Explanations for Visual Illusions (AQA GCSE Psychology): Revision Note

Ambiguity

• A visual illusion is a perception that does not match physical reality – what we see conflicts with what is actually there, because the brain has misinterpreted the visual information

• Illusions happen because the perceptual system applies its usual rules, i.e. depth cues, constancies, past experience to an image where those rules don't fit, so it draws the wrong conclusion

• Ambiguity is one type of visual illusion: it occurs when a single image can be interpreted in two different ways, and the image contains equal evidence for both, so the brain cannot settle on one interpretation

• Because there is no single 'correct' answer, perception flips between the two interpretations – the viewer can see one or the other, but not both at the same time

• Often only one interpretation is noticed at first; the other appears when the viewer shifts their focus, e.g. from the central shape to the background

• Examples of ambiguity include Rubin's vase and the Necker cube

Can you make out both an old woman looking down and a young woman turning her face away in this ambiguous image?

Misinterpreted depth cues

• Some illusions occur because the brain applies depth cues to a flat image as if it were a real 3-D scene, and then misapplies  size constancy 

• In the Ponzo illusion, two converging lines act as a depth cue (linear perspective) – the brain reads the narrowing lines as parallel lines receding into the distance, like railway tracks

• Two identical horizontal lines are placed across them; the upper line sits where the tracks are 'further away'

• The brain reasons: the upper line is further away but produces the same-sized retinal image as the lower one, so it must actually be bigger – and it is perceived as longer

• The depth cue is genuine and the rule is normally correct; the illusion happens because the cue has been misinterpreted in a 2-D image where there is no real depth

Misinterpreted Depth Cues Diagram

A variation of the Ponzo illusion to show misinterpreted depth cues. The red figures are the same size but the nearer figure appears smaller than the figure further away.

Fiction

• Fiction is a type of visual illusion in which we perceive a figure that is not actually present in the image, as the brain creates something that has not been drawn

• The brain does this because the arrangement of the shapes is most easily explained by assuming another object is sitting on top of them, blocking part of them from view; the brain 'fills in' that object

• This produces illusory contours, which are edges we perceive even though there is no line or change in colour on the page

• An example is the Kanizsa triangle: three black circles with wedges cut out and three V shapes are arranged so that the brain perceives a white triangle lying over them, with clear edges and often appearing brighter than the background, yet no triangle has been drawn

• The effect is almost impossible to switch off, even when you know the figure isn't there

Fiction Example Diagram

A visual illusion using ‘fiction’ - can you see a cube when in fact no cube has been drawn?

Size constancy

• Size constancy is the tendency to perceive an object as staying the same size even though the image it projects onto the retina changes as its distance from us changes

• As a person walks away from you, the image of them on your retina gets smaller – but you don't perceive them as shrinking. As they walk towards you the image grows, but you don't perceive them as getting bigger

• The brain achieves this by combining the size of the retinal image with depth cues: if the image is small but the cues say "far away", the brain scales the perceived size up to compensate

• Size constancy is a normal, useful feature of perception, as it is what allows us to judge real sizes in a 3-D world

• However, illusions such as the Ponzo and Müller-Lyer, misleading depth cues make the brain apply size constancy where it isn't needed, so identical objects appear to be different sizes (see misinterpreted depth cues above)

• The Ames room works the same way: the room's odd shape removes the normal depth cues, so the brain assumes both people are the same distance away and perceives one as a giant and one as tiny

Size Constancy Diagram

The leaning tower of Pisa is being kept upright by these tourists…or is it?