What is memory?

What would a memory actually look like?
20 November 2018

Interview with 

Dr Amy Milton, Cambridge University


Whether it’s reminiscing about that baking hot beach holiday, forgetting your keys or reciting that work to do list, our memories are never far from our minds. But what is this mysterious system we call memory? Katie Haylor spoke with Cambridge University's Amy Milton.

Amy - The brain is made up of millions billions of brain cells and these all talk to each other, if particular sets of brain cells talk to each other again and again and again, they get more efficient at talking to each other and they lay down what we call a memory trace, which allows them to communicate more efficiently next time Some of that information is presented.

This change in communication efficiency. For that to persist there has to be some kind of structural change in the way that these brain cells talk to each other and the only thing that the brain cells have really got to build with is proteins. So what you would see if you could look down a microscope and see a memory, is a difference in proteins that are being produced by these individual brain cells, particularly the one that's receiving the signal which we call the post-synaptic neuron. You'd see lots of proteins basically coding for receptors to receive the signal from the pre-synaptic neuron, the one before the synapse.

Katie - And by coding, it’s making a product, making a protein?

Amy -  That's right. So when brain cells communicate with each other, the pre-synaptic neuron releases chemicals which is detected by the post-synaptic neuron using receptors. So there's a little protein that receives the signal. When the change in communication efficiency happens, there’s more of these proteins to receive that signal.

Katie - So if we were to summarize then a memory is a change in the behaviour in neuron our selection of neurons that can communicate with each other much more efficiently than they could do before?

Amy - That's right. So it's a change in behavior following an experience. And that works even at the level of individual neurons, having received this signal again and again and again the second neuron becomes much more efficient in detecting that, so it changes its behavior based on its prior experience.

Katie - And I guess that's a lovely definition because it also works at the level of the individual. If I remember that right cup of coffee is particularly good at that café compared to that cafe, I might change my behaviour and go to the other cafe.

Amy -  That's right. So it's a very broad definition and there are a few problems with it. But as a working definition it's not a bad one.

Katie - Okay so say before I came to see you, I got my cup of coffee it's particularly nice, that memory is being made in my brain. What happens afterwards, where does it go? Does it get shuttled off to a different part of my brain?

Amy - So that type of memory would actually be laid down in a number of different memory stores. We often think of memory as being a single thing but it's not. There's lots of different types of long term memory. We can have memories for individual events, so you remember that this morning you went to this location and you bought your cup of coffee and you might remember the person who served you, you might remember your individual order and so on. That's an event memory that we sometimes call episodic memory and that depends on a particular brain area called the hippocampus.

Alongside that you will probably have formed an implicit memory, a sort of unconscious much more motivationally relevant memory that the coffee from that shop is good. That location is a good location and you may find that next time you’re just wandering past that you feel drawn into that location because you’ve had something good there before. And that type of memory is stored in a different part of the brain which we call the amygdala and that kind of unconscious or implicit memory you can’t pass that memory on in words.

Katie - Now Amy explained that these implicit memories tend to stay put in the part of the brain where they’re made but episodic memories can over time wander off.

Amy - We know that event memories are initially stored in the hippocampus but from studies of patients such as Henry Molaison or the patient H.M., he had damage to his hippocampus, he had it removed surgically to stop very severe epilepsy. And it was found that he could recall events from his early childhood. In fact he could still draw the layout of his childhood house well into his 80s but the last couple of years before he had his hippocampus removed he couldn't remember.

And of course you couldn't lay down any new event memories because he didn't have a hippocampus after the surgery. So that suggests that over two to three years those hippocampus memories are becoming independent of the hippocampus and they're moving elsewhere.

Katie - Where do they go?

Amy - So the idea is that they're moving to cortical areas so it's like the hippocampus teaches the cortex over a very long period of time what those memory traces are. And then once that's been achieved the hippocampus is no longer necessary to recall those memories. They now live in the cortex if you like and they can be recalled directly from there.

Katie - The cortex overlays a lot of the regions in the brain and different cortical regions house different bits of a memory, like what something looked like or sounded like. The hippocampus, tucked away in the brain nearest to the ears on the inside of the head, is kind of like a puppet master pulling all the strings of the memory together from different bits of the cortex. And it teaches the cortex how to put the memory together. Sounds rather complex huh?.

Amy -It is rather, and we now think of memory as being much more distributed that lots of different brain areas contribute to memory a little bit like the internet. So there are key hubs there are key points that need to be working but actually the information is much more distributed much more like the World Wide Web.

There was a view back in the 80s and 90s that the hippocampus was like the index card system and the cortex was like the books on the shelves. But it's interesting that views of memory seem very much to mirror how we store information at the time. So as quantum computing develops it will be interesting to see how the theories of memory evolve.

Katie - Quantum memory? The thought of it makes my brain hurt so let's move on. Now it's all very well to store a memory in this great vast internet of the brain. But what happens when you need to go in and actually find it?

Amy - So the idea is that you get activity again within that memory trace. So you might only switch on a few neurons within that trace but because they've become so efficient at signalling to each other they all then become active together and that gives you the memory again.

Katie - Almost like a map of neurons, a map unique to that memory?

Amy -  Exactly exactly but under certain conditions where maybe a few extra neurons are active that can then become wired up to that original memory trace.

Katie -So that's the unexpected add-on information. Say you’ve got something slightly wrong, or something else has come along to add to that memory. Those neurons can sort of tack on to make up a new map?

Amy - That's right. Under certain conditions of retrieval where there's new information incorporated, if that's replacing some old information maybe that you got wrong. You can also unpick some of that original information. You would be taking some of those receptors out of the neuronal membrane. So you put those receptors in when you made the memory, maybe now you need to fill them out and that will then allow you to rewire that memory trace.

And of course if you're doing this again and again, and the more you recall that memory, the stronger those connections are getting. You can see how doing this over a period of time could lead to two people who had an original memory that was pretty similar actually having two memories that are now quite different.

Katie - So memory is pretty flexible. We update memories all the time but with flexibility comes vulnerability to suggestion and for instance police, Amy says, have to be really careful of this when questioning witnesses. But is it really fair to expect us to be able to recall events absolutely? And is it even really necessary? I'll give the last word to Amy.

Amy - We often think about memory as being about recording the past. Well we know memory is not an accurate recording of the past. We do reconstruct quite a lot. Memory is actually more about knowing what to do next time you're in a similar situation, so it doesn't need to be 100 percent accurate. It just needs to be good enough to predict what's going to happen in the future. And that's one of the ways that we use to mark what's important.


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