Electroretinography measures the electrical responses of various cell types in the
retina, including the
photoreceptors (
rods and
cones), inner retinal cells (
bipolar and
amacrine cells), and the
ganglion cells.
Electrodes are usually placed on the
cornea and the skin near the
eye, although it is possible to record the ERG from skin electrodes. During a recording, the patient's eyes are exposed to standardized
stimuli and the resulting signal is displayed showing the time course of the signal's amplitude (voltage). Signals are very small, and typically are measured in microvolts or nanovolts. The ERG is composed of electrical potentials contributed by different cell types within the retina, and the stimulus conditions (flash or pattern stimulus, whether a background light is present, and the colors of the stimulus and background) can elicit stronger response from certain components.