retinal

Meaning: Retinal is the adjective for the retina, the light-sensitive layer lining the back of the eye. Its photoreceptors transduce light into neural signals. Rods work in dim light and do not give color. Cones work in bright light, give color and fine detail, and are concentrated in the fovea. Signals pass from photoreceptors to bipolar cells to ganglion cells, whose axons form the optic nerve. Where the nerve leaves the eye there are no photoreceptors, which produces the blind spot. In perception, retinal disparity is the small difference between the images on the two retinas. The brain compares them to judge distance, and nearer objects produce greater disparity.

On the MCAT: Expect a question asking which depth cue is lost when one eye is closed. Retinal disparity is binocular, so it fails with one eye, while monocular cues such as relative size, interposition and linear perspective still work. Other items ask which photoreceptor explains a finding, such as poor color vision in dim light or loss of fine detail in peripheral vision.

Common confusion: Retinal is also a noun in biology and biochemistry, a vitamin A derivative in rhodopsin that changes from 11-cis to all-trans when it absorbs light. In a perception passage, retinal almost always means "of the retina." Do not mix retinal disparity with convergence. Convergence is also binocular, but it relies on feedback from the eye muscles as the eyes turn inward, not on a difference between the two images.