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

238 | HOW THE MIND WORKSproperties have since been found in the brains of monkeys and cats. Thedifferent kinds of sterenblindness appear to be genetically determined,suggesting that each pool of neurons is installed by a different combinationof genes.Stereo vision is not present at birth, and it can be permanently damagedin children or young animals if one of the eyes is temporarilydeprived of input by a cataract or a patch. So far, this sounds like thetiresome lesson that stereo vision, like everything else, is a mixture ofnature and nurture. But a better way of thinking about it is that the brainhas to be assembled, and the assembly requires project scheduling overan extended timetable. The timetable does not care about when theorganism is extruded from the womb; the installation sequence can carryon after birth. The process also requires, at critical junctures, the intakeof information that the genes cannot predict.Stereo vision appears abruptly in infants. When newborns arebrought into a lab at regular intervals, for week after week they are unimpressedby stereograms, and then suddenly they are captivated. Close tothat epochal week, usually around three or four months of age, thebabies converge their eyes properly for the first time (for example, theysmoothly track a toy brought up to their nose), and they find rivalrousdisplays—a different pattern in each eye—annoying, whereas beforethey had found them interesting.It is not that babies "learn to see in stereo," whatever that wouldmean. The psychologist Richard Held has a simpler explanation. Wheninfants are born, every neuron in the receiving layer of the visual cortexadds up the inputs from corresponding locations in the two eyes ratherthan keeping them separate. The brain cant tell which eye a given bit ofpattern came from, and simply melts one eye's view on top of the other'sin a 2-D overlay. Without information about which eye a squiggle camefrom, stereo vision, convergence, and rivalry are logically impossible,Around the three-month mark each neuron settles on a favorite eye torespond to. The neurons lying one connection downstream can nowknow when a mark falls on one spot in one eye and on the same spot, ora slightly shifted-over spot, in the other eye—the grist for stereo vision.In cats and monkeys, whose brains have been studied directly, this isindeed what happens. As soon as the animal's cortex can tell the eyesapart, the animal sees stereograms in depth. That suggests that when theinputs are first tagged "left eye" or "right eye," the circuitry for stereocomputation one layer downstream is already installed and functioning.