Steven Pinker -- How the Mind Works - Hampshire High Italian ...

The Mind's Eye 233shapes materialize, looking as if they are shrink-wrapped in leaves orflowers.Why did natural selection equip us with true cyclopean vision—anability to see shapes in stereo that neither eye can see in mono—ratherthan with a simpler stereo system that would match up the lemons andcherries that are seeable by each eye? Tyler points out that our ancestorsreally did live in Ames' leaf room. Primates evolved in trees and had tonegotiate a network of branches masked by a veil of foliage. The price offailure was a long drop to the forest floor below. Building a stereo computerinto these two-eyed creatures must have been irresistible to naturalselection, but it could have worked only if the disparities were calculatedover thousands of bits of visual texture. Single objects that allow unambiguousmatches were just too few and far between.Julesz points out another advantage of cyclopean vision. Camouflagewas discovered by animals long before it was discovered by armies. Theearliest primates were similar to today's prosimians, the lemurs and tarsiersof Madagascar, who snatch insects off trees. Many insects hidefrom predators by freezing, which defeats the hunter's motion detectors,and by camouflage, which defeats its contour detectors. Cyclopeanvision is an effective countermeasure, revealing the prey just as aerialreconnaissance reveals tanks and planes. Advances in weaponry spawnarms races in nature no less than in war. Some insects have outwittedtheir predators' stereo vision by flattening their bodies and lying flushagainst the background, or by turning into living sculptures of leaves andtwigs, a kind of three-dimensional camouflage.now does the cyclopean eye work? The correspondence problem—matching up the marks in one eye with their counterparts in the other—is a fearsome chicken-and-egg riddle. You can't measure the stereodisparity of a pair of.marks until you have picked a pair of marks to measure.But in a leaf room or a random-dot stereogram, there are thousandsof candidates for the matchmaker. If you knew how far away the surfacewas, you would know where to look on the left retina to find the mate ofa mark on the right. But if you knew that, there would be no need to dothe stereo computation; you would already have the answer. How doesthe mind do it?