Questions: Depth Perception: Monocular and Binocular Cues

Question 1

A person loses all vision in one eye. Which depth cue would they permanently lose as a result?

• A. Motion parallax — requires coordinated movement of both eyes
• B. Stereopsis — depends on retinal disparity between two eyes viewing from different angles
• C. Linear perspective — requires binocular comparison of converging lines
• D. Occlusion — the brain needs both eyes to determine which object is in front

Answer: (B)

Stereopsis (retinal disparity) is a binocular cue: it works by comparing the slightly different images on each retina, which comes from the 6.5 cm separation between the two eyes. With one eye, there is no disparity to measure. Motion parallax, by contrast, is a monocular cue — it works by tracking how objects move across the visual field as your head moves, which requires only one eye. Occlusion and linear perspective are also monocular and remain available.

Question 2

A painting of a long hallway creates a vivid sense of depth despite being a flat canvas. Which of the following best explains this?

• A. The visual system suppresses binocular cues and relies entirely on the depicted depth
• B. Binocular cues like convergence are enhanced by framed images
• C. Monocular cues such as linear perspective and relative size are richly present even though binocular cues reveal a flat surface — the brain partially resolves this conflict
• D. Retinal disparity is zero for a flat surface, so the brain defaults to treating it as three-dimensional

Answer: (C)

Paintings and photographs carry abundant monocular depth cues — converging lines, occlusion, relative size, texture gradient, shading — which the brain interprets as depth signals. Meanwhile, binocular cues correctly signal a flat surface. The brain partially resolves this conflict, producing the experience of depth in an image we simultaneously know is flat. Option A is wrong: binocular cues are not suppressed, they are simply outweighed by the richness of monocular cues in familiar artistic contexts.

Question 3

Retinal disparity is greatest for nearby objects because they project to very different locations on the two retinas.

• T. True
• F. False

Answer: True

This is the correct geometric relationship. Because the two eyes are about 6.5 cm apart, a close object creates a large angular difference between the two retinal images — high disparity. A distant object is so far away that the two views are nearly identical — low disparity. Neurons in V1 and V2 are tuned to specific disparity values, allowing the brain to read relative depth from the disparity map with extraordinary precision at close distances.

Question 4

Motion parallax is a binocular depth cue because it requires coordinated eye movement.

• T. True
• F. False

Answer: False

Motion parallax is a monocular cue — it works with one eye. It arises when you move your head: nearby objects sweep rapidly across your visual field while distant objects appear nearly stationary. This differential motion provides depth information without any comparison between two eyes. Photographers and people with monocular vision both exploit motion parallax by moving slightly to judge distances. Only stereopsis and convergence require two eyes.

Question 5

Why does the visual system use multiple overlapping depth cues rather than relying on a single definitive depth sensor?

Model answer: No single cue is reliable in all conditions. Each cue has limits — stereopsis works best for near distances, convergence only within arm's reach, motion parallax requires movement. By integrating multiple cues probabilistically and weighting more reliable ones more heavily, the visual system produces robust depth estimates across a wide range of environments. When cues conflict (as in depth illusions), the weighting process reveals which cues dominate in a given context.

The probabilistic integration model explains both why depth perception works so well under normal conditions (multiple consistent cues reinforce each other) and why illusions occur (conflicting cues force the brain to weight some over others, sometimes incorrectly). A system with only one depth sensor would be fragile — blinded by any single failure condition. Multiple redundant cues are a form of perceptual robustness.