Photo Gallery The Mind's Eye
Optical illusions enable us to determine how the brain renders a picture of the world, based on projections cast on our retinas. We can make sense of the play of light and shadow when they combine with experience and knowledge in our brain, and we use this information to calculate our own version of reality.
Evolution dictates that the species with the advantage is not the one that can recognize two tones of brown, but the one that can determine how dark a predator's skin would be in the shadows.
Equally as important is the ability to recognize the actual form of an object in space. Take the example of these four figures: In two-dimensional form, on the screen or on paper, they might look like simple right angles.
But additional information -- like shadows, light and perspective -- show that each of these angles would be very different in three dimensions.
This example may be hard to believe: The tables are the same shape, though they're set at perpendicular angles.
The yellow and blue lines show actual dimensions. But the brain interprets each table using perspective, so the green surface looks rectangular and the red surface appears to be square.
In his new book, Sacks describes a blind spot caused by a tumor in his right eye as a "gaping nowhere." He drew a picture of it (shown), and says it has "more or less the shape of Australia. It causes me a lot of trouble when I bump into people or lamp posts, but it sometimes fills me with wonder, too."
