Transcranial magnetic stimulation (TMS) is more than just a new way to treat mental disorders. It also allows scientists to probe tiny areas of the human brain to find out what they do.
Harvard neurologist Alvaro Pascual-Leone says images from brain scans can't do this. And it's essential for scientists trying to understand how the brain works.
"The only way we're able to really disentangle the functions of this very complicated organ is by tapping here or there," Pascual-Leone says.
Transcranial magnetic stimulation gives researchers two different ways to tap on parts of the brain.
One is by using an electromagnetic frequency that temporarily disables brain cells.
Eric Wassermann runs a brain stimulation unit at the National Institutes of Health. He says researchers there suspected they had found an area of the brain that was essential to speech. But they didn't know for sure until they used stimulation on volunteers to disable that part of the brain.
"Suddenly speech would become garbled and stop," he says. "And when the stimulation was turned off, the speech would resume suddenly. It's very dramatic. I've had it done to me; it's a strange sensation."
By using a different frequency, electromagnetic stimulation can also make brain cells work better, Wassermann says.
"It's a little analogous to the enhancing effects of certain drugs like amphetamine or methylphenidate," he says, referring to drugs used to treat attention deficit disorder.
But these drugs tend to affect the entire brain, Wassermann says. Stimulation can be limited to precise areas, indicating exactly which ones improve our ability to focus.
One interesting experiment with brain stimulation involved blind people reading Braille. Pascual-Leone says their brain scans show activity in places that usually process information from the eyes.
"The question was, is this activity in the visual areas really contributing to the ability of these subjects to read?" he says.
Brain scans couldn't answer that question. So Pascual-Leone and other scientists tried brain stimulation.
"If you use magnetic stimulation to block the activity in the visual areas of the brain in these congenitally blind subjects," he says, "they make more errors when reading Braille by touch.
That meant the visual areas were critical.
To find out why, the team did another experiment. It involved waiting until just after a blind volunteer's finger touched a Braille symbol.
Then, Pascual-Leone's team blocked activity in the brain's visual area.
"The subject knows that they have touched the finger, but can't come up with the Braille symbol that was presented," he says, "suggesting that the activity in the visual cortex was contributing to the decoding of the Braille symbol."
Something a mere picture of the brain could never have shown.