Jewish World Review Dec. 11, 2001 / 26 Kislev, 5762
Why you don't need to worry a lot about losing important capacities
A Hindu teaching asserts that one has only so many words to utter in a lifetime, therefore one had best make measured use of them. This spare calculation might make a sort of sense applied to the human heart, as when we say, with a certain grim accuracy, that each heart has only so many beats to beat before it expires. Being a mere muscle, exercise increases its capacity only up to a point. But is the mind or brain limited in that way?
One has simply to think of how certain great composers, philosophers, scholars or literary figures developed their minds right up to the end of their lives to get a sense of how the brain, an organ with a hundred trillion connections, increases its capacity through use. A current debate in the neurosciences touches on this matter in a most interesting way.
Overwhelming evidence demonstrates that repeated use of our brain leads to nerve-cell growth of various kinds, and alteration and growth of our sensory brain maps. A sensory brain map is simply a group of nerve cells that respond to sensory stimulation from the external world. Studies have shown that London cabbies have a thicker part of their brains involved in visual mapping than other people do. Though the brain maps of children with certain kinds of reading disabilities do not light up normally on brain scans, if they are given exercises to treat dyslexia, they begin to develop a normal brain map pattern. A concert pianist practising piano eight hours a day develops the area of the brain that maps for music.
Whether this occurs at the expense of another area of the brain map is a serious question for a hyper-specialized society that promotes the idea that it is good to devote oneself almost exclusively to a narrow range of activities. As a psychoanalyst who is also an accomplished composer once responded when I remarked, with naive surprise, that a world-class musician I had met was a total bore at a party and hadn't troubled to read a book in a decade: "You obviously haven't met many first-rank musicians who do nothing but spend all day, every day, practising."
The idea that we can change our brain map is actually a radical and new one. Until about 15 years ago, the conventional wisdom in neuroscience was that we are born with every nerve cell we will ever have, and that nerve cells, unlike other cells, don't divide, grow or regenerate. But starting in the 1960s, a series of experiments began to show this wasn't true. These experiments indicated that, at all sorts of levels, there is "neuroplasticity" in the brain, i.e., the very structure of the brain not only can change, but seems to have evolved to change. For instance, it was discovered that children born with damage to the left hemisphere (which is used for language processing) could learn to use language with the help of their right hemisphere. Recently, Elizabeth Gould at Princeton and Fred Gage of the Salk Institute in California demonstrated that there are stem cells in the brain -- baby cells that can grow into adult cells.
But the twist is this. Along with the discovery of stem cells, there is an increasing awareness that brain cells die all the time, and their branches are "pruned back." As well, just as some parts of brain maps can enlarge, other parts can diminish.
A number of experiments have shown that the brain map can be manipulated. Tactile stimulation of the skin can lead to electrical activity in the tactile brain map, which can be measured by plant-ing electrodes in the brain. Michael Merzenich and colleagues, from the University of California, San Francisco, measured the size of the brain map for a monkey's finger. Then they surgically amputated the finger, and noted that the brain map for that area soon decreased in size. As well, they found that the brain map for adjacent fingers expanded into the space of the brain that had once mapped the amputated digit. These are momentous findings, and led Merzenich to argue that brain maps are in a constant, competitive relationship with each other and are ever-responsive to environmental stimuli.
While Merzenich argued that the brain could develop radically new brain maps in response to novel environments, others argued for a somewhat more conservative hypothesis -- that when environmental stimuli change, pre-existing circuits, not previously activated, are selected and turned on.
Gerald Edelman, winner of the Nobel Prize in Physiology for describing antibodies and the immune system, has argued for a version of this selection hypothesis. According to Edelman, brain maps consist of populations of cells that function as the immune system does. (Indeed, aspects of the immune system and the nervous system have similar evolutionary roots.) Early theories of the immune system saw it working in the following way. When an invading organism entered the body, our immune system's molecules wrapped themselves around it, getting an imprint, and then instructed the immune system to watch out for it. If that naughty organism ever came back, the immune system was ready to attack it. But that view was wrong.
The current view of the immune system is modelled on Darwinian principles. When an invading organism enters us, with our tremendous variety of types of cells, it sooner or later bumps into those few cells that it can easily attach to. When that happens, these immune cells are turned on, and begin to reproduce themselves. The key point, according to Edelman, is that the immune system isn't instructed by its encounters. Rather, as in Darwinian natural selection, the cells already fit for dealing with the invader are turned on, and become fruitful and multiply. The system only works because there is already so much variation in it.
According to Edelman, something similar happens in the brain. It isn't instructed by new stimuli either. Rather, when external stimuli change, populations of neurons that are adapted for incoming stimuli gradually take over, out-competing those cells that are not already adapted.
The good news is that no matter whether cortical maps are radically responsive to
external stimuli and can be totally rearranged to greet any new stimulus or whether
new stimuli work by selecting those pre-adapted nerve populations at the expense of
others, one needn't worry excessively about losing important capacities. This is
because we tend not to lose the abilities that we use in an ongoing way, only the
ones we seem to have no need for. One hundred trillion neuronal connections aren't
11/08/01: The terrorists and their last-night temptresses