In the late 1700s, Italian anatomist Luigi Galvani made a dead frog's muscles twitch when struck by a spark, a discovery that paved the way for the modern understanding of electricity's role in living things. It is the basis for countless medical technologies like the pacemaker.

But electricity does not travel easily through the skull to the brain, the organ responsible for every purposeful twitch and altered mood. So when a group of British scientists in 1985 used magnetic pulses from outside the head to induce an electrical field inside the brain - and got a subject's hands to move - their colleagues clamored for a chance to zap themselves.

That breakthrough, known as transcranial magnetic stimulation (TMS), led to the Food and Drug Administration's approval last month of the first noninvasive, non-pharmacological treatment for depression.

As a practical matter, approval of the device made by Neuronetics Inc., a five-year-old Malvern company, is intended for patients with major treatment-resistant depression who do not respond to any one medication. Millions of Americans fail to benefit from antidepressants, and millions more quit because of side effects.

Symbolically, the federal action is a big deal - another advance in a group of emerging fields that involve electrical stimulation of the brain.

"Our view of the brain is changing," said Mark S. George, a professor of psychiatry, radiology and neuroscience at the Medical University of South Carolina.

Just 10 or 15 years ago, scientists thought of the brain as a single entity - what he called "the brain-as-soup" model. "But really you want to treat specific regions in the brain."

George is editor in chief of a year-old journal named Brain Stimulation, and he is a champion for the cause. After decades of success with psychiatric drugs, he said, "we had forgotten that the brain is really an electrical organ."

Researchers worldwide are testing therapies ranging from highly invasive electrical implants to hardly noticeable magnetic fields on dozens of psychiatric and neurological disorders. Success has been limited - but so are current treatment options.

When a major depression enveloped Ernie Mercer in the late 1980s, Prozac had just come on the market, and it worked. When depression struck again five years ago, it didn't. Neither did Effexor or a third drug. Worse, they made him nervous and constipated.

For Mercer, a retired engineer who lives near Atlantic City, depression was withdrawal from life. "Nothing was fun anymore," he said.

He answered an ad seeking research volunteers for an experimental treatment in 2005.

The clinical trial of transcranial magnetic stimulation went like this: He'd show up at a University of Pennsylvania clinic five times a week, answer the same set of questions about his mood, and then sit in what resembled a dentist's chair for 40 minutes with earplugs in his ears and an apparatus strapped to the top left of his head. He heard loud clicking sounds but felt nothing.

After four weeks, a sensation suddenly matched the clicking - "kind of like somebody tapping on your scalp like 10 times a second," he said - and his depression began to lift. He had been initially assigned to the sham (placebo) group; now he was getting TMS. After several weeks of the real thing, he felt fine. He still does.

Mercer, 65, paid nothing for either the treatment series or twice-monthly maintenance sessions ever since. The research grant ends this month, however, and the clinic will charge him $150 on its sliding scale if health insurance doesn't cover it; most of the clinic's patients are likely to pay at least $200. Insurers are just now beginning a review.

The new treatment is not a panacea. An unrelated study two years ago found that, of patients who failed to benefit from one antidepressant medication, just one-third responded adequately to a second. TMS produced a similar response rate (as does talk therapy, according to other studies), although the effect was described as greater.

The biggest difference is side effects, which cause many patients to stop taking antidepressants. The most commonly reported side effects to the brain stimulation were headaches and scalp irritation, both temporary.

TMS poses a slight risk of seizure. No incidents were reported in data on 10,000 sessions submitted to the FDA.

Neuronetics didn't seek approval to treat all major depression; when antidepressants work well, they are hard to beat. Still, the FDA rejected the initial application last year to use the NeuroStar TMS device for treatment-resistant cases generally.

A reanalysis of data on the 301 patients in the multicenter trial found the strongest response among those who had tried and failed with just one drug, and that's what the agency approved. Patients like Mercer, who gave up on three, can be treated "off label," which may be less likely to qualify for reimbursement.