glossary

The center of the fovea is the foveola ? about 0.2 mm in diameter ? where only cone photoreceptors are present. The central fovea consists of very compact cones, thinner and more rod-like in appearance than cones elsewhere. Starting at the outskirts of the fovea, however, rods gradually appear, and the absolute density of cone receptors progressively decreases.

A technique of recording electrical impulses from the contraction of a muscle. It is used to confirm the correct position of the needle when botulinum toxin is used to correct strabismus.

A test that measures the electrical activity of the retina when exposed to flashes of light of varying intensity. Abnormalities in the electroretinogram typically occur in conditions that affect the photoreceptor cells (e.g. retinitis pigmentosa).