Short Wave - Time Cells Don't Really Care About Time

Episode Date: January 3, 2023

Time is woven into our personal memories. If you recall a childhood fall from a bike, your brain replays the entire episode in excruciating detail: The glimpse of wet leaves on the road ahead, that moment of weightless dread and then the painful impact. This exact sequence has been embedded into your memory thanks to some special neurons known as time cells. Science correspondent Jon Hamilton talks to Emily about these cells — and why the label "time" cells is kind of a misnomer.Concerned about the space-time continuum? Email us at [e-mail] — using science, we might be able to set you at ease in a future episode.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Happy 2020, shortwavers. Personally, I love the new year because, one, parties, two, permission to entertain all of my existential issues about the meaning of life and what I'm doing on this planet. The passage of time does that kind of thing to a person, which is why this month, we are bringing you a batch of episodes on time itself reported by our time. talented colleagues on the science desk. And today, we are talking about how the brain tracks the passage of time with NPR's very own neuro savant, John Hamilton. Hello, John. Hello, Emily. Happy New Year. To you too. So, as you know, I'm perpetually late to everything. That is very Kim Kardashian. Okay, okay, but only by like four minutes, or maybe 10 minutes sometimes.

But I know from talking to you that even if I lose track of the time, my brain is still monitoring the passage of time. Can you break down for us how that works? Two words, Emily, time cells. Time cells. In our brains, we have got these special neurons that put timestamps on events and experiences in our lives. And these timestamps are what allow us to remember things, say, in the order they happen. Let me give you an example. Let's take something really important, a really important event in your life like brewing coffee.

I feel a little trolled, but coffee is religion to me. It's the event that makes the rest of my day even possible. Right. So let's think back to this morning. You staggered into the kitchen and then presumably a whole bunch of stuff happened. You ground the beans. Yep, having a moment just hearing this sound. Then you started the brewing process.

Sweet. sweet, sweet, sweet music to my sleepy ears. And finally, you reached for the carafe and... My precious. Easy there, Gollum. The point here is that as your coffee experience was unfolding, your time cells were firing away in a specific sequence. They were putting down all these time stamps. Those stamps are now embedded in that memory.

And now, if you choose to replay that coffee making in your mind, those time cells will fire in the same order. Grind triple. That is so cool. Genius, actually. Today on the show, how the brain keeps track of time. And why it often doesn't. Classic John Hamilton-Bait and switch. I'm Emily Kwong and you are listening to Shortwave, the Daily Science Podcast from NPR. Okay, John, so scientists think the brain has these time cells that keep our memories in the right order. It sounds so sci-fi and far-fetched, but nonetheless appears to be true. But how does scientists even know this? Yeah, I mean, until 10 years ago, it was really just a hunch. Oh.

Scientists had been studying what they called episodic memories, and those are memories of personal events and experiences. There are kind of three parts of an episodic memory. There's what happened, where it happened, and when it happened. Okay. So what happened is obviously the event or experience itself. But the question has been how do you sort of imprint on that both the where and the when it occurred? The where part actually got figured out back in the 1970s. Researchers were studying rats and they had them go through this maze.

And they figured out that there were certain cells in the hippocampus of these animals that would fire every time one of them reached a specific place. So they called these cells place cells. It made sense that there would also be special cells. to indicate when a certain amount of time had passed. But these cells were really only formally identified and named time cells in 2011. That's when a team at Boston University showed that certain cells in a rat hippocampus would fire at specific time points. Even when a rat wasn't moving, it was just waiting for something to happen. And finally, time cells were found in people in 2020.

First, they were found in the hippocampus, but it appears time cells may be in other parts of the brain too. So this is a very recent puzzle that's kind of clicked into place. What have scientists learned about how time cells work now that we know that they are real? I talked about that with a researcher named Mark Howard. He's a mathematical psychologist, whatever that is, at Boston University. And Boston University, I should say, is the place where time cells were discovered. So he explained to me this way. He said, let's say something notable happens.

Like I clap my hands, right? That was notable. Howard says that's all it takes to start these time cells firing and in this really highly choreographed pattern. What we find is that cells fire in a sequence, right? So cell one might fire right immediately after the clap, but cell two waits a little bit and only fires a little later and cell three and cell four and so on. And there's this sequence of cells that unfolds over time such that if you can see which cell is firing, you know how long ago the clap was. Hmm. So is the brain kind of like a stopwatch ticking away? Not really.

Okay. A stopwatch follows what you would call a linear time scale. The ticks and talks occur at the same rate, regardless of how much time has passed. Uh-huh. Howard says time cells don't work that way. Instead, they seem to follow a logarithmic time scale. The sequence doesn't unfold at the same rate. The sequence unfolds slower and slower and slower.

In effect, your ability to distinguish time. decreases as things get further into the past. Oh, that doesn't sound like a stopwatch at all. This sounds really different. Yeah, it's more like, you know what happens when popcorn hits hot oil? You know, the kernels are there, and at the beginning, they're popping all the time. But later on, there's only the occasional pop. And so kind of like that, your brain is using fewer timestamps to record an event as it goes

long. Oh. And the reason for that seems to be because your brain is really good at relative time, telling you, you know, whether one event took twice as long as another one. But it's not so good at keeping track of a set period like seconds. It's paying attention to proportional time, not the absolute time or the linear time. Oh, that's so cool. Does anyone know why our brains even work this way?

Yeah, there are definitely some theories. It's probably for efficiency. So a logarithmic approach is a really efficient way to manage huge data sets, you know, like a lifetime of episodic memories. What it does is it lets us compress all that information. A linear system, which doesn't have any compression, would just overload the brain. Okay. And are there other advantages to this way that our brain tracks the passage of time?

Howard thinks that there probably are. He says this system seems to make it easy for the brain to compress and expand our percentage. of time. And that turns out to be really useful. So, for example, he says the network of time cells in our brain, they let us understand words, even when those words are spoken very slowly. If I say the word seven, you can recognize that as seven perfectly well, because the relative shape of the syllables is the same. And this network generalizes to things that are faster or slower than it was trained on. So my time cells are sort of replaying the word at normal speed in my head so I knew what it was, even though it was slowed down. Right. Like we were talking about in the brain, time is relative and we can manipulate it.

Oh, that makes me feel so powerful. Indeed. But wait, wait, there's more. Remember how time cells and play cells keep track of where and when something happened to us, right? Yes, they organize those episodic memories you were talking about, allow me to make coffee the right way in the moment. morning. Exactly. But in the past few years, scientists have shown that sometimes time cells and place cells are exactly the same neurons. What? I know. I talked about that with Dr. Yuri Buzaki at New York University. He's done a lot of research on episodic memory. When you look for time cells, you will find that it's exactly the same as the play cells. We had a paper in science where we showed that 100% of neurons can be play cells if you want, and 100% of the brain cells. And 100% of the

can be time cells depending on how you set up the experiment. So you put an animal in a maze and a given cell will respond to a specific place, you know, like a place cell. Put it on a stationary treadmill and the same cell will fire at a specific time, like a time cell. Wait, so some of these cells are tracking both space and time. That is so cool. It makes me wonder if you were alive what Einstein would say about all of this. It's getting very space-time continuum up in here.

It is. I mean, I think he might say that it makes sense because our brains need to navigate both space and time to get us through the day. And of course, all these cells are found in the hippocampus, which is the brain's navigation center. Buzhaki also pointed out that we humans often use time and distance interchangeably. So if I ask you, you know, how far is it from Portland to Seattle? You might tell me, well, it's 175 miles. Or you might tell me it's a three-hour drive. Either way, I'd know what you meant.

Yeah, my sister and I got into an actual fight about this, John, in the last mile of our marathon. She wanted to track it based on distance. And I was like, no, no, no, we have like 10 minutes of running left. And we were both right. You were both right. And I am not getting in the middle of that fight. But I should mention that, you know, whenever I talk to Dr. Bishaki, he says something that really totally makes my brain hurt. And in this interview, it was this kind of cosmic question he had. What do we want to have with time cells? the brain doesn't generate time.

Also, the brain cannot sense time because it's immaterial. Is he undermining the importance of his research? I mean, that if time isn't real, why do time cells even matter? I think his point is more that I don't think he really feels like we should call time cells time cells because they are not really about clock time or any sort of absolute time. You know, what they're doing is to help us track change and to keep our memories in order. You know, we need to remember that you grind the coffee beans before you start brewing. The precise duration of each of those steps, you know, it's less important.

So we should call them, we should almost call them life cells or change cells. I think change cells has actually been proposed and it's not a bad idea. The thing is, our brains need external signals to keep track of time with any real precision. They're not getting it from time cells. And so, you know, historically we've used the sun, of course, but that's not super precise if you're trying to like start a meeting at an exact time like 11 a.m. So Buzaki told me that what we did as a species, we started to rely on these artificial signals like church bells. And the bells were soliciting people for prayer, but also for lunchtime and, you know, in the fields and so on. So there was a joint timer that helped everybody to do pretty much the same thing at about the rather the same time.

That was all fine until sailors started crossing the big oceans. And they realized they needed to know the exact time of day in order to use celestial navigation to tell exactly where they were. Sure. So in the early 18th century is when we got the first marine chronometers. Now, of course, we've got super precise time signals coming at us from every. every place, including the National Institute of Standards and Technology in Colorado. So our time cells are getting a whole lot of help. Nowadays, they certainly, certainly are.

Well, John, thank you so much for coming on the show to talk about this. I find this incredibly fascinating. And I'm looking forward to hearing the rest of the stories in the time series. Me too. This story was edited by Human Clown, Giselle Grayson, produced by Dilettante Rebecca Ramirez, and fact-checked by Britt Hansen, the only grown-up in this group. Brendan Crump is our podcast coordinator, Beth Donovan is our senior director, and Anya Grundman is our senior vice president of programming. I'm Emily Kwong. Thanks for listening to Shortwave, the Daily Science Podcasts from NPR. See you tomorrow.