Examples of Visual Illusions (AQA GCSE Psychology): Revision Note

The Ponzo illusion, the Muller-Lyer illusion, Rubin’s vase & the Necker cube

The Ponzo illusion – misinterpreted depth cue

• Two identical horizontal lines are drawn across a pair of converging lines

• The converging lines act as a depth cue (linear perspective): the brain reads them as parallel lines receding into the distance, like railway tracks

• The upper line sits where the 'tracks' are narrowest, so the brain assumes it is further away

• The brain then applies size constancy: if the upper line is further away but produces the same-sized image on the retina, it must really be bigger – so it is perceived as longer

• The depth cue is real and the rule is normally correct; the illusion occurs because the brain has applied a 3D rule to a flat image

The Ponzo illusion. The blue rectangles are the same size, but the top one appears larger because the converging lines make it look further away.

The Müller-Lyer illusion – misinterpreted depth cue

• Two identical vertical lines are drawn, one with outward-pointing fins (>–<) and one with inward-pointing fins (<–>)

• Gregory explained this using our experience of a built environment of straight edges and corners:

  • The line with outward fins resembles the inside corner of a room (the far edge, e.g. where two walls meet), while the line with inward fins resembles the outside corner of a building (the near edge, pointing towards us)

• The brain therefore assumes the outward-fin line is further away

• Size constancy is then misapplied: the 'further away' line produces the same retinal image as the 'nearer' line, so the brain scales it up and it appears longer

• As with the Ponzo, the fins are a depth cue that has been misinterpreted in a 2D image

The Müller-Lyer illusion. The red lines are the same length, but the line with outward-pointing fins appears longer.

Rubin's vase – ambiguity

• The image can be perceived as either a vase (the central shape) or two faces in profile looking at each other (the outer shapes)

• The image contains equal evidence for both, so the brain cannot decide which part is the figure (the object) and which is the ground (the background)

• Perception flips between the two interpretations; the viewer can see the vase or the faces, but never both at the same time

• Nothing about the image changes – only the brain's interpretation of it

Rubin’s vase. Can you see a black vase and also two faces in profile? Try switching between them.

The Necker cube – ambiguity

• A wire-frame drawing of a cube with no shading, no hidden lines and no other depth cues to show which face is nearer

• Because the drawing is equally consistent with two different 3D cubes, the brain cannot settle on one:

  • The lower-left square can be seen as the front face (cube viewed from above) or the upper-right square can be seen as the front face (cube viewed from below)

• As with Rubin's vase, the perception flips between the two; the brain tries each interpretation in turn because the image gives it no reason to prefer one

The Necker cube. There are no depth cues to show its orientation, so either the lower-left or the upper-right square can be seen as the front face.

The Ames Room, the Kanizsa triangle

The Ames Room - misinterpreted depth cues

• The Ames Room is an illusion based on misinterpreted depth cues and size constancy

• The room is built as a trapezoid: the back wall slants away so that one back corner is much further from the viewer than the other, and the floor and ceiling slope to match

• Viewed through a peephole with one eye, these distortions are invisible

  • The binocular depth cues that would reveal the true shape are removed, and the room looks like an ordinary rectangular room

• The brain therefore assumes the two people standing in the back corners are the same distance away

• But the person in the far corner produces a much smaller retinal image than the person in the near corner; because the brain believes they are equally distant, it does not apply size constancy to compensate

• The result is that one person appears to be a giant and the other tiny – and if they swap places, they appear to change size

The Ames Room. The room is a trapezoid, but through the peephole it appears rectangular, so the brain assumes both people are the same distance away and misjudges their size.

The Kanizsa triangle - fiction

• The Kanizsa triangle is an example of a fiction illusion as we perceive a figure that has not actually been drawn

• The image consists of three black circles with wedges cut out of them and three V shapes, arranged so that the gaps line up

• The brain decides the simplest explanation for this arrangement is that a white triangle is lying on top, covering parts of the circles and lines – so it 'fills in' the triangle

• This creates illusory contours: we perceive the triangle's edges, and it often looks brighter than the background, even though there is no line or change in colour on the page

• The effect is almost impossible to switch off even when you know the triangle is not there

The Kanizsa triangle. A white triangle appears to lie over the shapes, but no triangle has been drawn – the brain creates it.