The Passage of Time and Your Brain
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I’m sure that you’ve noticed, on various occasions, that time flies by too quickly. Like when it’s a sunny day and your kid steps up to the plate at the Little League game. On other occasions, each minute drags on like an hour. Like when the rain clouds roll in and the other eight kids take their turns at bat. There’s a lot that transpires in your brain (spanning far flung regions, involving unexpected brain cells and dependent on the production of a unique protein), which makes the passage of time so subjective.
Timekeeping regions of your brain
Do you believe that the universe had a long ago beginning and will one day end? Or do you think that we’re living through a brief moment, which is an infinitesimal part of an infinitely repeating cycle? I don’t know if that argument will ever be settled. But I can tell you that your suprachiasmatic nucleus and your entorhinal cortex would fall on opposite sides of the debate.
Time as Ouroboros: You’ve almost certainly seen an image of a serpent eating its own tail. It's a symbol of infinite time, a unification of beginning and end. The Ouroboros relates to how a part of our brain experiences the passage of time: The circadian rhythm.
The circadian rhythm is the body’s internal clock. It’s set to a twenty four hour cycle, which corresponds to the earth spinning on its axis during the passage of a twenty-four-hour day. Like the symbol of the Ouroboros, the circadian rhythm is an endless cycle: one day’s end is the next day’s beginning. The circadian rhythm goes on and on. It began when our earliest evolutionary relatives walked the earth and will continue when our distant descendants explore the furthest reaches of the galaxy.
The circadian rhythm is not your body’s only internal clock, though. Another internal clock is set to a monthly rhythm, which corresponds to the lunar journey around the earth.
A lot of things in our lives happen again and again on a daily or monthly repeating pattern. For example, becoming sleepy or hungry or experiencing a menstrual cycle.
The suprachiasmatic nucleus marks the passage of circadian time. The suprachiasmatic nucleus is a grain-of-rice sized region in the hypothalamus. The hypothalamus is a marble-sized structure near the center of the brain. You can think of the hypothalamus like Goldilocks, it keeps things just right. When you get too warm, it coordinates multiple systems to cool you down. When you get dehydrated, it makes you thirsty. If your stomach is empty, it makes you hungry. If you’ve just finished an all nighter while preparing for a Physics exam, it makes you sleepy.
Time as River: As opposed to an infinitely repeating cycle, some view time like a river. Every river begins as a trickle in the hills, flows along its unique course and terminates when it spills into the sea. Time, like each individual human in the world and the world itself, may have a beginning and an end.
The ability to experience linear time is crucial for planning ahead or remembering distinct past events. An example that takes both past and future into account in a linear way would be: I need to get to the airport on the 18th of August at 4PM, the plane boards at 4:15 and doors close at 4:45. Last year, I arrived late. The plane took off without me and missed my meeting in Denver.
The entorhinal cortex marks the passage of linear time. The entorhinal cortex stretches along the middle part of the temporal lobe of your cerebral cortex. The cerebral cortex is the thinking part of your brain. The entorhinal cortex is intimately associated with the hippocampus, which is the hub of memory formation and memory storage.
The role of the entorhinal cortex in memory formation was elucidated in 2018, by Chicago scientists. The researchers resorted to the classic: Rodents running a maze. The Chicago mice were trained to pause the race for exactly six seconds in order to receive an award. The scientists measured the rodents’ brain activity. The entorhinal cortex, with the assistance of the hippocampus, marked the passage of time.
In 2021, a multinational group of scientists discovered where the site of linear timekeeping takes place in the human brain. The volunteers for the study were undergoing testing and treatment for epilepsy, which required recording electrodes to be placed in the temporal lobes of their brains. The subjects learned multiple tasks and marked a ten-second gap between them. In 2020, a multi institutional group of American researchers performed a similar set of experiments with twenty-seven epilepsy patients volunteering to have electrical brain signals recorded. The scientists demonstrated a partnership between the entorhinal cortex and the hippocampus. Neurons in the entorhinal cortex gradually ramped up electrical activity corresponding to the passage of time. Neurons in the hippocampus had electric activity spikes, an all or nothing phenomenon, which recorded the memory that occurred during the time passage.
Why do the two parts of the brain mark time in such different ways?
Marking the passage of time is crucial for survival. Even the most primitive creatures have a need to rest when they’re tired and eat when they’re hungry and mate in the proper season. Marking this type of time is a task which is perfectly suited to the ever repeating rhythm of the suprachiasmatic nucleus. It’s also befitting that the suprachiasmatic nucleus is located in the most primitive part of the brain, often called the reptilian brain, because its structure hasn’t changed much for millions of years and across multiple species of animals.
On the other hand, planning for the future and remembering distinct events in the past are the domain of more evolutionarily advanced creatures. So, it makes sense that marking the passage of linear time is in the domain of the entorhinal cortex and hippocampus. These structures straddle the limbic system and neocortex, which evolved much more recently and aren’t present in primitive classes of animals such as fish or reptiles.
Which Brain Cells Keep Time?
Your brain is comprised of four different cell types, each of which has long been believed to have specific functions:
-Neurons: The gray matter is where the cell bodies of the neurons reside. The neurons send electrochemical signals to one another, which enables thought, sensation and action.
-Microglia: The microglia don’t do much, unless they become agitated by inflammation or trauma. But once microglia get angry, they stalk around the central nervous system and dispense cellular justice.
-Oligodendrocytes: The oligodendrocytes snuggle up to neurons and wrap the axons (part of the neuron) in myelin (a fatty substance). This process is like wrapping a wire with insulation and allows the efficient transmission of electrochemical signals.
-Astrocytes: The astrocytes provide behind-the-scenes support to the rest of the brain and maintain the barrier between the central nervous system and the rest of the body.
Giving proof to the notion that the science is never really settled, novel research has demonstrated that the known roles of different brain cells (neurons, astrocytes, oligodendrocytes and microglia) are constantly evolving. It had been previously believed that the lofty neurons performed all of the heavy lifting (regarding marking our conscious perception of time) and their subservient astrocytes were relegated to a caretaker role.
In 2019, researchers in the United Kingdom turned the thinking about the role of Neurons and Astrocytes upside down. The British scientists discovered something unexpected in the suprachiasmatic nucleus in the hypothalamus of mice. The neurons, as expected, maintained a circadian rhythm and marked time on a twenty-four-hour clock. They devised a genetic engineering method, which disrupted the function of neurons and astrocytes. When the researchers blocked neuronal function, they proved that the astrocytes (all by themselves) were capable of maintaining a biological clock. Astrocytes danced to the beat of their own drummer, though. Instead of a twenty-four-hour cycle, the astrocytes maintained a twenty-three-hour day.
The Protein that Marks Time
In 2020, Bostonian scientists discovered that the brain’s production and the neuronal distribution of a small protein may determine how the passage of time is experienced. Pigment-Dispersing Factor (PDF) was isolated from the brains of fruit flies. The scientists then devised a method to block this neuropeptide. The disruption of the neuronal PDF physiology upset the circadian rhythms of the flies and caused a misalignment between the insects’ internal clocks and their external environmental conditions.
Fruit flies aren’t people. Fruit fly brain is the size of a poppy seed and is comprised of a mere 100,000 neurons. There are 86,000,000,000 neurons, on the other hand, between your ears. In 2021, a multinational group of scientists determined that PDF may be important in the timekeeping function of the human brain. Their evidence was indirect, but powerful. The researchers determined that phthalates, commonly used in the production of plastics, skin care products and pharmaceuticals, are disruptive to DNA function. Phthalates block the expression of a group of genes shared by humans and fruit flies. The genes in question control circadian rhythm and one of the protein products of these genes is PDF.
How does this affect your life?
The Circadian rhythm gets out of whack in certain people. This may cause weird types of insomnia and difficulty coping with the rest of the world. Learning about how the suprachiasmatic nucleus maintains our internal clock may lead to therapeutic interventions, which might help restore proper time alignment.
Boredom is a joy killing state of mind, which may be characterized by a perception that time is flowing too slowly. A deeper understanding of how the entorhinal cortex marks the passage of time may allow for greater introspection. Rather than wishing the hours to rush by, we can enjoy the temporary sensation of time dilation. Because when the PDF production kicks back in (which it always does), hold onto your hat. The river of time will yank you into the rapids and, before you know it, a ten-year-long waterfall will be behind you. Cherish every moment of the ride.