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

The Mind's Eye \ 215benign illusion. Smears of ink or flickering phosphor dots can make uslaugh, cry, even become sexually aroused. Humans have made picturesfor at least thirty thousand years, and contrary to some social-sciencefolklore, the ability to see them as depictions is universal. The psychologistPaul Ekman created a furor in anthropology by showing that isolatedNew Guinean highlanders could recognize the facial expressions in photographsof Berkeley students. (Emotions, like everything else, werethought to be culturally relative.) Lost in the brouhaha was a more basicdiscovery: that the New Guineans were seeing things in the photographsat all rather than treating them as blotchy gray paper.The picture exploits projection, the optical law that makes perceptionsuch a hard problem. Vision begins when a photon (unit of light energy)is reflected off a surface and zips along a line through the pupil to stimulateone of the photoreceptors (rods and cones) lining the curved innersurface of the eyeball. The receptor passes a neural signal up to thebrain, and the brain's first task is to figure out where in the world thatphoton came from. Unfortunately, the ray defining the photon's pathextends out to infinity, and all the brain knows is that the originatingpatch lies somewhere along the ray. For all the brain knows, it could be afoot away, a mile away, or many light-years away; information about thethird dimension, distance from the eye, has been lost in the process ofprojection. The ambiguity is multiplied combinatorially by the millionother receptors in the retina, each fundamentally confused about howfar away its stimulating patch lies. Any retinal image, then, could havebeen produced by an infinite number of arrangements of three-dimensionalsurfaces in the world (see the diagram on p. 9).Of course, we don't perceive infinite possibilities; we home in on one,generally close to the correct one. And here is an opening for a crafter ofillusions. Arrange some matter so that it projects the same retinal imageas an object the brain is biased to recognize, and the brain should haveno way of telling the difference. A simple example is the Victorian noveltyin which a peephole in a door revealed a sumptuously furnishedroom, but when the door was opened the room was empty. The sumptuousroom was in a dollhouse nailed to the door over the peephole.The painter-turned-psychologist Adelbert Ames, Jr., made a careerout of carpentering even stranger illusory rooms. In one, rods and slabswere suspended from wires higgledy-piggledy throughout the room. Butwhen the room was seen from outside through a peephole in a wall, therods and slabs lined up into a projection of a kitchen chair. In another