Why Do People Forget Memories?

Hey, please take a second and leave us a review on Apple Podcasts, Spotify, or wherever you listen to the podcast. Thanks a lot. Hey, welcome to sign Stuff, a production of iHeartRadio I'm More cham and today we're looking into amnesia. How does it happen, why does it happen, and what are some of the most dramatic cases of it in history. It's part of a two part series on memory. So if you haven't checked out the first episode about super memory,

Yes, thank you for having me. I am Sarah Robbins. I'm a philosopher and cognitive scientist of memory. So I study and in particular the science of memory.

That's good that you remember all that I.

No longer have to write it down takes a little bit.

So that's an interesting mix between psychology and philosophy. Can you tell us a little bit about that area in between.

Yeah, and it includes neuroscience very deeply as well, So I think of it as sort of different ways of researching and thinking about the mind and the brain. Historically they were very intertwined, so early psychology was often called natural philosophy, and I often see the philosopher's role as someone who's paying attention to what every one of those is doing and talking about the bigger questions that started the science in the first place.

Feels a little like inceptions. It's like your researching what we think of, how we think about, what we're learning about, how we.

Think, yes, exactly, that's fun.

Okay. According to doctor Robins, scientists have been trying to figure out how the brain records memories for a long time. It's been a search for what scientists call the n gram.

So an ingram is really the neural mechanism that makes remember impossible or by which memories are supported so that they can later be remembered.

Okay, like how the brain stores information? Yeah, and I guess for a long time people have been wondering like, how does that work in the brain?

Exactly? Yeah, how is it? Where is it? Like what could possibly be the case? So the zoologists that coined the term, Richard Demmon, she first wrote the book where he coined the term in nineteen oh four in German. He's calling it like it's somewhere in the irritable substance, which is how people thought about the kind of material of the brain at the time, which I just disagree.

That's well, unfortunately I have a lot of irritable substance in my brain we all, especially in the mornings.

Yes, it's especially irritable.

Then, absolutely there.

So we've been looking for how the brain stores memories for over a hundred years now. You might think, why has it taken so long? Can we just open up a brain and see what's going on? Well, it's not that easy.

One is a challenge that is, like it's hard to know what's in your memory outside of actually like being able to look in there. Right, So if you can't remember something, it's kind of a question of like is it in there but you can't get it out?

Or is it not in there?

But like how do you tell Like how do we know what's really in there until we get it out.

Yeah, it just made me realize what a difficult problem this is because I think we're all familiar with memories. We all have memories, we all remember things, but like, how do you do an experiment on to figure out how it works inside a brain as it's happening.

Absolutely, And I think that's why some of these memories, like when they come to you kind of out of nowhere, are so delightful. Like when you hear a song you haven't heard for twenty years and you remember all the word. You can have this kind of like oh my gosh sort of reaction to that. Oh it was in there, and you sort of had no idea that it was there. Right, It's not like I can like, oh, what other fun stuff isn't there? Like I'm not going to know until that happens again.

Yeah, that is so tricky. In other words, it's hard to study how the brain stores memories because you can't just poke around someone's brain while they're making a memory or trying to remember something. But all of that changed about ten to fifteen years ago with the development of a technology called optogenetics. I'll explain what that technology is a little later. But first, here's what we've learned after a century of science, but how the brain records a memory.

I think it's now the case that it's possible to identify the set of neurons that are active in storing memories.

We'll step me through this. So I am walking around and something incredible happens. The unicorn jumps out of the manhole and then grows wings and flies off into the sky. I'm definitely going to remember that. What's happening in my brain?

Yes, So what's happening?

I mean, I'll give you a second to process that image. I just.

Well, the unicorn area of processing and the manual air and not there yet. So there's a set of cells in the hippocampus and surrounding structures that are kind of responsible for being the ones that react to that experience and kind of encode information about that experience in somewhay, there's a whole lot of neurons in that area, and a subset of them are going to say, like we've

All right, So I see the unicorn. There are some cells among the many, many, many many in my hippocampus that are like I'm ready, I'm ready to remember things. And then I see the unicorn. And it's what happens to those cells. They sort of like change, they something what happens.

So they like become better connected to each other. I'm not sure how far I can go with this metaphor, but this is just like a sea of willing people to play on the team for whatever sport you have. Each time there's an experience, you like send a team out and like they play together and they kind of form a bond that makes them a team that is about the experience.

They lock hands, they're like, we got this memory. We're going to remember that unicorn. We are the unicorn in your head.

We got it exactly. And then they kind of go back into the sea and you bring out a new set to play the next game and they do their connecting.

So that, in a nutshell, is what we know about how the brain records a memory, or at least a memory of an event. There's an area in your brain called the hippocampus, which sort of looks like two small jalapeno peppers buried deep inside your with millions of neurons in it, all connected to each other. And when something memorable happens, some of those neurons, maybe a few hundred of them, kind of get activated and they step up to remember the event, and they do it by strengthening

And it's possible over time that some of these cells will play on different teams.

Right, It's like I'm the unicorn cell with these group of cells, but with these group of cells, I am the I am the purple crocodile, or is another.

Day exactly exactly And.

Two that's what we see happening in the hippocampus during an event, but we don't actually know if that's where the memory is stored. Is a hippocampus kind of like the hard drive of the brain, or is it more like the relay of the brain, or like what's happening?

I think that's a question of live debate.

So it could be that it's individually storing each memory and holding them and they stay in the hippocampus long term. That is a view that some people hold. It could be that it is just kind of a relay, like it's the place that keeps tags or recipes, right, but all the content is distributed elsewhere.

Okay, that's what we know about how memories are formed. Now the question is how do memories get forgotten and what's happening in people with amnesia. We'll get to that, but first I wanted to dig deeper into how we know this is how memories are formed, because the answer is pretty cool. It involves a fairly recent technology called optogenetics, which basically lets you control memories with the push of a button.

And so that's changed in the last few decades thanks to a range of sort of technological tool innovations. A central one of which is something called optogenetics, which is a technique for making neurons responsive to light. So, colloquially speaking, you're putting a light switch on neurons. You make it so that you can turn them on and off in a living, behaving organism. So you can make certain neurons fire action potentials, make them active, or you can inhibit

Okay, we can probably spend the whole episode just on this technology because it's pretty wild. Basically, scientists can now hack into the DNA of neurons and make it so that when the neurons are activated, they produce a protein that makes the neurons sensitive to light, and scientists can time when this happens so that it only happens to

It's absolutely incredible to see what sorts of work could be done in this way. And there are people that have been using these similar techniques to sort of figure out which neurons are trying to be a part of the memory, and you can kind of follow them around and again find the ngram in this way and then activate and manipulate it.

Okay, that is how we make memories. When we come back, we'll get to the ession of how we forget memories and what's happening in the brain of people with amnesia and something called severely deficient autobiographical memory. We'll get to all of that, So don't blink out. Lock in this memory. We'll be right back. Hey, welcome back. We're talking about the size of amnesia, and so far we've talked about how the brain makes memories. Now we're going to talk

Yeah, my pleasure.

So I thought it'd be interesting to talk about the absence of memories what are some of the different ways that we can lose memories.

Yeah, that's a really good question. So I guess I'll start by saying that any of us can lose our memories.

Okay. According to doctor Palombo, there are several kinds of memory loss. First is the casual kind that we all experience every day in our memory slips.

So in our day to day lives, you know, whether we're young or older, we see that we experience memory slips where we think we should have access to something we don't, and that happens to all of us all

I see, are you still friends with this person or that relationship?

They did forgive me thanks leady.

They forgot about it. Well, this makes the question about in the situations where we can get at a memory, but later on we remember, like what happens there? Like where does that memory go that I can't excess it?

So sometimes it's simply due to interference. So we're in a situation where we're just not searching for the memory in the right way, and then later on something in our environment might trigger that memory. Sometimes it's because we're feeling very stressed. It can be feeling a bit anxious in the moment.

So that's a memory slip where we have temporary trouble accessing a memory. There's also just forgetting something over time. Is there just plain forgetting? Like is it possible for me to have stored a memory in my brain cells but over time they just sort of, you know, like the hard drive gets corrupted or erased.

Yes, so forgetting is a pretty loaded term. But you're right that we can form memories and then over time those memories become difficult to access. And one hypothesis is that that's because there's lots of interfering new information that we're encoding and needing to retrieve, and so the more memories we form over our lifetime, the more competition there is.

Right.

Another hypothesis is that just as our lives change and we grow, et cetera, the retrieval cues that we use to access those really old memories are not as close to our fingertips anymore. So you've moved on from your childhood home, et cetera, so it's harder to access those memories.

I see, you sort of lose the path to those memories.

Yeah, exactly.

So some things you can lose a memory because you lose a connection to it. It's still in your brain, but because it's simply not relevant to you, you never find your way to it again. Now, memory slips and losing connections to your memories over time are both natural processes. They happen to anyone. But now we get to the kind of memory loss that's due to something specific.

And that can happen in the context of disease. So you've probably heard about different forms of dementia, like Alzheimer's disease, where you can see quite profound, albeit progressive memory loss. And then there's another condition called amnesia. Then this is often due to a brain injury of virus. It can happen in the context of a stroke, and here.

We get to amnesia. Now, the thing about amnesia is that it typically happens because of an event. It could be a stroke or a traumatic brain injury, or sometimes it's a virus that infects your brain. Whatever the cause, most cases of amnesia have something in common.

Yeah, lots of different causes to amnesia, but one thing that they have in common is that they affect really similar networks in the brain. And there's one area of the brain in particular that's quite vulnerable to whether it's a virus or certain kinds of stroke, certain kinds of traumatic brain injury, et cetera, epilepsy, and that's a region of the brain called the hippocampus. And so when that area of the brain is affected, regardless of the cause, you see a profound loss of memory.

Yes, the thing most cases when anesia have in common is that they affect the hippok campus. Now, once the hippocampus's damaged, two basic things can happen. One, you can lose past memories that you had before the event the damage to hippocampus. For example, childhood memories, the names of people, those could all be gone. And two, you can also lose the ability to make new memories after the event, and both of these kinds of anesia have different names.

Yeah, so this is a question that a lot of memory scientists have what does it actually mean to have loss of memory? And so the one hypothesis is that the places in the brain where the memories are stored are damaged and so those memories are gone. And another hypothesis that you're getting at is that the memories are still stored somewhere in the brain, but via damage to the hippocampus, those memories cannot be accessed.

Wow, So the memories are still.

In there potentially potentially Yeah, Because remember last time we spoke, I sort of described this idea that the hippocampus in particular is a pointer to different parts of the brain where the memories are kind of stored within a network, and so when that index pointer is not working anymore, those memories are not accessible. Now we don't know for sure, because there's a lot about memory and the brain that we don't yet understand, but that is one idea that's been put out there.

Yes, if you have retrograde amnesia, your memories might still be intact in your brain and what you lose is actually the ability to access them, although that doesn't mean you could get them back.

Some cases, like in soap operas for example, sometimes it does involve an injury to the head, and so you'll see a situation where somebody got bonked on the head. They can't remember anything, including who they are, for a period of time, and then oftentimes they get bonked on the head again and that sort of shakes them out of the amnesic syndrome. And so that's not an accurate depiction of what looks like.

Oh no. Now, for people with anterograde amnesia, or the inability to make new memories, there are two fascinating things about them. The first is that because they can't make new episodic memories, they live in a constant state of surprise, as if they are just waking up every so often, because anything that's happened to them recently they don't remember.

What we tend to see is this kind of profound loss of episodic memory. And that's kind of the most striking feature of amnesia. And what I mean by that is if you're sitting with an m music participant and you're having a conversation and you leave the room for a few minutes and you return, they're not necessarily going

One of the most extreme examples of this kind of amnesia is a man named Clive Wearing. He's still alive today. He's eighty seven years old now, but when he was forty seven, a virus got into his brain and destroyed a lot of his hip hook campus. This prevents him from remembering anything that happens beyond about seven seconds. So you can have a conversation with him, but after a short while, he won't remember who you are, what you were talking about, or even how he got to the

Yes, exactly. So he was studied for many decades and this first began in the nineteen fifties. So he had a surgery in nineteen fifty three to remove parts of the brain that are really important for memory, such as the hippocampus as well as the surrounding medial tempor lobe structures, and all of that was removed in HM in this

He was profound me in music.

So Henry Mullison had severe and terror grade amnesia, just like dor and finding Nemo or Clive Wearing the music conductor I talked about before, if you met Henry, he would forget about you within a few minutes of meeting you, which meant he lived in a constant state of not knowing where he was or how he got there, But scientists, namely a researcher called Brenda Milner, soon figured out something amazing. Henry could learn new skills.

Come into the laboratory, and he'd be given a new skill task, but he wouldn't remember ever having been exposed to the task. He'd come in day after day and he would perform it as though it was a brand new task for him, even though he was getting better over time.

So scientists learned they could teach Henry new skills, but he wouldn't remember having learned those skills. For example, you could teach him to play the piano or to play tennis, and he wouldn't remember taking the lessons, But the next time he would sit in front of a piano or pick up a tennis racket, he'd be amazed that he knew how to use them. And this is because, as we talked about before, there are different kinds of memory

So I did some work on what we refer to as severely deficient autobiographical memory. And my work on STAM was done in collaboration with doctor Brian Levine, and we studied a group of three individuals who contacted us and shared with us that they had profound difficulty or even an inability to vividly recollect events from their past wow,

Okay, if you remember, in our last episode about super memory, we talked about people with something called highly superior autobiographical memory or h SAM, who could remember everything that's ever happened to them in their lives. Well, people with severely deficient autobiographical memory or as DAM, are sort of the opposite of that condition. They can't remember anything that happens to them.

And so I'll say early days with us as it is with h SAM, And so our scientific understanding of SDAM is quite limited. But what we do know so far is that it seems like individuals with SDAM access their past in a different way. And so rather than reliving episodes from their past in vivid and rich detail, they tend to access their past more through semantic remembering.

I see it. Could you describe the condition, like what is it like for them?

So it kind of depends on the person, and so some people will describe having difficulty accessing visual images through their memory. And so it's mainly that this sort of conscious access to reexperiencing the past is limited or absent. But yet nonetheless they can access their knowledge about the world and through facts. And so oftentimes you'll hear individuals with SDAMS say that they just know their past, but they're not reliving it in that sort of rich episodic format.

Like I remember I went to college at the University of Toronto, but I have no memories of being there or what it was like exactly.

Yes, the episodes that can be relived.

Yes, there are people out there that can remember facts about their lives, but they don't actually remember living it. They don't seem to be able to record the kind of memory that lets you relive your past. They know they were there, but they don't remember being there. Uh. It's like I know I had a kid, and I know they were really young, but I have no memories of those early days taking care of newbore.

Yes, exactly.

And so you have lots of information about your lives, about your loved ones, et cetera, but it's just you're not accessing that information through episodic memory.

Uh, like my record button just wasn't on.

Possibly it could be that the record button was on, but it's a different pathway towards accessing those memories. So that could be right from the get go, that memories are being laid down in a more factual conceptual way. That's a very open question.

Oh, I see they might have the record but on, but maybe the camera works a little bit different.

That's right exactly.

I see, what do you think and this might not be a scientific question, but what is it like too?

Yeah, so I think you're asking a little bit about sort of what the lived experience of ASBM is.

Yeah.

I think that's such an important question. And I think as scientists sometimes we get wrapped up in studying the nuts and bolts of what memory might look like in a particular group of individuals, and I think it's important to take a step back and think about what those lived experiences are like for our participants. Anecdotally, I guess I'll say that for some individuals, lacking this re experiencing component of memory can be hard for them and for

This brings us to the last question I had for doctor Palumbo, which is, why do we forget in the first place. If it's possible to remember everything that happens to you in your life, as some people would super memory prove, why didn't we all evolve disability to remember everything? What's the point of forgetting?

Yes, exactly. And that might seem strange because we would hope that our memory systems would be a perfect record the past. That would be really helpful in many situations. But in fact, memory researchers would argue that it is actually adaptive to forget, and we sort of have this

Yeah, I guess the idea is that your memory is meant to be a little fuzzy so that you can see the larger patterns in your life.

Yeah, that's a really good way to put it.

So you don't miss the forest for the trees.

Kind yes, exactly exactly, because we all didn't have to make really quick decisions, especially if we're being threatened by something, and so we want to know how things generally occur, not necessarily how they occurred just once.

I see, like, we don't get hung up on whether the tiger running at you have the same stripes as the last tiger you saw a few weeks ago. You just want to remember it it's a tiger. At least you run away.

That's exactly right.

Yes, oh interesting, So there's a reason I'm forget that. What we're saying, there's there's a good reason, very.

Good reason you're forgetful.

Yeah, yeah, all right. That was a lot of information about memory, amnesia and forgetting. But the good news is that if you don't exactly remember the things we talked about, you can just get the replay button and you'll get to relive these conversations all over again. It's like our phones and our computers are now part of our extended memory. Thanks for joining us, don't forget to come back next week.