A new neuropsychological memory test may help explain why many Parkinson’s disease patients develop depression.
The test helps to understand how Parkinson’s disease can alter people’s ability to learn about the consequences of the choices they make.
Test developer Dr. Mark Gluck found that nonmedicated patients in the early stages of Parkinson’s were selectively impaired at learning from reward. His findings will be published in the journal Brain.
“What we are seeing in recently diagnosed patients is that prior to being put on any medications, they exhibit a selective impairment in their ability to learn from positive (rewarding) outcomes while their sensitivity to learning from negative (or punishing) outcomes is normal,” says Gluck, director of Rutgers’ Center for Collaborative Research on Cognition and Parkinson’s Disease.
This selective deficit in learning from reward is not surprising, says Gluck, because scientists have long known that dopamine is used to carry reward information throughout the brain. Parkinson’s patients, however, have lost most of their dopamine-producing cells by the time they are first diagnosed with the disease.
This decrease in their ability to process rewarding outcomes could be one reason why many Parkinson’s patients experience depression, says Gluck. “It’s not just that they have an awful disease, but they have lost the ability to process the rewarding aspects of life.”
Gluck and colleagues found that this reward-learning deficit in unmedicated Parkinson’s patients is in direct contrast to what is seen in patients once they begin treatment with dopamine agonists, a standard therapy for treating the disease’s motor symptoms.
On dopamine agonists, a patient’s ability to learn from positive rewarding outcomes improved to normal levels. But there was a catch – their ability to learn from negative (or punishing) outcomes, which had previously been normal, was now impaired.
Gluck explains that an increased sensitivity to learning from events that results in positive outcomes, coupled with a decrease in the ability to learn from negative outcomes, could explain why some Parkinson patients treated with dopamine agonists develop impulse-control disorders, including pathological gambling, hypersexuality, alcoholism, and compulsive eating and shopping.
All of these behaviors can be understood as reward-seeking behaviors in the absence of appropriate sensitivity to their negative consequences.
“For example,” notes Gluck, “if your ability to learn from negative outcomes is reduced and you play the slot machines and win $10 for a few rounds but lose many more times in between, what you may recall best is the thrill of winning. As such, you will be hampered in your ability to learn that gambling can also have negative consequences.”
The ability to test the effects on feedback learning in early onset Parkinson’s disease could provide additional insight into the impact of dopamine loss on cognition and behavior. It also could pave the way for identifying which Parkinson’s patients are most likely to experience agonist-related feedback problems so they can be treated with alternate medications.