Far field correlates of thalamic activity in cortical and cervical somatosensory evoked potentials

Median nerve cortical and cervical somatosensory evoked potentials (SEPs) were recorded from a group of normal adults in order to study far field reflections of thalamic activity. In the cortical SEP, the primary response (N20) was preceded by two inflections (N16 and N18). It is shown that these two potentials bear a close temporal relationship to a positive trough on the downward slope of the principal negativity (N13) in the cervical SEP. This potential, which is often bifurcated into two subcomponents, is labeled P17 and it has previously been suggested that it originates in or near the thalamus. The coincidence between the P17 complex and N16 and N18 inflections is consistent with the hypothesis that all three responses are generated in either the thalamic relay nucleus or the thalamo-cortical radiations. Possible clinical applications of recording far field thalamic potentials are discussed.