Measuring Dopamine in Real-Time Reveals New Information About Reward
Abstract
Data obtained from microsensors implanted into the striatum suggest dopamine integrates the differences in actual and expected outcomes along with what outcomes might have occurred if a different action was taken.
In a landmark study, scientists have managed to measure and track real-time dopamine activity in the human brain—providing an unprecedented look at a neurotransmitter that is known to be involved with learning and decision making and whose absence or dysfunction is associated with a range of neurological and psychiatric disorders.
A total of 17 patients with Parkinson’s disease (PD) scheduled to have electrodes implanted for deep brain stimulation (DBS) volunteered to also have researchers implant carbon fiber microsensors into the striatum. The striatum is one of the brain’s reward centers and a region rich with dopamine neurons. These cells are gradually destroyed during the progression of PD, impairing motor function and resulting in other symptoms of the disease.
After the sensors were implanted, researchers measured the striatum dopamine levels of the participants as they played a financial investment game that mimicked betting on the stock market. This allowed the researchers to observe dopamine activity at the microsecond level as the patients achieved virtual rewards and losses.
Using an innovative microsensor on 17 volunteers undergoing deep brain stimulation, Read Montague, Ph.D., and his colleagues have taken the first highly detailed dopamine measurements in living humans.
As senior author Read Montague, Ph.D., director of the Human Neuroimaging Laboratory at Virginia Tech Carilion Research Institute described, the resulting measurements painted a more complex picture of dopamine’s role in decision making than was previously believed.
“We had thought that dopamine release during decision making represents reward error—that is, the difference between the expected and actual outcome. But this was an incomplete story,” he said. “Rather, these dopamine pulses integrate the differences in actual and expected outcomes along with what outcomes might have happened with a different decision.”
The idea that “what could have been” (counterfactual reward) influences how an individual feels about an outcome is not new, but the idea that dopamine regulates both of these reward pathways—actual reward and counterfactual reward—is, according to the study authors.
After decades of using animal models and less sensitive tools to gauge dopamine levels in humans, the findings highlight the advantage of being able to measure real-time dopamine activity in humans. Such insight could serve to advance understanding of the neural mechanics behind reward processing, including the relationship between dopamine misfiring and psychiatric disorders such as depression and addiction, as well as in impulse-control problems that are often reported in PD patients on dopamine therapy.
Montague acknowledged that since all the participants in the current study had PD, these results may not represent how dopamine operates in a healthy individual.
However, because previous studies suggest that decision making remains largely unaffected in patients with PD, he said that he believes the findings may be similar in people without PD.
The study by Montague and his colleagues was published December 4, 2015, in Proceedings of the National Academy of Sciences. The study was funded by the Wellcome Trust, the Kane Family Foundation, and Virginia Tech. ■
An abstract of “Subsecond Dopamine Fluctuations in Human Striatum Encode Superposed Error Signals About Actual and Counterfactual Reward” can be accessed here.
