Deep Brain Stimulation (DBS) is an implantable brain device used with Parkinson’s disease patients to help them control their movements better.

Parkinson's disease is caused by the degradation of nerve cells in the brain that produce dopamine, a signaling chemical that is necessary for smooth and controlled muscular movement. The neurostimulator, which is similar to a heart pacemaker, delivers electrical stimulation to a specific area of the brain, called the subthalamic nucleus, or STN, effectively blocking the abnormal nerve signals that cause tremors. This side of the brain is responsible for controlling impulsive behavior.

Researchers believe that when they are switched on, they not only control excessive body movements, but also block our ability to stop and weigh our options (hesitate, take time to decide).

In a new study to be published in the journal “Science,” researchers found that when Parkinson's patients received brain stimulation, they had trouble making hard decisions. However, when the stimulation was turned off, patients responded like the healthy individuals in the control study.

Study co-author Scott Sherman, a professor of neuroscience at the University of Arizona says: “From a scientific point of view, this research provides light on how some of the circuitry involved in decision making works in the brain.”

He added, “Even though patients know they have balance problems and are at risk for falling, they may still act impulsively. Patients may gain more mobility with deep brain stimulation, only to experience more falls.”

The study included 46 participants, of which 15 were Parkinson's patients who were taking dopamine drugs, 17 had received the DBS implant while the other 14 were healthy elderly adults (did not have Parkinson's). They had to tackle specialized computer games which tested their decision making behavior.

In the experiment, participants first went through a learning phase where they chose between two different Japanese characters on a screen and received positive or negative feedback about their choices. For example, they learned to always choose A over B and to choose C over D.

During the next phase of the study, all symbols were combined. Healthy patients weren’t sure which one to choose because they knew both letters were rewarding. In this situation, they knew a decision is hard to make, so they slowed down to deliberate before choosing an answer.

Patients taking medication for Parkinson’s disease were unable to learn from their past mistakes. “Medication can also cause impulsive behavior such as pathological gambling habits,” says study co-author Michael Frank, an assistant professor of psychology at the University of Arizona at Tucson.

“These patients have the ability to slow down, but the medication prevents them from learning from negative feedback about their decisions. They show insensitivity to losses and don't learn from their past mistakes,” Frank added.

Patients with Deep Brain Stimulation did not hesitate at all. In fact they answered these questions more quickly. “We found that one specific aspect of decision making is impaired,” says Frank. “The 'hold-your-horses' aspect of being able to slow down when faced with conflicting information is inhibited in these patients.”

This study will lay the foundations for further clinical studies to try to eliminate the impulsive side effects of Deep Brain Stimulation. Doctors recommend these patients to talk to their physicians in case they experience any impulsive side effects.