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The "caution light" that goes on in our brains when we're facing tricky and important decisions can turn off in patients with brain implants designed to control Parkinson's disease, a team of University of Arizona researchers has shown.

Known as deep brain stimulation, or DBS, the treatment involves implanting electrodes in the part of the brain that affects the physical mobility — especially the tremors — of Parkinson's patients.

Although the treatment can dramatically improve those physical symptoms, some patients suffer from a strange side effect — a type of impulsive behavior that can be dangerous. In short, the implant can block the "go slow" signal that helps us make good decisions.

"The benefit of the (DBS) therapy was so powerful when we first started doing this, it took us some time to understand the more subtle cognitive side effects," said Dr. Scott Sherman, a UA neurologist who has been working since 1998 with local patients receiving the treatment. He is a co-author of the UA study, which was published this week in the journal Science.

"But even with what we know about the impulsive behavior now, the effects of deep brain stimulation can be really amazing for patients and far outweigh the side effects," Sherman said. "So this is not a reason to avoid the therapy. But as we understand how this affects the brain, we are more aware of what can happen, and we can make adjustments to benefit the patient's quality of life."

Scientists have long known that anti-Parkinson's medications — the first line of treatment — occasionally spark impulsive forms of behavior, including addictive gambling, hypersexuality or compulsive shopping in extreme cases.

But the UA team found that deep brain stimulation — often used after the drugs stop working — can trigger a completely different kind of impulsiveness, through a different mechanism in the brain.

While the drugs affect levels of the brain chemical dopamine — crucial to physical movement — that decline in Parkinson's patients, deep brain stimulation targets a smaller area of the brain that affects motor function, but also detects conflict in decision-making.

"It is the area of the brain that says 'pause, slow down,' so we can make the best choice," said Michael Frank, a UA neuroscientist who worked with Sherman on the study.

"So when DBS takes out the processing of this area, we are not able to slow down," Frank said. "In fact, some patients actually speed up their rush in high-conflict situations."

Most often, this effect has been noticed in physical situations, they said.

"One patient, clearly a sharp guy, was in a wheelchair, and I asked him if he would be more comfortable in another chair. He reacted immediately, got up without assistance and fell," Frank said.

"Putting holes in the drywall" is how Sherman described the most common behavior in patients who make rash and unsafe judgments about how much they can do physically.

"I have seen patients with dramatically improved mobility, but they don't use it in a safe way," he said. "So we really need to watch out for patients when they start making these kinds of bad decisions."

And when they do, the strength and frequency of the voltage delivered through the implant can be adjusted to lessen the effects.

"But there is a trade-off," Sherman said. "Better movement comes with higher voltage, and that means more side effects. When you lower the voltage, you lose some of the mobility. There's really no free lunch in medicine."

One Tucson-area patient who has greatly benefited from deep brain stimulation has experienced a fairly mild trade-off. When Oro Valley resident Doug Link is getting peak results from his stimulator, he begins to suffer slight speech difficulty.

"So if I reduce the stimulation a bit to improve my speaking, then I notice some difference in my motor skills," said Link, 70, who has battled Parkinson's disease for about 20 years.

But the benefits have far outweighed that problem, he said, noting that the treatment shut down his tremors almost completely, allowing him to do simple things he couldn't do before — pick up his coffee cup, drink from a glass, negotiate a buffet table and take a 16-day cruise.

"The only one who hasn't benefited from this is my little schnauzer, Katie," he said. "She used to get under the table and scoop up all the food I spilled. I don't do that anymore, and I don't think she's too happy about it."

The UA-based study used specialized computer games to probe decision-making in 15 Parkinson's patients taking dopamine drugs, 17 others who received deep brain stimulation, and 14 healthy older adults.

First, participants were shown pairs of Japanese characters and were told to pick the "correct" one. It was baffling — what makes one symbol better, especially if you don't know Japanese? But as the computer screen beamed back "Correct!" or "Incorrect!" their brains learned to prefer some characters over others.

After that, the symbols were paired differently: "Correct" ones together to simulate "win-win" decisions; "incorrect" pairings to model choosing the lesser of two evils; and easy "right-wrong" pairs.

Healthy people and Parkinson's patients on the drugs hesitated when faced with win-win or lose-lose choices, allowing time to weigh options. But deep-brain-stimulation patients didn't hesitate with lose-lose choices — and actually sped up win-win decisions.

Remarkably, when the brain implant was switched off, the patients with deep brain stimulation quit rushing the close calls.

Rushing those calls likely will have effects that go beyond an ill-advised dash across a room, Frank said.

Say your job offers a range of 401(k) options. Sure, any one is better than no investment, but just grabbing the first one might not be the most lucrative.

"It makes a lot of sense," Dr. Valerie Voon, a psychiatrist with the National Institutes of Health's neurology center, said after reviewing the research.

"Because they don't have those brakes in place, you need to teach someone to slow down" when faced with certain decisions, she said.

As many as 1.5 million Americans have Parkinson's disease, suffering increasingly severe tremors and periodically stiff or frozen limbs as brain cells quit producing dopamine. There is no cure. Standard treatments include medications to stimulate dopamine, and once those fail, deep brain stimulation surgery to control tremors.

It hasn't been obvious that different treatments cause different types of impulsive behavior, said Dr. Kathleen Shannon of Chicago's Rush University Hospital.

"They all seem to make bad decisions and have trouble making decisions," she said. Now, "I'll start to look at my patients differently."