By Tim Mickleburgh
Among the senses, vision is the one that rules them all. More than 80% of our sensory input comes through our eyes, and we celebrate the truth of this in terms like “eye-witness”, “see it with my own eyes” and “the eyes are the windows of the soul”.
Most of this visual input is processed unconsciously by the brain, because we simply don’t have time to look at everything. This unconscious, or more correctly pre-conscious, activity is amazingly efficient, but once in a while an optical illusion can set off a conscious alarm, a whoa-did-you-see-that moment.
Like the bricks illusion, shown here:
The horizontal lines are perfectly parallel, but it is impossible to see them that way. Our visual system is very good at making quick scans, and detecting sudden changes in contrast that might signify danger, but in this case it is unable to fully resolve the input information, resulting in the shifting illusion that we see. The illusion persists, not quite so strongly, when we use a colored pattern:
It is only when we lower the contrast that things settle down, and we see the simple pattern of squares in parallel bands:
My favorite demonstration of this kind of visual ambiguity is Schrodinger’s Cat, the final picture, because it actually demonstrates the conundrum that it describes:
Wikipedia will fill in the background for this artful combination of graphic design, typography and visual ambiguity. And if you think this stuff is only for design artists, you should see what your brain gets up to while you are asleep. Yep, your visual cortex is pulling another all-nighter, spinning out the improvisations we call dreams.
[ I borrowed my title from Margaret Livingstone, whose book Vision and Art: The Biology of Seeing is the very best layman’s guide to the science of seeing ]