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 Priz