What Types of Memory do we Have?

I want to talk about memory. You can argue that actually possibly it's memory that makes us who we are. Without our memories, perhaps we become a different person. So memory is clearly crucial to the sense of being a person. It's also, of course, very crucial as we design user interfaces. There are different kinds of memory, and what I've done is put them on a time scale.

So you can think of ones that operate very, very quickly to ones that operate over a very long time scale. The fastest kinds of memory are sensory memory. When you hear something, you can often repeat it back even if you didn't think you'd heard it properly. It sort of stays around in your head for a little bit. You might not even think of that as memory. It's below the point that you normally think of as real memory. And then there's what's called short-term memory.

This is for when you've... perhaps if you think about an interface you've read a number and you have to type it in somewhere else. I constantly have to do that with the two-factor authentication where you get the little text message that tells you a number that you've got to type into your user interface. And you look at the number and then you type it in. Even ten minutes later, let alone three days later, you won't remember that number. You don't want to remember that number; it's just there for a very short amount of time in remembrance.

There's also the kinds of memory you've got of your childhood. This is long-term memory, things that go back many years. Hopefully, the things you learn about in these videos will become part of your long-term memory, but probably not all of it will. They're the things that you remember pretty much forever. And in between... I call this mezzanine memory. It's an under-studied bit of memory, there's some memory that fits between the two.

So whereas your long-term memory is things that are there your whole lifetime, there's the bit about remembering that, this morning, what did you have for breakfast? Now you probably won't remember that in ten days' time, let alone in ten years' time, but you probably will remember it now. And that in-between memory I call *mezzanine memory*, and there isn't a standard word for it. And it's under studied, so the things that we say about it are going to be a little bit vaguer than things like short-term memory and long-term memory,

which are very well studied and you will see talked about a lot in books. Focusing on those three sort of more mental memories, the short-term memory, the long-term memory and that middle mezzanine memory, we can think about their different properties. So first of all, look at capacity. How much? Short-term memory, that bit where you read the number and type it in, is very constrained. People talk about seven plus or minus two chunks, five to nine chunks.

Basically, it's not a lot. You just remember very small bits for a very short amount of time. Your long-term memory, that lifetime memory is big. You can remember all sorts of things. You could tell me about your childhood, perhaps the first film you ever watched, or certainly the most recent film you ever watched. And tell me all sorts of details about it. Perhaps things that you learned in school. How big it is, it's hard to know. There's a limit in terms of the number of neurons in your head,

but that's a very, very big number. And this bit in between, this mezzanine memory is so under-studied, I won't even guess how large it is because this just isn't being studied well enough. The short-term memory, it's both small, you don't remember much, and it decays very rapidly. So you pick it up... So you look at that number on your phone, you type it in, and unless you keep repeating it in your head, you will forget it within 30 seconds or so. It goes.

So it's very, very fast. So if you want to keep it around, you'll have done this, just like repeating something to yourself: 596243, 596243, 596243. And you repeat it until you type it in. So if you're scared of forgetting it. The lifetime memory, the long-term memory – you can remember your childhood. You probably can't remember your childhood when you were very, very young, like under about three or four years. That's called infantile amnesia, there are different words for it. But that's for different reasons. Not that you can't remember it.

It's I think, because your brain changes in different ways. This mezzanine memory, this bit in the middle, is the minutes-to-hours bit. It's the remembering why you're watching this video in the first place, that you started watching it, the things I said a few moments ago. But also perhaps your breakfast, the things that lay over that period. Both the short-term memory and the mezzanine memory, most of the time it's implicit. You not even aware you're doing it. These longer-term memories, some of those are implicit, but actually, you probably got

a whole load of strategies that you learnt at school about committing things to long-term memory, a lot of which is about rehearsal and repetition. So if you think about the same thing, the same memory you've had, if you talk to somebody, you will often put them into long-term memory. There are some dreams I can remember, but they're normally because I told somebody about them straight afterwards. And so by telling them that I rehearsed them and laid them down into long-term memory.

So that's about how you remember it. And then there's the mechanism, the underlying neurological mechanisms that store it. Short-term memory is about electric changes happening in your brain. So they're not currents in the sense of wires, like they are in a computer. They're ionic changes, but they're basically electrical activity in the brain. It's the things you detect if you put one of those brain caps on with lots of little electrodes on. And the short-term memory is governed by that. Your moment-to-moment thinking, but also short-term memory is driven by those.

When those go away, short-term memory goes away. So it's neuron firing. Long-term memory is about physical changes in the connection patterns within your head. That's why when you have an anaesthetic, which basically knocks out most of that electrical activity, when you wake up again, your long-term memories are intact because they're about actually where the connections between the neurons are and the synapses, which is where they connect in, how big they are. And actually the synapse,

the point where one neuron touches another neuron actually gets physically bigger to make them stronger connections. So there's a physical layer to the way in which long-term memory is stored. That's why it's long-term, and there's arguments that you never forget it; it's always there. So even if you think you've forgotten something, it's actually just that you don't have the right path to it. And this bit in between, again, this mezzanine bit – under studied, but it's probably stored by something called long-term potentiation, which is the fact that when a neuron fires its chemical structure changes

and that change can take a while to revert back to, shall we say, a neutral state. That "while-ness" can be anything from minutes to hours. My guess is, I said this is widely unknown because it's not been well studied, is that would be the mechanism by which those are stored. So it's something that if you're not under anaesthetic, actually you often can remember things immediately before. You might forget a little bit, but you remembered it before

because it's in that chemical change which lasts quite a period. Whereas the electrical activity all goes. So three kinds of memory, very, very different properties. And in an interface actually those properties you'll use very differently. From copying that number in short-term memory to perhaps learning how to use an interface, which is a long-term memory thing.