What colour can YOU see?
Dizzying optical illusion creates a different shade for every viewer - but no-one knows why

Illusion named Benham's top after Essex-born toymaker Charles Benham
Some people see green, a few yellow or red and others no colour at all
Exactly why illusion looks different to different people is still a mystery
Benham's top is being researched for use as a tool to identify eye disease

Stare at this black and white wheel long enough and you may see a colour appear out of nowhere.

But incredibly, different people see different colours - and some may not see anything at all.

Many people say they see green, others see yellow and a few see red, but exactly why this is the case has baffled scientists for decades.

The wheel is called Benham's top and it creates an illusion of colour when black and white patterns rapidly change, as reported by Casey Chan on Sploid.

It is named after the Essex-born toymaker Charles Benham, who, in 1895, created a top painted with the pattern shown.

When the wheel is spun, arcs of subtle colour, called Fechner colours, are visible at different areas on the surface.

The higher the speed, the more obvious the colour effect and reversing the wheel can also change the shade.

One theory about why people see different colours is that the receptors in the human eye respond at different rates to red, green, and blue.

The retina of the eye is composed of two types of receptors sensitive to light: cones and rods. Cones are important for colour vision and for seeing in bright light.

There are three types of cones, each of which is most sensitive to a particular wavelength of light.

‘It is possible that the colours seen in spinning Benham disks are the result of changes that occur in the retina and other parts of the visual system,’ according to Washington University.

‘For example, the spinning disks may activate neighbouring areas of the retina differently. In other words, the black and white areas of the disk stimulate different parts of the retina.’

This response may cause a type of changes within the nervous system that creates colours, scientists believe.

Another theory is that different cones stay activated for different amounts of time.

This means, when the wheel is spun, the white areas activate all three types of cones, and then the black deactivates them.

The sequence may cause an imbalance because the different types of cones take different times to respond, resulting in the brain producing different colours.

While these theories do not explain the illusion completely, Benham's top is currently being researched for use as a diagnostic tool for diseases of the eye.