For decades, doctors have used pacemakers to regulate the heart. Now they're implanting similar devices into the brain.

Thousands of patients with the most serious cases of Parkinson's disease and epilepsy have received the devices since they obtained approval in 1997 from the Food and Drug Administration. Hundreds more are slated to take part in clinical trials to see if the pacemakers' electrical impulses can control chronic pain, depression and even obesity.

In America, over 1.2 million people suffering from depression have found that traditional medications don't work for them, say makers of the device. About 250,000 epileptics are in the same situation, as are nearly 100,000 sufferers of Parkinson's disease.

So far, early results are positive. Dr. Rajesh Pahwa, an associate professor of neurology at University of Kansas Medical Center, has implanted hundreds of the devices, finding that 80 percent of his pacemakered patients with Parkinson's showed significant improvement – with a 40 percent reduction in need for medication and a 40 to 50 percent reduction of symptoms.

Before receiving her pacemakers, Carole Carey, a 48-year-old retired teacher in Leavenworth, Kansas, was "tremoring all of the time" because of her advanced-staged Parkinson's. Driving a car – even cutting up her own food – had become impossible for Carey.

"We called it the jackhammer, because one side would be going up and down so bad," she said.

Medicines designed to reduce the tremors would send her into dyskinesia – uncontrolled twitching – and into bouts of hyperactivity.

Surgeons implanted two pager-sized battery packs near her collarbone and wires into her brain's subthalamic nucleus. Electrical pulses from these devices have quieted her tremors enough to allow her to cut her medical intake in half, and to start driving and eating on her own again.

These electricity-based treatments work because "all brain activity is basically electrical chattering between cells," said Dr. Rodolfo Llinas, chairman of the physiology and neuroscience department at the New York University School of Medicine.

Diseases such as Parkinson's, epilepsy, obsessive-compulsive disorder (OCD) and depression all occur when particular cell groups start "talking" too slowly, developing a sluggish, sleep-like rhythm. This drowsy conversation sets off a chain reaction, which can lead to tremors, seizures or depressive episodes, depending upon what part of the brain is affected.

"Parkinson's, depression, OCD, tinnitus (loud, incessant ringing in the ears), central pain – they're all the same disease. The difference is their brain location, not the mechanism," Llinas said.

By emitting electrical impulses into the brain – a process known as Deep Brain Stimulation (DBS) – the pacemakers, such as the ones implanted into Carole Carey, are supposed to interrupt these cellular yawn-fests, waking up the cells and minimizing the episodic frequency of seizures or other symptoms that afflict the wearer.

The pacemaker device sends electrical impulses through one of the brain's 10 billion neurons, triggering the release of chemicals, among them serotonin and dopamine, that in turn "convince" neighboring cells to send out new electrical impulses to other neurons.

DBS is currently approved in the U.S. only for controlling the tremors caused by Parkinson's and "essential tremor," a common, unidentifiable (yet benign) tremor exacerbated by stress, anxiety or the use of stimulants such as caffeine.

But trials to investigate the effects of the pacemaker implants on all of the symptoms of Parkinson's and on chronic pain are currently underway at the University of Kansas, the Cleveland Clinic and elsewhere.

At Brown University, studies of the effects of DBS on OCD are slated to begin later this year. Around the same time, surgeons at Cornell University will begin to use the pacemaker to try to revive people in minimally conscious states.

When implanting the DBS device, researchers must be painstakingly precise – a few millimeters off can make a huge difference. A misplaced electrode in the treatment for Parkinson's, say, can induce depression.

And these 5-to-10-hour surgeries must be done while the patient is awake – and unmedicated.

"I could feel them putting the wires in my brain," Carey said. "It's like a long hair going down your throat."

According to Pahwa, Carey's neurologist, these devices are implanted incorrectly about 20 percent of the time, necessitating repeat surgery. Another 10 percent of the time, the hardware fails. There's also about a 3 percent risk of bleeding in the brain, which in very rare circumstances can cause a stroke or a coma.

For these reasons and others, Llinas says he's against electrode implantation.

Another model of pacemaker attempts to sidestep these complications by implanting the wires into the vagus nerve – one of the brain's major pathways to and from the upper torso, which is located around the neck, near the brain stem.

Known as Vagus Nerve Stimulation (VNS), this procedure has been approved domestically for the treatment of epileptic seizures.

Researchers at 20 labs across the country are now investigating the effects of VNS on people with long-term, medication-resistant depression. The last of 235 subjects was implanted with the device last month. (The procedure has already been approved for such purposes in Europe and Canada.)

The effects of VNS on other conditions are being examined, too. For example, since many scientists believe the vagus nerve passes along information from the stomach – to indicate to the brain when the stomach is full – researchers at Lenox Hill Hospital in New York are looking into VNS as a treatment for obesity.

But the precise mechanics behind why VNS works are still largely a mystery.

"We're only guessing at what the chemical basis for this might be," said Dr. Mitchel Kling, a psychiatry professor at the University of Maryland School of Medicine, where a trail for VNS use against depression is taking place.

In contrast, Llinas said, much more is known about chemical therapies. Drugs like dopamine are "incredibly precise" because they only interact with the cells that have receptors for the chemical.

So while brain pacemakers are promising, Llinas feels they should "only be a choice as a last resort."

But for people like Carey, sometimes a last resort is all that's left.

"This was my last hope. I was getting pretty desperate by then," she said. "And I figured, if they're opening my head up and they slip, well, it's OK."