How to make a memory

What is actually happening in the brain when we make a new memory? Dr Dean Burnett is a neuroscientist from the University of Cardiff and he's here to give Kat Arney the lowdown. He started by explaining how memories first form...

Dean - Let's say someone's just written down their phone number or your number phone for you on a piece of paper and it's an Inspector Gadget situation and it's going to self-destruct in ten seconds so you've got to try and remember it from recall only. So, the number itself when you first experience it will enter your short term memory, which is like part of your brain which is actively processing information at any one time.  People have a misconception that short term memory is like a few minutes, few hours.  No, it's actually 30 seconds to a minute, maximum.  It's very short - anything more than that is technically a long term memory.  It's supported by activity in the frontal cortex, the frontal lobes...

Kat - That's the front bit of your brain, right?  The bit at the front of the head?

Dean - Yes. The bit at the front straight on - it looks like a pair of Ds back to back. It's very active, it's very ongoing, it's processing information all the time.  It's not for storage so I liken it to writing your name in a sparkler - the information's there briefly but then things happen it fades.  Information is coming in all the time so the space is needed for other things so you've got maybe a minute to get it into the long term memory.

Kat - Kind of like taking a photo of that word written with the sparkler.  You have to find some way of capturing it and keeping it for longer.  What happens next then?

Dean - Exactly.  The phone number, if that's what we're talking about, enters your short term memory and it's like a chunk of information - chunking is what it's called.  We can remember up to four chunks at any one time in our short term memory.  Now we need to attach a significance to that and make it salient enough that it becomes important enough to be laid down in the long term memory by the hippocampus.  So the information feeds through into the hippocampus which is sort of the central hub of the memory system.

Kat - Just give us a quick description of the hippocampus.  If I picture my brain, my head, where's the hippocampus and what does that do?

Dean - Where is it?  It's in the temporal lobe which, if you think of the brain as a boxing glove, it's the thumb.  So it's like the inside of the thumb toward the centre near the...

Kat - Right in the middle...

Dean - It's an older area and it's right next to the olfactory area too which is one theory as to why smell is such evocative of memory.

Kat - Ah, yes..

Dean - And it's shaped a bit like a seahorse in cross section.  Hence the name hippocampus - it's not about a holiday camp for large mammals.  It is like the processing centre of all long term memory.  Sensory information is fed into it and the hippocampus takes this and binds all the important stuff and forms new memories by connecting neurocells together in a certain way to form synaptic connections, and each of these is what represents a memory.

Kat - So memory is basically nerve cells (nerve connections), that are just knitted together as the hippocampus decides 'okay we need to remember this, this is a phone number we need to remember', and it will knit together neighbouring nerve cells and that's your memory?

Dean - That's generally the accepted wisdom at the moment in that there's a certain combination of connections forms a memory, in the way that certain patterns in ink on paper form a word.  So that's what we think memories are at the moment, and these are being constantly made by the hippocampus all the time because, obviously, we are constantly experiencing things.

Kat - So say we've got as far as my phone number, it's gone into my hippocampus.  How then does it become something I can remember?  I can still remember my parent's phone number from the house I grew up in.  Is that 'it's just still there' or is that then stored in a different way, those very, very long term memories?

Dean - There still there essentially.  The very first one, you could argue, is still there but every time it's retrieved or reactivated they might be forming a new memory for the incident itself where that memory is retrieved.

Kat - It's kind of writing on top again?  It's kind of colouring it in a bit firmer?

Dean - Yes, or forming new connections with it.  Every time you use that number you are experiencing it in a different way.  So you're telling someone you want to call you - someone you've taken a fancy too - and you say well my number is this.  Then that memory is going to be a lot more vivid so it's connected to that as well.  So it's constantly being shored up and reinforced all the time if it's a common memory.

Kat - Could my brain get full?  Say, there's lots of people I fancy and I collect all their phone numbers, is my brain going to get full of them?

Dean - Logically, eventually it would but, as far as we know, no-one's lived long enough yet for that to happen.  It's actually got a fantastic storage capacity the human brain has.  We don't know what it is yet but it's way beyond our current scope to really calculate it so... Yes it could, but don't worry about it is the general jist.

Kat - And then, that's all the information that's in there.  So when I want to retrieve a memory - what's happening?  Are those nerve cells just firing in the pattern that it originally went in?

Dean - Sort of, yes, unless it's been modified since.  That's another thing about human memory, if you remember things in a different context it can constantly tweak your memories and adjust them and that's arguably a failing of biological memory, it's not essentially rigid.  So when you're in a certain situation where you need to retrieve some memory like in your frontal cortex is doing all the conscious thinking again.  It will think 'what happened there then', and then the patterns of connections is triggered and the link between what you think about now and the memory you need is activated and the more connections there are to that memory, then the more likely and the more able you are to retrieve it.

Kat - And very, very briefly.  We've talked about remembering things like phone numbers or you can imagine a fact for an exam, is there a fundamental difference between remembering those kind of things and then, for example, I can remember the birth of my friends child?

Dean - Yes.  It's believed the brain does categorise these in certain different ways or at least we think of them in different ways.  One of them is semantic memory - that's memory for just information.  So knowing pythagoras' theorem, knowing your niece's birthday - that is semantic information.  It's just information you have access to, whereas the memory for learning pythagoras' theorem or memory of the birth itself, that would be an episodic memory, which is memories of actual events from your life themselves in which the information is contained, and that's constantly ongoing.