How do you make a tough choice? For most people, the answer is to pause and deliberate, as the brain puts the brakes on its initial impulses. Deep brain stimulation (DBS) used to treat Parkinson’s disease, however, can interfere with this braking action, according to new research that is starting to reveal the anatomy of choice.
Parkinson’s disease, which affects at least half a million Americans, is a progressive neurological disorder that is characterized by tremors and difficulty initiating and controlling movement, and can lead to dementia. Deep brain stimulation — which uses a device implanted in the brain like a pacemaker — can reduce symptoms when drug treatments fail.
DBS affects a part of the brain called the subthalamic nucleus (STN), which in turn affects multiple systems, according to Michael Frank, professor of cognitive, linguistic and psychological sciences at Brown University. During decision-making, it inhibits impulsive urges originating in a brain region known as the striatum. This allows a “pause,” during which slower deliberation can take place in an area called the medial prefrontal cortex (mPFC).
The STN also affects movement in Parkinson’s patients. “If the STN is really active, people have a hard time initiating movement,” Frank says, which helps explain why people with Parkinson’s may suffer from physical immobility or undirected shaking or become trapped in this “pause.”
Consequently, DBS is used to regulate STN activity to ease movement. If it overshoots, however, it may allow impulses from the striatum to slip past the mPFC. That can sometimes lead to impulsive, unwise decision-making — and an increased risk of addictive disorders like gambling problems.
(Interestingly, the specific activity of the STN may also explain why Parkinson’s patients experience tremors: these occur at the same frequency as the activity seen in this brain region. The computer model used in the study predicts that tremor might increase in frequency during decision conflicts as STN activity rises.)
To figure all these connections out, Frank and his team recruited 65 healthy participants and 14 people with Parkinson’s being treated with DBS for the study. The volunteers were given a decision-making task on a computer in which they had to choose between pictures that they had learned to associate with different levels of reward.
When the choice was difficult — for example, when faced with two pictures whose values were nearly equal — mPFC activity increased in all participants. Both healthy participants and people with Parkinson’s, who had their DBS turned off, took longer to make up their minds.
But when DBS was activated in Parkinson’s patients, they made decisions quickly and impulsively — even though mPFC activity was still seen. The mPFC wanted to deliberate, but deep-brain stimulation prevented the STN from hitting the brakes on impulsive urges.
While impulsivity as a side effect of DBS can currently be managed when it occurs, Frank’s research suggests that tweaking the stimulation to better allow communication between the STN and the mPFC could improve treatment.
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Activity in the STN may also be involved in the “paradox of choice,” in which people become overwhelmed by having too many options. In one famous study of this phenomenon, researchers found that people were easily able to choose between four types of jam in a supermarket, but when faced with 12 different options, they couldn’t bring themselves to decide at all.
“We think this mechanism is at play, where too much conflict [puts] a brake on the decision-making system. The brain did not evolve in a world where we are given a myriad of options in a supermarket,” Frank says. “The sort of decision paralysis observed in that case might be a byproduct of a the healthy tendency to pause in the face of conflict.”
This kind of mental incapacitation may also play a role in various psychiatric disorders. For example, people with depression often feel “stuck” because their inability to experience pleasure makes it impossible for them, when faced with any kind of choice, to judge one option as better than the other.
In people with addiction, the reward systems involving the striatum are trained to respond as people repeatedly experience pleasure by taking a drug. To quit successfully, the STN needs to prevent the striatum from initiating the behaviors needed to get high. “Any way in which that circuit can be trained up or enhanced could potentially be helpful for people with addictions,” says Frank.
The study was published in Nature Neuroscience.