Time is woven into our personal memories.
Recall a childhood fall from a bike and the brain replays the entire episode in excruciating detail: the glimpse of wet leaves on the road ahead, the moment of weightless dread, and then the painful impact.
This exact sequence has been embedded in the memory, thanks to some special neurons known as time cells.
When the brain detects a notable event, time cells begin a highly orchestrated performance, says Marc Howard, who directs the Brain, Behavior, and Cognition program at Boston University.
"What we find is that the cells fire in a sequence," he says. "So cell one might fire immediately, but cell two waits a little bit, followed by cell three, cell four, and so on."
As each cell fires, it places a sort of time stamp on an unfolding experience. And the same cells fire in the same order when we retrieve a memory of the experience, even something mundane.
"If I remember being in my kitchen and making a cup of coffee," Howard says, "the time cells that were active at that moment are re-activated." They recreate the grinder's growl, the scent of Arabica, the curl of steam rising from a fresh mug – and your neurons replay these moments in sequence every time you summon the memory.
This system appears to explain how we are able to virtually travel back in time, and play mental movies of our life experiences. There are also hints that time cells play a critical role in imagining future events.
Without time cells, our memories would lack order.
In an experiment at the University of California, San Diego, scientists gave several groups of people a tour of the campus. The tour included 11 planned events, including finding change in a vending machine and drinking from a water fountain.
Afterward, the participants were asked to recall their experiences. People with typical brains tended to remember the events in chronological order. But those with damage to the hippocampus – where many time cells are found – recalled events without regard to the order in which they occurred.
Time cells were identified in the rat hippocampus in 2011. The cells' presence in human brains was confirmed in 2020.
But despite their name, time cells do not behave like a clock. Their ticks and tocks appear to follow rules that are independent of units like seconds and minutes.
At the beginning of any new event or experience, time cells fire like popcorn kernels hitting hot oil. That creates lots of time stamps in rapid succession. As seconds pass, Howard says, the firing becomes less and less frequent.
"The sequence doesn't unfold at the same rate," he says. Instead, the interval between each firing gets larger and larger, leaving fewer and fewer time stamps. "In effect, your ability to distinguish time decreases as things get further into the past," Howard says.
It's still not clear how the brain decides precisely how many time stamps to put down, or how far apart they should be. But Howard is among the scientists who believe it takes a mathematical approach.
Time cells appear to maintain a logarithmic timeline, Howard says, which allows them to represent time in a compressed form. He thinks the brain also uses a mathematical tool called a Laplace transform to navigate between the real-world firing of neurons and their representation in memory.
Howard and a team of scientists devised this explanation before the existence of time cells was established. Since then, studies have shown that time cells' actual behavior is consistent with the theory.
"It's worked out pretty well so far," Howard says.
But even a mathematical approach to time can be affected by biological changes.
There's evidence that the behavior of time cells, like other neurons, is influenced by factors including emotion. When we experience an intense or terrifying event, the brain appears to put down more time stamps than it does during a mundane experience. That may be why skydivers tend to overestimate the duration of a remembered freefall.
Networks of time cells can even stretch or compress time as needed, Howard says.
Howard thinks that's how we are able to recognize a word, even when it is spoken very slowly. For example, imagine hearing the word s...e...v...e...n stretched out over several seconds.
"You can recognize that as seven perfectly well because the relative shape of the syllables is the same," Howard says. What's different is the duration of the syllables, something a network of time cells can adjust.
Of course, time cells don't work alone.
They're just one part of the brain's system for organizing episodic memories, which are "our personal egocentric memories – what happened to me, where, and when," says Dr. György Buzsáki, Biggs professor of neuroscience at New York University.
Episodic memories are for events and experiences. They are distinct from semantic memories, which deal with ideas and facts, like the name of your friend's pet ferret.
Time cells keep track of the when in an episodic memory. Meanwhile, another group of cells called place cells keep track of where you were when the episode occurred.
Both types of cells were first discovered in the hippocampus, an area of the brain involved in both memory and navigation. More recently, they've been found in other brain areas.
Neither place cells nor time cells are as straightforward as their labels imply, Buzsáki says. For example, if the brain is paying attention to where instead of when, a time cell may behave like a place cell.
"We had a paper in science where we showed that 100% of neurons can be place cells if you want and 100% can be time cells, depending on how you set up the experiment," Buzsáki says.
So it makes sense that people often use time and distance interchangeably in conversation, Buzsáki says. Ask someone how far to the next town and they may say ten miles or a ten-minute drive.
But there's a more cosmic problem with the very notion of time cells, Buszaki says.
"The brain doesn't generate time," he says. "Also, the brain cannot sense time because it's immaterial."
Time is an illusion, according to Albert Einstein. And, in any event, time doesn't really matter to the brain, Buzsáki says. What's important is detecting change and the sequence of events.
Put another way: You need to remember that you heard the snake's rattle before you felt its fangs. The exact amount of time that the whole experience took is ultimately much less important.
This story is part of our periodic science series "Finding Time — a journey through the fourth dimension to learn what makes us tick."