Electrophysiologic testing is useful in diagnosing a variety of inherited retinal diseases, toxic drug exposure, intraocular foreign bodies, and retinal vascular occlusions. Electrophysiologic testing is performed most often in large referral centers which have expertise in obtaining and interpreting these data. Photoreceptors are nerves in the eye which are sensitive to light. Cones provide central reading vision, and are responsible for color vision. There are 6 to 7 million cones in the retina of which about 650,000 are concentrated in the foveola for central vision.
Rods provide night vision and detect motion.
There are about 120 million rods in the retina of which none are located in the foveola. Minute electrical voltages are produced by the eye. Small electrodes placed about the patient's eye record these micro-voltages as different lighting conditions are presented. The differences in voltages are analyzed to differentiate diseases which affect the rods from those which affect the cones. As an example, patients with retinitis pigmentosa initially have problems with their rods, causing them to complain of poor peripheral vision and reduced night vision.