What are memories?

What do we actually mean by “memory”? Izzie Clarke has been investigating and began by asking some people she bumped into on the street to recollect their earliest memories...

I remember dancing on my dad’s feet while listening to old 60s music and singing at the top of our voice.

It’s a really hot day and the Sun’s streaming through and I’m checking on stones that we collected from the seaside and we painted images on them and faces.

Amy - Probably the first memory I can remember very distinctly was playing in my parent’s garden.

Izzie - That’s Amy Milton; she’s a psychologist at the University of Cambridge…

Amy - They used to have, I think it must have been a concrete fence post that I used to stand on to look over and chat to the kids in the neighbours garden, and one day I fell of it and fell back onto my foot and then ended up giving me an enormous bruise. I can still remember that quite clearly.

Izzie - Recounting her first memory, Amy not only remembers the pain of the concrete block, but also the event itself. So do we have different types of memory? Well… we have short term, which is really short like the ability to remember a phone number for just a few seconds while you need to dial it and then, poof, it’s gone. But our long term memory is a bit more complicated...

Amy - There are two very broad types of long term memory that we have. There’s declarative memories and these are memories that you can pass on in words, and that would include memories for events. We refer to this as episodic memory because it’s talking about episodes, but it would also include semantic memory which is memory for facts.

Izzie - Episodic memory or event memory is the ability to account things like where you were when you passed your driving test, or the day you graduated…

Amy - We think about episodic memories as having three components: a “what,” a “where”, and a “when.” All three of those in one memory would be considered an episodic memory.

Izzie - How does a semantic memory differ from this?

Amy - A semantic memory is really the memory for “what.” We tend to think of that as being fact based. These memories aren’t necessarily entirely distinct from each other. Memories can move from one type to another.

The kids have started back at school fairly recently so they will be learning, for example, capitals of the world. You may have a small child coming home fairly soon telling you that they know that Paris is the capital of France and they could tell you who told them that information and where they were when the learned it. But, actually, we all learned that at school at some point but we don’t hold on to the where I learnt it and who told me because it’s no longer relevant.

Izzie - If you can describe it in words then it’s a declarative memory…

Amy - Alongside those we also have non-declarative memories which would include things like emotional memory, so memories that give rise to particularly emotional states and skill learning. So things like learning to ride a bike is clearly a memory but, they’re called non-declarative because you can’t pass those things on in words.

Izzie - So, we’ve got different types of memory but do we know where they’re stored?

Amy - We do know that different types of memory depend upon different parts of the brain. Event memory refers on the parts of the brain that sit right at the side by the ears; we call that the temporal lobe and, in particular, it depends on structures in the temporal lobe called the hippocampus. We know skill learning, for example, depends upon areas within your motor system, so the little tiny bit at the back of the brain called the cerebellum. But also, your motor cortex, for example, will show changes as you acquire a new skill.

Izzie - One of the most influential studies into this involved a patient called Henry Molaison who was known as patient HM until he passed away in 2008. Henry suffered from severe epilepsy and no medication seemed to help. So in the 1950’s surgeon, William Scoville, removed the front part of Henry’s hippocampus.

Amy - This was really good from the perspective of the seizures, they reduced markedly but Henry, from that point onwards, couldn’t form any new event memories. So he had this very, very profound amnesia which gave a very, very strong steer to the hippocampus being important for those type of memories.

Izzie - But surprisingly, there was a breakthrough from a psychologist called Brenda Milner…

Amy - What she managed to show was actually a lot of his memory was fine and skill learning was absolutely fine with Henry. So one of the tasks that she used was mirror drawing. He had to trace around within a very small outline, so something like a star, but he could only see his hands in a mirror. He had a screen that stopped him from seeing his hands directly, which is quite tricky but, with practice, you get much better at it. She found that he improved with practice on a single day, and she brought him back in the next day and he was better again, and if she brought him back in the next day he was better again.

Izzie - Even though HM didn’t remember doing this task he was still able to learn a new skill, showing that any skill based memory like drawing wasn’t associated to the hippocampus. But, when it comes down to it, where we actually store memories is an area that still has a few unknowns…

Amy - Looking at HMs case, for example, he had the surgery when he was in his late twenties. Events up to his early twenties he could remember pretty well - his teenage years, his childhood. He could still draw the floorplan of his parent’s house well into his 80s. From the moment of surgery he had no memory, and for a couple of years before that his memories were starting to be much less reliable. So, we know that very old memories don’t require the hippocampus because otherwise HM wouldn’t be able to remember them. We know that those memories go to the cortex so the question then becomes once they’re in the cortex, are they really episodic memories still or are they semantic memories that have become anecdotes about ourselves?

So, maybe I don’t really remember falling off the concrete post and hurting my foot, I’ve just told that story so many times that’s now part of my history and that’s now a fact about me and it’s now a semantic memory.