How our memory develops
- Memories are formed by neurons that fire in our brains, creating or changing networks of connections.
- Human brains aren’t fully developed at birth.
- As our brain develops in infancy and early childhood, so does our capacity to remember.
- There are changes in the brain’s prefrontal cortex during puberty and adolescence, with corresponding changes in our memory abilities.
Have you ever wondered why you can’t remember being a baby? Or why you can easily remember all the words to a song you learnt as a teenager—even if that was 20 (or more) years ago?
The answers to these questions may lie in the way our memory system develops as we grow from a baby to a teenager and into early adulthood. Our brain is not fully developed when we are born—it continues to grow and change during this important period of our lives. And, as our brain develops, so does our memory. Let’s wander down memory lane and take a look.
Um … What’s memory again?
First, a quick recap of the basics.
- Memory isn’t a video camera
Many of us think of our memory as being a bit like a recording device—a video camera, say. We imagine it faithfully recording events in detail which we can, at some later stage, retrieve by simply pressing the ‘play’ button.
But this video-camera idea of memory isn’t really accurate. That’s because memories aren’t just static recordings which are ‘there’ to be accessed. Rather, memories are dynamic—they’re always changing. They can become stronger or weaker over time. They can become distorted, and they can be manipulated. What we remember and how we remember it depends on when we do the remembering, and what meaning and experience we bring to that memory. In fact, every time we remember something, we alter that memory a little bit.
- Memories are made when neurons fire
Neurons are nerve cells which send electrochemical signals to each other. As a person processes an event, neurons in the brain pass information through synapses (tiny gaps between neurons). This invites surrounding neurons to start firing, creating a network of connections of various strengths. It’s this persistent change in the strength and pattern of connections that is a ‘memory’.
- There are different kinds of memory
There are a number of different kinds of memory. It can be explicit (consciously remembered) or implicit (unconscious). Good at remembering facts and figures? That’s what’s known as your semantic memory. Able to thrash your partner at Pacman without even thinking, even though you haven’t played for years? You can thank your procedural memory, which is all about learned motor skills.
- Memories are stored in different, interconnected parts of the brain
- Memories aren’t just stored in just one place in the brain. Rather, different (interconnected) parts of the brain specialise in different kinds of memories. For example, an area of the brain called the hippocampus is important for storing memories of particular things that happened in your life, known as episodic memories.
From birth to adolescence
Infancy and childhood
Can you remember your first birthday? Your second? If not, don’t panic—you’re not alone. Adults rarely remember events from before the age of three, and have patchy memories when it comes to things that happened to them between the ages of three and seven. It’s a phenomenon known as ‘infantile amnesia’.
So why is it so hard to remember being a baby or toddler? Is it simply because our first, third, and even seventh birthdays happened a long time ago, and our memories have naturally faded? Not necessarily. In fact, a 40-year-old adult will usually have very strong memories of adolescence (more about that later) which, for them, happened more than 20 years ago. A 15-year-old, on the other hand, will be unlikely to remember something that occurred when they were two, even though it happened only 13 years ago.
What do babies remember?
It used to be thought that the reason we can’t remember much of our early childhood is because, as young children, we just aren’t able to make stable memories of events. You can’t access a memory, the logic goes, if it’s not there!
But it turns out that infants and small children can and do form memories. This includes both implicit memories (such as procedural memories, which allow us to carry out tasks without thinking about them) and explicit memories (like when we consciously remember an event that happened to us).
Our ability to remember things for long periods of time does, however, progressively get better throughout childhood. In experiments in which young children were taught to imitate an action, for example, six-month-olds could remember what to do for 24 (but not 48) hours, while nine-month-olds could remember what to do one month (but not three months) later. By 20 months of age, infants could still remember how to do a task which they were shown a whole year earlier.
Interestingly, recent research in rats has revealed that, despite the apparent loss of early episodic memories, a latent trace of the memory of an early experience remains for a long period of time—and can be triggered by a later reminder. This may explain why early trauma can influence adult behaviour and increase the risk of future mental disorders.
Our changing brain
Neuroscientists studying memory in animals (such as rats and monkeys) have discovered that it’s not just people who experience infantile amnesia. It seems to be common to animals whose brains, like ours, keep developing after they’re born. At birth, a human baby’s brain is only a quarter of its adult size. By the age of two, it’ll be three-quarters of the size of an adult brain. This change in size correlates with the growth of neurons and the testing and pruning of connections (more about that later). So what does the fact that our brains are still developing in infancy and early childhood mean for our memories?
Let’s take a look at the hippocampus—that part of the brain which is especially important in the formation of episodic memories (memories of events that happened to us). While many parts of the brain keep developing and changing after we’re born, it’s one of only a few regions that keeps producing new neurons into adulthood. When we’re little, for example, a part of the hippocampus called the dentate gyrus is in overdrive, making neurons at a great rate. These new neurons are then integrated into hippocampal circuits. Although the production of new neurons continues in adulthood, the rate of activity slows down.
Scientists think that this rapid rate of neuron production in childhood could contribute to our higher rate of forgetting when we’re young. How? By forming new connections with memory circuits, the masses of new neurons may disrupt existing networks of already-formed memories.
While early childhood has long been recognised as an important time for brain development, it used to be thought that it was all over long before we hit puberty. But it’s now known that our brain keeps developing and changing during puberty and adolescence. In particular, our prefrontal cortex, which is important for executive functions like controlling our behaviour, shows important changes at this time. And, as these areas of our brain continue to change and develop, so does our memory.
The reminiscence bump
The words to a corny love song, the moves to the Macarena, even the boring, everyday stuff—if it was part of our adolescence, we’re more likely to remember it 20, 30 or even 40 years later. A number of studies have shown that adults over the age of about 30 have more memories from adolescence and early adulthood than from any other time of their lives, before or after—a phenomenon known as the ‘reminiscence bump’.
It’s thought that this is because, when we form a new self-image, we encode robust and lasting memories that are relevant to that self. In other words, we are most likely to favour memories that reinforce our ideas of who we are. Since adolescence is a key time for the emergence of a stable and enduring self, it’s also the period we tend to remember most strongly.
You’ve probably heard of ‘grey matter’. Often used as a kind of shorthand for the cells of the brain, grey matter is largely composed of densely packed neurons.
But go beneath this ‘topsoil’ of the brain and you’ll discover, filling nearly half of it, a mass of communication cables ( axons GLOSSARY axonslong, slender projections of a neuron that typically conduct electrical impulses away from the neuron's cell body ) which connect neurons in different parts of the brain. This is white matter. The cables are coated in a fatty substance called myelin, which give them the white colour that shows up on an MRI GLOSSARY MRImagnetic resonance imaging, a technique that uses a magnetic field and radio waves to create detailed images of the organs and tissues in your body . Myelin acts like insulation around the axons, allowing messages (in the form of electrical signals) to be carried more quickly between areas of the brain. The more myelin, the quicker the messages will travel.
Thanks to MRI technology, scientists have been able to observe what happens to myelin in our brains during childhood and adolescence. They’ve found that, while sensory and motor brain regions become fully myelinated (coated in myelin) in first few years of life, myelination in our frontal cortex continues well into adolescence.
Synaptic growth and trimming
In our first few months of life, our brains get busy making lots and lots of synapses (connections between neurons), until we end up with many more than we’ll eventually have as adults. Over the following few years, these connections are gradually pruned. Depending on our experiences, some connections are strengthened while others disappear until, eventually, the density of our synapses reaches adult levels.
But, in our prefrontal cortex, it seems that this happens a second time. As we hit puberty, corresponding with a turbulent time of growth and learning in the rest of the body, there is another wave of synaptic proliferation in the brain. Then, as we move through adolescence, these connections are again pruned back and reorganised. This pruning makes the existing connections more efficient, so it’s essential to cognitive processes such as memory.
Because our frontal and prefrontal cortex continue to develop in these ways during puberty and adolescence, we might expect to see a corresponding improvement in executive functions to do with memory which are associated with these frontal regions of our brain. And indeed, this has been found to be the case: experiments have shown that our performance on complex working memory tasks continues to improve in adolescence, as does our prospective memory (our ability to remember to do things in the future).
We all know that our childhood and adolescence is a time when our bodies go through huge changes. What might not be so obvious are the hidden changes in that dynamic, and sometimes mysterious, place that is our brain. And, as we’ve seen, as our brain develops, so does our memory.