As cryptic as the concept of time may be, the average human mind is adept at making sense of it in ways presently unknown to scientists. The search for the answer to how our mind seamlessly decodes time, and sometimes even twists it, has unmasked many surprising findings over the years, while also leaving several mysteries unresolved. Could it be individual cells that track time tirelessly? Or complex circuits working in unison that dial in temporal information to our conscious experiences? Or both? In this article, we explore the journey of science as it strived to understand this fundamental function of our brain and recount a few of the hallmark experiments in the field.
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Have you ever felt that you could lose time?
If you look into a mirror and move your eyes back and forth, alternating your gaze between your left and right eyes, do you really see your eyes move? When you shift your eyes from one position to the other, intuitively, we know that this movement should take some time. But you feel that there is no perceivable break in time between the gaze shifts.
Now ask a friend to shift their gaze very fast. You will be able to see their eye movement quite clearly. This implies that the time gaps between the gaze shifts are, in fact, (theoretically) perceptible. So, what just happened?
Humans have always puzzled over the evasive nature of time. Egyptian sundials dating back to 1250 BCE show that even ancient civilizations were aware of the flow of external time. Practices of timekeeping with calendars and clocks were well established by the 16th century. While most cultures believed in a linear passage of time, some schools of thought like Hinduism or Buddhism espoused a cyclic view of time: death leading to rebirth and renewal. Observations of plants responding to sunlight led to early notions of living organisms having an internal clock directing their biological rhythms (Falk, Lutz, and Shmahalo, 2020). These so-called circadian rhythms, cycling over 24 hours, help organisms adapt to a change in the day-night cycle. But these rhythmic phenomena are not the sole contributor to our perception of time.
STOPWATCHES IN THE BRAIN
In a deep structure embedded in our brain, called the hippocampus, there are some special cells that form a signal to track precise timespans. They perform a crucial role in integrating space, time, and memory. How do researchers map these cells? In these experiments, a rat is made to run on a treadmill for a certain period (10 or 15 seconds, say) and rewarded upon successfully completing the sprint. The rodent eventually learns to track this time interval as it performs the task again and again. Recording the neural activity of the rat’s brain in real-time enables scientists to develop a “code” of cells that fire at each second. Defined sets of neurons get activated sequentially, behaving like a plastic stopwatch. Plastic, because they can be easily reprogrammed to track a different time interval, say by making the rat run for 30 seconds instead of 15. And, if the rat is allowed to explore an unknown arena at its own free will, these time cells take on a different role as other variables come into play. They now start mapping space and location instead of time (Singer, 2016).
Experimental set-up showing a rat subject on a treadmill, with neural firing patterns being recorded. The hippocampal “time cells” have a discrete activity pattern.
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However, things are rarely this simple in nature. A fundamental question that remains to be addressed is whether there are cells allocated solely for tracking time. What the experimenter construes as “time tracking” might be nothing more than a bunch of cells firing in sequential order, one after the other. Drawing a parallel with the space-time continuum in Physics, where the universe is viewed in a fused four-dimensional framework, the human brain’s representation of time may also be a similar continuum.
A TWIST IN TIME
The idea of time cells has been around for several years now, earning their discoverers the Nobel Prize in 2014. More recent research by a Norwegian team of scientists led by Albert Tsao has devised the correlation between how we perceive time and the formation of unique memories (Nilsen, 2018). In a nutshell, our brain receives information from the different senses and encodes memories in a continuous fabric of space and time. This does seem rather confusing. Let’s think of the times when our brain seemed to warp time strangely. Like in the example at the beginning of this story where you looked into the mirror and tried to follow your gaze. Time flies when you are having fun. It drags while waiting in an endless queue. And it can stop entirely in the mere moments before a fatal accident.
Neuroscientist David Eagleman and his team at Stanford University had designed an experiment to test if time can slow down in dangerous situations. Volunteers were subjected to a simulation where they were forced into a hyper-perceptive state – dropping them from 150 feet in the air. While they fell, they were strapped with a piece of gear called the perceptual chronometer. This elegant device had an LED display of rapidly changing numbers. The hypothesis? Well, if time did actually slow down in that life-threatening situation, the volunteers should be able to discriminate the changing numbers easily. But as it turned out, none of them were able to actually “see in slow-motion” (Eagleman, 2015).
TIME AND MEMORY: TWO SIDES TO A COIN
In addition to the hippocampus, another interesting brain region called the amygdala exists, which functions as a secondary memory formation system. It comes into action in such dire circumstances, recruiting all the available brain machinery to tackle the impending doom. Memories are captured in more vivid detail, allowing the brain to have more information when things get scary. But the default state of the brain is not to have such fine-tuned memories. And so, when you recall the catastrophic event (assuming you live to tell the tale, of course), it seems that your reality played a trick on you. Just because the memories were laid out so “memorably,” you interpret it as having taken a longer time.
Schematic of the human brain showing locations of the structures implicated in the experience of time in the amygdala, and the hippocampus.
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So, time can also be interpreted as a function of the information entering our brain. We may experience time using our timer cells as it passes. Or it could be a correlate of the energy spent to represent sensory information (making memories). The greater is the quantity of information bombarding the brain, the slower time passes. Reduce that information and time races.
Our understanding of time perception is still blurry, with several questions waiting to be probed. Will the quest to find the exact cellular players, brain circuits, and mechanisms controlling time be successful? Perhaps, only time will tell.
 Eagleman, D. (2015). The Brain: The Story of You (pp. 71-77). New York: Pantheon Books, Penguin Random House LLC.
 Falk, D., Lutz, E., & Shmahalo, O. (2020). Arrows of Time. Retrieved 22 May 2021, from https://www.quantamagazine.org/what-is-time-a-history-of-physics-biology-clocks-and-culture-20200504/
 Nilsen, R. (2018). How Your Brain Experiences Time - NTNU. Retrieved 22 May 2021, from https://www.ntnu.edu/how-your-brain-experiences-time
 Singer, E. (2016). New Clues to How the Brain Maps Time | Quanta Magazine. Retrieved 21 May 2021, from https://www.quantamagazine.org/new-clues-to-how-the-brain-maps-time-20160126/
Author: Sukanya Chakraborty
5th year MS student at Indian Institute of Science Education and Research, Berhampur majoring in Biological Sciences.
Editors: Shreyas Gadge, Project Encephalon