Click play and keeping staring at the dot.
After 15 seconds the image will change to black and white, but you will see it in full colour!

How does the illusion work?

Our visual system is complex and includes multiple steps. Our colour vision is first determined by our cone cells. There are three types of cone cell in the normal eye, responding mainly to red, green and blue light.

Key point: Because white light is a mixture of all the other colours of light, all three types of cone will respond to white light.

Trichromatic (tri = three, chromatic = coloured) information from the cone cells is then collected by the ganglion cells, which compare red vs green, as well as yellow vs blue. The ganglion cells send colour-opponent information to the brain, where it is processed further.

To illustrate, let’s use a simple example of a red image.

Step 1) First look

When you look at a red image, the red cones send strong signals to the ganglion cells. Because the ganglion cells are comparing red vs green, red will “win”.

Step 2) Continued staring

As you continue to stare steadily at the image, the red cones will become less sensitive to the light and will send weaker signals to the ganglion cells. It’s as if they have been working too hard and have tired themselves out!

Step 3) Black-and-white image

When the red image is replaced by the black-and-white image, the red light is replaced by white light. Remember – all cones respond to white light! The green cones will send strong signals to the ganglion cells but, because the red cones are still tired, they will not be able to send strong signals. Because the ganglion cells are comparing red vs green, green will now “win”!

This illusion is called a negative afterimage, and demonstrates that the colour we “see” does not exist in the external world, but rather is created through a complex interaction between the cells in our eyes and our brains.