivity, P22; a parieto-frontal P27-N30 complex; a central negativity, N33; a central-parietal positivity, P45. The first four components were optimally explained by two EDs: a tangential one, modelling both the N20-P20 and the P27-N30 complexes, was located close to the posterior bank of the central sulcus; a radially oriented dipole, explaining the P22 and N33 peaks, was situated in peri-rolandic cortex (crown of pre/postcentral gyrus). In all patients but those with rolandic spikes, these early cortical SEP components were identified: depending on the patient, either tangential or radial components alone or both components were enhanced. EDs were more easily distinguishable as compared with normal SEPs and the radial one was located anterior to the central sulcus, in the crown of precentral gyrus. The two patients with tapping-evoked spikes showed an extreme enhancement of component N60, with a temporal-frontal field explained by a tangential dipole, never observed in normal subjects. Giant SEPs mainly originate from the same cortical sources as normal SEPs.