Time seems to fly when you are having fun, while some days stretch endlessly. Scientists have succeeded in explaining this phenomenon by identifying for the first time the neurons that modulate judgments of elapsed time in the brains of mice.
Past studies in the field of psychology have shown that certain emotions could disrupt our perceptions of time. For example, being stressed out or bored may makes time appear longer than it really does but often feels like our weekend and holidays go by rapidly too.
In the new study published in the journal Science, researchers investigated the question of how the brain produces such variable, subjective estimates of time.
Because the passing of time is such an elusive concept, there were concerns that they would not be able to study the phenomenon from a neurobiological perspective.
It is indeed impossible to associate time perception to a particular organ – unlike vision or audition, time judgement cannot be traced back to the eyes or the ears, so finding the corresponding neurons is a complex task.
However, the scientists, from the Champalimaud Centre for the Unknown, in Lisbon, decided to look at a type of dopamine-releasing neurons in a part of the brain called the 'substantia nigra pars compacta', because these neurons are linked to the emotions that can change time perceptions.
How mice judge the passing of time
The scientists started by training mice to perform a task that involved estimating whether the duration of the interval between two sounds was shorter or longer than 1.5 seconds. To give their answer, the mice had to place their snout either right if the interval was shorter, or left, if it was longer. If they answered correctly, they received a reward.
Next, the researchers measured the activity of dopamine neurons in the substantia nigra pars compacta during the task, and identified an increase in neural activity during the two sounds, indicating that this area of the brain was involved in estimating how much time elapsed.
However, it wasn't clear at that point if these neurons actually controlled subjective perceptions of time or just objective estimations. "The neurons seemed to reflect information about the estimation of duration by the animals. But might they actually be controlling their sense of time?" asked lead author Joe Paton.
The scientists performed another experiment, using the technique of optogenetics lights that flicker at a certain frequency – to stimulate or silence neurons. It appeared that manipulating the neurons in the substantia nigra pars compacta in this way changed the mice's ability to succeed in the task, suggesting changes in their perceptions of time.
"We found that if we stimulated the neurons, the mice tended to underestimate duration, and if we silenced them, they tended to overestimate it", Paton explains. "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. This was the major result of our study."
This study had only been conducted in animals, so it is not clear whether these findings could be applied to humans, although the scientists believe possible that a similar circuit may be at work in our brains.