retinal disparity

Meaning: Your eyes view the world from slightly different positions, so each retina receives a slightly different image of the same object. The brain compares the two images and reads the amount of difference as distance. Nearby objects have a large disparity, and faraway objects have a small one. This comparison gives stereoscopic (three-dimensional) depth perception. It is also called binocular disparity.

On the MCAT: Retinal disparity is classed as a binocular cue, along with convergence, the inward turning of the eyes, which is greater for close objects. It is contrasted with monocular cues such as relative size, interposition (overlap), linear perspective, texture gradient, and motion parallax, which work with one eye. A passage may describe a person who has lost vision in one eye, and you are asked which depth cues remain. Binocular disparity is lost, but monocular cues still work. Another item may ask why a 3-D movie works. Two slightly offset images are shown, one to each eye, which mimics disparity. Questions also test the trend: closer object, greater disparity.

Common confusion: Students mix up retinal disparity with convergence. Disparity is a difference between the two retinal images. Convergence is the muscular signal from the eyes turning inward. Both are binocular, but only disparity depends on comparing images. Another error is calling it a monocular cue. It needs both eyes.