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 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).
A general term for color vision abnormalities. Heritable dyschromatopsias include red (protan), green (deutan), and blue (tritan) color vision deficits. These deficits may be partial (-omaly) or complete (-opia). Red and green color vision defects are X-linked while loss of the blue receptor (tritanopia) is inherited in an autosomal recessive fashion.