hemispheric organization and the 10% myth

In response to a request on ozarque's journal, an attempt to dispell a couple of popular myths about brain structure.

Let's start with hemispheric organization. Pop psychologists can spend hours discoursing on right-brain/left-brain dichotomies. Ostensibly, the left hemisphere is responsible for analytical, intellectual, rational thought. The left side of your brain is staid and conservative, unemotional, and quite possibly a tool of the patriarchy. The right hemisphere, by constrast, is said to be creative, holistic, intuitive and emotional. It is probably planning the overthrow of all conventional ideas about politics and art at this very moment. Furthermore, everyone's thought processes reflect the dominance of one hemisphere over the other. In order to increase your creativity, you must practice your right-brain skills (it's assumed that most people in patriarchal American culture have ended up left-brained, or at least that there aren't any artists with a desperate yen to improve their mathematical abilities). Obviously there's an assumption here that hemispheric dominance is learned. You can be whatever you want, with enough effort. Or you can just spend all day taking internet tests that tell you if you're right-brained or left-brained.

My lovely Intro to Learning textbook (An Introduction to Theories of Learning, by B.R. Hergenhahn & Matthew H. Olson) calls this "dichotomania." It's based on a kernel of truth, but that kernel has gotten pretty buried.

Your cerebral cortex is, indeed, divided into two hemispheres. The do have functional assymmetries, the most obvious of which is that each is responsible for the opposite side of the body. The right hemisphere takes sensory input from, and sends motor output to, the left side of the body, and vice versa. This is so weird that it's been suggested that in the distant evolutionary past, vertebrate heads somehow mutated and flipped around 180 degrees. If you are strongly right- or left-handed, the motor cortex on the opposite side of your brain will be slightly more developed in the section responsible for your arm and hand.

The major difference between the right and left hemispheres is that, in most people, the left hemisphere performs most language functions. All speech production and much of comprehension originate in this hemisphere. This is where you store vocabulary, put together sentences, piece out meanings. The equivalent areas of the right hemisphere are responsible for what Ozarque refers to as non-verbal communication--particularly the "tune" of your speech. Damage this area, and you'll speak with completely flattened affect, or be unable to tell the difference between "You ate the last donut" and "You ate the last donut?!!!" Musical tunes also seem to be more a right-hemisphere thing.

For some people (about 15% of left-handers), language is in the right hemisphere instead. Another 15% of left-handers have language evenly divided across hemispheres. This doesn't appear to have any effect on your thought processes--it's only important if you go in for brain surgery.

Overall, the left hemisphere does seem to be more detail-oriented, and obviously more verbal. Processing in the right hemisphere does seem to be more holistically-oriented. However, there is no basis for the idea that people have any sort of hemispheric dominance. None. Most thought processes involve both hemispheres to some extent. Under normal circumstances, they communicate constantly, so that everything gets both broken down into details as well as perceived as a whole. You've got to be able to do both of those things in order to, for example, understand that the thing in front of you right now is a computer.

In fact, there's some evidence that creativity is a function, not of right-brain activity (where would that leave writers?), but of cooperative activity between the two hemispheres. The primary connection between the two is a wad of axons called the corpus callosum; there's been some demonstration that people who score high on tests of creativity have slightly thicker CCs. (Standardized tests of creativity: a rant for another time).

Now, certainly, some people tend to think more intuitively, some more analytically. It's the difference between someone who enjoys painting and someone who really loves computer programming or calculus. There are also people who are pretty good (or pretty bad) at both. What you like to do, or what you're good at, is a more complicated issue than which hemisphere is lording it over the other one.


The 10% myth explanation is, happily, shorter. You use all of your brain. You don't the whole thing all the time (that would be severe and probably fatal epilepsy), but everything that's in there right now has a function, and gets called on on a semi-regular basis. We know this because any connections that don't get used become weaker over time and eventually go away (we're talking about the learning and reasoning areas here--what happens to old memories is a more complicated question). This is especially notable in infants. A baby starts with an enourmous number of connections, and much of their initial learning is a matter of pruning connections that don't actually reflect the universe they find themselves in. It's like carving a statue from a block of marble, chipping away those bits of stone that aren't part of the image of a horse (or whatever you feel like carving). The only difference is that you also regularly grow new connections, as you learn things you didn't know before. The phrase "use it or lose it" is an apt one here. Skills that are used become stronger; skills that are neglected fade.

We know, roughly, what most of your brain does. That is, we can point to a section that's responsible for movement, and another that does something with language, and another that lights up when you do crossword puzzles and we're still arguing over what that means. Obviously the map is more detailed in some places than in others. However, if someone makes an argument for psychic abilities that starts with "scientists only know what 10% of your brain does--you must be doing something with the rest of it," then you probably want to look around for a better class of parapsychologist.