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

The Mind's Eye | 249the flesh and bone as having a more ideal shape. Dark blush on the sidesof the nose makes them look as if they are at a shallower angle to thelight, which makes the nose appear narrower. White powder on theupper Up works the other way: the lip seems to intercept the light sourcehead-on as if it were Fuller, bestowing that desirable pouty look.The sh ape-from-s hading analyzer has to make other assumptions, too.Surfaces in the world are made of thousands of materials, and lightbounces off their slanted surfaces in very different ways. A matte surfacelike chalk or dull paper follows a simple law, and the brain's shading analyzeroften seems to assume that the world is matte. Surfaces withglosses, patinas, fuzz, pits, and prickles do other, stranger things withlight, and they can fool the eye.A famous example is the full moon. It looks like a flat disk, but ofcourse it is a sphere. We have no trouble seeing other spheres from theirshading, like ping-pong balls, and any good artist can sketch a spherewith charcoal. The problem with the moon is that it is pockmarked withcraters of all sizes, most too small to be discerned from the earth, andthey combine into a surface that behaves differently from the matteideal that our shading analyzer takes for granted. The center of the fullmoon faces the viewer flat-on, so it should be brightest, but it has littlenooks and crannies whose walls are seen edge-on from the viewer'searthly vantage point, making the center of the moon look darker. Thesurfaces near the perimeter of the moon graze the line of sight andshould look darker, but they present their canyon walls face-on andreflect back lots of light, making the perimeter look lighter. Over thewhole moon, the angle of its surface and the angles of the facets of itscraters cancel out. All portions reflect back the same amount of light,and the eye sees it as a disk.if we had to depend on any one of these analyzers, we would be eatingbark and stepping off cliffs. Each analyzer makes assumptions, but thoseassumptions are often contradicted by other analyzers. Angle, shape,material, lighting—they're all scrambled together, but somehow weunscramble them and see one shape, with one color, at one angle, in onekind of light. What's the trick?Adelson, together with the psychologist Alex Pentland, used his zigzag