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

The MinJ's Eye | 239In monkeys it's all over in two months: by then each neuron has afavorite eye and the baby monkeys see in depth. Compared with otherprimates, humans are "altricial": babies are born early and helpless, andcomplete their development outside the womb. Because human infantsare born earlier than monkeys in proportion to the length of their childhood,the installation of their binocular circuitry appears at a later age asmeasured from the date of birth. More generally, when biologists comparethe milestones of the maturation of the visual systems of differentanimals, some born early and helpless, others born late and seeing, theyFind that the sequence is pretty much the same whether the later stepstake place in the womb or in the world.The emergence of the crucial left-eye and right-eye neurons can bedisrupted by experience. When the neurobiologists David Hubel andTorsten W r iesel raised kittens and baby monkeys with one eye covered,the input neurons of the cortex all tuned themselves to the other eye,making the animal functionally blind in the eye that was covered. Thedamage was permanent, even with only brief deprivation, if the eye wascovered in a critical period in the animal's development. In monkeys, thevisual system is especially vulnerable during the first two weeks of life,and the vulnerability tapers off during the first year. Covering the eye ofan adult monkey, even for four years, does no harm.At first this all looked like a case of "use it or lose it," but a surprisewas in store. When Hubel and Wiesel covered both eyes, the brain didnot show twice the damage; half the cells showed no damage at all. Thehavoc in the single-eyepatch experiment came about not because a neurondestined for the covered eye was starved of input hut because theinput signals from the uncovered eye elbowed the covered eye's inputsout of the way. The eyes compete for real estate in the input layer of thecortex. Each neuron begins with a slight bias for one eye or the other,and the input from that eye exaggerates the bias until the neuronresponds to it alone. The inputs do not even have to originate in theworld; waves of activation from intermediate way-stations, a kind ofinternally generated test pattern, can do the trick. The developmentalsaga, though it is sensitive to changes in the animal's experience, is notexactly "learning," in the sense of registering information from the world.Like an architect who hands a rough sketch to a low-level draftsman tostraighten out the lines, the genes build eye-specific neurons crudely andthen kick off a process that is guaranteed to sharpen them unless a neurobiologistmeddles.