Lex Fridman PodcastCharan Ranganath: Human Memory, Imagination, Deja Vu, and False Memories | Lex Fridman Podcast #430
EVERY SPOKEN WORD
150 min read · 30,037 words- 0:00 – 1:03
Introduction
- CRCharan Ranganath
... the act of remembering can change the memory. If you remember some event and then I tell you something about the event, later on when you remember the event, you might remember some original information from the event as well as some information about what I told you. And sometimes, if you're not able to tell the difference, that information that I told you gets mixed into the story that you had originally. So now I give you some more misinformation, or you're exposed to some more information somewhere else, and eventually your memory becomes totally detached from what happened.
- LFLex Fridman
The following is a conversation with Charan Ranganath, a psychologist and neuroscientist at UC Davis specializing in human memory. He's the author of Why We Remember: Unlocking Memory's Power to Hold On To What Matters. This is the Lex Fridman Podcast. To support it, please check out our sponsors in the description. And now, dear friends, here's Charan Ranganath.
- 1:03 – 14:44
Experiencing self vs remembering self
- LFLex Fridman
Danny Kahneman describes the experiencing self and the remembering self, and that happiness and satisfaction you gain from the outcomes of your decisions did not come from what you've experienced, but rather from what you remember of the experience. So, uh, can you speak to this interesting difference that you write about in your book of the experiencing self and the remembering self?
- CRCharan Ranganath
Danny really impacted me 'cause I was an undergrad at Berkeley and I got to take a class from him long before he won the Nobel Prize or anything, and it was just a mind-blowing class. But this idea of the remembering self and the experiencing self, I got into it because it's so much about memory, even though he doesn't study memory. So we're right now having this experience, right?
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
And people are, can watch it presumably on YouTube or listen to it on audio. But if you're talking to somebody else, you could probably describe this whole thing in 10 minutes, but that's going to miss a lot of what actually happened. And so the idea there is, is that the way we remember things is not the replay of the experience. It's something totally different, and it tends to be biased by the beginning and the end. And he talks about the peaks. There's also the, you know, the, the best parts, the worst parts, et cetera. And those are the things that we remember. And so when we make decisions, we usually consult memory, and we feel like our memory is a record of what we've experienced, but it's not. It's this kind of very biased sample, but it's biased in an interesting and I think biologically relevant way.
- LFLex Fridman
So in the way we construct a narrative about our past, you say that, uh, it gives us an illusion of stability. Can you explain that?
- CRCharan Ranganath
Basically, I think that a lot of learning in the brain is driven towards being able to make sense... I mean, really memory is all about the present and the future. The past is done, so biologically speaking, it's not important unless there's something from the past that's useful. And so what our brains are really optimized for is to learn about the stuff from the past that's going to be most useful in understanding the present and predicting the future, right? And so cause-effect relationships, for instance. That's a big one. Now, my future is completely unpredictable in the sense that, like, you could r- you know, in the next 10 minutes pull a knife on me and slit my throat, right?
- LFLex Fridman
Hm. I was planning on it, but like... (laughs)
- CRCharan Ranganath
(laughs) Exactly. But having seen some of your work and just, uh, you know, generally my expectations about life, I'm not expecting that. I have a certainty that everything's gonna be fine and we're gonna have a great time talking today, right? But we're often right. It's like, okay, so I go to a, a, see a band on stage, you know? I know they're gonna make me wait, the show's gonna start late (laughs) and then-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... you know, come, they come on. There's a very good chance there's gonna be an encore. I have a memory, so to speak, for that event before I've even walked into the show, right? There's gonna be people holding up their camera phones trying to take videos of it now 'cause this is kind of the world we live in. So that's like everyday fortune telling that we do, though it's not real. It's imagined. And it's amazing that we have this capability, and that's what memory is about. Uh, but it can also give us this illusion that we know everything that's about to happen. Um, and I think what's valuable about that, that illusion is when it's broken, it gives us the information, right? So, I mean, y- I'm sure being an AI, you know about information theory, and the idea is the information is what you didn't already have. And so those prediction errors that we make based on, m- you know, we make a prediction based on memory and the errors are where the action is.
- LFLex Fridman
The error is where the learning happens.
- CRCharan Ranganath
Exactly. Exactly.
- LFLex Fridman
Well, just to linger on Danny Kahneman and just this whole idea of experiencing self versus remembering self. Uh, I was hoping you can give a simple answer of how we should live life. Uh... (laughs)
- CRCharan Ranganath
(laughs)
- LFLex Fridman
Based on the fact that our memories could be a source of happiness or could be the primary source of happiness, that an event when experienced bears its fruits the most when it's remembered over and over and over and over. And maybe there is some wisdom in the fact that we can control to some degree how we remember it. How we evolve our memory of it such that it can maximize the long-term happiness of that repeated experience.
- CRCharan Ranganath
Okay. Well, first I'll say I wish I could take you on the road with me (laughs) because that was such a great description. (laughs)
- LFLex Fridman
Can I be your opening act or...
- CRCharan Ranganath
Oh my God, no. I'm gonna open for you, dude.
- LFLex Fridman
(laughs)
- CRCharan Ranganath
Otherwise, it's like, you know, everybody leaves after you're done. (laughs)
- LFLex Fridman
(laughs)
- CRCharan Ranganath
Believe me, I did that in c- in Columbus, Ohio once. It wasn't fun. Like, the opening acts, like, drank our bar tab.
- LFLex Fridman
(laughs)
- CRCharan Ranganath
We spent all this money going all the way there. There was only the... Everybody left after the opening acts were done, and there was just that stoner dude with the dreadlocks hanging out. And then next thing you know, we, we blew, like, our savings on getting a hotel room. (laughs)
- LFLex Fridman
So we should, as a small tangent, you're a legit touring act.
- CRCharan Ranganath
... when I was in grad school, I played in a band. And yeah, we traveled, we would play shows. It wasn't like we were in a hardcore touring band, but we did some touring and, and had some fun times. And yeah, we did, we did a movie soundtrack.
- LFLex Fridman
Nice.
- CRCharan Ranganath
Henry Portrait of a Serial Killer. So that's a good movie. We were on the soundtrack for the sequel, Henry 2: Mask of Sanity, which is a terrible movie.
- LFLex Fridman
Yeah. How's the soundtrack? It's pretty good?
- CRCharan Ranganath
It's badass.
- 14:44 – 24:16
Creating memories
- CRCharan Ranganath
on you.
- LFLex Fridman
Is there some insight into the human brain that explains, um, why we don't seem to remember anything from the first few years of life?
- CRCharan Ranganath
Yeah. Yeah. In fact, actually, I was just talking to my, uh, really good friend and colleague, Simona Gedi, who studies, uh, the neuroscience of child development. And so we were talking about this. And so there are a bunch of reasons, I would say. So one reason is, is there's an area of the brain in the... Called the hippocampus, which is very, very important for remembering events or episodic memory. And so the first two years of life, there's a period called infantile amnesia. And then the next couple years of life after that, there's a period called childhood amnesia. And the difference is, is that basically in the lab and, you know, even during childhood and afterwards, children basically don't have any episodic memories for those first two years. The next two years, it's very fragmentary and that's why they call it childhood amnesia. So there's some, but it's not much. So one reason is, is that the hippocampus is taking some time to develop. But another is the neocortex. So the whole folded stuff of gray matter all around the hippocampus is developing so rapidly and changing, and a child's knowledge of the world is just massively being built up, right? So... And I'm gonna probably embarrass myself, but it's like if you showed, like... You know, you trained a neural network and you give it, like, the first couple of patterns or something like that. And then you bombard it with another, like, you know, year's worth of data, try to get back those first couple of patterns, right? It's like everything changes. And so the brain is so plastic, the cortex is so plastic during that time. And we think that memories for events are very distributed across the brain. So imagine you're trying to get back that pattern of activity that happened during this one moment, but the roads that you would take to get there have been completely rerouted, right? So I think that's my best explanation. The third explanation is a child's sense of self takes a while to develop. And so their experience of learning might be more learning what happened as opposed to having this first-person experience of, "I remember. I was there."
- LFLex Fridman
Well, I think somebody, uh, once said to me that, uh... Kind of loosely, philosophically, that the reason we don't remember the first few years of life, infantile amnesia, is because how traumatic it is.
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
Basically, the, the error rate that you mentioned, when your brain's prediction doesn't match reality. The error rate in the first few years of life, your first few months certainly, is probably crazy high. It's just nonstop freaking out.
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
The, the collision between your model of the world and how the world works is just so high that you want whatever the trauma of that is not to linger around. I always thought that's an interesting idea because, like, just imagine the insanity of what's happening in a human brain in the first couple years. Just you, you don't know anything and there's just this stream of knowledge and we're somehow... Given how plastic everything is, it just kind of molds and figures it out. But it's- it's like an insane waterfall of information.
- CRCharan Ranganath
I wouldn't necessarily describe it as a trauma. And we can get into this whole stages of life thing, which I just love. Basically, those first few years, there are... I mean, you know, I mean, think about it. A kid's internal model of their body is changing, right? It's like just learning to move. I mean, like, you know... If, if you ever have a baby, you'll know that, like, the first three months they're discovering their toes, right? (laughs) and it's just nuts. So everything is changing. But what's really fascinating is... And I think this is one of those... This is not at all me being a scientist, but it's like one of those things that people talk about when they talk about...... the, you know, positive aspects of children is that they're exceptionally curious and they have this kind of openness towards the world. And so, that prediction error is not a p- a negative, traumatic thing. I think it's, like, a very positive thing because it's what they use. Th- they're seeking information. One of the areas that I'm very interested in is the prefrontal cortex. It's an area of the brain that w- I mean, I could talk all day about it, but it's, it helps us use our knowledge to say, "Hey, this is what I want to do now. This is my goal. So this is how I'm going to achieve it." And focus everything towards that goal, right? The prefrontal cortex takes forever to develop in humans. The connections are still being tweaked and reformed, like, into late adolescence, early adulthood, which is when you tend to see mental illness pop up, right? So it's being massively reformed. Then you have about 10 years, maybe, of prime functioning of the prefrontal cortex, and then it starts going down again. And you end up being older and you start losing all that frontal function. So, uh, look at this, and you'd say, okay, from... You sit around episodic memory talks and always say, "Children are worse than adults at episodic memory. Older adults are worse than young adults at episodic memory." And I always say, would say, "God, this is so weird. Why would we have this period of time that's so short when we're perfect, right?" Or optimal. And I, I like to use that word optimal now because there's such a culture of optimization right now. And it's like, I realize I have to redefine what optimal is because for most of the human condition, I think, we had a series of stages of life where you have basically adults saying, okay, young adults saying, "I've got a child and, you know, I'm part of this village and I have to hunt and forage and get things done. I need a prefrontal cortex so I can stay focused on the big picture and long-haul goals." Now, I'm a child. I'm in this village, I'm kind of wandering around, and I've got some safety and I need to learn about this culture because I know so little. What's the best way to do that? Let's explore. I don't want to be constrained by goals as much. I want to really be free, play and explore and learn. So you don't want a super tight prefrontal cortex, 'cause you don't even know what the goal should be yet, right? It's like, if you're trying to design a model that's based on a bad goal, it's gonna... (laughs) It's not gonna work well, right? So then you go late in life, and you say, "Well, why don't you have a great prefrontal cortex then?" But I think... Y- I mean, if you go back and you think how many species actually stick around naturally long after their child-bearing years are over, r- after reproductive years are over? Like menopause. From what I understand, menopause is not all that common in the animal world, right? So why would that happen? And so, I saw Alison Gopnik was, uh, said something about this, so I started to look into this, about this idea that, you know, really when you're older, in most societies, your job is no longer to form new episodic memories. It's to pass on the memories that you already have. This knowledge about the world, or what we call semantic memory. To pass on that semantic memory to the younger generations, pass on the culture. You know, even now in indigenous cultures, that's the role of the elders. And they're respected. They're not seen as, you know, people who are past it and losing it. And I thought that was a very poignant thing, that memory is doing what it's supposed to throughout these stages of life.
- LFLex Fridman
So, it is always optimal in a sense.
- CRCharan Ranganath
Yeah.
- LFLex Fridman
It's just optimal for that stage of life.
- CRCharan Ranganath
Yeah, and for the ecology of the system. So you've got... So I looked into this, and it's like, another species that has menopause is orcas. Orca pods are led by the grandmothers, right? So th- not the young adults, not the parents or whatever. The grandmothers. And so they're the ones that pass on the traditions to the, I guess the younger generation of orcas. And if you, you know, if you look, from what little I understand, different orca pods have different traditions. They hunt for different things, they have different play traditions. And, uh, that's a culture, right? And so, in social animals, evolution, I think, is designing brains that are really around, you know... It's, it's obviously optimized for the individual, but also for kin. And I think that the kin are part of this, like... And when they're a part of this intense social group, the brain development should parallel that, the nature of the ecology.
- LFLex Fridman
Well, it is just fascinating to think of the individual orca or human throughout its life in stages, doing a kind of optimal wisdom development. So, uh, in the early days you don't even know what the goal is, then you figure out the goal and you kind of optimize for that goal and you pursue that goal, and then all the wisdom you collect through that, then you share with the others in the system, the other individuals. And as a h- as a collective, then you kind of converge towards greater wisdom throughout the generations, so in, in that sense, it's optimal. Us humans and orcas got something going on. It works.
- CRCharan Ranganath
Oh, yeah. Apex predators. (laughs)
- LFLex Fridman
(laughs) Uh, I just got a megalodon tooth, speaking of ace p- apex predators.
- CRCharan Ranganath
(laughs) Oh, man. (laughs)
- LFLex Fridman
It's, uh... Just imagine the size of that thing.
- CRCharan Ranganath
(laughs)
- LFLex Fridman
(inhales deeply) Anyway, uh,
- 24:16 – 31:53
Why we forget
- LFLex Fridman
how does the brain forget, and how and why does it remember? So maybe some of the mechanisms. You mentioned the hippocampus. What are the different components involved here?
- CRCharan Ranganath
So we can think about this on a number of levels. Maybe I'll give you the simplest version first, which is, we tend to think of memories as these individual things and we can just access them maybe a little bit like, you know, photos on your phone or something like that. But in the brain, the way it works is you have this distributed pool of neurons. And...... the memories are kind of shared across different pools of neurons. And so, what you have is competition, where sometimes memories that overlap can be fighting against each other, right? So sometimes we forget because that competition just wipes things out. Sometimes we forget because there aren't the biological signals which you can get into that would promote long-term retention. And lots of times we forget because we can't find the cue that sends us back to the right memory.
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
And we need the right cue to be able to activate it, right? So, um, you know, for instance, in a neural network, there is no, you wouldn't go and you'd say, "This is the memory," right? It's like the whole network, or I mean the whole ecosystem of memories is in the weights of the neural network. And in fact, you could extract entirely new memories depending on how you feed...
- LFLex Fridman
You have to have the right query, the right prompt to access that, whatever the part you're looking for.
- CRCharan Ranganath
That's exactly right. That's exactly right. And in humans you have this more complex set of ways memory works. There's, as I said, the knowledge or what you call semantic memory, and then there's these memories for specific events, which we call episodic memory. And so there's different pieces of the puzzle that require different kinds of cues. So that's a big part of it too, is just this kind of what we call retrieval failure.
- LFLex Fridman
You mentioned episodic memory, you mentioned semantic memory. What are the different separations here? What's, uh, working memory, short-term memory, long-term memory? What are the interesting categories of memory?
- CRCharan Ranganath
Yeah. And so memory researchers, we love to cut things up and-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... say, you know, is, is memory one thing or is it two things or it's two things or it's three things? And so one of the things that... There's value in that, and especially experimental value in terms of being able to dissect things. In the real world it's all connected. Speak to your question, working memory is a term that was coined by Alan Baddeley. It's basically thought to be this ability to keep information online in your mind right in front of you at a given time, and to be able to control the flow of that information, to choose what information is relevant, to be able to manipulate it, and so forth. And one of the things that Alan did that was quite brilliant was he said there's this ability to kind of passively store information, you know, see things in your mind's eye or hear your internal monologue. But, um, you know, we have that ability to keep information in mind, but then we also have this separate, what he called an, uh, a central executive, which is identified a lot with the prefrontal cortex. It's this ability to control the flow of information that's being kept active based on what it is you're doing. Now, a lot of my early work was basically saying that this working memory, which some memory researchers would call short-term memory, is not at all independent from long-term memory. That is that a lot of executive function requires learning and you have to have, like, synaptic change for that to happen. And, um, but there's also transient forms of memory. So one of the things I've been getting into lately is the idea that we form internal models of events. The obvious one that I always use is birthday parties, right? If you go to a child's birthday party, once the cake comes out and they start, you just see a candle, you can predict the whole frame, you know, set of events that happens later. And up till that point where the child blows out the candle, you have an internal model in your head of what's going on. And so if you follow people's eyes, it's not actually on what's happening. It's going where the action's about to happen. Um, which is just fascinating, right? So you have this internal model, and that's a kind of a working memory product. It's something that you're keeping online that's allowing you to interpret this world around you. Now, to build that model though, you need to pull out stuff from, uh, your general knowledge of the world, which is what we call semantic memory. And then you'd want to be able to pull out memories for specific events that happened in the past, which we call episodic memory. So in a way they're all connected, even though it's different. Um, the things that we're focusing on and the way we organize information in the present, which is working memory, will play a big role in determining how we remember that information later, which people typically call long-term memory.
- LFLex Fridman
So if you have something like a birthday party and you've been to many before, you're gonna load that from disk into working memory, this model, and then you're mostly operating on the model. And if it's a new task, you're, you don't have a model, so you're more in the data collection?
- CRCharan Ranganath
Yes. One of the fascinating things that we've been studying, and this is, we are not at all the first to do this. Um, Jeff Sax was a big pioneer in this, um, and I've been working with many other people. Ken Norman, um, Leila Davachi at NYU, or at Columbia has done some interesting stuff with this, is this idea that we form these internal models at particular points of high prediction error. Or points of, I believe also points of uncertainty, points of surprise or motivationally significant periods. And those points are when it's maximally optimal to encode an episodic memory. So I used to think, oh, well, we're just encoding episodic memories constantly. Boom, boom, boom, boom, boom. But think about how much redundancy there is in all that, right? It's just a lot of information that you don't need. But if you capture an episodic memory at the point of maximum uncertainty for the singular experience, right? You're just, it's only gonna happen once. But if you capture it at the point of maximum uncertainty or maximum surprise, you have the most useful point in your experience that you've grabbed. And what we see is that the hippocampus and these other networks, uh, that are involved in generating these internal models of events, they show a heightened period of connectivity or correlated activity during those breaks between different events, which we call event boundaries. These are the points where you're, like, surprised or you cross from one room to another and so forth. And that communication is associated with a bump of activity in the hippocampus and better memory. And so if...... people have a very good internal model. Throughout that event, you don't need to do much memory processing. You're in a predictive mode, right? And so then at these event boundaries you encode, and then you retrieve and you're like, "Okay, wait a minute. What's going on here? Ranganath's now talking about orcas. What's going on?" And maybe you have to go back and remember reading my book to pull out the episodic memory to make sense of whatever it is I'm babbling about, right? And so there's this beautiful dynamics that you can see in the brain of these different networks that are coming together and then de-affiliating at different points in time that are allowing you to go into these modes. And so to speak to your original question-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... to some extent when we're talking about semantic memory and episodic memory and working memory, you can think about it as these processes that are unfolding as these networks kind of come together and pull apart.
- 31:53 – 42:22
Training memory
- CRCharan Ranganath
- LFLex Fridman
Can memory be trained and improved, this beautiful connected system th- that you've described? What aspect of it is a mechanism that can be improved through training?
- CRCharan Ranganath
I think improvement, it depends on what your definition of optimal is. So what I say in the book is, is that you don't want to remember more. You want to remember better-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... which means focusing on the things that are important, and that's what our brains are designed to do. So if you go back to the earliest quantitative studies in memory by Ebbinghaus, what you see is that he was trying so hard to memorize this arbitrary nonsense, and within a day, he lost about 60% of that information. And he was using, he was basically using a very, very generous way of measuring it, right? So as far as we know, nobody has managed to violate those basics of having people forget, you know, most of their experiences. So if your expectation is that you should remember everything and that's what your optimal is, you're already off, because this is not what human brains are designed to do. On the other hand, what we see over and over again is that the brain does... Basically, one of the cool things about the design of the brain is it's always less is more, less is more, right?
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
It's like, I mean, I've seen estimates that the human brain uses something like 12 to 20 watts, you know, in a day. I mean, that's just nuts, the low power consumption, right? So it's all about reusing information and, and making the most of what we already have. And so, um, that's why basically, again, what you see biologically is, you know, neuromodulators, for instance, these chemicals in the brain like norepinephrine, dopamine, uh, serotonin. These are chemicals that are released during moments that tend to be biologically significant, surprise, fear, stress, et cetera. And so these chemicals promote lasting plasticity, right? Essentially some mechanisms by which the brain can say, prioritize the information that you carry with you into the future. Attention is a big factor as well, our ability to focus our attention on what's important. And so, uh, there's different schools of thought on training attention, for instance. Um, uh, so one of my colleagues, Ane- Amishi Jha, she wrote a book called Peak Mind and talks about mindfulness as a method for improving attention, uh, and focus. Uh, so she works a lot with military, like Navy SEALS and stuff to do, do this kind of work, um, with mindfulness meditation. Um, Adam Gazzaley, another one of my friends and colleagues has work on kind of training through video games actually as a way of training attention. And so, uh, it's not clear to me... You know, one of the challenges though in training is you tend to overfit to the thing that you're trying to optimize, right? So you tend to... If I'm looking at a video game, I can definitely get better at paying attention in the context of the video game, but do you transfer it to the outside world? That's very controversial.
- LFLex Fridman
Uh, the implication there is that attention is a fundamental component of remembering something, allocating attention to it, and then attention might be something that you could train.
- CRCharan Ranganath
Yeah.
- LFLex Fridman
How you allocate attention and how you hold attention on a thing.
- CRCharan Ranganath
I can say that, in fact, we do in certain ways, right? So if you are an expert in something, you are training attention. So we did this one study of expertise in the brain. And, uh, you know, so people used to think, let's say if you're a bird expert or something, right? People will go like, if you get really into this world of birds, you start to see the differences in your visual cortex is tuned up and it's all about-
- LFLex Fridman
Yeah.
- CRCharan Ranganath
... plasticity of the visual cortex. And vision researchers love to say everything's visual, (laughs) you know? But, but it's like, we did this study of attention and working, or, uh, working memory and expertise, and one of the things that surprised us were the biggest effects as people became experts in identifying these different kinds of just crazy objects that we made up. As they developed this expertise of being able to identify what made them different from each other and what made them unique, we were actually seeing massive increases in activity in the prefrontal cortex. And this fits with some of the studies of chess experts and so forth, that it's not so much that you learn the patterns passively. You learn what to look for. You learn what's important and what's not, right? And you can see this in any kind of expert professional athlete. They're looking three steps ahead of where they're supposed to be. So that's a kind of a training of attention, and those are also what you'd call expert memory skills.
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
So, um, if you take the memory athletes, I know that's something we're both interested in.
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
And, you know, so these are people who train in these competitions and they'll memorize, like, a deck of cards in, like, a really short amount of time. Um, there's a, a great memory athlete. Her name I think is pronounced Genya Winter soul, but she... Uh, so I think she's got, like, a giant Instagram following. And so she had this YouTube video that went viral, um, where she had memorized an entire IKEA catalog, right?
- LFLex Fridman
(laughs)
- CRCharan Ranganath
... and so how do people do this? By all accounts from people who become memory athletes, they weren't born with some extraordinary memory. But they practice strategies over and over and over again. The strategy that they use for memorizing a particular thing, it can become automatic and you can just deploy it in an instant, right? So again, it's not necessarily gonna ... One strategy for learning the order of a deck of cards-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... might not help you for something else that you need, like, you know, remembering your way around Austin, Texas. But it's gonna be these, whatever you're interested in, you can optimize for that. And that's just a natural byproduct of expertise.
- LFLex Fridman
There's certain hacks. There's something called the memory palace-
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
... that I've played with. I don't know if you're familiar with that-
- CRCharan Ranganath
Yeah, yeah.
- LFLex Fridman
... whole, uh, technique. And it works. It's interesting.
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
So, uh, another thing I recommend for people a lot is I use Anki a lot every day. It's a app that does spaced repetition.
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
So I think medical students in ... Like, students use this a lot to remember a lot of different things.
- 42:22 – 54:10
Memory hacks
- CRCharan Ranganath
- LFLex Fridman
We should maybe linger on this memory palace thing just to make obvious 'cause when people were describing to me a while ago what this is, it seems insane. (laughs)
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
I just ... You literally think of a place like a childhood home or a home that you're really visually, uh, familiar with and you literally place in that three-dimensional space facts or people or whatever you want to remember and you just walk in your mind-... along that place, visually.
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
And you can remember, uh, remind yourself of the different things. One of the limitations is there is a sequence to it.
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
So it's... I think your brain somehow... You ne- you can't just, like, go upstairs right away or something.
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
You have to, like, walk along the, the room. So it's really great for remembering sequences, but it's also not great for remembering, like, individual facts out of context. So-
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
... the full context of the tour, I think-
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
... is important. Uh, but it's, uh, it's fascinating how the mind is able to do that, uh, when you ground these pieces of knowledge into something that you remember well already, especially visually.
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
Fascinating. And you can just do that for any kind of sequence. And I'm sure she used something like this for the, for Ikea catalog. Something-
- CRCharan Ranganath
Oh, yeah.
- LFLex Fridman
... of this nature.
- CRCharan Ranganath
Absolutely, absolutely. Um, and I think the, the principle here is... Again, I was telling you this idea that memories can compete with each other, right? Well, I like to use this example, and maybe someday I'll regret this, but I've used it a lot recently-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... is, like, imagine... If this were my desk, it could be cluttered with a zillion different things, right? So imagine it's just cluttered with a whole bunch of yellow Post-it notes.
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
And on one of them, I put my bank password on it, right? Well, it's gonna take me forever to find it. I might, you know... It's just gonna be buried under all these other Post-it note. But if it's, like, hot pink-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... it's gonna stand out, and I find it really easily, right? So that's one way in which if things are distinctive, if you've processed information in a very distinctive way, then you can have a memory that's gonna last. Um, and that's very good, for instance, for name/face associations. If I get something distinctive about you, you know, that it's like... That you've got very short hair, and maybe I can make the association with Lex Luthor that way, or something like that, right? You know, but I, I get something very specific, that's a great cue. But the other part of it is, what if I just organize my notes so that I have my finances in one pile, and I have my, like, uh, reminders, my to-do list in one pile, and so forth. So I organized them. Well, then, I know exactly if I'm going for my banking, you know, pa- my bank password, I could go to the finance pile, right? So the method of loci works, or memory palaces work, because they give you a way of organizing. Um, there's a school of thought that says that episodic memory evolved from this, like, kind of knowledge of space and, you know, basically this primitive abilities to figure out where you are. And so people explain the method of loci that way. And, you know, whether or not the evolutionary argument is true, the method of loci is not at all special. So if you don't... you're not a good visualizer, um, uh, stories are a good one. So a lot of memory athletes will use stories, and they'll go, like, if you're memorizing a deck of cards, they have a little code for the different, like, uh, um, like the king and the jack and the-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... 10 and so forth. And they'll make up a story about things that they're doing, and that'll work. Songs are a great one, right?
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
I mean, it's like a... I could still remember, there's this obscure episode of the TV show Cheers. They sing a song about Albania that he uses to memorize all these facts about Albania. And I can still sing that song to you (laughs) .
- LFLex Fridman
(laughs) .
- 54:10 – 1:03:29
Imagination vs memory
- LFLex Fridman
for that.
- CRCharan Ranganath
One of the cool things that, that I found is, is that some people really just revolutionize a field by cre- by creating a problem that didn't exist before. It's kind of like why I love science is like I, I... engineering is like solving other people's problems and science is about creating problems.
- LFLex Fridman
(laughs)
- CRCharan Ranganath
I'm just much more like I want to break things and-
- LFLex Fridman
Yeah.
- CRCharan Ranganath
... you know, create problems.
- LFLex Fridman
Yeah.
- CRCharan Ranganath
Uh, not necessarily move fast though but-
- LFLex Fridman
(laughs)
- CRCharan Ranganath
(laughs) But one of my former mentors, Marsha Johnson, who in my opinion is one of the greatest memory researchers of all time, she comes up, young woman in the field, in this mostly guy field, and she gets into this idea of how do we tell the difference between things that we've imagined and things that we actually remember? How do we tell... I get some mental experience, where did that mental experience come from, right? And it turns out this is a huge problem.... because essentially our mental experience of remembering something that happened and our mental experience of thinking about something, how do you tell the difference? They're both largely constructions in our head. And so it is very important, and the way that you do it is, uh, I mean, it's not perfect, but the way that we often do it and succeed is by, again, using our prefrontal cortex and really focusing on the sensory information or the place and time and the things that put us back into when this information happened. And if it's something you thought about, you're not gonna have all of that vivid detail as you do for something that actually happened. But it doesn't work all the time. But th- that's a big thing that you have to do. But it takes time, it's slow, and it's, again, effortful, but that's what you need to remember accurately. But what's cool, and I think this is what you alluded to about how that was an interesting experience, is imagination is exactly the opposite. Imagination is basically saying, "I'm just gonna take all this information from memory, recombine it in different ways, and throw it out there." And so for instance, um, Dan Schacter, um, and Donna Attas have done cool work on this. Demis Hassabis did work on this with Eleanor Maguire, um, in UCL. And this goes back actually to this guy Frederick Bartlett who was this revolutionary memory researcher. Bartlett, he actually, like, rejected the whole idea of quantifying memory, and he said, "There's no statistics in my book." He came from this anthropology perspective. And short version of the story is he just asked people to recall thing, he would give people stories and poem, ask people to recall them, and what he found was people's memories didn't reflect all of the details of what they were exposed to, and they did reflect a lot more, they were filtered through this lens of prior knowledge. The, the cultures that, that they came from, the beliefs that they had, the things they knew. And so what he concluded was that, he called remembering an imaginative construction, meaning that we don't replay the past, we imagine how the past could've been by taking bits and pieces that come up in our heads. And likewise, he wrote this beautiful paper on imagination saying when we imagine something and create something, we're creating it from these specific experiences that we've had and combining it with our general knowledge. But instead of trying to focus it on being accurate and getting out one thing, you're just ruthlessly recombining things without any, you know, any necessary kind of goal in mind. Um, I mean, or at least that's one kind of creation.
- LFLex Fridman
So imagination is, um, fundamentally coupled with memory, in, in both directions.
- CRCharan Ranganath
I think so. I mean, it's not clear that it is in everyone, but one of the things that's been studied is some patients who have amnesia, for instance, they have, uh, brain damage, say, to the hippocampus, and if you ask them to imagine things that are not in front of them, like, "Imagine what could happen after I leave this room," right, they are, find it very difficult to give you a scenario of what could happen, or if they do, it would be more stereotype, like, yes, this would happen, this would...
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
But it's not like they can come up with anything that's very vivid and creative in that sense. That's partly 'cause when you have amnesia, you're stuck in the present, because to get a very good model of the future really helps to have episodic memories to draw upon, right? And so that's, that's the basic idea. And, in fact, one of the most impressive things... When people started to scan people's brains and ask people to remember past events, what they found was there's this big network of the brain called the default mode network. It gets a lot of press because it's, like, thought to be important, r- it's engaged during mind wandering-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... and if I ask you to pay attention to something, it only comes on when you stop paying attention, you know? So people go, "Oh, it's just this kind of, you know, d-daydreaming network." And I thought, "This is just ridiculous research. Who cares," you know? Um, but then what people found was when people recall episodic memories, this network gets active. And, uh, so we started to look into it, and this network of areas is really closely functionally interacting with the hippocampus, and so, in fact, some would say the hippocampus is part of this default network. And if you look at brain images of people, or brain maps of activation, so to speak, of people imagining possible scenarios of things that could happen in the future, even things that couldn't really be very plausible, they look very similar, I mean, you know, to the naked eye, they look almost the same as maps of brain activation when people remember the past. According to our theory, and we've got some data to support this, we've broken up this network into various sub-pieces, is that basically it's kind of taking apart all of our experiences and creating these little Lego blocks out of them, and then you can put them back together if you have the right instructions to recreate these experiences that you've had, but you could also reassemble them into new pieces to create a model of an event that hasn't happened yet, and that's what we think happens, and when I'm... Our common ground that we're establishing in language requires using those building blocks to put together a model of what's going on.
- LFLex Fridman
Well, there's a good percentage of time I personally live in, in the imagined world. I think of, I have, I do thought experiments a lot. I, you know, take the, uh, the absurdity of human life as it stands, and, uh, play it forward-
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
... in all kinds of different directions. Sometimes it's rigorous thoughts, thought experiments, sometimes it's fun ones, so I was... I imagine that that ha- has an effect on how I remember things. (laughs) And, um, I suppose I have to be a little bit careful to make sure stuff happened versus stuff that I just imagined happened. And this also...... I mean, some of my best friends are characters inside books that never even existed. And I'm, you know, there's some degree to which they actually exist in my mind. Like, these characters exist, authors exist. Dostoevsky exists, but also, uh, Brothers Karamazov. I-
- CRCharan Ranganath
I love that book.
- LFLex Fridman
Yeah. (laughs)
- CRCharan Ranganath
It's one of the few books I've read. (laughing)
- LFLex Fridman
(laughing)
- CRCharan Ranganath
One of the few literature books that I've read, I should say. I read a lot in school that I don't remember, but Brothers Karamazov.
- LFLex Fridman
(laughs) But they exist.
- CRCharan Ranganath
Alyosha.
- LFLex Fridman
They exist, and I have almost kind of like conversations with them. It's interesting. It's, uh, it's interesting to allow your brain to kind of play with ideas of the past, of the imagined, and see it all as one.
- CRCharan Ranganath
Yeah, there was actually this famous mnemonist, he's kind of like back then the equivalent of a memory athlete except he would go to shows and do this, uh, um, that was described by this, uh, really famous neuropsychologist from Russia named, uh, Luria. And so this guy was named Solomon Shereshevski, and he had this condition called synesthesia that basically created these weird associations between different senses that normally wouldn't go together. So, that gave him this incredibly vivid imagination that he would use to basically imagine all sorts of things that he would need to memorize, and he would just imagine, like just create these incredibly detailed things in his head that allowed him to memorize all sorts of stuff. But it also really haunted him by some reports that basically it was like he was at some point, you know, and again who knows if the drinking was part of this (laughs) , but he at some point had trouble differentiating his imagination from reality, right? And this is, this is interesting because it's like, I mean, that's what psychosis is in some ways is you, you know, first of all you're just learning connections from prediction errors that you probably shouldn't learn. And the other part of it is, is that your internal signals are being confused with actual things in the outside world, right?
- LFLex Fridman
Well, that's why a lot of this stuff is both feature and bug. It's a double-edged sword.
- CRCharan Ranganath
Yeah, I mean, it might be why there's such an interesting relationship between genius and psychosis. (laughs)
- 1:03:29 – 1:13:18
Memory competitions
- LFLex Fridman
Can we just talk about memory sport a little longer? There's something called the USA Memory Championship.
- CRCharan Ranganath
Mm.
- LFLex Fridman
Like, what are these athletes like? What does it mean to be like elite level at this? Have you interact with any of them or reading about them? What have you learned about these folks?
- CRCharan Ranganath
There's a guy named, uh, Henry Rodinger who's studying these guys, and there's actually a book by Joshua Foer called, uh, Moonwalking with Einstein where he talks about he actually as part of this book just decided to become a memory athlete.
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
They often have these life events that make them go, "Hey, why don't I do this?" So there was a guy named Scott Hagwood who I write about who, um, uh, thought that he was, uh, he was getting chemo for cancer, and so he decided, like because chemo there's a- a th- well-known thing called chemo brain where people become like they just lose a lot of their sharpness. Um, and so he wanted to fight that by learning these memory skills, so he bought a book and this is the story you hear in a lot of memory athletes is they buy a book by other memory athletes (laughs) -
- LFLex Fridman
(laughs) Yeah.
- CRCharan Ranganath
... or other memory experts so to speak, and they just learn those skills and practice them over and over again. They start by winning bets and so forth, and then they go into these competitions, and the competitions are typically things like memorizing long strings of numbers or memorizing, you know, orders of cards and so forth. So they're tend to be pretty arbitrary things, not like things that would be able, you'd be able to bring a lot of prior knowledge. But, they build the skills that you need to memorize arbitrary things.
- LFLex Fridman
Yeah, it's fascinating. I've, uh, gotten a chance to work with something called, uh, n-back tasks.
- CRCharan Ranganath
Mm. Oh, yeah.
- LFLex Fridman
So there's all these kinds of tasks, memory recall tasks-
- CRCharan Ranganath
Yeah, yeah.
- LFLex Fridman
... that are used to kind of load up the quote unquote "working memory"-
- CRCharan Ranganath
Yeah, yeah.
- LFLex Fridman
... and to see, and psychologists use it to test all kinds of stuff like to see how well you're good at multitasking. We use it in particular for the task of driving, like if it, if we fill up your brain with-
- CRCharan Ranganath
Mm.
- LFLex Fridman
... intensive working memory tasks, how good are you at also-
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
... not crashing?
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
That kind of stuff. So it's fascinating, but, again those tasks are arbitrary, and they're usually about recalling a sequence of numbers in some kind of semi-complex way.
- CRCharan Ranganath
Mm-hmm.
- LFLex Fridman
Are you, uh, do you have any favorite tasks of this nature in your own, uh, studies?
- CRCharan Ranganath
I've really been most excited about going in the opposite direction and using things that are more and more naturalistic.
- LFLex Fridman
Mm.
- CRCharan Ranganath
And the reason is, is that we've really moved, we've moved in that direction because what we found is, is that memory works very, very differently when you study it, when you study memory in the way that people typically remember. And so it goes into a much more predictive mode, and you have these, um, event boundaries for instance, and you have, uh, but a lot of what happens is this kind of fascinating mix that we've been talking about, a mix of interpretations and imagination with perception, and so, um, and the new direction we're going in is understanding, uh, navigation and our memory for places. And the reason is, is that there's a lot of work that's done in rats which is very good work. They have a rat and they put it in a box and the rat goes chases and cheese in a box-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... and you'll find cells in the hippocampus that fire when a rat is in different places-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... in the box. And so the conventional wisdom is, is that the hippocampus forms this map of the box, and I think that-... probably may happen when you have, like, absolutely no knowledge of the world, right?
- 1:13:18 – 1:28:33
Science of memory
- LFLex Fridman
So you mentioned, uh, fMRI. What is it and how is it used in studying memory?
- CRCharan Ranganath
This is actually the reason why I got into this whole field of science. When I was in grad school, fMRI was just really taking off as a technique for studying brain activity. And, uh, what's beautiful about it is you can study the whole human brain and, uh, there's lots of limits to it, but you can basically do it in a person without sticking anything into their brains, and very noninvasive. I mean, for me, being in an MRI scanner is like being in the womb, I just fall asleep. (laughs)
- LFLex Fridman
(laughs)
- CRCharan Ranganath
If I'm not being asked to do anything, I get very sleepy, you know? Um, but you can have people watch movies while they're being scanned or you can have them do tests of memory, like giving them words and so forth to memorize. Uh, but what MRI is itself is just this technique where you put people in a very high magnetic field. Typical ones we would use would be three tesla, to give you an idea. So a three-tesla magnet, you put somebody in, and what happens is you get this very weak but, you know, measurable magnetization in the brain, and then you apply a radiofrequency pulse, which is basically a different electromagnetic field. And so you're basically using water, the water molecules in the brain as a tracer, so to speak. Um, and, uh, part of it in fMRI is the fact that these magnetic fields that you mess with by y- by manipulating, um, these radiofrequency pulses in the static field and you have things called gradients which change the strength of the magnetic field in different parts of the head. So they're all... We tweak them in different ways, but the basic idea that we use in fMRI is that blood is flowing to the brain, and when you have blood that doesn't have oxygen on it, it's a little bit more magnetizable than blood that does 'cause you have hemoglobin that carries the oxygen, the iron basically in the blood that makes it red. And so that hemoglobin, when it's deoxygenated, actually a- um, has different magnetic field properties than when it has oxygen. And it turns out when you have an increase in local activity in some part of the brain, the blood flows there and as a result, you get a lower concentration of hemoglobin that is not oxygenated and then that gives you more signal. So I gave you a j- I think I sent you a GIF, as you like to say.
- LFLex Fridman
Yeah. We had an off-record-
- CRCharan Ranganath
(laughs)
- LFLex Fridman
... intense argument, uh-
- CRCharan Ranganath
(laughs)
- LFLex Fridman
... about if it's pronounced JIF or GIF, but that's... we'll j- we shall set that aside as friends.
- CRCharan Ranganath
Uh, we could have called it a stern rebuke perhaps, but... (laughs)
- LFLex Fridman
Re- rebuke, yeah. I drew a hard line. Uh, it is true the creator of GIF said it's pronounced JIF, but that's the only person that pronounces it JIF.
- CRCharan Ranganath
(laughs)
- LFLex Fridman
Anyway, s- yes. You sent a GI- uh, a GIF of...
- CRCharan Ranganath
Oh, this would be basically a whole... a movie of fMRI data. And so when you look at it, it's not very impressive. It looks like these, like, very pixelated maps of the brain, but it's mostly kind of like white. But these tiny changes in the intensity of those signals that you probably wouldn't be able to visually perceive, like about 1%, can be statistically very, very large effects for us. And that allows us to see, hey, there's an increase in activity in some part of the brain when I'm doing some task like trying to remember something, and I can use those changes to even predict, is a person going to remember this later or not? And the coolest thing that people have done is to decode, um, what people are remembering-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... from the patterns of activity from... 'Cause maybe when I'm remembering this thing, like I'm remembering the pl- house where I grew up, I might have one pixel that's bright in the hippocampus and one that's dark, and if I'm remembering, uh, you know, something like more like a, a, the car that I used to drive when I was 16, I might see the opposite pattern where a different pixel's bright. And so all that little stuff that we used to think of noise, uh, we can now think of almost like a QR code for memories, so to speak, where different memories have a different little pattern of bright pixels and dark pixels. And so this really revolutionized my research. So there's fancy research out there where people really... I mean, not even that fa- I mean, by your standards this would be Stone Age, but... (laughs)
- LFLex Fridman
(laughs)
- CRCharan Ranganath
You know, applying machine learning techniques to do decoding and so forth and now there's a lot of forward encoding models and you, you can go to town with this stuff, right? And-
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
... I'm much more old school of designing experiments where you basically say, "Okay, here's a whole web of inter... of memories that overlap in some way, shape, or form. Do memories that occurred in the same place have a p- similar QR code-"
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
"... and do memories that occurred in a different place have a different QR code?" And you can just use things like correlation coefficients or cosine distance to measure that stuff, right? Super simple, right? And so what happens is you can start to get a whole state space of how a brain area is indexing all these different memories. And it's super fascinating because what we could see is this little, like, separation between how certain brain areas are processing memory for who was there and other brain areas are processing information about where it occurred or the situation that's kind of unfolding, and some are giving you information about what are my goals, uh, that are involved and so forth. And so... And the hippocampus is just putting it all together into these unique things that just are about when and where it happened.
- LFLex Fridman
So there is a separation between spatial information, concepts. Like, literally, there's distinct...
... as you said, QR codes for these?
- CRCharan Ranganath
So to speak. Let me try a different analogy too, that might be more accessible for people.
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
Which would be, like, uh, you've got a folder on your computer, right? And you open it up, there's a bunch of files there. I can sort those files by, you know, alphabetical order.
- LFLex Fridman
Mm-hmm.
- CRCharan Ranganath
And now things that both start with the letter A are lumped together, and things that start with Z versus A are far apart, right?
- LFLex Fridman
Mm-hmm.
Episode duration: 3:10:49
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