The Human Beast

Why we do what we do

If you had half a brain

Do we need only half our brains?

Folk wisdom can be useful but it is often erroneous. One of my favorite pieces of pseudo science is the oft repeated claim that we use only half of our brains, or less.

What makes this so intriguing is that the evidence might go either way. Whether it is true or not, it is intriguing to try and figure out where the theory came from.

The origin of the half-a-brain theory
I feel that pioneer neuroscientist Karl Lashley deserves some of the credit, or blame. Lashley was interested in what part of the thinking brain (i.e., the cortex) stores memory. His procedure was to teach rats to run in a maze, Then he would remove various segments of the cortex in an effort to cut out the memory.

To Lashley's intense frustration, he found that it didn't seem to matter what part of the rat's cortex he removed. He did find that the more cortex he took out, the worse his subjects performed. Subsequent researchers found that the most important structure for spatial learning in rats is the hippocampus that lies beneath the cortex and was unaffected by his scalpel.

Evidence for the half-a-brain theory
There is surprisingly good evidence that people can function fairly well with half of a brain. One clue was provided by split brain patients where the main information highway between the two halves of the brain (the corpus callosum) is severed. This allows researchers to address queries to one half of the brain or the other.

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This operation is generally performed in the case of people suffering from uncontrollable seizures: it keeps the seizure activity on one side of the brain allowing the other half to function more normally. Sometimes it may be necessary to take the more radical step of removing the seizure-prone half of the brain entirely, an operation that is more likely to be performed on children due to the greater plasticity of their brains (1).

Astonishingly, people with half a brain manage remarkably well with some qualifications. If the left side of the brain is removed, they may lose the ability to speak given that most people have limited language capacity on the right side. They also have problems with depth perception, movement coordination, and localization of sound. Some patients experience a steep decline in cognitive function. Others, make it through graduate school.

Evidence against the half-a-brain theory
This Lashley-type natural experiment would seem to close the case in favor of the half-a-brain theory. Yet, there is much evidence that we need everything we have -- and more.

To begin with, there are the failures of evidently healthy brains. Most of us experience occasions where some piece of information that we know is temporarily unavailable, whether it is the name of an exotic plant, a trivia answer, or the name of a person we had not met in years. However one explains such laboring of the cerebral engine, it suggests that we scarcely have the brain power needed for what we do each day.

Then the brain is vulnerable to all kinds of impairment from cold viruses, or slight inebriation, to concussions, strokes, or rabies. Some disorders involve tiny structures of the brain, such as Parkinson's disease being caused by loss of cells in the substantia nigra. The normal brain also deteriorates with age that renders people occasionally incapable of thinking or remembering.

We are thus left with the paradox of half a brain functioning almost as well as the entire organ whereas the complete brain often lets a person down. This seeming contradiction might have a fairly simple explanation. Having two sides to the brain is like having two kidneys. Most people function fine with a single organ. Yet, they travel in pairs, as Woody Allen would say.

1. Choi, Charles Q. (2008). Do you need only half your brain? Scientific American, 298, 104.

Nigel Barber, Ph.D., is an evolutionary psychologist as well as the author of Why Parents Matter and The Science of Romance, among other books.

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