depth cues

Meaning: The retina is a two-dimensional surface, so the brain infers depth from cues in the image. Two groups are tested.

Binocular cues need both eyes. Retinal (binocular) disparity: each eye sees a slightly different image, and the closer an object is, the larger the difference. Convergence: the eyes turn inward more for near objects, and the brain uses the muscle tension as a distance signal.

Monocular cues work with one eye. Examples: relative size, interposition (overlap, where the object that blocks another looks closer), linear perspective (parallel lines appear to meet in the distance), texture gradient, relative height, shadow and shading, motion parallax (near objects seem to move faster than far ones when you move), and aerial perspective (distant objects look hazier).

On the MCAT: A passage may describe a person with vision in one eye only, or a picture on a flat page. Predict which cues remain. With one eye, retinal disparity and convergence are lost, but monocular cues still work. Flat paintings and photographs rely on monocular cues alone. Questions may also ask you to classify a described cue as binocular or monocular.

Common confusion: Students assume depth perception needs two eyes. Binocular cues add precision, mainly for near objects, but monocular cues still give good depth information. Also, motion parallax is monocular even though it involves movement. Depth cues are part of bottom-up processing, but the brain's interpretation of them also draws on experience.