Instant Genius - The neuroscience of remembering and forgetting

Episode Date: April 25, 2024

We all forget things from time to time, it’s a normal part of everyday life. But according to the latest research in neuroscience, it is forgetting, not remembering that is the brain’s default action. So why is this? In this episode I speak to Prof Charan Ranganath, director of the Dynamic Memory Lab at UC Davis and author of the book Why We Remember: The Science of Memory and How it Shapes Us. He tells us how memories form in our brains, how they are intimately linked to our emotions and imagination, and why we often walk into another room and forget why we went there in the first place. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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Music just as the artist intended. Visit name audio.com. to learn more. Hello and welcome to Instant Genius, a bite-sized masterclass in podcast form. Each week you'll hear world-leading scientists and experts talking about the most fascinating ideas in science and technology today. I am Jason Goodyear, commissioning editor, a BBC science focus. We all forget things from time to time, as a normal part of everyday life.

But according to the latest research in neuroscience, it's forgetting, not remembering that's the brain's default action. So why is this? In this episode, I speak to Professor Charan Ranganath, director of the Dynamic Memory Lab at UC Davis, an author of the book, Why We Remember, The Science of Memory and How It Shapes Us. He tells us how memories form in our brains,

how they're intimately linked to emotions and imagination, and why we often walk into another room and forget why you went there in the first place. So welcome to the show. Thank you. Thanks for having me. So first off, can you tell our listeners a little bit about who you are and what your background is? Yes, I'm Professor Charan Rankinath. I'm at the Center for Neuroscience at UC Davis, and I direct the Memory and Plasticity Program and the Dynamic Memory Lab.

So we're talking about all things memory today then based on your new book, Why We Remember. And at the start of the book, you say we're constantly bombarded with information, but we only remember some of it. essentially we're designed to forget. So I think that might come as a relief to a lot of people, but exactly what do you mean by that? Well, what I mean is that if you look at every study of memory where you actually quantify how much people forget, what you find is that at least when you're talking about meaningless information,

over, you know, about 60% of that information would be lost within 24 hours. And as you delay more and more, you're talking about maybe 20 to 30% of information being retained. So what that tells us right away is that forgetting is the norm. And remembering is actually the minority case in our experiences. So having said that, what's the sort of Cliff's Notes version of what we know about how the memory is formed in the brain? Oh, it's very hard to give a Cliffs Nodes version, but I'll try. So at the most abstract level, if you're talking across species, across brain areas, is that we think that memory is driven by neural plasticity,

meaning that we have some experience and there's a change in the strength of connections between certain neurons. And so a way to talk about it in the book is that neurons might form a coalition or what Donald Heb called a cell assembly. So if we think about the cell assembly or the coalition of neurons that are active, say when I even just tell you a word for the first time, that word now will activate some of those neurons that were active the first time. But because they form the stronger coalition, now those neurons can recruit the other neurons that were part of the original experience.

And so the general theory goes that if you can reactivate many of the same neurons that were active during the original experience, then you will have a little twinge of that mental experience again, and that's what we typically would call memory or learning. So that's why people often say memories are created anew each time we remember them. Is that right? Well, in some sense, yes, because what happens is each time we recall them, the memories get updated further. So the most important connections between neurons and an assembly might be strengthened, the less important ones might be weakened, and they might drop out of the cell assembly altogether. And that process of updating the memory each time we recall it allows us to take the information that we recall most

often and have it more accessible each time we do it. And that's the basis for something that I call error-driven learning in the book. So a lot of times when people are talking about memory, things come up which are different types of our memories. So it's usually episodic and semantic memory. So what's the difference between those two types of memory? Well, so I'll pause by saying that in the real world, these kinds of memories that memory researchers like to differentiate between or actually intertwined and harder to separate outside the lap. And I'll come back to that point later. But episodic memory and semantic memory is an important distinction in my work. And we can think of semantic memory as your knowledge about, you know, facts. And even sometimes

people will talk about knowledge about particular kinds of events as being semantic memory. So, I'll give you an example that I can think of if I think of some facts relevant to my time when I was doing my sabbatical in England. One would be that I grew to love tea and I can tell you the brands of tea that I like. I tend to like Builders tea, but I know what Builders T is after having lived in England. And I can tell you I like M&S gold, I think it was. And I like Yorkshire Gold. I tend to like the gold, strong teas. But getting back to the point, that's different than remembering

the specific events that I experienced when I was in England, knowing about the fact of what a good beer is, let's say, in a particular pub, that doesn't tell me about the specific experience I had during my last day in England when I went to the pub with some colleagues and being in that pub and getting some crisps and, you know, talking about all the fun times we had over the course of that 10 months. So I apologize for that long story, but the point is that the specific events are what we would call episodic memory. And the knowledge that we have about different kinds of events, the sort of ingredients that go into building an event would be what we call semantic memory. And we know they're different because we can look at patients who have damage to a brain area called

the hippocampus. And actually, Farina Varna Varga-Qadem at University College London, show that kids, for instance, who grow up with damage to the hippocampus, say if they have a near-drowning event, can still build that semantic memory, but they have very little to know episodic memory for their life experiences. And we've shown in our work and others that the hippocampus is active when people recall episodic memories, but not if they just pull out semantic knowledge. You mentioned the hippocampus there. So what do we know about the role other areas of the brain play in the memory process? Well, one area that I feel is very important is the prefrontal cortex. So the prefrontal cortex is about one third of the human brain, really.

It's a gigantic area in humans, not so much in other species. And in some sense, actually, you should talk about it as a set of areas. But the general collective function of the prefrontal cortex is to regulate what information is going to be prioritized based on what is relevant to your goals. Not what's just kind of shiny or loud on the outside world, but rather, are what's important based on what you want to get done, right? So a patient with prefrontal damage can remember, but what they remember isn't necessarily going to be what's important to them in the moment. So people with prefrontal damage can often look like they have memory problems, and they'll tell you they have memory problems. But it's not because they have no episodic memory, but their

episodic memory is just associated with all of these things that are tangential to what they need. So in the book, you mentioned the role of attention and intention in the memory process. So what do you mean by that? Well, the prefrontal cortex is actually pretty key to this, right? So let me give you an example, right? So if I walk into the house and I notice that the TV was on very loud, my attention will be captured by that. In other words, there's something in my environment that captured my attention. but I can redirect my attention intentionally to the fact that I have to take the keys out of the lock of my door and make sure to put it in a place while I'll remember it later.

In other words, I'm using my attention to focus on what's relevant to my goal, which is to be able to find my keys later on when I'm looking. Now, if for whatever reason I'm tired, I'm stressed out, I'm otherwise preoccupied, I'm distracted by other things, my attention will be grabbed by things. my attention will be grabbed by things in my environment, but I won't be able to direct that attention and use it to be able to focus on what matters to me. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals because we're built for what you're building. Fit for your ambition for Citizens Bank. Lots of places can expose you to identity theft. Oh, no. That's why LifeLock monitors hundreds of millions of digital.

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expertise, Name and Focal have been bringing music to listeners just as the artist intended. Since day one, this mantra has shaped every innovation in hi-fi design, technology and acoustic engineering, balancing craftsmanship and tradition with pioneering thinking. Name Audio pushes cutting-edge technology to ensure digital precision whilst sustaining Pratt, pace, rhythm and timing. The elusive quality that makes music feel alive, and gives it emotional texture. Today, in partnership with French acoustic specialist focal,

name audio creates systems that deliver exceptional sound and unforgettable listening experiences at home. Try it for yourself at a focal powered by name boutique. Visit focal powered by name.com for more information. When we talk about modern neuroscience, we often talk about imaging techniques such as functional magnetic resonance imaging. So if we used this to study memory at all?

Oh, yes. That's been much of my career, actually. So part of why I got into memory research was I was doing work in the clinic and doing testing with people with brain damage. But the work that we were doing in the clinic was based on a very rudimentary understanding of the brain. And so functional MRI was this technique that was just taking off at that time around the 90s where it allowed us to use standard MRI scanners to track.

blood oxygenation, which really relates to brain activity because essentially when neural activity goes up in a region, you'll see a change in blood oxidation that you can measure with MRI. And we call that functional or fMRI. We can use fMRI to measure what happens in particular brain areas while people are remembering things. And one of the things we've shown in my lab and others is when people remember the context of a past event. So in other words, if they can go back not only to what happened, but they can remember when it happened and they can remember all these details that happened in the background of that event, like a song that might have been playing or something that you might have been thinking about as far as your goals during that event, that's when the hippocampus seems to come online.

And we can even use techniques where I describe in the book that we can look at particular patterns of activity in the hippocampus, almost like a QR. code that you can read out with just some very simple statistical techniques and read out the pattern of brain activity that's associated with particular kinds of memories. So if you're thinking about two people that you saw at the same picnic, you might be able to read out a very similar QR code, so to speak, from the hippocampus with Ephrae. But if you're thinking about events that involved the same person, but in two different events, one that occurred last week and another that occurred last year, those QR codes will be different. And so what that tells us is that the hippocampus is sorting out our experiences and storing

them according to time as opposed to storing them according to the content or what happened. And in semantic memory regions in the brain, we see quite the opposite, that it's about what happened, but not necessarily the context when it happened. So in the book, music comes up an awful lot. Personally speaking, I can't remember the last time I read. a book on neuroscience that had quotes from Iggy Pop and Richard Hell in it. And obviously listeners won't be able to tell, but I can see your impressive collection of instruments behind you.

One thing about musicians is that they seem to have an outstanding ability to remember long passages of music, you know, whether it's a classical performance or a 90-minute long set of pop songs. And as we're so used to seeing this, it might seem normal. But if you stop and think about it, it's pretty amazing, really. how are they able to do this? Are they just special people or can we all learn how to do it? No, I would argue that not only can we all learn how to do it, but that we all do it routinely every day. We're all experts in something. So if you look at chess experts, what you find is that they have this

enormous ability not only to memorize the complex pattern of, let's say, pieces on a chessboard in any given moment, but they can memorize an entire match and the sequence. of moves that progresses throughout a match that could occur over an hour, let's say. You know, it's absolutely incredible. And then you can look at basketball players or hockey players who can, and I imagine this is true of footballers, I haven't run across any first-person accounts of this, but that they can recount an entire sequence of plays across a game that they might have played years ago.

Bird experts can recall a bird that they might have seen in extraordinary detail from one particular viewing that they had. So all of these factors suggest that what happens is when you have a very rich body of expertise, which you can think of as a kind of semantic memory, all you have to do is lay on a small amount of information on top of that pre-existing bed of knowledge to form very detailed, very rich episodic memories. And so we do this all the time in everyday life, because everybody's an expert in something. So in the book, you mentioned the idea of something called chunking. So personally, I'd never heard of that before.

So could you explain that for us? Yes, so chunking is based on the idea that our attention, which we talked about earlier, is limited to a small amount of information. And so there's this general principle of economy in the brain, where we try to do as much as possible with as little information as possible. So I might be able to hold in my, three or four chunks of information. But what a chunk is is just based on the unit of knowledge that I'm trying to think about, right?

So I might be able to hold in mind two or three numbers while there's a lot going on in the background. But I could also potentially memorize one string of like as a three-digit number. And so the amount of attention that's taken up by the numbers one, two, and three, might be much more than the amount of attention that's taken up by the number 123. So it's that ability to reduce the amount of information that we're holding in mind. So if we're talking about a phone number, let's say, in the U.S., our phone numbers have a three-digit sequence, another three-digit sequence, and then a four-digit sequence.

Now, my time in Europe, the sequences are arranged in a completely different manner, and I find it extraordinarily difficult to memorize European phone numbers as a result. But in the US, I can chunk it into three chunks that are very easy to memorize. So let's move on from that to forgetfulness. So one thing that you talk about in the book, which I thought was really interesting, and most people will have had this experience, all of us, I'd imagine. It's when we walk from one room into another and immediately forget why we went there in the first place. So why does that happen?

This actually relates to many of the concepts that we've been talking about so far. I was telling you that episodic memory is based on this idea of context, and that includes where we are. And so what happens is when I'm in this room right now and I form an intention to go to the kitchen, I say, oh, I have to pick up my phone. My phone is in the kitchen. So now I walk out of this room, but my sense of where I am has changed, right? and so that context has changed.

And so we actually call that point where my mental context has shifted, my sense of where I am, my sense of what I'm doing, my sense of what's going on in the background. We call those event boundaries. And we say event boundaries because the idea is that one event has ended and another has begun in our heads. And so once I reach the kitchen, I've crossed an event boundary, so to speak. And now my mental context is different. So to remember why I came into the kitchen, I have to pull up the old context.

What was I thinking about when I left my room? And that's effortful because I have to mentally travel back in time. I have to retrieve a completely different episodic memory to do this. So what happens is we often find that very difficult, and you end up just grabbing some food from the kitchen, consuming some empty calories, you go back, and then you have that aha moment where you realize what you forgot. So let's go back to creative pursuits for a moment. In the book, you talk about the link between memory and imagination. I thought that's really interesting. What do we know about that?

Well, some of the early work that link between memory and imagination was done by a memory researcher named Frederick Bartlett at Cambridge. And Bartlett argued, based on his work, that remembering isn't just playing back a memory. It's not just reactivating an assembly of cells per se. It's really an active imagination. He called it an imaginative construction, meaning that we might remember some bits and pieces of a past event, but then we use that as fuel to simulate or imagine how the past could have been, right?

So we're not replaying the past, we're imagining how the past could have been. And he wrote a separate paper where he argued that what happens when we imagine, imagine something new or recreate something new, we actually synthesize pieces of memory and recombine them into new bits. And so more recently, Demis Hasebis and Eleanor McGuire working at UCL, now Demis is actually head of deep mind. He's an absolutely brilliant guy, or Sir Demis, I should say, just got knighted. And as well as Dan Shactor in the U.S., Donna Addis and Canada, they all came together around the same time and they started reporting that you can look at people with brain damage that affects memory, and they have trouble imagining new things that haven't happened yet. And they also found that if you look at brain activity while people imagine things that have never happened, it actually highly resembles brain activity that you see when people remember things that did happen.

To the point where even an expert couldn't tell the difference in these brain. activity maps without some detailed statistics to analyze it. So sort of coming off the back of that then, what role can emotions play in our memories? You know, why do emotional memories seem to stick so much? Well, our emotions are linked to experiences typically that are biologically important. So if you take a moment to think about memories in your life, as you pointed out, most of the memories we would pull out would have emotional significance, actually. And so when we think about what's the content of those memories, well, they might be experiences where you were scared or you

were stressed out. And again, if we think about the importance of the biology there, we can see that there's certain chemicals in the brain like noradrenaline, like cortisol and other glucocorticoids that are released during states of fear or anxiety. And evolutionarily speaking, And it makes sense because when you're under threat, you want to be able to remember what the threats are in your environment. You want to remember not only that you saw the bear, but where you saw the bear and be able to pull that information up very, very easily, right? And likewise, you can see this for rewards. Like, I'm hungry and I get a great slice of pizza somewhere. Or you can see it for things like attachment experiences.

so the reward would be associated with dopamine, attachment experiences with chemicals like serotonin and so forth. All of these chemicals promote plasticity. They allow the changes that happen in the strength of connections between neurons during the encoding of an event. They allow them to last much, much longer. And so that's something that researchers call consolidation. So how about the effect of aging on our ability to form memories? You know, a lot of people say as they got older, they find it more difficult to remember things. Is there any truth in that? Yes, there is. Actually, this is something that comes up. I get asked this a lot because right now in the United States, we have two of the oldest candidates that we've ever had in history, I think,

for the presidential election. And one of the things that comes up a lot is memory. And as it turns out, as we get older, on average, we tend to forget more. Some people do a lot better than others at retaining their memory abilities if you follow individuals up. But we lose that capability to remember sometimes because we're losing that plasticity or we have changes in the hippocampus. But a lot of it relates to changes in the prefrontal cortex. So one of the things when people tell me they have memory problems, they'll say, I'm trying to remember the name of that actor.

And I just can't pull it up for the life of me. And then an hour later, when I don't need it, it pops into my head. And a memory researcher would say, that's not even a memory problem. That's just a problem retrieving a fact that you have. But the memory is still there, right? The knowledge is still there. And we have this problem, too, where you have more of those forgetting points at event boundaries that we talked about, like, why am I in the kitchen, forgetting people's names.

And those are factors that often relate to this loss and this ability to focus memory on your goals. And so you remember the inane at the expense of the important. So having said that, can we improve our ability to remember? And should we even really be worried about that? Well, I think it's natural to worry about that. And what I would say is that you can make it a priority, but I wouldn't worry about it unless you have problems that are indicative of a serious memory disorder, which we can get into. But what I would say is, is that the lesson from the research is not to remember more, but to focus on remembering what you want to take with you. And that could be information that's relevant to your tasks that you're

doing or relevant to your work, or it could be something that's more like remembering a family vacation that you've taken. And so I think we should try to remember that. And I think the first step in that is to accept that we won't remember everything. And therefore, to try to focus your intention on what you want to take away, whether it's something as simple as putting down your keys and remembering where you put your keys to something more interesting, as I mentioned, like the family vacation. Yeah, so you mentioned their memory disorders. That was going to be my next question. So what happens in our brains when somebody has one of these disorders? What's going on with the internal wiring, etc.? Well, we can definitely see a number of different kinds of

disorders that happen, especially with aging. So one is that you can see changes that are related to Alzheimer's disease pathology. That's like one of the most common. And so it starts off in a brain area called the enterrinal cortex, which is an area that basically connects up many areas all over the brain with the hippocampus. And so what happens is that pathology will spread to the hippocampus, but it will also spread to all of these areas in the cortex. that are storing little bits and pieces of a memory.

And so as this pathology spreads, more and more you see this kind of path of devastation in its wake. And that's why Alzheimer's progresses to some issues with forgetting things like people feeling disoriented because they don't remember where they are or losing track of not only the content of a conversation, but that they're forgetting that they had a conversation. And those would be the signs that you go, okay, this person's got a memory disorder. then it progresses eventually to not remembering even people who they are very, very close to and misrecognizing them.

So that's one kind of what we call a neurodegenerative disease. There's other things like you can have vascular dementia, which is caused by tiny strokes that change the white matter of the brain and result in an inability for brain areas to talk to each other. Those are just two, but you can see this with strokes, with epilepsy. see, even clinical depression can mimic the early effects of Alzheimer's disease and older adults. And this is something I saw a lot in the clinic. So we've covered an awful lot in the last 30 minutes or so there.

And it's clear that we know quite a lot about memory. But as a memory researcher, what would you like to learn next? You know, what are the big unanswered questions? Oh, I think actually what we know is far eclipsed by what we don't know. And in fact, one of the things that I find in my research is progress is often linked with a 1% change in what we know and a 99% increase in a realization of what we don't know. So one of the things that we've looked into in our research is rather than asking how do we remember a little bit of information like a word, how do we remember a lot of information like the content of this conversation that we've been having over. the past half hour. And our ability to take that information and recall it is not a replay,

as I said, of that experience, but rather we'll often summarize a 30-minute conversation in about five minutes. And if I did that, you could say that I remember it in extraordinary detail. And so what we're finding is that people will often say, well, if I'm remembering information that occurred over my lifetime, how do I not run out of storage space? And so one of the things that we've started to see in the brain is when we look at these very long events that are more in the scale of how we remember the real world, there's actually much less episodic memory encoding than we would have thought, meaning that we're not encoding a video, let's say, of our entire experience, but we're forming these memories for the most informative, the most surprising, the most novel

parts of those events. And that gives us the little bits and pieces that we need to synthesize the event altogether. And so right now we're trying to build computer models to explain that and see if we can use the same computer models to understand how I might form a mental map of a particular city and so forth. And so I think the big question we're asking is, how does memory play out in the real world? And of course, it's always more complicated than what you see when you can reduce it to these little bits. Thank you for listening to this episode of Instant Genius, brought to you from the team behind BBC Science Focus.

That was Professor Charon Ranganath. To discover more about the topics we've just discussed, check out its latest book, Why We Remember, The Science of Memory, and how it shapes us. If you liked what you just heard, please consider subscribing to Instant Genius on your preferred podcast platform. The current issue of BBC Science Focus magazine is out now. pick up a copy wherever you buy your favourite magazines or download a copy on your app store of choice. You can also find us online at sciencefocus.com. This podcast is sponsored by Name, Audio and Focal. The texture and emotional depth of music can be lost through digital sources or poor signal.

Name audio believes you can have digital precision with analogue warmth. Alongside French acoustic specialist focal, name creates high-end audio systems, combining innovation with craftsmanship, so you can listen to music, just as the artist intended. Discover more at name audio.com. Some follow the noise. Bloomberg follows the money, whether it's the funds fueling AI or crypto's trillion-dollar swings. There's a money side to every story. Get the money side of the story. Subscribe now at Bloomberg.com.