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Tom Siegfried, Randomness

When evaluating textbook dogmas about the brain, keep an open mind

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SN Prime, March 12, 2012 | Vol. 2, No. 10

Scientific textbooks are repositories of accumulated wisdom, gathered by painstaking investigation and verification producing the distilled essence of humankind’s knowledge of nature. They’re also loaded with undocumented dogma.

Much conventional scientific wisdom can be readily verified by consulting a textbook. But sometimes nobody has bothered to verify the verifier. Textbooks often contain assertions based on nothing more than a similar assertion in an older textbook. When you try to track down the original evidence for some scientific “facts,” you’ll find out that they may not be so factual.

Several current examples from the field of neuroscience show up in a recent paper by Brazilian neuroscientist Roberto Lent and colleagues at the Universidade Federal do Rio de Janeiro. Starting with one of the simplest questions about the human brain — how many nerve cells it contains — the Brazilian researchers show how much of what neuroscientists think they know simply isn’t so.

Consult textbooks, for instance, and you’ll usually find the answer to the above question to be “100 billion.” If you keep looking, you may find other estimates. Some sources like to hedge with an “estimate” of “between 10 billion and a trillion,” an implicit recognition that the 100 billion number isn’t very precise. After all, it’s hard to count the number of nerve cells in a brain; estimates rely on indirect methods that might be valid for some parts of the brain and not others.

Citing new work using advanced technologies, Lent and coauthors say the correct answer is 86 billion (an average for male human brains between 50 and 70 years old). That’s not so far off from the common conception. But textbooks also suggest that the brain contains 10 times as many “support” cells (known as glia) as nerve cells (or neurons). New quantitative methods debunk that dogma also.

In fact, for the brain as a whole the numbers of neurons and glia are roughly equal. In the cerebellum, supposedly the not-so-bright part of the brain, neurons outnumber glia by about 3-to-1. In the cerebral cortex, the brain’s wrinkled outer layer where high-level thinking occurs, glia are the majority, but still less than four times as numerous as neurons.

Oh, and about that high-level–thinking cerebral cortex. Another common textbook dogma holds that it is the pinnacle of evolution. One observation driving this view is the size of the cortex relative to the brain as a whole — in humans, it’s about 80 percent or so of the brain. That’s a much higher ratio than in “lower” animals. On the other hand, the cerebellum makes up only 10 to 15 percent of brain size regardless of an animal’s rung on the evolutionary ladder. Since the cortex seems so much more prominent in the smarter human species, conventional textbook wisdom suggests, it must be evolution’s repository of cognitive capability.

But if you use some of that ability to think about it a little, you’d realize that the computational power of the brain resides in neurons. So maybe the best thinking happens not where the most volume is, but where the most neurons are. And that’s the cerebellum — it contains 80 percent of a human brain’s neurons. As it happens, over the course of evolution the growth in neuron number seems to have been coordinated in the cerebellum and the cortex, suggesting that greater human thought power depends on both regions.

“Each neuron in the cerebral cortex corresponds to about four in the cerebellum, irrespective of the species considered,” Lent and colleagues write in a recent issue of the European Journal of Neuroscience. “Such a coordinated evolution of the cerebral cortex and cerebellum fits well with recent clinical and experimental evidence suggesting an important role of the cerebellum in cognitive and affective functions.”

Among the other dogmas deserving less reverence is the view that human brains are considerably more complex than those of other primates. That belief relies on measures of complexity based on brain size relative to body size, but such measures may be misleading. Human brains are bigger than those of other primates, Lent and colleagues show, but not exceptionally more complex.

Debunking such dogmas is important for more than improving textbook accuracy ratings. Knowing the right number of neurons in the brain provides a helpful baseline for comparison with diseased brains, or for studying the effects of aging on the brain.

And replacing dogma with data aids efforts to answer questions for which textbooks have no dogma to offer. Why, for example, do studies indicate that Neandertal brains had more neurons than modern humans? Hard to figure that out if you don’t know how many neurons modern humans have to begin with.

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