The study, “Deep brain stimulation of the subthalamic nucleus and the temporal discounting of primary and secondary rewards,” was published in the Journal of Neurology.
Although DBS of the subthalamic nucleus (STN) — a brain region hyperactive in Parkinson’s and implicated in motor control — is an effective way to ease the disease’s hallmark motor symptoms, it may induce confusion and changes in behavior and in decision-making processes, such as those regarding food. This may lead to a tendency toward risky decision-making in gambling tasks and increased impulsivity.
Aiming to better understand impulsivity in this patient population, a research team from Italy used a task called delay discounting, in which the patients had to decide whether they preferred an immediate, smaller reward over a larger one later. A preference for the immediate reward would be expected from impulsive participants. However, studies using monetary rewards in people with Parkinson’s who underwent DBS have led to contradictory results.
While money is a secondary reward, food is a primary reward as it is necessary for one’s survival. In rats, stimulation of the STN increased the motivation to obtain food, but reduced motivation toward cocaine, a secondary reward.
In accordance, Parkinson’s patients undergoing DBS attributed increased value to food rewards, which was associated with weight gain, a common non-motor side effect of the surgery.
The 45 participants were divided into three groups: 15 Parkinson’s patients who underwent bilateral DBS into the STN (mean age 62.8 years, 4.4 years since the surgery), 15 patients without DBS on dopaminergic replacement therapy (mean age 69.1 years) and 15 healthy controls (mean age 67.7 years).
All patients were tested under their best medical treatment condition, which means on stimulation in DBS and on medication for dopaminergic treatment.
All participants performed three computer tasks where they had to choose a smaller reward immediately or a reward that was larger but delayed — by two days, two weeks, one month, three months, six months, or one year.
“In these tasks, the choice usually depends on the time that passes between one option and the other: if it is very short, delayed gratification is chosen and vice versa,” Marilena Aiello, the study’s lead author, said in a press release.
In the food task, the participants were asked to choose from six foods, three sweet and three salty. When using discount vouchers as rewards, they had to choose from six hobbies or activities presented through pictures — a theater trip, a restaurant visit, book purchases, gardening equipment, sewing tools, or tools to paint — for which they would like to get a discount voucher.
The results revealed no differences in food or discount voucher choice comparing the three groups, even in patients who experienced weight gain and/or eating alterations after DBS.
The data further showed greater preference for immediate rewards — greater impulsivity — in patients with fewer years since DBS, on higher doses of levodopa, and with better memory.
“Our study confirms that patients with DBS are no more impulsive in this kind of situation and they do not try to find gratifications more hastily than the others,” Aiello said.
The investigators said, “In addition, our results also extend the current literature by showing, for the first time, that this result is independent of the type of reward.”