Researchers have begun to understand the neurobiological basis for our differing time perceptions.
Some studies in psychology have shown that emotions impact the way we perceive time. Fear and stress distort time and make it appear longer than it really is, while days seem to fly by when we are on holiday.
Why does this happen? How does the brain "trick" us into thinking time is longer or shorter than it actually is? And could we locate the areas of our brain responsible for how we experience time?
In an attempt to answer these fundamental questions, a team of neuroscientists at the Champalimaud Center for the Unknown in Lisbon, Portugal, has investigated the neural activity in certain parts of the mouse brain.
The study - led by Joe Paton, head of the Learning Lab at the Champalimaud Neuroscience Programme - has been published in the journal Science.
Hypothesizing the role of dopamine in time perception
Dopamine is a brain chemical, or neurotransmitter, commonly associated with the brain's pleasure centers. Dopamine is involved in reward, motivation, learning, addiction, as well as in movement and attention.
Researchers at the Champalimaud Center hypothesized that dopamine-releasing neurons may also play a role in how we perceive time. They assumed this because dopaminergic neurons are found in a deep structure of the brain called substantia nigra pars compacta, and damage in this area has been noticed in neurological conditions where time perception is affected.
Substantia nigra is a large pigmented cluster of neurons located in the midbrain, and it is divided in two parts: the pars reticulata and the pars compacta. The cells in the latter part synthesize dopamine and "send" it to other areas of the brain responsible for movement, such as the striatum.
The substantia nigra pars compacta plays a key role in temporal processing. Its influence on time perception and movement can be seen in Parkinson's disease, a condition associated with the death of dopamine neurons in the pars compacta.
The research team had previously studied the striatum in rodents in an attempt to understand how the brain estimates and keeps track of time. Their research had revealed that removing the input of dopamine neurons to the striatum "can cause a selective deficit in timing."
Examining time perception in mice
For their new study, the scientists trained mice to perform a task that involved timing.
Assessing perception in animals is challenging, as animals cannot report on their experience. So for decades, scientists have trained animals to make categorical judgments instead, and they have used these judgments to draw conclusions on what the animals might be experiencing.
In order to assess the perception of time, researchers trained mice to "estimate whether the duration of the interval between two tones was shorter or longer than 1.5 seconds." Paton explains that the mice became "pretty good" at this after months of training.
Mice indicated their choice by placing their snout either at a right port for a shorter interval, or a left port for a longer interval.
The interval between tones was made to vary during the test, and scientists rewarded the mice when they estimated the time correctly.
In the second part of the research, the team was concerned with examining the electrical activity of dopamine neurons in the substantia nigra pars compacta.
To do this, they used modern molecular and genetic tools to measure and manipulate dopamine neurons on a fast timescale.
Using fiber photometry, scientists measured the signals that reflected the electrical activity of dopamine neurons. More specifically, they made the neurons fluoresce when active and measured the intensity of the emitted light.
Dopamine neurons associated with time perception
"[Fluorescence] is an indicator of the electrical activity of a number of these neurons around the optical fiber tip, so this allowed us to indirectly monitor the variation of those neurons' activity during the task," explains Paton.
Researchers noticed that the neural activity increased at the start of the first and second tones. This indicated that the dopamine neurons were actively involved in the task.
More importantly, the neural activity varied in intensity. Scientists were able to correlate the amplitude of neural activity with the animals' judgment of time.
"What we saw was that the bigger the increase in neural activity [at the first and the second tone], the more the animals tended to underestimate the duration of the interval," says Ph.D. candidate and study co-author Sofia Soares. " And the smaller the increase, the more the animal overestimated duration."
Researchers were thus able to establish a clear association between dopamine neurons and how the brain perceives time.
However, researchers also needed to establish causality.
Judgment of time controlled by dopamine neurons
To do this, they had to see if the neural activity can actually induce changes in how time is perceived.
With the help of a technique called optogenetics, researchers selectively and quickly manipulated dopamine neurons in order to see how they impacted the animals' ability to assess time intervals.
In optogenetics, brain cells are genetically modified to become light responsive. The technique allows scientists to target and control specific cells using light sensitivity. The technique is most commonly used in neuroscience.
Having modified the mice's neurons, the team then "silenced" and "activated" dopamine neurons using light.
"We found that if we stimulated the neurons, the mice tended to underestimate duration, and if we silenced them, they tended to overestimate it. 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."
Results likely to apply to humans
It is very likely that these results can be extrapolated to humans, Paton explains, as the neural circuitry is probably similar.
However, he cautions that given the limitations of rodent studies, we cannot make definitive claims about what the mice "perceived."
Animals cannot report on their own perceptions, so what scientists have measured in this study is not a percept per se.
"When we study animals, the only thing we can measure is the animal's behavior. But we are never sure of what they perceive," he says. "We interpret this as 'a subjective experience of the animal,' but it is no more than an interpretation. And that is the best we can do."
If the same mechanism does apply to humans, however, this could have significant implications for the treatment of disorders that involve dysfunctions in the dopaminergic system. Such conditions include attention deficit disorder and substance addiction.