Scientists may be one step closer to explaining why, neurologically speaking, time seems to fly during good times and crawl during bad, and they have the humble mouse to thank for it.
Neuroscientists from Portugal's Champalimaud Centre for the Unknown report in a new study in the journal Science that they have discovered neurons in the mouse brain that can be manipulated to tinker with the rodent's judgment of elapsed time.
The scientists came by their finding by training mice on tasks that depended on their sense of time. In a statement, the study's lead author Bassam Atallah remarked on the difficulty of a getting a mouse to indicate its assessment of time duration: "There was real doubt whether it could even be done."
But the team was able, over a period of months, to get a group of mice to estimate the length of time that elapsed – shorter or longer than 1.5 seconds – between two tones that were played. Correct answers won them a treat.
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Then, the researchers zeroed in on something in the animal's brain they thought might bear fruit: dopamine-releasing neurons in a section called the substantia nigra pars compacta, an area previously known to be involved with the processing of time. In people with Parkinson's disease, a condition that includes hampered perception of time, the section is destroyed.
"Dopamine neurons are implicated in many of the psychological factors and disorders associated with changes in time estimation," the team explained in their study.
In early experiments, the scientists noticed a seeming link between the neurons' activity and the mice reacting to the tones that were played.
Later they found that they could stimulate the neurons with light to affect whether the mice underestimated or overestimated the duration between the tones.
"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," said study co-author Joe Paton.
The researchers think it's likely something similar may be going on in the human brain, when people perceive the passage of time, but they caution against excessive enthusiasm.
"When we study animals, the only thing we can measure is the animal's behavior. 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."
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