Figure 1.
One way to support this theory is the Muller-Lyer illusion. This illusion is a pair of lines, as shown in Figure 2.
Figure 2.
According to Gregory (a psychologist), the arrows with ingoing fins provides linear perspective cues which suggests it could be the out side corner of a building; the ingoing fins are seen as walls receding away from us so that the shaft looks closer to us. The arrow with the outgoing fins, is e reverse of that idea. Perspective cues suggests it could be the inside corner of a room and that the outgoing fins are seen as walls approaching us, in which case the shaft looks to be further away from us. However, the retinal images produced by the shafts are equal and according to size constancy, if the two lines produce equal-sized images, one of which is perceived to be further away than the other, then the line which is further away must actually be longer.
However not everyone can see the illusion. The Zulu Tribes of Africa live in a very small insular environment. They are born, grow up, work and die within a tiny radius, so they have little or no linear perspective, the world they live in is small and round. This means their interpretations of the Muller-Lyer illusion in Figure 2. is not the same as your or my interpretation their visual past experience has not allowed their depth perception to develop in the same way as our has. Visual illusions are designed to have depth cues. As a result due to having lived in an environment that’s rich in depth cues, we perceive these depth cues, apply size constancy and perceive the illusion.
When we are in a familiar situation we expect to see or hear something based on our past experience of that situation. Subconsciously we form a hypothesis of what we will see or hear and we select the stimuli to fit that hypothesis, for example, whilst writing this essay, I read what I expect to se based on what I thought I had written, rather than what has actually been written. We perceive what we expect to perceive!
Visual illusions are designed to have depth cues. Unlike Zulu tribes of Africa we live in an environment, which is rich in depth cues, and this has allowed our linear perspective to grow and develop. We see the arrows, perceive the depth cues, apply size constancy and as a result, see the illusion. The suggestion is that Gregory’s Top-Down Theory supports the Muller-Lyer illusion in Figure 2.
It is now thought that the brain misapplies size constancy to see the illusion therefore does not need depth cues. One way to illustrate this is to adapt the Muller-Lyer illusion by taking away the fins and replacing them with circles as shown in Figure 3.
Figure 3.
We have manipulated the illusion by taking away the depth cues, but our brain uses past knowledge and experience and still perceives the illusion. Line A still looks shorter than B even with the replacement of the fins with circles. In conclusion, research using visual illusions has led to knowledge of how depth cues cause 3D perception and how our brains apply perceptual constancies. Our past experience and expectations affect the way we see ambiguous drawings!
Gregory’s (1972) Top-Down Theory – The top-down process involves the brain ‘sending down’ stored information to the sensory system as it receives information from the stimulus, enabling a plausible hypothesis about the scene to be made without the need to analyse every feature of the stimulus.
2 Muller-Lyer illusion (1972) – an illusion to show perception, involving two lines the same size with different depth cues.