After providing all the funding for The Brain from Top to Bottom for over 10 years, the CIHR Institute of Neurosciences, Mental Health and Addiction informed us that because of budget cuts, they were going to be forced to stop sponsoring us as of March 31st, 2013.

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Bruno Dubuc, Patrick Robert, Denis Paquet, and Al Daigen

Monday, 4 February 2013
Adelson’s Checkerboard

Optical illusions are always a humbling experience for people who think that they see the world “the way it really is”. Often, when such people are confronted with optical illusions induced by context, the context has to be removed and then restored several times before these people can be convinced that, for example, two lines that appear to be different lengths are actually the same.

The world that we see is often ambiguous, and our visual system tries to give it a meaning on the basis of certain recurrent clues. Scientists today are familiar with some of these clues and can combine them to produce some truly astounding optical illusions. One famous example is Adelson’s checkerboard, in which one square is perceived as black and another as white, even though both are actually the same shade of grey!

This fact is very real, but people initially refuse, with every shred of their logical, rational beings, to accept it. Many ingenious attempts have therefore been made to show that the two shades of grey are truly the same and that no trickery is involved. The strongest recent contender is a video in which a giant Adelson’s checkerboard has actually been built and is lit by actual spotlights. A woman comes on screen, picks up a tile from one square on the checkerboard and moves it to another. As she does so, we see the tile apparently change from “white” to “black”, even though we have to admit that it is still the same tile!

Of course, with today’s digital effects, anything is possible, but since we already know the great power of each of the physical phenomena that contribute to this illusion, no such explanation is necessary. Our visual system simply has not evolved to be a good exposure meter, because that would not afford us any particular adaptive advantage. Its function is to subdivide what we see into objects that have meaning for us, and at that task, it is remarkably effective, even if it sometimes makes errors of “overinterpretation”, as in the case of optical illusions!

i_lien The squares marked A and B are the same shade of gray

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