The selections are easily made, on the basis of what the world it's landed in requires. Japanese babies can tell apart the "l" and "r" sounds which Japanese adults struggle with, and English babies can distinguish all consonantal contrasts in Hindi. Waiting circuits that aren't reinforced by being heard weaken, detach, are harvested by prowling housekeeping cells, and then get dumped in the bloodstream for removal.
Brain cells which survive the Great Cull get nourished, and wrapped in fatty tubes to speed their electoral communications with each other. Since cholesterol is an excellent insulator, easily made by the body, it becomes one of the main components of the brain. By age seven, the cull is over, and although the remaining circuits can hook up in fresh configurations, it's hard to learn totally new skills - such as a language at native fluency - as well after this age as before. The apparatus of extra circuitry is metabolically just too expensive to keep on lugging around.
From the late teens on the brain has slowed so much that it's taking only 20 per cent of the body's oxygen supplies - half a four-year-old's level. There aren't going to be any new brain cells produced from here on, so the ones inside are well guarded. Blood entering the cranium gets filtered with a thoroughness matched only within the testicles - another container whose internal protection is recommended for species survival.
It's tempting to think that the insides of this fortress brain are divided into distinct areas, with separate parts for reason, emotions and vision, and a controlling sector on top of it all. But everything's much more dispersed. Vision, for example, isn't just a matter of carrying miniature copies of what the eye registers deeper into the brain. Even while still at the retina, the signal gets a first processing to boost the intensity of angles and edges. Then, once it's cabled to the main visual analysis centre at the back of the head, the truly surreal separations begin. Signals dealing with movement are ripped away and shuttled to one area; signals dealing with overall shape, colour and depth are pushed on to others. Similar deconstructions take place with our memory, word recognition, and other functions of the brain.
This is why strokes can be such an odd catastrophe. Destructions in the visual movement centre will make a street full of speeding cars seem desolately empty, with only the occasional parked cars being noticed. One woman with a lesion in this area could see tea when it was in an open pot, but when someone started pouring, it suddenly seemed to disappear for her. Only once it was settled in her cup, static enough for functioning brain groupings to take over, would it pop back into existence.
Increase the magnification down to the individual cells that carry our dispersed self around, and an even stranger landscape appears. Any vision of a controlling centre has vanished. Gnarled, long brain cells stretch forward in the darkness, laboriously pumping their electrical signals along. Since the cell endings don't quite touch, bubbling molecules are constantly being shuttled across the gaps to start up the circuit on the next side. These are the famous neurotransmitters, often of a simple, three-dimensional shape, which caffeine, crack, Prozac, nicotine and other chemicals can notoriously speed, duplicate, or slow.
The whole frantically-firing structure lasts a surprisingly long time, as most creatures our size have lifespans much shorter than ours. But even in a young adult it's quietly falling apart, with the brain's weight shrinking slightly from about age 25. By age 40, an estimated 5 per cent of the brain cells in the networks installing memories die each decade. Later other cells will shrink, or get hemmed in by decaying substance.
It sounds enough to justify Yeats's lament about being yoked to a dying animal, but it's not as one-way a decline as once thought. New connections form in the memory- installation centres, apparently by surviving cells trying to make do for the ones being lost. The cloud-like dispersal of our personality helps here. Although it may take longer to find a memory, its fragments will be so widely scattered that at least large parts of it are likely to still be around, somewhere, to be reached.
There's even comforting evidence that the mere effort of learning something new can accelerate the rate at which the surviving brain cells make fresh connections. A DIY course, anyone?
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