Visual Depth Cues and the Six Named Illusions
Your eyes send a flat signal, but your brain builds a world with depth. Learn the monocular and binocular depth cues, and the six named illusions that reveal how perception can be fooled.
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How you see depth
Each eye sends the brain a flat image, yet you see a world with depth. The brain works out that depth using clues called depth cues. Sometimes the brain is fooled and we see an illusion, through things like ambiguity or a misinterpreted depth cue. This module covers the depth cues and the six named illusions you must know.
Depth cue words
Four terms run through this topic. Learn them first.
One eye or two?
Depth cues come in two families.
Match the depth cue
- Occlusion
- Relative size
- Retinal disparity
- Convergence
- A nearer object blocks part of one further away
- Objects that look smaller seem further away
- Each eye sees a slightly different view
- The eyes turn inwards to focus on near objects
One eye or two?
Which of these depth cues needs BOTH eyes?
- Retinal disparity
- Occlusion
- Relative size
- Linear perspective
The six named illusions
Learn all six: the Ponzo (equal lines across converging rails look unequal), the Muller-Lyer (equal lines with inward or outward fins look unequal), the Rubin vase (seen as a vase or as two faces), the Ames room (a distorted room that changes apparent size), the Kanizsa triangle (a triangle you see that is not really drawn), and the Necker cube (a wireframe cube that seems to flip).
Match the illusion
- The Ponzo
- The Muller-Lyer
- The Rubin vase
- The Necker cube
- Equal lines on converging rails look unequal
- Equal lines with inward or outward fins look unequal
- An image seen as a vase or as two faces
- A wireframe cube that appears to flip
Monocular cues
Which TWO of these are MONOCULAR depth cues? Choose all that apply.
- Occlusion
- Linear perspective
- Retinal disparity
- Convergence
From light to seeing
Put the stages of seeing into a sensible order.
- Light enters the eye
- The eye sends a sensation
- The brain reads the depth cues
- A perception is formed
Sensation, perception and depth
The raw signal from the eyes is _____. How the brain interprets it is _____. A depth cue needing one eye is a _____ cue. A depth cue needing both eyes is a _____ cue. When a nearer object blocks a further one, the cue is _____.
Spot the binocular cues
Tap the TWO binocular depth cues.
- Retinal disparity
- Convergence
- Occlusion
- Relative size
Vase or faces?
Which illusion can be seen as either a vase or two faces?
- The Rubin vase
- The Ponzo
- The Necker cube
- The Ames room
Name the illusion
For each description, choose the illusion it matches.
- Two identical lines have fins pointing inwards or outwards, making them look different lengths. Which illusion?
- A specially built room makes two people look very different in size. Which illusion?
- You clearly see the edges of a triangle that has not actually been drawn. Which illusion?
Explain depth cues
Explain the difference between monocular and binocular depth cues.
- Define monocular and binocular depth cues
- Give an example of each
- Explain why binocular cues need both eyes