The origin of the N20 component of the somatosensory evoked potential in man

Somatosensory evoked potentials (SEPs) were recorded from a 63 year old male who had sustained severe right sided cortical damage including infarction of the primary sensory hand area. When SEPs were recorded following right median nerve stimulation, both cervical and cortical waveforms were of normal configuration. When SEPs were recorded following left median nerve stimulation, the cervical potential was intact but the cortical waveform was absent and replaced with a positive-negative complex which was probably a far field reflection of thalamic activity. These findings suggest that the first negative component of the cortical SEP (N20) is not generated in the diencephalon but represents the initial response of the somatosensory cortex.