Indistinguishables

Can you see a difference between the colours of the figures?

Compare the adjacent figures in terms of their shades of colour. Then slide the display surface over them to check their true colour.

In two of the pairs, the colour of the figures is the same, even though they appear to be different. In one pair, the colours are different even though they seem to be the same.

This is called the colour contrast phenomenon, in which the shade of colour we observe changes in response to the colour surrounding it. When surrounded by a darker colour, the grey lightens, and when surrounded by a lighter colour, it darkens. A red background gives the figure a greenish tint, green produces a reddish tint and a blue background, a yellowish tint. The cause of the phenomenon is not fully known. On the cellular level it involves a stimulation of the sensory cells on our retinas that prevents the activity of the adjacent cells. However, the illusion is also the result of other higher level perceptual processes.

Normally, changes in colours result from changes in the quality or amount of light entering our eyes. With the three types of cone cells on our retinas we perceive short wavelengths of light as the colour blue, medium wavelengths as green and long wavelengths as red. Other colours are created through the simultaneous activity of these three cell types. Colours are, thus, a product of our sight and our brain. The different materials in our surroundings simply reflect or absorb light in different ways.

How we see things, depends up many things. If a born blind person gets his ability to see later, he usually can not interpret his vision, because there are no seeing memories in his brain.