by Anthony J. Brown, MD

Last Updated: 2007-10-25 14:00:07 -0400 (Reuters Health)

NEW YORK (Reuters Health) – Deep brain stimulation (DBS) of the subthalamic nucleus is known to improve the motor symptoms of Parkinson’s disease, but it can also cause impulsivity. Now, new research suggests that this is because DBS interferes with the normal ability to slow down when making a difficult decision.

"DBS is widely known to cause behavioral disinhibition and other cognitive deficits, and as a result does not always improve patients’ quality of life despite dramatic improvements in motor symptoms," Dr. Michael J. Frank, from the University of Arizona in Tucson, told Reuters Health.

"Our findings point to a mechanistic basis for these observations, and suggest that patients should be monitored for their susceptibility to impaired decision making, and that these factors should be given greater attention preoperatively," he added.

The findings, which appear in the October 25th online issue of Science, also show that while dopaminergic agents do not impair the slowing down process, they do interfere with a patient’s ability to learn from bad decisions.

In the study, Parkinson’s disease patients and controls completed various computerized decision-making tasks. The difficult decisions typically involved win/win outcomes, whereas the easy decisions involved win/lose outcomes. Normally, difficult decisions take longer to make than easy ones.

Patients were evaluated while DBS was on and off and while they were on and off dopaminergic medication. The investigators found that with DBS on, the speed with which difficult decisions were made was increased and was actually faster than the speed with which easy decisions were made. With DBS off, the patients’ response speeds were comparable to those of controls.

L-Dopa use did not hasten the difficult-decision-making process, but it did impair learning from negative decision outcomes.

Areas for future research on DBS in relation to behavior, according to Dr. Frank, include looking at "whether there is a dose-response relationship (i.e., do the same effects hold at 50% voltage?)" and "whether there are particular subregions within the subthalamic nucleus that can be targeted by DBS and be selective to motor function while sparing effects on conflict-based decision making."

Science 2007.