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Time, that frustratingly finite resource, and the ruling construction of our lives, still confuses neuroscientists. What’s especially puzzling is why time can seemingly go by at a frigid pace when you’re doing something like waiting in line, but hours seem to pass in minutes during enjoyable activities, like going to a concert.
In other words: Why does time fly when you’re having fun?
In a recent study, researchers went one step closer to answering this perplexing question. By studying people’s brain activity during time-manipulating tasks, scientists discovered that there are time-sensitive neurons that fire in response to a specific period of time. These time-oriented neurons are found in the supramarginal gyrus (SMG), part of the right parietal cortex, which is also involved in the perception of space and movement.
When exposed to repeated stimuli that last the same amount of time, these time-sensitive neurons can wear out. Meanwhile, other neurons continue to function normally, creating an imbalance that ultimately skews the perception of time.
Based on these findings, if someone is shown the same five-second video 50 times, for example, they may have trouble estimating a later clip, even if it is much longer or shorter.
These findings were published Monday in the Journal of Neuroscience.
Masamichi Hayashi is the co-author of the study. He is a researcher affiliated with the Center for Information and Neural Networks, the National Institute of Information and Communications Technology, the University of Osaka, and the University of California, Berkeley. Hayashi has long investigated the neural basis of time.
Hayashi emphasizes Reverse that this discovery is “only the first step” towards a full understanding of the neural mechanisms of subjective temporal experiences. Still, he says there are some practical conclusions.
He says one message he can give is: “Don’t trust your sense of time, especially after you’ve been exposed to stimuli with a constant interval that can lead to neuronal fatigue of the time-sensitive neurons in your brain.”
To discover how the brain creates the experience of time, Hayashi and his team used functional magnetic resonance imaging (fMRI) to measure brain activity while 18 healthy adults participated in a time comparison task. This technology measures small changes in blood flow that occur with brain activity.
“While humans have sensory organs designed to perceive the lights of vision or the pitch of sounds, there is no specific organ for the perception of time,” explains Hayashi. “That means our sense of time is probably the product of our brain activity.”
In the time comparison task, participants viewed a gray circle on a screen for a set period of time, 30 times in a row. This was considered the adaptation period. The participants then calculated how long it lasted, indicating the “duration of the adapter.” After this initial adaptation period, the group was shown a test stimulus (either a gray cross on a screen or a white noise sample) and again the estimated time.
This adaptation procedure allowed researchers to manipulate perceived duration by keeping physical duration constant.
How does the brain perceive time?
When the duration of the adapter (the time of exposure to the gray circle) was long, the participants underestimated the time of the test stimulus; if it was short, they overestimated the time of the test stimulus.
When the adapter and the test stimulus were of similar duration, the activity in the SMG decreased, suggesting that the person’s neurons had become fatigued. People’s time distortion was correlated with how much activity decreased in their SMG.
the more tired neurons seemed to be, the the worst people were in the estimated time.
“Our finding suggested that time-sensitive neurons in the parietal cortex fatigue after exposure to a repetitive presentation of a specific time interval and that results in a distortion of time perception,” Hayashi explains.
Specifically, the results suggest that activity in the correct SMG reflects humans’ subjective experience of time.
Interestingly, these time-sensitive neurons operate similarly to other neurons that underpin how humans subjectively experience space.
“It was already known that basic spatial characteristics, such as the orientation of a bar or its movement, are represented by a population of neurons that are sensitive to a specific orientation and direction of movement,” Hayashi explains.
“Surprisingly, despite the fundamental difference between how we perceive lights and time, we found that time is likely encoded by time-sensitive cells, and that activity is associated with the way we humans perceive time.”
Someday, researchers may manipulate people’s subjective sense of time stimulating these time-sensitive neurons. Until then, people will have to find other ways to spend time in waiting rooms and boring work presentations and hope their brain cells don’t get too tired.
Summary: It has been hypothesized that the perception of duration in the range of subseconds is mediated by the response of the population of duration-sensitive units, each tuned to a preferred duration. One line of support for this hypothesis comes from neuroimaging studies showing that cortical regions, such as the parietal cortex, exhibit a duration adjustment. It is not clear whether this representation is based on the physical duration of the sensory input or the subjective duration, an issue that is important given that our perception of the passage of time is often not true, but is skewed by various contextual factors. Here we use fMRI to examine neural correlates of subjective perception of time in human participants. To manipulate perceived duration by keeping physical duration constant, we employed an adaptation method, in which, before judging the duration of a test stimulus, participants were exposed to a train of adaptive stimuli of fixed duration. Behaviorally, this procedure produced a pronounced negative side effect: a short adapter predisposed participants to judge stimuli as longer, and a long adapter predisposed participants to judge stimuli as shorter. Duration tuning modulation, manifested as an attenuated BOLD response to stimuli of similar duration to that of the adapter, was only observed in the right supramarginal gyrus (SMG) of the parietal lobe and the middle occipital gyrus, bilaterally. In all individuals, the magnitude of the behavioral side effect was positively correlated with the magnitude of modulation of duration tuning in SMG. These results indicate that the duration-adjusted neuronal populations in the right SMG reflect the subjective experience of time.
