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Visual illusions and visual constancies
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Sometimes the brain gets it wrong. Visual illusions show that perception is more than just seeing, while constancies show how the brain keeps the world stable.
Three kinds of illusion
Fictions: we see something that is not there. In the Kanizsa triangle, three 'Pac-Man' shapes make us see a white triangle that has no edges drawn.
Ambiguous figures: one image can be seen in two ways, and perception flips between them. Examples: Rubin's vase (a vase or two faces), the Necker cube (which face is at the front?) and the young woman/old woman picture.
Distortions: we see something wrongly, such as the wrong length or size. In the Muller-Lyer illusion, two equal lines look different lengths because of the arrow-heads on their ends. In the Ponzo illusion, two equal lines between converging lines look different sizes.
Ambiguous figures: one image can be seen in two ways, and perception flips between them. Examples: Rubin's vase (a vase or two faces), the Necker cube (which face is at the front?) and the young woman/old woman picture.
Distortions: we see something wrongly, such as the wrong length or size. In the Muller-Lyer illusion, two equal lines look different lengths because of the arrow-heads on their ends. In the Ponzo illusion, two equal lines between converging lines look different sizes.
Visual constancies
Constancy means seeing an object as unchanged even when the image on the eye changes.
Size constancy: a person walking away does not seem to shrink, even though their image on the retina gets smaller.
Shape constancy: a door still looks rectangular when it swings open, even though the image becomes a trapezium.
Colour constancy: a red apple still looks red in dim or yellow light.
Size constancy: a person walking away does not seem to shrink, even though their image on the retina gets smaller.
Shape constancy: a door still looks rectangular when it swings open, even though the image becomes a trapezium.
Colour constancy: a red apple still looks red in dim or yellow light.
Why they matter
Illusions suggest the brain uses prior knowledge and makes guesses, which supports Gregory's constructivist theory.
Constancies help us live in a stable world. But they can also cause illusions: size constancy may explain the Ponzo illusion, because the brain treats the upper line as further away, so 'corrects' it to look bigger.
Constancies help us live in a stable world. But they can also cause illusions: size constancy may explain the Ponzo illusion, because the brain treats the upper line as further away, so 'corrects' it to look bigger.
Two lines are exactly the same length, but one has arrow-heads pointing in and one has them pointing out, and they look different. Name the illusion and its type.
- This is the Muller-Lyer illusion.
- We perceive the lengths wrongly, so it is a distortion.
Answer: The Muller-Lyer illusion, a distortion.
Fictions show things that are not there (Kanizsa triangle); ambiguous figures can be seen two ways (Rubin's vase, Necker cube); distortions make us misjudge size or length (Muller-Lyer, Ponzo). Constancies keep size, shape and colour stable even when the image on the eye changes.
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