In John D. Pettigrew's lab, there is less to human experience than meets the eyes. Over the past several years, dozens of test subjects have stared through goggles and pressed keys while the neuroscientist squirted ice water into the volunteers' ear canals, fired strong magnetic pulses into their heads or told jokes that made them giggle. These unusual experiments, which were reported in part last March in Current Biology and presented more fully in November at a neuroscience conference in New Orleans, confirmed that people often cannot see what is plainly before their eyes. More important, the studies suggest that many optical illusions may work not by deceiving our visual system, as long suspected, but rather by making visible a natural contention between the two hemispheres of the human brain. If Pettigrew's theory is correct, then the reason an optical illusion such as the Necker cube outline, which seems to turn inside out periodically, works is that, in some deep biological sense, you are of two minds on the question of what to see.

Reversible figures, such as the Necker cube and drawings of a white vase between black faces, have been curiosities for centuries. And it was in 1838 that Charles Wheatstone first reported an even more peculiar phenomenon called binocular rivalry. When people look through a stereoscope that presents irreconcilable patterns, such as horizontal stripes before one eye and vertical bars before the other, most don't perceive a blend of the two. Instead they report seeing the left pattern, then the right, alternating every few seconds. "Every couple of seconds something goes ¿click' in the brain," Pettigrew says. "But where is the switch?" The answer is still unknown.

This article was originally published with the title Side Splitting.

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