DEAR DOCTOR K:

I've heard it said that we use only 10 percent of our brains, but I'm skeptical. Could it be true that we use only this small percentage of our brain capacity?

DEAR READER:

Many parts of our bodies have some extra capacity built in. You can have an entire lung or kidney removed and get along fine with the one that remains. Your body can spare skin and bone marrow. If your appendix, gallbladder or spleen needs to go, so be it -- you can live without these organs if you need to.

But we need all of our brain. Regardless of what we're doing, even if we're asleep, all parts of our brain are working. New brain-imaging techniques, such as functional MRI scans, can measure how much energy is being used in each part of the brain. They show that we regularly use all of our brains. They also show that during any particular task, certain parts of the brain are working harder than others.

Take your memory as an example. Your memories aren't stored in a single spot. Instead, they are widely distributed in networks throughout your brain. Different areas of your brain process different kinds of information.

Say you want to sing a song you know. The words are stored in one part of your brain, but the melody is stored somewhere else. Calling up the memory of a song initiates a search that travels through many parts of your brain.

So the notion that we use just 10 percent of our brains isn't true. A lot of my patients seem attracted to that idea. I think it's because they think it means they could be a lot smarter: If they're using only 10 percent of their brains, they'd be twice as smart if they could figure out how to use 20 percent!

Could we be smarter? That's a controversial question. There are some people who have truly remarkable memory. They can remember in detail things that happened years ago -- the number of the hotel room where they stayed decades ago, for example.

Other people have remarkable computational powers: They can multiply two large numbers instantly. They don't have physically bigger brains. Do they have unique inherited brain chemistry? If so, and if we figure out what chemistry gives them such mental powers, could the rest of us someday also have such added powers? I think it's possible.

Another common belief is that really smart people have more brain cells than others. That, too, is probably incorrect. But it probably is true that really smart people have more connections between their brain cells.

While the "10 percent theory" is not true, it is true that challenging our brains makes us sharper. Particularly in the last half of our lives, the brain is like a muscle: The more it's used, the better it will continue to do its job.

(Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.)

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