Spatio-temporal characteristics of SEP to tibial nerve stimulation

Source derivation techniques have recently been applied to analyse the spatial distribution of brain potentials. Somatosensory evoked potentials (SEPs) to tibial nerve stimulation recorded at 8 sites were analysed by using 2 source derivation techniques: Laplacian and criterion, presented by Kossev et al. (1988). First SEP recorded as a function of time were averaged and thus their basic components P39, N49, P58 were enhanced. In most of the subjects these components had maximal amplitudes at Ez rather than Cz. These sites were taken as nodes and the source derivation techniques were applied for all SEP components. Spatial derivatives (potential gradients) between adjacent leads were also computed. The current density at the nodes for different time points of SEP was obtained using Laplacian technique. For the chosen electrodes places and subjects it was found that the maximal source density did not correspond always to the SEP peak amplitudes. According to the criterion used it might be supposed that the generators of P39, N49 and P58 were located close to the scalp surface. Because of the significant individual variety of the results obtained more research is needed for choosing the appropriate scalp sites and for the contribution of the base line when measuring the SEP amplitudes.