Illusions

You need to know:

* Types of illusions
* Examples of those illusions
* How they are explained
* Which theories can they be explained by

## Types of Illusions

**Visual illusion:** a conflict between reality and what we perceive

**Fiction:** an illusion caused when a figure is perceived even though it is not present in the stimulus

**Ambiguous figure:** a stimulus with two possible interpretations, only possible to see one at each time

**Distortion illusion:** where our perception is deceived by some aspect of the stimulus, affecting shape/size of the object.

**Illusory contour:** a boundary that is perceived in a figure but is not present

**Motion after-effect**: an illusion that is caused by paying more attention to movement in one direction and perceiving movement in the opposite direction immediately afterwards

**Colour after-effect:** an illusion caused by focusing on a coloured stimulus and perceiving opposite colours immediately afterwards

## All illusions from Topic A

**NAME: HERING ILLUSION**

TYPE: DISTORTION

EXPLANATION: In the Hering illusion, the radiating lines look like a linear perspective cue. The people in the illusion opposite appear bigger towards the back of the scene. But they are actually all the same height. If the radiating lines act as linear perspective cues then we would use constancy scaling as if the scene really had depth. According to **Gregory’s theory**, The person who appears ‘furthest away’ would be scaled up so they look bigger. The person who appears closest would be scaled down, and look smaller. This is exactly what we perceive.

**NAME: NECKER’S CUBE**
ILLUSION TYPE: Ambiguous

EXPLANATION: You can see this cube in two different ways. You can see it as a see through box with a red dot at the back or a solid box with the red dot on the front. According to **Gregory’s theory**, You think that the side ‘closer’ to you is closer to you because we scale down things that are near us. This means that the vertical line on the outside of the square will be scaled down to make it look smaller.

**NAME: LEEPER’S LADY**

ILLUSION TYPE: Ambiguous

EXPLANATION: You can see both a young lady and an old lady in the picture. It uses binocular depth cues. The eye’s retina receives to stimulus, and the brain makes a hypothesis and guesses what the picture is.

**NAME: The Ponzo Illusion**

ILLUSION TYPE: Distortion

EXPLANATION: The top bar looks bigger than the bottom one. Following **Gregory’s theory**, If the railway tracks are used as cues to linear perspective, the top bar would seem further away. As it is perceived to be more distant, it is scaled up so seems bigger than the bottom bar. They are actually all within the same distance from you.

**NAME: The Muller Lyer illusion**

ILLUSION TYPE: Distortion

The depth cue of the lines that appear to meet is linear perspective. This gives us an idea of distance. According to **Gregory’s theory**,  we recognise that one line is closer, we scale it down and perceive it as smaller. They are actually all the same length.

**NAME: The Kanizsa Triangle**

ILLUSION TYPE: Fiction

When you look at this image you can see a white triangle in the centre.

According to **Gestalt theory**, when we see a figure which is incomplete, our perception makes a ‘whole’ shape, using closure. We complete the edges to make a regular or familiar shape. This is the ‘figure’ of the figure-ground relationship. The arrow heads and the space around them become the ground.

**NAME: Rubin’s Vase**

ILLUSION TYPE: Ambiguous Figure

You could see this as 2 heads or a vase.

This is an illusion because we cannot tell whether the black or the white area is the figure. Both are meaningful objects.

According to **Gestalt theory**, when we identify an object it is the ‘figure’ and we separate it from the ‘ground’. This is a situation where something could be either figure or ground, so it becomes an illusion.