For more than a decade, doctors have created tiny electric fields deep within the brain to treat the effects of neurological disorders.

But one common issue with the pacemaker-like devices that deliver electricity into the brain is that it can be difficult for doctors to know precisely how much current is being applied at the end of a wire in the brain, because of the varying levels of resistance created by the tissues near the electrode tip. Precisely regulating such current is important to achieving the desired symptom relief without creating unwanted side effects.

This month, the U.S. Food and Drug Administration approved a new kind of deep-brain stimulation (DBS) system called the Vercise, made by Boston Scientific, that borrows electrical engineering concepts from neural implants used to treat hearing problems. The Vercise's proprietary electric system is designed to give doctors and patients better control over the current applied at the individual electrodes buried deep in the brain, regardless of the resistance at the end of the wire.

The Vercise system was approved to treat the symptoms of Parkinson's disease, and it's the third major-brand entry into a field long dominated by pacemaker pioneer Medtronic and its line of Activa devices. Parkinson's disease is a progressive movement disorder whose symptoms are caused by the death of brain cells that make dopamine, a neurotransmitter that helps regulate movement among other things.

Medtronic first got U.S. permission to sell DBS devices to treat advanced Parkinson's disease in 2002. In 2015, the FDA granted permission for the device to be marketed for patients who only recently started to experience movement problems.

Last year the FDA approved St. Jude Medical's Infinity Deep Brain Stimulation System, now sold by Abbott Laboratories. Doctors called the Infinity DBS the first major advance in deep brain stimulation devices in many years, partly because it featured directional deep-brain electrodes that can be activated in tandem to create nonspherical current fields, giving doctors better control over where the electricity stimulates the brain.

The global market for deep-brain stimulation devices is about $600 million and is growing around 10 percent a year.

About 10,000 patients are treated with DBS per year, but they are a minority. Most of the more than 1 million Americans who are living with the disease today take drugs like dopamine-boosting levadopa medications, in addition to behavior changes like getting more rest and exercise. Typically a DBS device would be considered only after those therapies have waned in effectiveness.

While drug therapies often consist of chemicals intended to change dopamine levels, implantation of a deep-brain stimulator is intended to electrically stimulate parts of the brain like the subthalamic nucleus. Success with a DBS device can mean that patients can lower their levadopa doses while still getting relief from tremors, muscle rigidity, slow movements and other symptoms.

The Vercise, which has been approved in Europe for years, is designed to use independently powered electrodes to increase precision and avoid the negative side effects of DBS therapy that affects the brain beyond the intended target. The device has an implanted battery that can be recharged through the skin.

"In basic terms, it really provides a constant optimized therapy for the patient," said Maulik Nanavaty, senior vice president with Boston Scientific's neuromodulation division. "And it's rechargeable through the skin, so the patient has 15-plus years of rechargeable battery life."

The FDA approved the device for U.S. patients after seeing the safety and effectiveness results of a 40-person study in Europe called the Vantage trial, plus the early safety results of an ongoing clinical trial with 292 patients at 23 sites called the Intrepid study. Boston Scientific said Intrepid reached its primary endpoint of change in symptom control during waking hours, but the exact data will be published next year.

Dr. Jerrold Vitek, head of the neurology department at the University of Minnesota and coordinating principal investigator for the Intrepid study, said the rounded shape of the Vercise's power generator may reduce surgical infections while the rechargeable battery is supposed to cut down on the number of generator-replacement surgeries.

"Clearly, there is a benefit," Vitek said. "How much? That's what we're going to be looking at."

The device seems to have also stimulated the DBS market, he said, by introducing new features and generating market competition, which should be good for patients. "It is really nice to have more competition in the marketplace right now," Vitek said.