Neural generators of N18 and P14 far-field somatosensory evoked potentials studied in patients with lesion of thalamus or thalamo-cortical radiations.

Somatosensory evoked potentials (SEPs) to electrical stimulation of the right or left median nerve were studied in 4 patients with hemianesthesia and a severe thalamic or suprathalamic vascular lesion on one side. The SEPs were recorded with a non-cephalic reference. The normal side of each patient served as his or her own control. The lesion consistently abolished the parietal N20-P27-P45 and the prerolandic P22-N30 SEP components. It did not significantly affect the P9-P11-P14 positive far fields, nor the widespread bilateral N18 SEP component. This allowed N18 features to be studied without interference from cortical components. It is proposed that N18 reflects several deeply located generators in brain stem and/or thalamus whereas N20 represents the earliest cortical response of the contralateral post-central receiving areas.