Dopamine evoked by reward cues helps us learn about our world, according to Concordia research

A new study in Nature Neuroscience was co-authored by three undergrads under the supervision of psychology prof Mihaela Iordanova
June 9, 2020
Mihaela Iordanova: “The extent to which organisms learn about their environment in a moment-to-moment basis naturally results in an ability to predict the next state of the world.”

Three students in the Faculty of Arts and Science have completed their undergraduate careers at Concordia with a prestigious feather in their caps: a research article published in the highly respected academic journal Nature Neuroscience.

Working under the supervision of Mihaela Iordanova, Concordia associate professor of psychology, Etienne Maes, Alexandra Usypchuk and Megan Lozzi conducted a series of experiments that clarified the role of the neurotransmitter dopamine in learning.

The researchers used a light-sensitive opsin to hamper the firing of dopamine neurons to reward-predicting cues. They found that cue-evoked dopamine signals function more as errors in predictions than as reward predictions.

Melissa Sharpe of UCLA and Chun Yun Yang, Matthew Gardner and Geoffrey Schoenbaum at the National Institute of Drug Abuse in Baltimore co-authored the paper.

Etienne Maes: “While we were disrupting the dopamine, we were not disrupting the prediction.” Etienne Maes: “While we were disrupting the dopamine, we were not disrupting the prediction.”

Getting without learning

By attenuating dopamine firing in the brains of rats, the researchers wanted to understand how the neurotransmitter affects learning about upcoming reward.

They used rats that had been genetically modified and expressed a light-sensitive opsin in dopamine neurons. This allowed them to limit the dopamine signal at precise points in time. The rats were placed in an environment where they were trained to expect a reward based on a specific cue, usually a kind of sound or flashing light. A cue that signals upcoming reward normally also evokes spiking in dopamine neurons. So, the researchers disrupted the firing of dopamine neurons by shining a light on them.

“We wanted to see if the rat still expected a reward,” Maes says. “And it did. While we were disrupting the dopamine, we were not disrupting the prediction. However, the disruption did prevent the rat from learning from that cue.”

This, they found, had a profound effect on associative learning: the cue with the attenuated dopamine signal would not reinforce learning to other cues that preceded it. This meant that the dopamine signal evoked by cues that predict reward did not represent a prediction about the upcoming reward but a prediction error based on successive temporal states. This, the researchers say, has implications for the way we learn on a moment-by-moment basis and how we predict our world.

“Our data provides evidence that each bit of time is predicting the next bit of time and this predictability results from the back-propagation of an error signal carried by the phasic firing of dopamine neurons across temporal intervals,” Iordanova explains.

“The extent to which organisms learn about their environment in a moment-to-moment basis naturally results in an ability to predict the next state of the world. This robs one of the excitement of surprise that comes with unpredictability. The extent to which individuals strive to reinstate the joie de vivre and engage in sensation seeking might correlate with this learning and the lack of dopamine response across predictable points in time.”

Maes will be pursuing his studies in the fall at McGill University, where he is enrolled in the MD-PhD program. His Concordia education, and specifically the minor offered by the university’s Science College in multidisciplinary science, provided him with a unique experience cooperating with fellow researchers from across multiple departments. The publication in Nature Neuroscience is a tremendous source of pride.

“To be published at all during undergrad is incredibly exciting and is a testament to the hours of hard work that my colleagues and I have devoted to this project,” Maes notes. “This publication from the Iordanova Lab further exemplifies the cutting-edge research being conducted at Concordia and the community spirit that encourages such collaborative projects.”

The study was supported by the Intramural Research Program at the National Institute on Drug Abuse (NIDA), grants from the Natural Sciences and Engineering Research Council of Canada (NSERC) and a Concordia Undergraduate Student Research Award.

Read the cited paper: “Causal evidence supporting the proposal that dopamine transients function as temporal difference prediction errors.”

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