The scalp topography of the human visually evoked subcortical potential.

Stimulus and analysis parameters have been adjusted to provide optimum conditions for producing and recording the early components of flash visual evoked potentials. A visual evoked subcortical potential (VESP) of mean latency P23-N28-P34 has been recorded in 86% of subjects. The triphasic wave was maximal at an electrode position slightly posterior to the Rolandic/sylvian fissure and topographically separate from the lid electroretinogram and the visual evoked cortical potential. Monocular stimulation shows bilateral reduction of the amplitude of the VESP, indicating that the wave is independent of the retina and optic nerve and must be arising from a postchiasmal site.