Reliability of visual perception : susceptibility to visual illusion

Most people have an orderly working visual system but why do we perceive certain pictures different from reality? And do we all perceive these so called visual illusions in the same way? In this report the Kanizsa figures, the Müller-Lyer illusion, the Shepard illusion and the Thatcher illusion will be discussed. In the Kanizsa figures illusory contours are perceived and the brightness of the central area is perceived as being different from the surroundings while they in fact have the exact same brightness. The susceptibility to this illusion differs between adults and infants. In the Müller-Lyer illusion the perceived length of a line is influenced by attaching arrows to the ends of a line. Sex-related differences affect the susceptibility to this illusion. Although the table tops of the Shepard figures have the exact same size, one table top is perceived as being longer and smaller than the other. In these Shepard figures individuals with the neurodevelopmental disorder autism have a different susceptibility to the illusion compared to individuals without autism. Finally in a Thatcherized face the eyes and the mouth region are turned upside-down. People can immediately observe these changes but when the face is inverted this strong perceptual effect is lost. In these Thatcherized faces not only individuals with autism but also individuals with Williams Syndrome have a different susceptibility to the illusion than typically developing individuals. Some of these visual illusions can be used to manipulate human behaviour and may help to create safer situations.