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

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The <strong>Mind</strong>'s Eye 233shapes materialize, looking as if <strong>the</strong>y are shrink-wrapped in leaves orflowers.Why did natural selection equip us with true cyclopean vision—anability to see shapes in stereo that nei<strong>the</strong>r eye can see in mono—ra<strong>the</strong>rthan with a simpler stereo system that would match up <strong>the</strong> 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 <strong>the</strong> forest floor below. Building a stereo computerinto <strong>the</strong>se two-eyed creatures must have been irresistible to naturalselection, but it could have worked only if <strong>the</strong> disparities were calculatedover thousands of bits of visual texture. Single objects that allow unambiguousmatches were just too few and far between.Julesz points out ano<strong>the</strong>r advantage of cyclopean vision. Camouflagewas discovered by animals long before it was discovered by armies. Theearliest primates were similar to today's prosimians, <strong>the</strong> lemurs and tarsiersof Madagascar, who snatch insects off trees. Many insects hidefrom predators by freezing, which defeats <strong>the</strong> hunter's motion detectors,and by camouflage, which defeats its contour detectors. Cyclopeanvision is an effective countermeasure, revealing <strong>the</strong> prey just as aerialreconnaissance reveals tanks and planes. Advances in weaponry spawnarms races in nature no less than in war. Some insects have outwitted<strong>the</strong>ir predators' stereo vision by flattening <strong>the</strong>ir bodies and lying flushagainst <strong>the</strong> background, or by turning into living sculptures of leaves andtwigs, a kind of three-dimensional camouflage.now does <strong>the</strong> cyclopean eye work? The correspondence problem—matching up <strong>the</strong> marks in one eye with <strong>the</strong>ir counterparts in <strong>the</strong> o<strong>the</strong>r—is a fearsome chicken-and-egg riddle. You can't measure <strong>the</strong> 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, <strong>the</strong>re are thousandsof candidates for <strong>the</strong> matchmaker. If you knew how far away <strong>the</strong> surfacewas, you would know where to look on <strong>the</strong> left retina to find <strong>the</strong> mate ofa mark on <strong>the</strong> right. But if you knew that, <strong>the</strong>re would be no need to do<strong>the</strong> stereo computation; you would already have <strong>the</strong> answer. <strong>How</strong> does<strong>the</strong> mind do it?

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