Time stands still when you're stuck in traffic. It flies when you're binge-watching Stranger Things.

A team of neuroscientists in Portugal has discovered a group of neurons in the brains of mice that may help explain why our perception of time passing — either slowly or quickly — is so subjective.

The results of the investigation, conducted at the Champalimaud Centre for the Unknown in Lisbon, were published Thursday in the journal Science.

The team says its findings are the first to pinpoint a neural circuitry that affects the judgment of elapsed time.

Teaching mice to judge time

In order to do so, however, the group spent several months training the mice to judge how much time passed between two sounds.

"We trained mice to estimate whether the duration of an interval between two tones was shorter or longer than 1.5 seconds," said principal investigator Joe Paton in a statement about the findings. The mice indicated their choice — longer or shorter — by placing their snouts either on a left or right port.

The next step was to examine what was going on deep in the brain when the animals made their estimates.

Using a technique called fiber photometry, the researchers began to measure signals that reflected the electrical activity of dopamine neurons in a part of the brain called the substantia nigra pars compacta, known to play a role in processing how time passes.

Scientists have understood for some time that, in humans, the substantia nigra is associated with our internal clocks. In fact, destruction of this part of the brain is a cause of Parkinson's disease, one of the symptoms of which is impaired perception of time.

Watching this area of the brain in their rodent subjects was illuminating for the researchers, who found that when neural activity was high the animals tended to underestimate the amount of time that had passed. In other words, time seemed to be passing quickly.

The team then performed a round of experiments to see if they could confirm a causal link between a higher level of neural activity and this apparent "faster" passing of time. They used light to stimulate neurons in the animals' brains and note how that impacted their performance on the task.

"We found that if we stimulated the neurons, the mice tended to underestimate duration, and if we silenced them, they tended to overestimate it," said Paton.

"This result, together with the naturally occurring signals we observed in the previous experiments, demonstrate that the activity of these neurons was sufficient to alter the way the animals judged the passage of time."

A similar phenomenon could be at work when, stuck in that traffic jam with nothing to occupy it apart from your to-do list and worrying about getting to your destination on time, your brain overestimates the amount of time passed.

The researchers' findings could help explain why time seems to pass so slowly when you're stuck in that traffic jam with nothing to do but ruminate on your to-do list and worry about getting to your destination on time. (Darryl Dyck/Canadian Press)

Paton said the research has never been more personally relevant to him than it was recently when two of his friends were in a serious accident.

"The few hours between when we knew about the accident and when we knew that they would be okay … felt like weeks. In retrospect, I wonder what role these neurons we have been studying might have played in that illusion," said Paton in the statement.

Perception of time plays a more critical role than simply determining if a brain is bored or entertained.

"Timing is important for extracting information from the environment and deciding when to expect something to happen or when to engage or disengage from an action," said Paton.

For example, the longer it lingers, a rabbit feeding in an open field increases its chances that a predator will sneak up.

What it means for humans

While the authors say it is very likely that a similar circuit is at work in the human brain, the findings are limited by the fact that the animals can't tell researchers what they felt.

"When we study animals, the only thing we can measure is the animal's behaviour. But we are never sure of what they perceive," said Paton.

"We interpret this as 'a subjective experience of the animal' but it's no more than an interpretation. And that's the best we can do."