binocular cue

Meaning: Binocular cues are depth-perception cues that depend on input from both eyes. Because the eyes sit a few centimeters apart, each retina receives a slightly different image. The brain (visual cortex) compares them to judge distance. Two are tested:

1. Retinal (binocular) disparity: the difference between the two retinal images. Nearby objects produce a larger disparity than distant ones, so greater disparity signals a closer object. 2. Convergence: the eyes rotate inward to fixate on a near object. The brain uses feedback from the eye muscles, and more strain and more inward turning signal a closer object.

Equation: None applies.

On the MCAT: Passages may describe a person with vision in one eye only, or an experiment with one eye covered, and ask which depth cues remain. Binocular cues are lost, but monocular cues such as relative size, interposition (overlap), linear perspective, texture gradient, motion parallax and shading remain. Stereoscopic 3-D displays and glasses work by giving each eye a slightly different image, which creates artificial disparity. Questions may also ask you to predict how disparity or convergence changes as an object moves closer.

Common confusion: Students often treat convergence and disparity as the same thing. Convergence is a muscle-based cue about eye position. Disparity is an image-based cue about differences between the two retinal images. Both need two eyes. Do not confuse "binocular" with "bilateral" or with the two visual fields. Binocular means both eyes are used, and monocular cues can still be used with two eyes open.