Scientists don't yet know exactly how the electrical stimulation works, but research in animal models gives some clues. "We think we're changing the excitability of neurons within the spared region of tissue," says Nudo.

Neurons communicate by sending electrical messages to each other. When a person moves his or her hand to pick up a cup, for example, the neurons in the motor cortex fire to tell the arm muscles to move. If neurons in a recuperating brain area are electrically stimulated at the same time that a patient tries to move the cup, it may become easier for these neurons to fire. Scientists theorize that with repeated practice and electrical stimulation, these neurons develop new neural connections that strengthen the patient's ability to pick up the cup, leading to a lasting change in motor ability.

In fact, experts say it is the pairing of stimulation and therapy that's the key to this treatment. "The technology is enhancing the effectiveness of the patient's own voluntary movement," says Carolee J. Winstein, a biokinesiologist and physical therapist at the University of Southern California in Los Angeles. "The combination of techniques seems to be more effective than any technique by itself," she says. Winstein is running a part of the current Northstar trial.

Northstar's cortical stimulator isn't the only treatment in development to boost neuroplasticity after stroke. Some neuroscientists are studying transcranial magnetic stimulation, a non-invasive method to stimulate specific brain areas. Others are developing drugs that boost neuroplasticity. But Nudo says that direct electrical stimulation may have some advantages. "We can control the location of the stimulation better, as well as other parameters, such as frequency," he says. "You don't have that control with a drug."

Northstar eventually hopes to develop the technology for a wide range of disorders, including brain injury, auditory and pain disorders, movement disorders, and neuropsychiatric disorders. The company is currently sponsoring clinical trials of aphasia (loss of speech), tinnitus (ringing in the ears), and hemiparesis (weakness on one side of the body.)

Chris Ware, a former police officer, had a stroke nine years ago that left him partially paralyzed in his right side. He learned to walk and talk again after the stroke but was still seriously impaired. After participating in a clinical trial of the Northstar technology in 2004, he says was able to do a lot more on his own, like tying his shoes and driving.

"The concept of neural plasticity has breathed new life into potential for more recovery after brain injury," says Winstein. "I think we will see the development of many new technologies and treatments that attempt to tap into this natural ability. It's a very exciting time in rehab."