Where do our early childhood memories go?
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What's your earliest memory? Is it getting ready for your first day of school?

Feeding the ducks in the park on your fourth birthday? Throwing up on the long car journey to grandma's? Or maybe you can go back even further.

When I give lectures on this topic, inevitably somebody raises their hand and says, I remember being born. It very well could be possible, at least in some cases and some people. But if I had to bet, I would say that it's very unlikely that humans have any accurate episodic memories through their first person experience before the age of three or four.

Nick Turk-Brown is a professor of psychology and neuroscience at Yale University. He's studying why we can't recall our early lives, even when we think we can. It very much is more about remembering stories they were told about themselves or scenes that they've constructed in their mind based on videos or photographs.

So why do we lack memories of our first steps? Drawing pictures in nursery or going on our first holiday? It's a mystery that scientists and parents have long puzzled over. But research exploring babies' brains is beginning to reveal what might be behind this gap in the story of our lives. So today, why can't we remember our early childhood? Where do all these memories go?

And would it ever be possible to retrieve them from the depths of our minds? I'm the Guardian Science Editor, Ian Sample, and this is Science Weekly. My earliest memory is at a park that had a little pool, a splash park, and I cut my foot on a piece of glass and had to go to the emergency room. And I know it's a real memory because I have a scar on my foot. That's my first one, age seven.

Nick's experience tallies with studies of when our first memories tend to date from, typically around four to seven years old. The lack of recollections before then is known as infantile amnesia, a term coined by a certain psychologist who was fascinated by our inability to remember infancy, better known as the father of psychotherapy. One of the earliest descriptions of this phenomenon was by Freud.

more than 100 years ago. And Freud had some ideas about early memories persisting and being repressed. That idea is not popular anymore, but it remains a mystery. And it has remained a mystery for the past several decades, whether this lack of early memories is because

we don't store memories when we're a baby, or whether we store them, we just can't access them later in life. And what kind of memories are we talking about here? Because babies, you know, they need to know who mum is and, you know, that they're more comfortable in this place rather than that place. And I mean, they must be learning a vast amount. They've got language, there's

all sorts of patterns, how to move their bodies. So they must be remembering some things, right? - Absolutely. This is one of the reasons we got so fascinated by this topic is that infancy is this incredible period of learning. You learn language better than you can for the rest of your life. You learn how to walk. You learn about social relationships. You learn the names of objects. You form concepts. And yet we don't remember specific experiences from when we were a baby.

And so this in the field of memory research is called episodic memory, memory for specific events in our past that happened at a certain moment in time and in a certain location in the world. So as you say, these are these contextual memories, these episodic memories, you know, that time you went to the zoo, your, maybe not your first, but your early birthday, and

I mean, in adults at least, where are these memories stored in the brain and sort of how are they stored? Do we know? We know a lot about this. One of the...

earliest discoveries in the field of neuroscience came from working with a patient named H.M. who had had a brain region removed surgically in both hemispheres that left him completely unable to form memories. His name's now known to be Henry Mollison. He's passed away. But the brain region at question is called the hippocampus, and it's a seahorse-shaped structure deep in the brain. And

And without this brain structure, you can't form memories. Synapses form in the hippocampus to connect cells into what's called an engram, which refers to a network of cells that are connected in a particular way to store specific experiences. What the hippocampus does is really the first stage in a much longer process of memory development.

formation and storage where it initially encodes the memory, but through a process called consolidation, those memories are played out to the rest of the brain, to the cortex, where new connections in the cortex are formed to store memories more permanently. So we know a lot about the neuroscience of memories in adulthood, but in early childhood, it's still a mystery.

Nick, interestingly, infantile amnesia is present across the animal kingdom, which means researchers like you have been able to study this phenomena and get some interesting clues as to what might be happening in the brain during this time. Correct. And it's likely a ubiquitous phenomenon that there is a relative dearth of memory for early life. And we know this from

really beautiful research, particularly in mice. So for example, if you have a baby mouse learn something, for example, you put them in a maze and they have to learn how to escape from the maze, you can set up an experiment such that the neurons that are active in the hippocampus as they're forming that memory of the maze get tagged in a certain way. And then when the mouse matures, if you put them back in the maze, they don't remember where the hole is. They

But using a technique called optogenetics, where you can activate neurons with light, you can reactivate those cells that were tagged in the baby mouse. And now the mouse remembers where to go and knows how to escape. And whether that's true in humans, we don't know. And that's what we're working on.

So Nick, let's get on to your research. You've just published this study led by cognitive neuroscience researcher Tristan Yates, exploring whether, like the mice, these episodic memories are actually being encoded in the brain, or if at this young age in humans they're not yet being processed in the developing hippocampus.

And you used fMRI to look at how babies' brains were activating to explore if memories were being formed. Tell me a bit more about what you did. We used functional MRI in infants ranging from four to 24 months.

And we gave them a very simple memory task. We show them photographs while they're lying in the machine. These are new photographs they haven't seen before, a person's face, a dog toy, a waterfall. And then after a delay and after they've seen several other things, a minute or so later, we give them a memory test. Now, you can't ask a baby, do you remember this? Most of these babies were pre-verbal babies.

So instead, we use an indirect measure of whether they remember something, which is how much they look at something. Their eyes sort of reveal what they remember. And we test their memory by showing them a photograph they've seen before versus a new photograph from the same category they haven't seen before. So if it's a face, we'd show a different face.

And we measure which of those two pictures they look at longer. And if they look at the picture they had seen before longer, we take that as evidence that they remember it. And I know babies are probably the worst research participants for fMRI studies where you're supposed to keep perfectly still. So well done on managing to even get images from their brains.

But tell me, what did you find? So in that task where we measure how much they look at a photograph, we can go back to when we first showed the photographs, when we're seeing them for the first time and say, what's different in the brain when you're seeing something for the first time that you later remember versus something where you later forget? And the difference in brain activity is a signature of successful memory formation.

And what we found in this study is that beginning around 12 months, the hippocampus in an infant's brain responds more strongly when it's having an experience that it'll later remember than one that it later forgets.

This was strongest in the babies over one year and not statistically reliable in babies under a year. So we think it kicks on around then. It was also strongest in the babies who had the biggest overall looking time preference for old pictures, you know, sort of a measure of their overall memory.

So for babies who had better overall memory, there was more hippocampal activity during encoding. Right. So babies that had a strong signal of activity in their hippocampus when they were looking at a picture for the first time, they were more likely to show signs of remembering that picture later on by looking at it for longer.

And this behavior emerged around a year old, suggesting that babies at least form these episodic memories. But how long might these memories last? What ultimately ends up happening to them? What we can conclude from this study on its own is that infants have the capacity to form episodic memories beginning around 12 months.

This leaves open many interesting questions that we're working on now. Each of these studies takes years, so we're well underway on several follow-up experiments. But one is exactly what you asked, which is, okay, they get in the system, but we can't access them later, so where do they go? And it could be that...

One of those other stages of memory formation, the later stages, is deficient. So, for example, I mentioned consolidation, the idea that memories need to be integrated with the rest of the brain in order to stick permanently.

That happens over a period of days and weeks. We don't know whether that's yet fully mature in a baby. The mouse studies I mentioned before suggest actually that that may not be the problem, rather that the memories may be formed properly, but when you go to retrieve them, you just can't access them. And so...

That's what we're looking at is trying to measure how long can babies access memories for. How are you trying to study that? We do that by having parents record home videos of their baby's experiences. So over the first two years, every week, they send us videos of the baby going to the park or going on a trip or meeting a grandparent or having a birthday party.

And then we bring the babies in every three months for an fMRI scan and we show them videos from their own life versus videos from another kid's life. And to the extent that their hippocampus is able to access an experience that they had, we expect to see greater activity in the hippocampus for videos from their own life than from another kid's life. And then we can measure how that changes over time. Is there better memory for recent experiences versus more remote experiences?

We have some very preliminary pilot results that suggest that episodic memories may persist possibly into early childhood, sort of a preschool age, three or four years. But we haven't yet been able to look beyond that. But it's a long path because you have to grow up with the children. And so we won't have answers for a little while, unlike the mice who develop much more rapidly. ♪

So our inability to remember our early life could simply be a byproduct of our developing brains, whether that's in storage or retrieval.

But could there be something adaptive about infantile amnesia? I mean, might there have been an evolutionary benefit to putting all our energy into remembering generalities, the important broader patterns in the world, rather than sort of specific isolated events? I think...

You could make the argument, if you had to place a bet, that it's more important for the baby to learn about the general properties of the world, the structure of the world, the language, the spaces, the social dynamics, etc. And that individual memories like this are kind of a luxury.

But we do know from adult research that episodic memories for single events actually have a more profound impact on our behavior than we might realize. It's not just for reminiscing or for telling stories. We use these memories to make decisions. We use these memories to make predictions.

to figure out where to pay attention. So my guess is that there is some adaptive value to episodic memory early in life, in addition to the value of learning about the general structure of the world. - And I know some researchers have speculated that perhaps the problem in encoding early memories lies in our lack of language during that time, that we need language to encode memories properly because that's how we report them back.

It sounds like your research actually challenges that thinking. I think that's right. Or that you need a sense of self, right? And episodic memories about what you do in the world. And so you have to be able to represent that you are somebody in the world that's distinct from other people and other things. And I think our data suggests that while that certainly may contribute, it doesn't really pertain to the encoding side of it. The only thing I'll say is that a version of that story may work at retrieval.

So imagine that you're a 12-month-old baby and you go to a grandparent's house and you're maybe lying on your back. You might be hearing sounds, chatter, but you can't make sense of it because you don't have language. You might be looking at objects but not able to name them or recognize them.

And then you go back to that same place, the same experience, you know, five years later, and you're now walking around, you're hearing the topics of conversation, you recognize the people and so on. So the baby might've formed a memory of what it experienced at 12 months, but now what you're experiencing at five years or at 25 years in the same environment is very different

in terms of how your brain is representing that experience. And so when that information gets sent to the hippocampus, it doesn't match the information that was stored in the hippocampus when you were a baby. And therefore, you fail to retrieve the memory. It's like if you search for the wrong term in a search engine. You're just using the wrong cue to pull up the memory. ♪

Just finally, Nick, what do you think figuring out infantile amnesia could tell us more widely about our memories or even human brains in general? I mean, what are researchers like you really keen to learn? I think there's many deep questions to ask if we feel that there's a chance that infants have memories.

then that might change the way in which we think about caregiving and early education. It's not that you have a blank slate starting at age three or four. You might be bringing to your future these experiences from infancy. And parents talk all the time about, you know, not wanting to take a once-in-a-lifetime vacation until their kid is old enough to remember it.

because it might be wasted in some way, but it's probably still there at some capacity and influencing how they experience the world. So this is an area of deep fascination about humanity. Memory is who we are. It's our personality. It's our fears. It's our dreams. And understanding when it comes about and how it comes about, I think is one of the most fundamental questions about what it means to be human. Nick, thanks so much for coming on. Thank you for having me and for your interest. Thank you.

Thanks again to Professor Nick Turk-Brown. And if you're looking for something to listen to next, I'd like to recommend today's episode of our sister podcast, Today in Focus. Five years on from the first lockdown, Helen Pidd hears stories of life with and after long COVID. Just search for Today in Focus wherever you're listening to this podcast.

This episode was produced by Madeleine Finlay. It was sound designed by John Cox and the executive producer is Ellie Burey. We'll be back on Thursday. See you then. Hello, I'm Jonathan Friedland from the Politics Weekly America podcast. Donald Trump has started his second term. It's going to be a turbulent period and The Guardian will be here covering it all.

This is The Guardian.

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