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Believe it or not, there is a speedometer out there for your brain, and it shows that smart people really do think faster. That's the conclusion of a study that also suggest people inherit the ability to think quickly.

NPR's Jon Hamilton has the story.

JON HAMILTON: A few years ago, scientists began using a special type of MRI machine to create images showing the pathways that carry information around the brain.

Dr. PAUL THOMPSON (Professor of Neurology, University of California School of Medicine): They're the most beautiful images of the brain you could imagine.

HAMILTON: Paul Thompson studied theses images in a lab at the University of California, Los Angeles.

Dr. THOMPSON: My daughter, who's five, says they look like flowers. They look little flowers at each point in the brain.

HAMILTON: Thousands of flowers connected by a network of spaghetti-like strands. Those strands are the pathways that carry messages from one part of the brain to another. They're made of the brain's so-called white matter. And Thompson says the images show not only where these pathways lead but how fast information is traveling.

Dr. THOMPSON: These images really give you a picture of the mental speed of the brain. And they link very well with IQ than other, you know, commonly relevant things for people having the scan.

HAMILTON: Thompson knows that because he was part of a team that studied the brains and intelligence of 92 people. The team conducted the usual IQ tests on each person. Then they did the scans that show information pathways. These scans measure the structural integrity of white matter. The greater the structural integrity, the faster nerve impulses travel.

Thompson says the link between brain speed and intelligence was pretty clear.

Dr. THOMPSON: You know, when you say someone is quick-thinking, it's genuinely true, that the impulses are going faster and they're just more efficient at processing information, and then making a decision based on it.

HAMILTON: The study also found that genetic factors play a big role in how fast your brain works. Thompson's team figured this out because the 92 people in their study were all twins. Some were identical twins, who share all the same genes. Others were non-identical twins, who share only certain genes.

This meant the researchers could tease out genes associated with the structural integrity of white matter. And it turned out many of these genes were also associated with intelligence.

Richard Haier, of the University of California, Irvine, says this may explain something scientists have been wondering about for a long time.

Dr. RICHARD HAIER (Professor Emeritus, University of California, Irvine): We know that intelligence has some genetic component. And what the Thompson study is showing is that a large part of the genetic aspect of intelligence has to do with the white matter tracks that connect different parts of the brain.

HAMILTON: Haier says the good news is we're not necessarily stuck with the brain or the brain speed we inherit. He says it's like running or weightlifting. It helps to have certain genes. But anyone can get stronger or faster by working out.

Dr. HAIER: Many people have conceptualized the brain as a kind of muscle for the purpose of this analogy. And the more you work it the more efficient it gets.

HAMILTON: So people who practice the violin, or do math problems, or learn a foreign language are constantly strengthening certain pathways in their brains. And Paul Thompson says unlike our bodies, our brains don't peak at an early age.

Dr. PAUL THOMPSON: The wires between the brain cells, the connections, are the things that you can modify throughout life. They change, and they improve through your 40s and 50s and 60s.

And there's a much more positive note than the sort of old, orthodox view of the brain, which was that you have all the brain cells you'll ever have by age of three, they can never be replaced, and you have to use it or lose it, so to speak.

HAMILTON: Thompson says there are practical, as well as academic, reasons to measure brain speed. The technique can spot problems like Alzheimer's disease, which slow down the brain. And because the scans are so sensitive, they can show whether new drugs for Alzheimer's are actually working. The research appears in the Journal of Neuroscience. Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.