Anatomy and physiology of visual evoked potentials and electroretinograms

ERGs are electrical potentials originating in the retina and recorded directly from the corneal surface of the eyes. Flash ERGs are elicited by high intensity flashes either on scotopic or photopic conditions. The origin of the A and B waves of ERG are a combination of photoreceptor potentials, K+ mediated current flow and DC potentials within Müller cells, whereas the C waves are altered K+ fluxes induced by light in the pigment epithelia. Oscillatory potentials are probably related to feedback circuits of the inner layer of the retina. Electroretinograms evoked by pattern are mostly originated in ganglion cells. The use of various visual stimuli allow the selective activation of different retinal structures. Flashes selectively activate retina luminance and color detectors, whereas small pattern stimuli preferentially activate contrast and edge detectors. Visual evoked potentials recorded from the scalp probably originate from multiple cortical visual areas. P-100 response of the visual evoked potential to pattern stimulation is a cortically originated wave either produced exclusively by area 17 or 18 or by a multiplicity of cortical neuronal pools. Visual evoked potentials have important application for the diagnosis of optic nerve disease, assessment in conjunction with pattern ERG of the prognosis of recovery of visual function, and the evaluation of visual physiology and physiopathology.