New research finds that the subjective experience of time is linked to learning, thwarted expectations and neural fatigue.

By Jordana Cepelewicz
Staff Writer

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Last month in Nature Neuroscience, a trio of researchers at the Weizmann Institute of Science in Israel presented some important new insights into what stretches and compresses our experience of time. They found evidence for a long-suspected connection between time perception and the mechanism that helps us learn through rewards and punishments. They also demonstrated that the perception of time is wedded to our brain’s constantly updated expectations about what will happen next.

“Everyone knows the saying that ‘time flies when you’re having fun,’” said Sam Gershman, a cognitive neuroscientist at Harvard University who was not involved in the study. “But the full story might be more nuanced: Time flies when you’re having more fun than you expected.”
Time to Learn

“Time” doesn’t mean just one thing to the brain. Different brain regions rely on varied neural mechanisms to track its passage, and the mechanisms that govern our experience seem to change from one situation to the next.

But decades of research suggest that the neurotransmitter dopamine plays a critical role in how we perceive time. Dopamine has myriad effects on how much time we think has elapsed in a given period, and these effects may conflict confusingly. Some studies have found that increasing dopamine speeds up an animal’s internal clock, leading it to overestimate the passage of time; others have found that dopamine compresses events and makes them seem more fleeting; still others have uncovered both effects, depending on context.

Dopamine’s association with time perception is intriguing, in part because the neurotransmitter is better known for its function in reward and reinforcement learning processes. When we receive an unexpected reward, for instance — in what’s known as a prediction error — we experience a rush of the chemical, which teaches us to continue pursuing that behavior in the future.

It’s likely more than a coincidence that dopamine is so fundamental to both time perception and learning processes. Drugs like methamphetamine and neurological disorders like Parkinson’s alter both processes and also involve changes in dopamine. And learning itself — the association of a behavior with its outcome — requires the linking of one event with another in time. “Really, at the very core of reinforcement learning algorithms is information about time,” said Joseph Paton, a neuroscientist at the Champalimaud Foundation in Portugal. (Paton was an investigator for the Simons Collaboration on the Global Brain, funded by the Simons Foundation, which also funds Quanta Magazine.)

But scientists have yet to disentangle just how and where reinforcement learning and time perception are integrated in the brain. Instead, “the two fields have traditionally stayed quite separate,” said Martin Wiener, a psychologist at George Mason University. “No one has asked, ‘How does reinforcement learning affect timing, or vice versa, if they’re both using the same neurotransmitter system?’”
The Power of the Prediction Error

The new Nature Neuroscience paper by Ido Toren, Kristoffer Aberg and Rony Paz examines that question more closely. Study participants saw two numbers flash on a screen, usually a zero followed by another zero. The second number was shown for a varying amount of time, and participants had to report which number lasted longer. But sometimes, randomly, a positive or negative integer was presented instead of that second zero: If it was positive, participants were rewarded with money, but if it was negative, money was taken away as a penalty.

For the participants, the consequences lined up with shifts in the perceived duration of the second stimulus. When something unexpected but good happened — a “positive prediction error,” as researchers called it — the stimulus seemed to last longer. The unwelcome surprises of negative prediction errors made those experiences seem shorter. “It basically tells us that our perception of time is going to be systematically biased by how surprised we are about outcomes,” said Matthew Matell, a psychologist at Villanova University who was not involved in the study.

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