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Sizing Up the Brain

By Sean Henahan, Access Excellence

brainPhiladelphia, PA (5/14/01)- From the time of Aristotle, humans have attempted to define the distinctions between themselves and all of the other animals. This has been accompanied by questions about the nature of human nature and the soul. In the intervening years, the theory of evolution has helped clarify some of the relationships between humans and other animals, while the field of neurology has explored the chemistry of the brain. A series of new studies of the brain suggest new ways to look at these fundamental questions.

Right: brain evolution in action brainmuseum.org

A long look at evolution shows a pattern of increasing complexity in the development of the brain and central nervous system from the very basic but remarkable structures seen in insects to the fully functional system that has produced everything from "The Origin of the Species" to "The Weakest Link". A new system developed by neuroscientists attempts to bring some order to the description of mammalian brain evolution. This system, called 'cerebrotyping', describes all brains based on the relative size of different components such as the cerebellum and the cortex.

"Intuitively, we know there is something about our brains that is extreme. What we have here is a direct measure of one way in which our brains are extreme," notes Sam Wang, assistant professor of molecular biology.

The principle underlying the new system is natural selection. Changes in cerebrotypes appear to correspond with the evolution of new species. A comparative analysis of primitive lemurs, monkeys, apes and humans, for example, reveals a ever increasing size of the neocortex relative to other parts of the brain. While the cortices of early primate insectivore make up only 16% of their brains, the human cortex makes up 80% of the whole. The researchers speculate that the increased relative size of the cortex could represent an adaptation to the demands of the complex social groups of the higher apes. The cerebrotyping system provides biologists with a new way to study vertebrate evolution.

Preliminary findings suggest that animals with similar cerebrotypes tend to be the most closely related by evolution. Studies of groups of related species sow remarkable constancy in cerebrotypes, even when brain size varies by as much as one hundred-fold. The new approach may be particularly helpful in the study of critical junctions in evolution where new groups emerge, with corresponding new cerebrotypes. This in turn could provide clues as to which genes are involved in these processes.

"It's quite clear that the structure of the brain is ultimately governed by natural selection," said Wang.

THE EYES HAVE IT

Researchers at the Salk Institute report another approach to understanding the evolution of the oversized human cortex. Charles Stevens and colleagues found that the number of cells in a given region of the brain also provides some intriguing clues. The researchers counted the cells in two regions of the cortex: the lateral geniculate nucleus (LGN), which receives the first nerve signals from the retina, and the primary visual cortex, also critical in the visual process. A comparison of various primate species revealed that the the number of cells in the primary visual cortex was proportional to the number of cells in the LGN raised to the power of 1.5. This means that a human employs four times as many neurons to process the visual input from each LGN neuron as a small primate called the tarsier. Stevens believes that this correspondence derives from from the evolutionary demand for more neurons to process the greater amount of information collected by larger eyes at an optimal level of resolution.

THE SEAT OF THE SOUL?

Self-awareness and all that goes with it are a key element of how we distinguish ourselves from one another, and other 'lesser' species. A UC San Francisco neurologist believes he has discovered the anatomic location of 'the self'.

"We think of our 'self' -- including our beliefs and values and even the way we dress -- as something we determine, not just an anatomical process. But this research shows that one area of the brain controls much of our sense of self, and damage to that area can dramatically change who we are," said Bruce L. Miller, MD, of the University of California, San Francisco, who presented his findings at the American Academy of Neurology's 53rd Annual Meeting in Philadelphia.

Miller and colleagues evaluated the cases of 72 people with frontotemporal dementia, a rare form of dementia can develops in people in their 50s. Using MRI and single photon emission computerized tomography (SPECT) imaging, they compared the brains of patients who had shown a change in their "self" (defined as changes in their political, social or religious values or style of dress) with those who had not. Changes in self were strongly associated with severe abnormalities in the brain's right frontal lobe.

"This suggests that normal functioning of the right frontal lobe is necessary for people to maintain their sense of self," Miller said. "It shows that a biological disorder can not only have profound effects on behavior, but it can even break down well-established patterns of awareness and self-reflection."

Miller began his studies after noticing that his patients with frontotemporal dementia sometimes demonstrated major personal changes. These changes could take the form of significant changes in political and religious beliefs, or changes in the clothes patients wore or the food they ate. One patient went from being a dynamic real estate agent with a taste for designer labels and French cuisine to a person who preferred thrift store clothing and fast food. Another patient went from being a short-tempered cheapskate fundamentalist to a relaxed hedonist.

The research of Drs.Wang and Stevens appears in the May 10, 2001 issue of Nature. Dr. Miller presented his research at the American Academy of Neurology's 53rd Annual Meeting in Philadelphia.

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