Electroretinography

Anatomically, the retina consists of primary photoreceptors occupying the outer layers, which relay visual information to second-order neurons (bipolar neurons) in the middle retinal layers. These synapse with ganglion cells (in the inner retinal layers), whose axons travel in the optic nerve. The photoreceptors are of two kinds, rods and cones. Rods primarily function in dim light; cones mediate color vision, and operate in bright light. Retinal function can be studied using flash electroretinography (ERG) and pattern electroretinography (P-ERG). Flash ERG is a means to evaluate photoreceptor and middle retinal layer function. It also permits distinguishing rod from cone abnormalities. The ganglion cell layer plays no role in the flash ERG response. P-ERG, however, permits ganglion cell evaluation. Function of the retinal pigment epithelium cells is difficult to study directly by ERG; instead, a variation of the saccade test can be used.