For those who suffer with the debilitating symptoms of Parkinson's disease, deep brain stimulation offers relief from the tremors and rigidity that can't be controlled by medicine. A particularly troublesome downside, though, is that these patients often exhibit compulsive behaviors that healthy people, and even those taking medication for Parkinson's, can easily manage.
Michael Frank, an assistant professor of psychology and director of the Laboratory for Neural Computation and Cognition at The University of Arizona, and his research colleagues have shed some light on how DBS interferes with the brain's innate ability to deliberate on complicated decisions. Their results are published in the Oct. 26 issue of the journal Science.
DBS implants affect the region of the brain called the subthalamic nucleus, known as STN, which also modulates decision making.
"This particular area of the brain is needed for what's called a 'hold-your-horses' signal," Frank said. "When you're making a difficult choice, with a conflict between two or more options, an adaptive response for your system to do is to say 'Hold on for a second. I need to take a little more time to figure out which is the best option.'"
The STN, he said, detects conflict between two or more choices and reacts by sending a neural signal to temporarily prevent the selection of any response. It's this response that DBS seems to interrupt. DBS acts much like a lesion on the subthalamic nucleus. Frank's hypothesis predicted that DBS would negate the "hold-your-horses" response to high-conflict choices. Surprisingly, it actually sped up the decision-making process – a signature, he said, of impulsive decision making.
The tendency toward impulsive behavior in Parkinson's patients is well-documented but only dimly understood. How is the STN involved in decision making and why should things go awry when you stimulate it?
For those taking them, medications did not slow down decision-making conflict. Regardless of whether these patients are on or off medication, for the purposes of the experiment they looked like healthy people or people who were off DBS.
But what Frank found was that medications prevent people from learning from negative outcomes of their choices. That could be one explanation for why some patients develop gambling habits. If you learn from the positive outcomes instead of the negative, it could cause you to become a gambler.
"Whereas the DBS had no effect on positive versus negative learning, it had an effect on your ability to 'hold your horses,' so it was a dissociation between two treatments which we think reveal different mechanisms of the circuit of the brain that we're interested in,” Frank said.
Frank said the results of his experiments are a test of a basic science mechanism for how the brain makes adaptive decisions. The same basal ganglia is involved in other disorders. People who are addicts, for example, are more likely to make impulsive choices, and DBS and medication used to treat Parkinson's have been shown to cause pathological gambling to some degree.
"We may be able to use this to understand a more basic sciences perspective. Maybe the same circuits are involved in gamblers who don't have Parkinson's," Frank said.
He also hinted that the study might offer clues to consumer behavior.
"I think that you can have the opposite effect, where the hold-your-horses signal is too strong in responding to decision conflict. One thing that has been shown in healthy people who have been presented with too many options is a kind of 'decision paralysis,'" he said.
For example, if shoppers are exposed to two dozen varieties of essentially the same product, research shows very few will actually make a purchase. Employees faced with too many options for 401k plans are less likely to invest in any of them, even though their employer is going to match their contributions.
Frank also is interested in whether impulsive decision making can be prevented in DBS patients. One long-range goal, he said, is to be able to test the STN during the implant surgery, avoiding the decision-making areas and targeting only the brain's motor function.
“We hope that in the operating room we can determine selective parts of the brain that respond to this conflict-based decision making and use that to avoid stimulating that area, and have it be selective to just the pure motor function,” he said.
Frank's collaborators include Johan Samanta of the UA neurology department and Banner Good Samaritan Medical Center in Phoenix; Ahmed A. Mousafa of the UA psychology department; and Scott J. Sehrman of the UA neurology department.
CONTACT: Michael Frank (520-626-4787; firstname.lastname@example.org)
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