Mind bender

Discuss this video, and watch other thought-provoking films on the OU YouTube channel.

The explanation

Our brains sometime seem a bit to clever for their own good. According to Edward H Adelson (the American academic who provided this illusion):

The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible".

This quote is taken from Edward H Adelson's research. You can read a fuller explanation and see some more illusions by visiting the Perceptual Science Group at MIT.

What could you do next?

• Can you think of any other illusions like this – post your clips on the 'Mind bender' You Tube page
• Check out the psychology courses at the Open University
• Learn more about Adelson's research and more illusions like this