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| |    Handedness and the Brain by Robert Needlman, M.D., F.A.A.P. reviewed by Robert Needlman, M.D., F.A.A.P.
For example, one theory suggests that ancient warriors who held their swords in their right hands and their shields in the left, were more likely to survive (because the shields covered their hearts). According to this theory, right-handedness became the norm over time by natural selection--those who were right-handed survived, and those who weren't didn't. However, the heart really sits pretty much in the middle of the chest, and there's archeological evidence that most people were right-handed long before swords were invented, so this premise seems unlikely. Smart use of the brain's surface area A more convincing reason, I think, has to do with specialization of the brain. Like large programs that eat up memory on your computer, complex hand movements use up a lot of brainpower. It would be a waste to have both hands be capable of these movements when just one hand will do. You can think of it in terms of real estate on the brain's surface. The surface of the brain is where most of the complex computing takes place. All the folds and crevices that make brains look like brains are there to increase the surface area and as a result, the brain's computing power. But even so, there is only a limited amount. Controlling all of the tiny hand muscles that are used in writing requires a lot of brainpower and thus a lot of brain-surface area. In the end, it would be wasteful to devote a lot of real estate on both sides of the brain to controlling fine hand movements that only really need to be done by one hand. Methods of locating brain functions We've known for more than 100 years that different brain functions take place in different parts of the brain. Believe it or not, in the old days, scientists found this out by surgically opening up the skull, poking at different parts of the brain surface, and asking the patient what he felt. Researchers later learned to put half of the brain to sleep by injecting anesthetics into the blood vessels that supply that half only. They could then study which functions stopped working and thereby locate them on one side or the other. For example, when the left side of the brain is put to sleep, most people temporarily lose the ability to talk, although they may be able to sing (a right-brain function). Nowadays, we have high-tech scans that can show where in the brain the nerve cells are most active. Computer-generated pictures of the brain then light up to show, for example, that listening to someone talking activates a particular region on the left side, while reading a book activates other regions in the back and sides. The right and left hemispheres As you may (or may not) remember from high school biology, the brain is made up of two distinct sides, or hemispheres, like the two sides of a sandwich cookie. The right hemisphere is almost completely separate from the left, with only a small band of nerve fibers connecting the two. The right hemisphere controls the muscles on the left side of the body, while the left hemisphere controls the muscles on the right. This switch-over is hard-wired in the developing brain. As the brain and spinal cord are forming, nerves originating on one side of the body send out long extensions, called axons, toward the midline of the body. Most of the axons proceed to cross over the midline and so end up connecting with the other side of the body. For right-handed people, the hand control center is much better developed on the left side of the brain than on the right. For those who are left-handed, the better-developed nerves live in the right hemisphere. Differences are physical, too But left-handed brains are not simply mirror images of right-handed ones. Functions such as verbal language are usually located in the left hemisphere regardless of handedness. However, among left-handers, there is a greater likelihood that the language centers turn out to be located on the right side or on both sides of the brain. In other words, the brains of left-handed people tend to be different, and to have a greater variety of configurations, than those of right-handed people. These differences have many implications for the different ways some left- and right-handers think. More information: Talk about:
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