"We have to get more nuanced understanding of how electricity impacts brain chemistry at the microscopic circuit level," says Helen Mayberg, a physician and neuroscientist at Emory University, in Atlanta, who was not involved in the research. "This type of technology gives us the opportunity to look precisely at very local changes in the chemical mix. As the technology is expanded to be able to detect an even wider range of neurochemical systems, it's going to really catapult what we can learn about the mechanisms of brain stimulation and the diseases we treat with it."

In addition to detecting dopamine, preliminary research shows the technology can also detect serotonin, a brain chemical implicated in depression. (Serotonin reuptake inhibitors such as Prozac target this neurotransmitter.) Deep brain stimulation is currently approved to treat Parkinson's, the movement disorder dystonia, and severe obsessive-compulsive disorder, and is under study for epilepsy, depression, anorexia, and other disorders.

Lee says his team has now been granted approval to test the system in a patient, which they aim to do in the next few months. Initially, it will be tested only during the implantation surgery to determine how moving the electrode alters the level of dopamine released. But the ultimate goal is to incorporate the sensor into the deep brain stimulation system. Researchers are currently developing new sensor electrodes that function effectively over the long term, as well as shrinking the device so that it can be packaged and implanted onto the skull. Once researchers better understand the link between deep brain stimulation and neurochemistry, the accompanying chemical data may help neurosurgeons to better place the electrode.

But some say this step may be premature. "The technology is very intriguing, but we need a lot more research before it can be applied in humans," says Ali Rezai, a neurosurgeon at Ohio State University, who was not directly involved with the research. He says that researchers need to show that using this technology alongside deep brain stimulation in animals with symptoms of Parkinson's disease improves outcomes.

In the long term, Lee and his collaborators want to develop a so-called closed loop system, allowing the stimulation device to detect the chemical changes in the brain and adjust its response accordingly. This approach is analogous to cardiac pacemakers, which stimulate the heart only when the instrument detects an abnormality. While abnormalities in heart rhythms are fairly straightforward to detect, "in the brain, it's much more complicated," says Rezai.