PARADOXES—in which the same information may lead to two contradictory conclusions—give us pleasure and torment at the same time. They are a source of endless fascination and frustration, whether they involve philosophy (consider Russell's paradox, “This statement is false”), science—or perception. The Nobel Prize winner Peter Medawar once said that such puzzles have the same effect on a scientist or philosopher as the smell of burning rubber on an engineer: they create an irresistible urge to find the cause. As neuroscientists who study perception, we feel compelled to study the nature of visual paradoxes.
Let us take the simplest case. If different sources of information are not consistent with one another, what happens? Typically the brain will heed the one that is statistically more reliable and simply ignore the other source. For example, if you view the inside of a hollow mask from a distance, you will see the face as normal—that is, convex—even though your stereovision correctly signals that the mask is actually a hollow, concave face. In this case, your brain's cumulative experience with convex faces overrides and vetoes perception of the unusual occurrence of a hollow face.