Happy Darwin Day!

To help celebrate, Rob Jenkins (University of Glasgow) and Richard Wiseman have teamed up to create an unusual optical illusion.

How to see the illusion
The picture below shows two monkeys. Set your computer monitor to maximum brightness and then stare at the centre of the picture for about 30 seconds without moving your eyes. Next, look to a white wall and blink a few times. The monkeys should suddenly transform into a perfect picture of Darwin!



How does it work?
The illusion combines two visual effects. First, staring at the picture produces a negative afterimage, in which the black and white pattern is reversed. Second, the 'resolution' of the afterimage is lower than the actual image, and so the thin white lines vanish, making it impossible to see the monkeys.

In biology, 'adaptation' refers to the gradual process by which a species becomes better suited to its environment. For example, humans and monkeys evolved from a common primate ancestor. Psychologists use the term adaptation to refer to rapid changes in perceptual sensitivity, including the brain’s adjustment to brightness that gives rise to the negative afterimage in the illusion. So, in both senses of the word, our commemorative Darwin illusion shows adaptation in action!

Basing the illusion around an afterimage also seemed especially appropriate as Erasmus Darwin (Charles Darwin's grandfather) carried out pioneering research into this curious optical phenomenon.

How was it created?
The image was created using a photographic portrait of Darwin. The image was first blurred to smooth the edges, and then reduced to pure black and white. To obtain a photo-negative, the black and white regions were then inverted. The outline was next isolated, to help visualize constraints on the shape.

We decided to use images of monkeys because the public associate them with Darwin's work. Then began the game of 'Monkey Tetris' - the attempt to fit simian anatomy to the fixed outline in a parsimonious fashion. With increasingly committed pencil sketches, the draft began to take shape. After being inked in, the image was scanned into the computer and made into a jpeg.

The 'evolution' of the illusion is shown below.

For more unusual psychology,
visit Prof Wiseman's blog here.