In Ancient Greece architects knew that their supporting stone columns needed to bulge in the middle if they were to appear straight to the observer, but why does your brain do this?
What is happening when you look at an optical illusion?
Light reflected from the illusion s focused by the lens in the eye to form an image on the retina. The retina consists of several layers of cells at the back of the eye, including the photoreceptors. The photoreceptors detect light and convert it into an electrical voltage that the nervous system interprets. Brighter points generate larger signals that darker ones. These messages are communicated to ganglion cells which are ‘excited’ by light in a small area of the retina and at the same time ‘inhibited’ by a ring shaped area surrounding this. This lateral inhibition confuses the senses when detecting edges and lines.
#1 Orbison’s Illusion - This is a square and a number of diagonal lines within a rectangle. They appear distorted.
#2 The Ehrenstein Illusion - The square in this illusion superimposed on a number of concentric circles appears like a four pointed star.
#3 Koffka Ring - The two semicircles are the same shade of grey.
#4 The McCollough Effect - Stare at the green and purple squares for up to 5 minutes, switching between them every 15 seconds or so, then look at the mixed square with white lines on it.
For more details look out for articles by the experimental psychologist Jamie Horder.