Speaker 1: What is memory in the brain? How is that different
Speaker 1: from the way a computer stores information? How do you
Speaker 1: put together billions of specialized cells and you store your
Speaker 1: home address in their activity? And what does any of
Speaker 1: this have to do with the happy Birthday song? Or
Speaker 1: with squirrels hiding acorns, or with bards memorizing epic poems,
Speaker 1: or with people who cannot forget any of the events
Speaker 1: in their life. Welcome to Inner Cosmos with me David Eagelman.
Speaker 1: I'm a neuroscientist and an author at Stanford and in
Speaker 1: these episodes we sail into our three pound universe to
Speaker 1: understand why and how our lives look the way they do.
Speaker 1: Today's episode is about memory. What is memory? How does
Speaker 1: it work? How do details get stored in your brains
Speaker 1: such that if I say, what was the name of
Speaker 1: your fifth grade teacher? You can retrieve that name even
Speaker 1: if you haven't thought about it in decades? And how
Speaker 1: is this totally different from the way that computers store memory?
Speaker 1: So in this episode, I'll give you a foundation into
Speaker 1: understanding the landscape of memory. Now, this is the first
Speaker 1: of a three part series about the way that our
Speaker 1: brains constantly unhook from the here and now, and they
Speaker 1: go somewhere else. We are time travelers, and only because
Speaker 1: we do this so constantly, we don't even notice how
Speaker 1: amazing this is. I mean, everything you might study in
Speaker 1: a textbook about the brain has to do with Okay,
Speaker 1: here's how vision works. Here's how the visual quartet analyzes
Speaker 1: photons captured at the retina and makes an assessment of
Speaker 1: what's in front of you. And yet we're able to
Speaker 1: travel back to previous times. You can put yourself back
Speaker 1: in that fifth grade classroom, or the first house you
Speaker 1: grew up in, or the moment of your first kiss.
Speaker 1: You can simulate the sights and sounds and smells. You
Speaker 1: can remember how you felt in certain moments. You can
Speaker 1: recreate and reexperience that. You may be able to recollect
Speaker 1: lots of the details that were around you. To do that,
Speaker 1: you just time traveled across years or decades to place
Speaker 1: yourself in another era. Your visual system is not simply
Speaker 1: looking at the photons in front of you. Instead, it's
Speaker 1: now involved in running a simulation of what had transpired
Speaker 1: and an earlier time wherever you physically are right now
Speaker 1: listening to this podcast, You just time traveled. When you
Speaker 1: thought about your fifth grade experience. Now it turns out
Speaker 1: that brains don't just time travel backwards, but they can
Speaker 1: also move forward. So in the next episode, Part two,
Speaker 1: we're going to talk about prediction, and then in part
Speaker 1: three we're going to pull pieces of the puzzle together
Speaker 1: to unlock some wild surprises about how we experience the
Speaker 1: world emotionally given our time traveling talents. So today we'll
Speaker 1: start with memory. And I just want to say this
Speaker 1: is a huge topic that I could teach you over
Speaker 1: the course of years. So for today's podcast, we're going
Speaker 1: to take a high level ride to get a feel
Speaker 1: for the landscape. So one way to approach any big
Speaker 1: topic is to look at the extremes. So a lot
Speaker 1: of memories come easily to us the day we moved
Speaker 1: into our new home, or the day we got the
Speaker 1: job offer, or the time that the phone rang with
Speaker 1: news of somebody's death. But imagine having a clear and
Speaker 1: immediate memory for all of the events of your day
Speaker 1: to day life, such as the dinner that you had
Speaker 1: on November thirtieth, twenty twenty three, or the friend who
Speaker 1: visited you on May twentieth, twenty nineteen, or the swimming
Speaker 1: pool party you went to on July eighth, twenty seventeen.
Speaker 1: Imagine having a vivid, movie like recollection of what you
Speaker 1: did on today's day ten years ago. Now imagine that
Speaker 1: these recollections dominate your thoughts. They run in parallel with
Speaker 1: the events of your waking life, as you commute to work,
Speaker 1: as you talk with your friends, as you shop for groceries,
Speaker 1: as you spend time with your family. You would be
Speaker 1: right that other people would marvel and probably envy your
Speaker 1: exceptional powers of memory, but you might find the process
Speaker 1: uncontrollable and overwhelming. Some memories are painful, or traumatic, or
Speaker 1: just annoying. Then you might wish that you could be
Speaker 1: rid of them, and exhausted by their presence, you might
Speaker 1: finally contact a neuroscientist, hoping that they could help you
Speaker 1: understand what's happening. And that is the real life story
Speaker 1: for a woman named Jill Price, who is a mneminist,
Speaker 1: which means she has an extraordinary memory and she can't
Speaker 1: forget things. So some years ago she went to the
Speaker 1: University of California at Irmine and described her experiences to
Speaker 1: two memory researchers there my colleagues Magaw and Cale, and
Speaker 1: at first they were skeptical, but they investigated her memory
Speaker 1: with a series of careful tests and interviews over a
Speaker 1: period of five years, and the results showed that Price
Speaker 1: indeed had a rare and astonishingly powerful memory, at least
Speaker 1: for certain types of information. Given a particular date, she
Speaker 1: could recall within seconds the day of the week that was,
Speaker 1: and the details of what she did on that day,
Speaker 1: and the news were the events that occurred on that day,
Speaker 1: without any preparations. She easily recalled the dates of every
Speaker 1: Easter from nineteen eighty to two thousand and three and
Speaker 1: her activities on those days. She also gave correct dates
Speaker 1: and personal anecdotes for randomly selected news events like the
Speaker 1: start of the First Gulf War, or the bombing of
Speaker 1: the Atlanta Olympic Games, or the death of Princess Diana. Now,
Speaker 1: while her recollections were detailed and consistent, they were generally
Speaker 1: specific to events of high personal relevance, like illnesses or relationships,
Speaker 1: or big news stories. Interestingly, her general intelligence was average,
Speaker 1: and her memory for other types of information was really
Speaker 1: no better than anyone else's. She never excelled in school
Speaker 1: and wasn't particularly good at memorizing dates and history books
Speaker 1: and at one point her interviewers asked her to close
Speaker 1: her eyes and describe their clothing, and she drew a blank.
Speaker 1: So her memory abilities were so powerful, but they were
Speaker 1: also strangely limited. So when these researchers published their first
Speaker 1: report on Jill Price, she became a news item. Her
Speaker 1: story drew headlines around the world, She was a guest
Speaker 1: on Oprah, and she published her memoirs. But by the way,
Speaker 1: Jill is not the only mneminist. The first official report
Speaker 1: ofneminism that I know about was a book from the
Speaker 1: Russian neuropsychologist named Luria about a newspaper reporter named Sharyshevski
Speaker 1: who had the gift of an untaxable memory, essentially total
Speaker 1: recall for all the moments of his life. Shiashevski first
Speaker 1: came to him in the nineteen twenties because he got
Speaker 1: in trouble at a meeting for not taking any notes
Speaker 1: when there was a speaker, and Sharishevski responded to his
Speaker 1: boss by recalling the speech word for word, and this
Speaker 1: blew everyone away there, and it also blew Sharyshevski away
Speaker 1: that the other people couldn't do that, and so this
Speaker 1: started this thirty year relationship with Luria, who tested him
Speaker 1: in all kinds of ways and had him memorize math
Speaker 1: formulas or huge blocks of numbers, or even poems and
Speaker 1: languages he didn't speak, and in all these cases, Sharyshevski
Speaker 1: could memorize these in a matter of minutes with perfect accuracy.
Speaker 1: Luria tested and observed him for three decades and then
Speaker 1: wrote a book called The Mind of a Mneminist. So,
Speaker 1: although they are rare, neminists exist, and as scientists continue
Speaker 1: to search for the source of their remarkable abilities, it's
Speaker 1: worth asking why their stories capture our imaginations so strongly.
Speaker 1: Perhaps it's because we'd all love to have supercharged memories,
Speaker 1: Or maybe we see how failing memory is so debilitating
Speaker 1: in old age and we hope to escape the fate
Speaker 1: of our grandparents. Or maybe we recognize how little we
Speaker 1: actually retain from our vast, rich set of life experiences
Speaker 1: that shape us. So in today's episode, we're going to
Speaker 1: dive into where memory comes from and how it works,
Speaker 1: and this is going to give us insight into both
Speaker 1: remarkable memories and bad memories. We'll see what memory is,
Speaker 1: how it functions, how it fails, and what purpose it serves.
Speaker 1: So the first thing I want to tackle is the
Speaker 1: mistake we make by calling the storage of zeros and
Speaker 1: ones in computer memory, because computer memory is actually nothing
Speaker 1: like human memory. Computers store in their memory exactly what
Speaker 1: you give them. So if I write a document, I
Speaker 1: expect my computer memory to function perfectly such that it
Speaker 1: gives me exactly that document back. I don't want something
Speaker 1: similar or a degraded version of it. But human memory
Speaker 1: is a completely different ballgame because you're not storing things
Speaker 1: in zeros and ones. Let's imagine that you speak two
Speaker 1: languages and I tell you a joke in English and
Speaker 1: you turn and tell it to someone else in your
Speaker 1: other language. So the joke is obviously not stored as
Speaker 1: zeros and ones. You're not memorizing my exact words and inflections. Instead,
Speaker 1: the joke is stored in a conceptual space in your mind,
Speaker 1: the way that the characters and events fit together, the
Speaker 1: gist of the action. It doesn't matter the language with
Speaker 1: which you output it. And note that this is the
Speaker 1: same thing with a tune. When you remember a tune,
Speaker 1: it's not about the individual notes. Instead, it's about the
Speaker 1: relationship between the notes, and that's why you can transpose
Speaker 1: the tune to any other key and you still have
Speaker 1: no trouble recognizing the tune. It's not about the exact notes,
Speaker 1: but about the relationship between them. And this is the
Speaker 1: same thing when you're recognizing your friend's face. What has
Speaker 1: been stored in your brain is the relationship between their
Speaker 1: nose length and lip thickness, and distance between their eyes
Speaker 1: and shape of their ears, and where their hairline is
Speaker 1: and so on. It's not about the individual features, but
Speaker 1: the relationships between them. And by the way, this is
Speaker 1: the only way we could have good recognition memory because
Speaker 1: other details change. For example, imagine that you learn a
Speaker 1: picture of the face where the lighting is on the left,
Speaker 1: and later you see the same face, but it's lit
Speaker 1: from the other side. If you are memorizing pixel by pixel,
Speaker 1: then the pattern is totally different and you would never
Speaker 1: be able to recognize it or account for all the
Speaker 1: lighting differences that might happen in the world, much less
Speaker 1: the size differences if you see a big picture or
Speaker 1: a small picture. So understanding the relationship between things is
Speaker 1: the only way to have a successful recognition and literally,
Speaker 1: for decades, computer scientists were chasing the wrong ideas by
Speaker 1: trying to get pixel by pixel recognition until they realized
Speaker 1: that it's all about the relationships. And this is not
Speaker 1: just about vision, but this applies to everything we learn.
Speaker 1: Like facts, your brain encodes new ideas with respect to
Speaker 1: other things you have learned. Just imagine two people looking
Speaker 1: at a list of important dates in Mongolian history. If
Speaker 1: one of them already has a richly developed Mongolian history timeline,
Speaker 1: then new facts are readily incorporated into that person's network
Speaker 1: of knowledge. We don't simply memorize facts. We fit new
Speaker 1: data into the relationship lattice of our internal model. Everything
Speaker 1: we've learned before based on what was relevant to us.
Speaker 1: And I talked about this in episode twenty seven. Everything
Speaker 1: you learn is represented in terms of what you already know.
Speaker 1: So when you listen to this podcast, the points made
Speaker 1: would be meaningless unless you all already we're living a
Speaker 1: human life and knew what a brain was and understood
Speaker 1: the particular language I am speaking. We do not encode
Speaker 1: things as little files of zeros and ones. What's happening
Speaker 1: in the human brain is much different. So that's the
Speaker 1: first thing. And there's another reason why human memory is
Speaker 1: not at all like computer memory. And this is because
Speaker 1: over the past several decades, we've come to realize that
Speaker 1: human memory is not one thing, but instead it's made
Speaker 1: up of lots and lots of different subsystems. There are
Speaker 1: many things going on under this one umbrella term that
Speaker 1: we call human memory. So for the next few minutes,
Speaker 1: let's dive into the nuts and bolts of that landscape.
Speaker 1: The first thing to know is that memory is divided
Speaker 1: between short term and long term. So short term memory
Speaker 1: is about in information that decays in a really short
Speaker 1: time if you don't use it, like seconds to minutes.
Speaker 1: This is also called working memory by neuroscientists. The idea
Speaker 1: is that short term memory uses information to address immediate situations.
Speaker 1: So let's say I say, oh, hey, your verification code
Speaker 1: to log into LinkedIn is six one, nine, five, three
Speaker 1: to two. You can remember that for some seconds or minutes,
Speaker 1: but you're probably not going to remember that number next week,
Speaker 1: or probably even in an hour from now. There's no
Speaker 1: strict time limit here, but short term memory typically lasts
Speaker 1: just for the duration of the task at hand. Now,
Speaker 1: there are lots of ways that your brain helps itself
Speaker 1: long with short term memories. Sometimes you do this by
Speaker 1: talking to yourself like six one nine five three two
Speaker 1: six one nine five three two, or sometimes you might
Speaker 1: hold on to information by visualizing something in front of you.
Speaker 1: But how whoever you do it, working memory has a
Speaker 1: limited capacity for holding information. In nineteen fifty six, a
Speaker 1: cognitive psychologist wrote a paper called the Magical Number seven
Speaker 1: plus or minus two. Now, why that weird title is
Speaker 1: because he found that most people can hold about seven
Speaker 1: items of information in short term memory, sometimes a little
Speaker 1: more so as a less It doesn't matter whether that's
Speaker 1: shapes or locations or colors or numbers or whatever. You
Speaker 1: can hold about seven items. And by the way, this
Speaker 1: has been proposed to explain why telephone numbers in most
Speaker 1: countries use seven digits, because back in the day people
Speaker 1: often needed to memorize phone numbers and if they were
Speaker 1: like sixteen digits, no one would be able to do
Speaker 1: it anyway, So that's short term memory. I ask you
Speaker 1: to memorize a combination lock or a log in code
Speaker 1: or a phone number for some task and you say, cool,
Speaker 1: got it. That lives in your short term memory. And
Speaker 1: that memory is built of the activity of millions or
Speaker 1: billions of neurons holding onto a loop of spikes that
Speaker 1: run around and around across giant swaths of territory. The
Speaker 1: activity is kept alive, and that represents the thing you
Speaker 1: are remembering now. Often, something you hold in your short
Speaker 1: term memory tends not to transfer to your long term memory.
Speaker 1: But long term memory is the really interesting thing in
Speaker 1: the brain. So that's what we're going to concentrate on
Speaker 1: from here on out. Because with long term memory, it's
Speaker 1: not about active neurons. Instead, there are changes to the
Speaker 1: physical structure of your brain such that you can pull
Speaker 1: up information that happened to you a long, long time ago.
Speaker 1: You're actually changing the stuff of your brain. I mentioned
Speaker 1: in a previous episode that if a rock hits your
Speaker 1: windshield and leaves a you could say poetically that your
Speaker 1: car remembers the rock even long after the rock is gone.
Speaker 1: There is a physical mark on your car, and that's
Speaker 1: the way to think about it. With the brain, an
Speaker 1: event happens to you, and it leaves a physical mark,
Speaker 1: and that mark can be read out later. So as
Speaker 1: opposed to short term memory, long term memory involves brain
Speaker 1: systems that in code and store and retrieve information over
Speaker 1: long periods of time, anywhere from minutes to a lifetime,
Speaker 1: and the capacity of these systems is much greater than
Speaker 1: short term memory, as we see with someone like Jill
Speaker 1: Price or Shashevski. Now, amazingly, long term memory is not
Speaker 1: one thing. Instead, we divide it into two groups. On
Speaker 1: the one hand, we have implicit memory and on the
Speaker 1: other explicit memory. Now, implicit memory is the stuff that
Speaker 1: you can't express or articulate or consciously recall, like riding
Speaker 1: a bike. You have no idea how you do it,
Speaker 1: but you learned it and you remember it. You just
Speaker 1: can't articulate how you do it. It is implicit. And this
Speaker 1: category of implicit memory also includes unconscious emotional memories, like
Speaker 1: an aversion towards snowboarding years after an accident, even if
Speaker 1: the accident itself is long forgotten. Okay, so all of
Speaker 1: that is implicit memory. On the other hand, explicit memory
Speaker 1: is the type of information that can be consciously recalled
Speaker 1: and described, like facts and events. So think about something
Speaker 1: that happened to you that you can recall and describe,
Speaker 1: like this is where you went to college, or this
Speaker 1: is the name of that great movie you saw, or
Speaker 1: that's what your neighbor said to you last week. Explicit
Speaker 1: memory also includes memories of facts like the fact that
Speaker 1: Amazon dot Com sells books or that penguins can't fly.
Speaker 1: If you can say some recollection or fact out loud,
Speaker 1: then that is explicit memory. Now, I wanted to clearly
Speaker 1: lay out these two sides of implicit memory and explicit
Speaker 1: memory before taking a deeper dive to see why these
Speaker 1: are understood to be separate systems. So now let's return
Speaker 1: for a closer look at this first one, implicit memory.
Speaker 1: So think about memories involved in how to perform skills
Speaker 1: or habits like reading or typing, or whimming, or juggling
Speaker 1: or playing piano. You acquire these kinds of memory slowly
Speaker 1: through a lot of practice and repetition. The way you
Speaker 1: learn to perfect these actions is from your brain changing
Speaker 1: its structure. It is remembering. So even though you might
Speaker 1: not typically think of things like reading and typing and
Speaker 1: walking and so on as memory, they are indeed a
Speaker 1: form of remembrance, just one that you cannot articulate, And
Speaker 1: there are many forms of implicit memory. When you might
Speaker 1: have heard of is called priming. That's what happens when
Speaker 1: some past experience influences your future response. So imagine I
Speaker 1: show you the letters S, blank, P, and I ask
Speaker 1: you to fill in the missing letter. If you have
Speaker 1: just had a shower, you'll probably put an A in
Speaker 1: there to make the word soap. If you've just seen
Speaker 1: a steaming hot bowl at the table, you'll probably put
Speaker 1: the letter you in there to make the word soup.
Speaker 1: Your brain just recently experienced something, and that memory influences
Speaker 1: your behavior in the next moment. Now what's fascinating is
Speaker 1: that people with amnesia, which means they can't write down
Speaker 1: new explicit memories, can nonetheless show these kinds of priming effects.
Speaker 1: So they have no conscious recollection of having seen this
Speaker 1: steaming hot bowl at the table, and they'll deny that
Speaker 1: they ever saw that, but nonetheless they'll choose soup. I'll
Speaker 1: come back to this point in just a bit, but
Speaker 1: first I want to finish painting the different types of
Speaker 1: implicit memory. So another type of implicit memory is one
Speaker 1: you might have heard of called classical conditioning. This was
Speaker 1: first described by the Russian physiologist Ivon Pavlov at the
Speaker 1: end of the nineteenth century, so you generally remember this story.
Speaker 1: Pavlov saw that dogs respond to food by drooling or salivating,
Speaker 1: so he said Okay, I'm going to teach the dog
Speaker 1: through experience that the ringing of a bell predicts the food.
Speaker 1: The bell itself is totally arbitrary, but it comes to
Speaker 1: tell the dog that some food is coming. Pavlov could
Speaker 1: just as easily have used a pat on the head
Speaker 1: to predict the food, or a purple dot flashing three
Speaker 1: times or whatever. It's just some random thing that you
Speaker 1: link to food arriving a moment later. So he teaches
Speaker 1: the dog to associate the ringing of the bell with
Speaker 1: the impending delivery of food, and after setting up the relationship,
Speaker 1: the dogs salivate when he rings the bell, even though
Speaker 1: there's no food there yet, because the bell now is
Speaker 1: as good as seeing the food. And again, this is
Speaker 1: implicit memory because you don't have to be able to
Speaker 1: tell the story of a connection consciously in order to
Speaker 1: form this kind of learning, this memory. So that's classical conditioning,
Speaker 1: and there's another form of implicit memory, which is operant conditioning.
Speaker 1: Here you learn to associate your own behavior like pressing
Speaker 1: a lever, with something rewarding like food, or something aversive
Speaker 1: like an electric shock, and you gradually do more or
Speaker 1: less of that behavior as a result. Of remembering the consequences. Now,
Speaker 1: I'm not going to go into the details of the
Speaker 1: brain regions that are involved in these different forms of
Speaker 1: implicit learning. If you're curious, to go to my textbook
Speaker 1: called Brain and Behavior. But I'll just mention that learning
Speaker 1: any of these types of responses depends on some brain
Speaker 1: areas like the amygdala and cerebellum, and some structures in
Speaker 1: the brain stem. The key thing I want you to
Speaker 1: appreciate is that all of these forms of memory I
Speaker 1: just described are forms in which your brain changes itself.
Speaker 1: It changes its detailed internal structure in response to things
Speaker 1: that it has experienced so that it be behaves differently
Speaker 1: in the future. And all of these things I just
Speaker 1: told you about, that's just implicit memory. Now let's switch
Speaker 1: back again to explicit memory, the things that you can
Speaker 1: consciously articulate, like events and facts. Even explicit memory can
Speaker 1: be divided into two different forms. The first is called
Speaker 1: episodic memory. This is memory for past events or episodes
Speaker 1: that you've experienced, like a birthday party or a surfing
Speaker 1: trip you took. These are called autobiographical memories. And the
Speaker 1: key point is that, unlike implicit memories, these Episodic memories
Speaker 1: can be consciously recalled and described. They can have a
Speaker 1: sort of cinematic quality to them, and they usually have
Speaker 1: a particular context, like in the living room on New
Speaker 1: Year's Eve. You're also generally able to pick out specific
Speaker 1: objects and features and the surroundings, like oh, yeah, the
Speaker 1: sofa was over here my left, and there was a
Speaker 1: television set over there, and my uncle was sitting over
Speaker 1: here on my right, And you can recall very particular
Speaker 1: sequences of actions like oh, we were wearing party hats
Speaker 1: and the wine was poured and then it clinked glasses
Speaker 1: with everyone. So those are episodic memories. Now there's another
Speaker 1: type of explicit memory, not episodic, but semantic memories, and
Speaker 1: these are things that your brain remembers about the outside world,
Speaker 1: like the fact that a sheep has four legs and
Speaker 1: it makes a noise that sounds like bah, and it
Speaker 1: has a wooly coat. Semantic properties are more general than
Speaker 1: single events that you saw or experienced, and because they're
Speaker 1: independent of any one particular kind of sensory input, they're
Speaker 1: generally useful for organizing the world into categories of related
Speaker 1: things Like these are vegetables and these are animals, and
Speaker 1: these are tools, these vehicles and so on. So that's
Speaker 1: semantic memory. Now, I just told you about a whole
Speaker 1: bunch of different subtypes of memories. But why do neuroscientists
Speaker 1: divide memory into all these different flavors. Well, it's because
Speaker 1: clinicians have observed for many, many decades that sometimes a
Speaker 1: person can damage one part of their brain and lose
Speaker 1: a very specific sort of memory while not losing other sorts. So,
Speaker 1: for example, you might see a patient with severe amnesia
Speaker 1: for personal experiences. She can remember essentially nothing about her
Speaker 1: personal life, but she retains a good general knowledge for
Speaker 1: facts that she learned before her brain injury. So that's
Speaker 1: how we know that episodic memory is different from semantic
Speaker 1: It's actually underpinned by different structures in the brain, and
Speaker 1: by the way, you see this often in forms of
Speaker 1: dementia like early Alzheimer's disease, where a person's episodic memory
Speaker 1: suffers dramatically. They can't remember the details of their own life,
Speaker 1: but this is long before their semantic knowledge begins to fail.
Speaker 1: And on the flip side, in a rarer illness known
Speaker 1: as semantic dementia, episodic memory is preserved, while even basic
Speaker 1: forms of semantic knowledge are lost, like what a sheep
Speaker 1: is or what sound it makes. I won't go into
Speaker 1: details here, but if you want to look up more,
Speaker 1: episodic memory generally depends on the medial temporal lobes, while
Speaker 1: semantic memory involves the anterior temporal lobes. Okay, so now
Speaker 1: how did people start to figure out what brain regions
Speaker 1: were involved in what functions. Well, like many things, the
Speaker 1: answer is that this generally happens when a person gets
Speaker 1: damage to a very specific part of the brain. So
Speaker 1: let's zoom in on a particular example. In nineteen fifty,
Speaker 1: a particular area of the brain called the hippocampus came
Speaker 1: to the center stage in the neuroscience of memory because
Speaker 1: there was a man named Henry Malaison, who, by the way,
Speaker 1: was known for decades in the literature as HM because
Speaker 1: the patient's real name is never used while he's still alive.
Speaker 1: So anyway, Henry had suffered a head injury at the
Speaker 1: age of nine, and after that he had epileptic seizures
Speaker 1: and these got more frequent and severe, and the doctors
Speaker 1: tried to control the seizures, but eventually they couldn't control
Speaker 1: them anymore. So at the age of twenty seven, Henry
Speaker 1: went in for a neurosurgery because of where the seizures
Speaker 1: were originating. The surgeon removed the hippocampus on both sides
Speaker 1: of his brain, on the right and the left, as
Speaker 1: well as some of the regions that surrounded the hippocampus. Now,
Speaker 1: after the surgery, the seizures were all better, but Henry
Speaker 1: became one of the most famous cases in the medical
Speaker 1: literature because while he was otherwise fine, he had a
Speaker 1: severe amnesia that means he couldn't remember things, and specifically,
Speaker 1: he had an antaro grade amnesia, which means he couldn't
Speaker 1: form new episodic memories. And so that meant he would
Speaker 1: function just fine if you were talking with him, but
Speaker 1: after say fifteen minutes, he couldn't recall anything about the
Speaker 1: conversation because he wasn't converting that into new long term memory.
Speaker 1: So just imagine being one of the people working with him.
Speaker 1: You walk into the room and you introduce yourself, and
Speaker 1: you bring a giant Saint Bernard dog with you. Then
Speaker 1: the dog leaves and you chat with him for about
Speaker 1: five minutes and you ask him, hey, when I came
Speaker 1: in here, did I have anything with me? And he says, yeah,
Speaker 1: you had a giant Saint Bernard dog with you. So
Speaker 1: you continue to have a nice conversation about things. He's
Speaker 1: a small guy. Now you distract him for about another
Speaker 1: fifteen minutes and you ask him, hey, when I came
Speaker 1: in here, did I have anything with me? And he
Speaker 1: draws a total blank. He says, I don't think So
Speaker 1: now you leave the room and you come back twenty
Speaker 1: minutes later and you say, hey, when I walked in
Speaker 1: the room before, did I have anything with me? And
Speaker 1: what does he say? He says, who are you? As
Speaker 1: far as he knows, he has never seen you before
Speaker 1: in his life. So the researchers who studied Henry for
Speaker 1: years had to reintroduce themselves each time they walked in
Speaker 1: the room, even if they'd only left the room briefly.
Speaker 1: So the issue was that Henry could not form new
Speaker 1: explicit memories. But here's the interesting part. His implicit memory
Speaker 1: was fine. He could practice and learn new tasks like
Speaker 1: tracing a five pointed star viewed through a mirror, but
Speaker 1: he had no recollection of the practice in learning, and
Speaker 1: he expressed surprise at how well, he could perform this
Speaker 1: task that, as far as he knew, he had never
Speaker 1: seen before. Also, his short term memory was fine, well
Speaker 1: within the range of seven plus or minus two items.
Speaker 1: And so the tragic outcome from Henry's surgery helped to
Speaker 1: define the taxonomy of memory systems that we know today.
Speaker 1: And by the way, we've learned these same lessons from
Speaker 1: hundreds of other patients with amnesia since then. For example,
Speaker 1: somebody who can't form any new explicit memories can nonetheless
Speaker 1: learn an implicit task like the video game Tetris, and
Speaker 1: they can get better and better at the game. But
Speaker 1: each time you place them in front of the game,
Speaker 1: they claim genuinely that they've never seen this before. And
Speaker 1: what's fascinating is that if you wake them up when
Speaker 1: they're sleeping, if you catch them in the middle of
Speaker 1: a dream, they'll say that they were just dreaming about
Speaker 1: colorful falling blocks, but they have no idea why, because
Speaker 1: they have no conscious memory of ever having seen that before.
Speaker 1: By the way, if you saw the movie Memento, you
Speaker 1: know that was about a man who had lost his
Speaker 1: explicit memory. He had had a head injury that gave
Speaker 1: him amnesia. In his case, both in the forward and
Speaker 1: backward direction and tarot grade and retrograde, and so the
Speaker 1: only way that he could keep track of his goals
Speaker 1: through time was to tattoo information onto his skin. And
Speaker 1: I'll be talking more about amnesia in a future episode,
Speaker 1: but all I want to say for now is that
Speaker 1: for most of us, we are lucky enough to tattoo
Speaker 1: the information directly into our vast neural forests, and we
Speaker 1: can do this again and again throughout every day of
Speaker 1: our lives, and we never run out of room. Now,
Speaker 1: we know from Henry and many other patients that normal
Speaker 1: autobiographical memory depends on the integrity of the hippocampus. But
Speaker 1: I mentioned in episode number one of this podcast that
Speaker 1: there's another important player in the medial temporal lob memory system,
Speaker 1: and that's the amygdala, which is an almond sized structure
Speaker 1: that's just in front of the hippocampus, and this is
Speaker 1: involved in emotional memory. The amygdala assigns value positive or
Speaker 1: negative to things that it sees or hears or smells
Speaker 1: based on past experiences. And because it's so well connected,
Speaker 1: it can coordinate all the different prongs of an emotional
Speaker 1: response to something. It can coordinate the autonomic responses like
Speaker 1: an increased heart rate, and the endocrine responses like the
Speaker 1: secretion of stress hormones, and behavioral responses like fear and avoidance.
Speaker 1: So it turns out that while normal memories are just
Speaker 1: taken care of by the hippocampus, emotional situations like something
Speaker 1: very stressful or dangerous, those kick the amigdala into gear
Speaker 1: and memories get written down on what is essentially a
Speaker 1: secondary memory pathway. And that's why emotional events are more
Speaker 1: likely to be remembered, because in a sense, those are
Speaker 1: the most important memories. When something really emotionally important happens,
Speaker 1: that's what you want to write down. That's why memory
Speaker 1: exists to keep a record of the really important stuff.
Speaker 1: By the way, I spoke in the first episode about
Speaker 1: time seeming to run in slow motion when you're in danger,
Speaker 1: when you're in fear for your life, And if you're interested,
Speaker 1: you can go back and listen to the experiments that
Speaker 1: my lab did. But the punchline is that when you're
Speaker 1: in a very stressful situation, your brain writes down denser
Speaker 1: memories than normal, and so when you think back on
Speaker 1: what just happened, your brain pulls up much more detail
Speaker 1: than you would normally have, and you interpret that as
Speaker 1: that must have taken a long time because I hit
Speaker 1: the brakes, but because of all the ice, I couldn't
Speaker 1: get traction, and so my car slid into the intersection.
Speaker 1: And I saw the blue Toyota coming and I saw
Speaker 1: the expression on the other driver, and she hit the
Speaker 1: front of my car, and I saw the hood crumple,
Speaker 1: and the glass spider web, and the rear view mirror
Speaker 1: fall off, and so on and so on, and so
Speaker 1: when you think what just happened, what just happened, your
Speaker 1: brain assumes, Wow, that must have taken a long time.
Speaker 1: That must have taken many many seconds for all of
Speaker 1: that to happen in order for me to see that
Speaker 1: much detail. But in fact, time does not actually run
Speaker 1: in slow motion for you, as we demonstrated with experiments. Instead,
Speaker 1: it's a trick of memory. You just have more memory
Speaker 1: that you're drawing up. As I mentioned in the first episode.
Speaker 1: You can convince yourself of this because if you had
Speaker 1: a passenger on the car seat next to you, you
Speaker 1: don't actually remember his voice as saying, which would have
Speaker 1: to be the case if time were actually slowed down.
Speaker 1: So it's a trick of memory, and the retrospective illusion
Speaker 1: happens because the amygdala has come online because something very
Speaker 1: salient is happening, and it wrote down denser memories than normal.
Speaker 1: So that's what I wanted to say about the amygdala.
Speaker 1: But now we're going to return to the hippocampus, which
Speaker 1: underlies most normal day to day memory. And there was
Speaker 1: a very interesting discovery which won a Nobel Prize some
Speaker 1: years ago, and that is that the hippocampus is involved
Speaker 1: in encoding your location, your position in space. In other words,
Speaker 1: it's involved in saying, I'm in this spot, and the
Speaker 1: room I'm in connects to that hallway which leads to
Speaker 1: the door of that lobby, which has an elevator to
Speaker 1: go up to that other room. Your sense of where
Speaker 1: you are relies on your hippocampus. Your hipp campus, in fact,
Speaker 1: has these specialized neurons that we call place cells, and
Speaker 1: these neurons become active they fire off spikes only when
Speaker 1: you're in a particular spot in your environment. And there
Speaker 1: are other neurons known as grid cells, and these have
Speaker 1: multiple receptive fields that are arranged in a grid pattern
Speaker 1: that covers your local environment anyway. So all these cells
Speaker 1: work together to give you a very precise sense of
Speaker 1: your position in a room. And the hippocampus is not
Speaker 1: only about your current position, but it's also crucial for
Speaker 1: spatial memory. For example, imagine that you're in a labyrinth
Speaker 1: and you have to go down this hallway and come
Speaker 1: back to the center, and then you have to remember
Speaker 1: that you've been there, and then you go down a
Speaker 1: different one and you come back, and then you have
Speaker 1: to figure out a different one and go back and
Speaker 1: so on. It's really easy for you to do this
Speaker 1: because even in a brand new environment, you have a
Speaker 1: spatial memory, as in, I've already been down that hallway.
Speaker 1: But if you have damage to your hippocampus on both sides,
Speaker 1: then you can't do that task. You need your hippocampi
Speaker 1: for spatial memory. Now, interestingly, you find hippocampus like structures
Speaker 1: throughout the animal kingdom, not only in mammals, but also
Speaker 1: in birds and in goldfish, and in all cases this
Speaker 1: is involved in spatial memory. And across the kingdom you
Speaker 1: see that the size of the hippocampus is related to
Speaker 1: the demands of the territory mapping and the spatial memory.
Speaker 1: For example, squirrels spend the autumn months hiding literally thousands
Speaker 1: of seeds and nuts in different locations throughout their territory
Speaker 1: so they can have a steady food supply through the winter.
Speaker 1: And during this period the volume of their hippocampus increases
Speaker 1: by fifteen percent. Or as another example, some species of
Speaker 1: birds hide their food and these species have larger hippocampi
Speaker 1: than the birds that don't do this, and when they're
Speaker 1: doing their food hiding, that stimulates growth of their hippocampus.
Speaker 1: And this is what you find in humans too. Our
Speaker 1: hippocampi give us map like spatial codes, and these regions
Speaker 1: guide navigations. So consider London taxi drivers. I don't know
Speaker 1: if they do this anymore, but pre GPS, they had
Speaker 1: to memorize an unbelievably detailed map of London with thousands
Speaker 1: of destinations encompassing something like twenty five thousand streets, and
Speaker 1: they had to be able to verbally recite the most
Speaker 1: efficient roots between any two points, and just working from memory,
Speaker 1: say points of interest along the way, like the names
Speaker 1: of all the theaters that they'd pass in sequential order.
Speaker 1: And when neu oh imaging studies were done on these
Speaker 1: cab drivers, it could be seen that their hippocampi had
Speaker 1: grown compared with novice taxi drivers. And interestingly, this doesn't
Speaker 1: happen for physicians, who have to master a similarly large
Speaker 1: body of knowledge. But it's not spatial. Okay, So I
Speaker 1: just told you a lot about how the hippocampus is
Speaker 1: involved in understanding this spatial layout of things. But why
Speaker 1: do I mention the hippocampus encoding space in an episode
Speaker 1: on memory? While first, the world is full of spatial things,
Speaker 1: and traditionally our memories had to care about that, just
Speaker 1: like the squirrels remembering where they had buried their seeds
Speaker 1: and nuts. Also, you may have noticed that we are
Speaker 1: generally very good at remembering spatial information, like all the
Speaker 1: important rooms in a very large building. And many people,
Speaker 1: when they're trying to memorize something like a list of items,
Speaker 1: will use what is called a memory palace, which is
Speaker 1: where you associate these different items with different locations. So,
Speaker 1: for example, let's say I had to memorize a long
Speaker 1: list of words like apple, baby, clock, dennis, exhibition, flaggirl, horse, ice, jester, ladder, machine, noos, ocean, pigeon, radio, sheep, theater,
Speaker 1: and so on. What I might do is I might
Speaker 1: picture my house and I visualize the first item apple
Speaker 1: at the front door, and then I imagine walking in
Speaker 1: and I visualize the second item, let's say a baby
Speaker 1: in this case in a crib just inside the door.
Speaker 1: And then just past the baby, I picture a giant
Speaker 1: pendulum clock on the wall, and on the couch over there,
Speaker 1: I visualize a dentist doing his work on a patient,
Speaker 1: and so on and so on. I leverage location. I
Speaker 1: take advantage of these cells in my hippocampus that care
Speaker 1: about place to tie other information to them. And when
Speaker 1: I need to remember this list much later, I simply
Speaker 1: imagine myself walking through the house, and I can recall
Speaker 1: an enormously long list of arbitrary objects this way. This is,
Speaker 1: in fact, the oldest memory technique called a mnemonic device,
Speaker 1: that is on record. The ancient Greek and Roman bards
Speaker 1: used to tell their epic tales this way. This was
Speaker 1: before the invention of the printing press, so they had
Speaker 1: to memorize these things. But happily this was after the
Speaker 1: evolution of the hippocampus. Now, although some people purposely leverage
Speaker 1: these kind of techniques to memorize for a small fraction
Speaker 1: of the population. This happens naturally. In episode four, I
Speaker 1: talked about synesthesia, and one of the most common forms
Speaker 1: of synesthesia involves imagining things with spatial locations. For example,
Speaker 1: you see the days of the week in a circle
Speaker 1: around you physically, like Monday is over here, and then
Speaker 1: tuesdays there, and then Wednesday is up here, and then
Speaker 1: Thursday's over there, and Friday off to the side, and
Speaker 1: so on. And for each person. This is idiosyncratic, meaning
Speaker 1: that it's a different pattern for everyone, and a cinnasthete
Speaker 1: might see the months of the year in spatial locations,
Speaker 1: or the years historical in future, like nineteen seventy one
Speaker 1: is over here, and then it goes up and up
Speaker 1: through nineteen ninety, and then it's flat over to two
Speaker 1: thousand and five, and then it goes a little up
Speaker 1: and down through twenty thirteen, and then cuts to the
Speaker 1: left at twenty twenty, and then it suddenly dives down
Speaker 1: and then curves around, and then the future goes off
Speaker 1: behind you. And it's different for each cynisthete, and I
Speaker 1: gave many more examples in episode four, but the point
Speaker 1: I want to make here is that This form of
Speaker 1: synesthesia is closely tied to memory because all of the
Speaker 1: things that I just mentioned, like weekdays and months and years,
Speaker 1: these are all sequences that you have to learn, you
Speaker 1: have to memorize, and so in many people these sequences
Speaker 1: get tied irreversibly to spatial location. It helps the brain
Speaker 1: to remember something by tagging it with a place. So
Speaker 1: some people suggest that the way to understand the hippocampus
Speaker 1: is as a cognitive map. The idea is that the
Speaker 1: hippocampus originally created and stored territory maps for orientation and
Speaker 1: navigation and finding resources, but eventually the system adapted to
Speaker 1: create and store episodic memories. Also, because events usually have
Speaker 1: a particular setting in space, and because movement cross locations
Speaker 1: also involves movement in time, the hippocampus would be naturally
Speaker 1: well suited to capture sequences of events in time, in
Speaker 1: other words, episodes. And I want to mention that there's
Speaker 1: a cousin theory about the hippocampus that proposes what it's
Speaker 1: doing is it's storing the associations between elements of the events,
Speaker 1: just like what we talked about with the spatial relations
Speaker 1: between objects. So the hippocampus stores the temporal relationship between
Speaker 1: events like this happened after this, or it happened close
Speaker 1: in time or far in time, and it stores the
Speaker 1: relationships between pieces of information. It stores the relationships rather
Speaker 1: than the specific items. So this gives us a way
Speaker 1: to see how the original spatial function of the hippocampus
Speaker 1: could have evolved into episodic memory. Okay, so now I
Speaker 1: want to turn to what's going on at the very
Speaker 1: tiny level to ask how does activity in the brain
Speaker 1: cause lasting changes such that we have learning and memory.
Speaker 1: So almost all theories of brain plasticity, that's what we
Speaker 1: call it, when the brain reorganizes itself. These use the
Speaker 1: idea that the strength of connections between cells, the strength
Speaker 1: of the synapses, can be modified by previous activity. Okay,
Speaker 1: but how does that work? Well, First of all, if
Speaker 1: you imagine one hundred years ago, microscopes didn't have the
Speaker 1: magnification power to actually see neurons in their inner connections.
Speaker 1: So a century ago, scientists believed that brain tissue was
Speaker 1: a continuous network like the blood vessels. Blood vessels are
Speaker 1: a system of tubes where you have stuff running along it,
Speaker 1: and people assumed that was what was happening in the brain.
Speaker 1: But this idea was overturned by a Spanish neuroscientist named
Speaker 1: Santiago Ramonicjl. He spent a lot of time at his
Speaker 1: microscope trying to look at brain tissue, and because he
Speaker 1: was a photographer also, he had all these chemicals in
Speaker 1: his workshop, these stains that he put on to thin
Speaker 1: slices of brain tissue to see if that would allow
Speaker 1: him to see anything better. And some of his stains
Speaker 1: leaped into a few of the neurons and turned them
Speaker 1: all black, and he was able to visualize them under
Speaker 1: the microscope that way. So he put forward this idea,
Speaker 1: which turned out to be right, that the brain, instead
Speaker 1: of being a subway system of tubes, was a massive
Speaker 1: collection of billions of discrete cells. He called this the
Speaker 1: neuron doctrine, and this was a massively important step in neuroscience,
Speaker 1: and it eventually won him the Nobel Prize. Now, this
Speaker 1: idea that the brain is made up of lots of
Speaker 1: individual cells, this ushered in an important new concept because
Speaker 1: people began to realize that these separate cells they have
Speaker 1: to influence each other through these little connections between them,
Speaker 1: these synapses. And Ramonic Cahol was the first to suggest
Speaker 1: that learning and memory might occur by changing these synapses,
Speaker 1: so several decades later, in nineteen forty nine, the neuroscientist
Speaker 1: Donald Hebb made a specific proposal for how synapses should
Speaker 1: adjust to underlie memory. He said that if one cell
Speaker 1: consistently participates in exciting another cell, then the connection between
Speaker 1: them is strengthened, and if the first cell consistently fails
Speaker 1: to excite the second cell, then the connection between them
Speaker 1: is weakened. So this rule is often as cells that
Speaker 1: fire together wire together, and most models of memory formation
Speaker 1: employ this kind of rule. Now, when Hebb proposed this hypothesis,
Speaker 1: there was no experimental evidence to support it, and it
Speaker 1: took until nineteen seventy three for two researchers to discover that,
Speaker 1: in fact, that seems to be what happens between neurons.
Speaker 1: So you stimulate some neuron over here, and you measure
Speaker 1: the very tiny electrical response that it causes in this
Speaker 1: neuron over here. Then you blast the first neural with
Speaker 1: a bunch of electrical stimulation for thirty seconds, and then
Speaker 1: you try that first experiment again, where you give this
Speaker 1: guy a little zapp and you look at the electrical
Speaker 1: signal that it causes in the second neuron, and what
Speaker 1: you find is that you now have a larger signal.
Speaker 1: The synapse has strengthened, and that strengthening lasts. The connection
Speaker 1: strength between these two neurons has changed, and that change
Speaker 1: is held on to through time. So the synaptic connection
Speaker 1: can be modified as a result of the cell's history
Speaker 1: of activity. Now, a typical neuron has ten thousand connections,
Speaker 1: and what's fascinating is that each individual connection can strengthen
Speaker 1: or weaken according to its history. So the way that
Speaker 1: activity flows through a network of billions of neurons can
Speaker 1: be completely changed. By dialing the strength of this connection here,
Speaker 1: in that connection there, and so on for all the
Speaker 1: connections across the brain. By dialing the strengths up and
Speaker 1: down and holding on to those, you can store information
Speaker 1: in the system. And just to give you a sense
Speaker 1: of the size of the parameter space here, the large
Speaker 1: language model GPT four has one point seventy six trillion connections,
Speaker 1: but the human brain has about one hundred times that.
Speaker 1: So there are a lot of parameters that can be
Speaker 1: tweaked in the brain to store information. Now, by the
Speaker 1: way the birth of artificial neural networks like GPT, for
Speaker 1: is rooted in these discoveries from the brain from the
Speaker 1: past century. You have a bunch of units, and you
Speaker 1: have connections between these units, and you change the strength
Speaker 1: of those connections. That's how large language models work because
Speaker 1: making small, subtle changes in the way that units communicate
Speaker 1: a network can change the entire network's output behavior. By
Speaker 1: tuning the parameters of the network just right, or in fact,
Speaker 1: letting a network adjust its own connections according to some algorithm,
Speaker 1: a network learns to associate inputs and remember what it
Speaker 1: has learned. This is how we build artificial learning and memory,
Speaker 1: and modern artificial neural networks are extraordinarily impressive. But I
Speaker 1: think it's really important to note that although these connections
Speaker 1: between neurons have gotten all the attention, both theoretically and experimentally,
Speaker 1: there are lots of other possible ways to store information
Speaker 1: in the brain. For the last century, people have assumed
Speaker 1: that synapses are the key to memory, but the story
Speaker 1: has been complexified by recent decades of research. Where it
Speaker 1: stands now is that synaptic changes are necessary for learning
Speaker 1: and memory, but we really don't know if they are sufficient.
Speaker 1: It's still unclear whether these changes that the synapse is
Speaker 1: will be the only or even the most important mechanisms
Speaker 1: involved in memory, or perhaps whether they're involved at all,
Speaker 1: because the fact is that a dense net of intertwined
Speaker 1: cells has to orchestrate a careful balance between excitation and inhibition,
Speaker 1: otherwise the whole thing blows up into epilepsy or it
Speaker 1: sinks down into non activity. And it could be that
Speaker 1: all these synaptic changes we see are just to keep
Speaker 1: the system away from epileptic overload or synaptic shutdown, and
Speaker 1: memory maybe is stored in an entirely different manner. Our
Speaker 1: science is still young, and it's possible that we're still
Speaker 1: missing the core mechanisms of memory, because after all, there
Speaker 1: are adjustable parameters throughout the brain. You change something here
Speaker 1: and the network behaves differently there. So there are many
Speaker 1: other possible mechanisms involved in memory in the brain. For
Speaker 1: the cognitionanty, this could involve neurogenesis or changes in the
Speaker 1: excitability of the neuron, or the distribution of ion channel
Speaker 1: or the shape of dendritic trees and their spines, where
Speaker 1: the phosphorylation states of intracellular proteins, where the epigenetic codes
Speaker 1: and on and on. Okay, this is all in my
Speaker 1: tech book and we're not going to go into that here,
Speaker 1: but I do want to say that with so many
Speaker 1: degrees of freedom in biological systems, the number of possible
Speaker 1: ways to storm memory in a brain is vast. So
Speaker 1: why do we look at the connection strength of synapse
Speaker 1: as well? It's partially because that's what we can measure
Speaker 1: most easily. It's extremely difficult, essentially impossible currently to measure
Speaker 1: in a living animal changes in channel distribution or dendritic
Speaker 1: spines or epigenetic codes in single neurons. So as a result,
Speaker 1: almost the whole field of neuroscience just measures changes at
Speaker 1: the connections between cells. And that might turn out to
Speaker 1: be right, But if you could read the textbooks one
Speaker 1: hundred years from now, it might turn out to be
Speaker 1: that we were like the drunk, looking for our keys
Speaker 1: under the street light because that's where the lighting was better.
Speaker 1: So while artificial neural networks are awe inspiring and they're
Speaker 1: rapidly changing the world, we don't know for sure that
Speaker 1: they're doing the same things algorithmically that the brain is doing.
Speaker 1: They in a sense simplify everything. They are a clever
Speaker 1: step backwards from biology. In other words, you take the
Speaker 1: complexity of a single cell with the entire human genome
Speaker 1: in it and millions of proteins trafficking around, and you
Speaker 1: just imagine that it's a unit with simple connections to
Speaker 1: other units. And as I said, that has turned out
Speaker 1: in large language models to be shockingly effective. But we
Speaker 1: really have no way of knowing right now how much
Speaker 1: more we could get out of artificial neural networks if
Speaker 1: we included the rich complexity of actual biology. Maybe it
Speaker 1: wouldn't add anything, but maybe it would unlock in entirely
Speaker 1: new levels of function, making artificial neural networks more like
Speaker 1: a human with a sense of what information is relevant
Speaker 1: to learn and incorporating needs and goals and strategies and
Speaker 1: enjoying the experience of consciousness. The thing to keep in
Speaker 1: mind is that Mother Nature has had billions of years
Speaker 1: to try quadrillions of experiments, and we've only been making
Speaker 1: artificial neural networks for a few decades. So if I
Speaker 1: were a betting man, I would say there's probably a
Speaker 1: lot more to be discovered. It is certainly possible that
Speaker 1: we have not yet found memories Rosetta Stone and I
Speaker 1: want to point to one more thing about real brains
Speaker 1: that's definitely not captured in artificial neural networks, and that
Speaker 1: is the concept of forgetting. Remember I mentioned at the
Speaker 1: beginning about the mneminist named Sharyshevski, It turns out that
Speaker 1: Sherishevski's enviable memory went hand in hand with a surprisingly
Speaker 1: handicapped personality. He was crippled by the fact that he
Speaker 1: could not forget. Because when you have a memory like his,
Speaker 1: all the moments in life are retained. Past vandettas and
Speaker 1: things people did to slight you, and embarrassing moments and
Speaker 1: situations you've outgrown, and heartbreaks you would rather let slip
Speaker 1: into the past. All of these remain present and emotionally
Speaker 1: salient for you. And remember I mentioned the mnemenist Jill Price,
Speaker 1: who also had an untaxable memory. Just think about what
Speaker 1: drove her to seek help in the first place. Her
Speaker 1: memories were deeply emotional and sprang up throughout her day.
Speaker 1: Jill is constantly pouring over her past, an obsessive detailing
Speaker 1: mementos from childhood onward and becoming distressed by changes like
Speaker 1: moving to a new home. So for both Sharashevski and
Speaker 1: Jill Price. Having a perfect memory impaired them as much
Speaker 1: as equipped them, and this unmasks one of the great
Speaker 1: values of the way memory typically exists in most people.
Speaker 1: We don't retain everything, but instead items fade. One of
Speaker 1: my favorite quotations is from the French novelist and playwright
Speaker 1: Honore de Balzac, who wrote, memories beautify life, but the
Speaker 1: capacity to forget makes it bearable. So when it comes
Speaker 1: to thinking about human memory, remember that of all the
Speaker 1: things we talked about that differentiate us from digital computers,
Speaker 1: a very important one is our capacity to not remember everything.
Speaker 1: And even that which we do remember doesn't last too long.
Speaker 1: So let's wrap up. We've seen that human memory is
Speaker 1: not quite the same as a computer's memory. We don't
Speaker 1: store zeros and ones, but instead we hold memories about
Speaker 1: the gist of things, the relationships between things, and we
Speaker 1: forget through time and memory for us is stored in
Speaker 1: lots and lots of subsystems. We have different mechanisms for
Speaker 1: short term memory and for long term memory, and within
Speaker 1: each of those categories we have subcategories. For example, implicit
Speaker 1: memories that you can't articulate, like how to ride a
Speaker 1: bike and explicit memories like facts and events that you
Speaker 1: can talk about. And in all of these cases you
Speaker 1: can lose some of these types of memory without any
Speaker 1: effect on the other types. And now that we've seen
Speaker 1: an overview of memory in the brain, we will now
Speaker 1: be ready to turn to the main reason we have memory,
Speaker 1: and that is to predict the future. We retain information
Speaker 1: in the detailed configuration of our forest of billions of
Speaker 1: cells and trillions of connections, and the point is to
Speaker 1: develop a better understanding of the world so that we
Speaker 1: can know what will happen next. And for that, please
Speaker 1: join me in the next episode where we look at
Speaker 1: prediction how we simulate possible future worlds. This is one
Speaker 1: of the main jobs of brains, and this is the
Speaker 1: reason they retain memory. So right now, if you're imagining
Speaker 1: tuning into the next episode and feeling the emotional joy
Speaker 1: of what you will learn and what you will see there,
Speaker 1: this is your brain doing what it was meant to do,
Speaker 1: simulating the future. I will see you there. Go to
Speaker 1: Eagleman dot com slash podcast for more information and to
Speaker 1: find further reading. Send me any questions to podcast at
Speaker 1: eagleman dot com and I'll be making episodes in which
Speaker 1: I address those until next time. I'm David Eagleman, and
Speaker 1: this is Inner Cosmos.