Scalp, basal epidural and intravascular far-field recordings after median nerve stimulation: evidence for a separate N18a potential

Far-field somatosensory evoked potentials (SSEPs) after median nerve stimul ation were recorded from scalp- (Fz), epidural( ED) and intravascular elect rodes (basilar artery [Bas]) to study the nature of the controversial N18a component of the widespread N18 potential. In healthy volunteers frequently an N18a potential was recorded at Fz. Simultaneous Fz and ED recordings at the pontomesencephalic junction as well as Bas-recordings at the caudal ba silar artery showed N18a components identical in latency and shape. With in travascular recordings the shapes differed between the top of the basilar a rtery and the caudal artery recordings. These findings support the existenc e of a separate N18a potential. The generator of the N18a is likely to be l ocalized within the upper brainstem.