The science of forgetting

What actually happens in the brain when we forget?
20 November 2018

Interview with 

Professor Michael Anderson - Cambridge University


Understanding post traumatic stress disorder.


Of course, severe memory loss can be devastating, but forgetting isn’t always a bad thing. Some memories are painful or distracting to remember, and an ability to suppress these memories can be useful. Katie Haylor met forgetting expert Professor Michael Anderson from Cambridge University.

Michael - We study people's ability to actively forget. So we believe that a lot of the forgetting that people experience is actually not accidental. It's not just due to the passage of time or to the crumbling of memory traces but things that we do to use our memory and also to protect ourselves. We have constructed a laboratory procedure which I think mimics the circumstances of motivated forgetting as they occur in the real world and we set people the task of trying to forget in the scanner and we watch their brain as they do this and we hope to document the brain regions involved in that process.

Katie - So we're not going to do a full test on me but say you were, what kind of things would you ask me to forget?

Michael - Well we basically are studying a situation in which you confront a reminder to something that you'd rather not think about. We've all had that situation right, you walk around the corner and you see a car from your ex and you just put up the mental and you say oh no I'm not going to think about that I'm going to stop thinking about that.

Katie - Would traumatic memories and things come under a similar sort of category?

Michael - You know traumatic memories for sure certainly. Basically a lot of the memories that we have are stored in our brain or of things that we'd rather not think about, whether it's trauma or embarrassment, shame, anxiety or any kind of negative emotions we'd rather not re experience, sometimes reminders in the world call those memories back into your mind. People usually are not very well disposed to that happening and so they try to push the unwanted memory out of awareness. What our research is focused on is that process of pushing? What is it that you are doing?

Katie - Well, the time has come I can't put it off any longer...

After making sure I had no metal on which could be influenced by the magnet, I was led into the MRI room. I took my shoes off, was given some earplugs and laid down on a rather comfy bed, which together with a funky headset and a mirror angled up at a computer screen, I slid slowly into a very large doughnut whose walls were no more than a few inches from my face. Luckily I’m not claustrophobic.

[MRI sounds]

Michael - Katie Haylor, meet Katie Haylor’s brain. Before we look at this I just want to emphasize the specialness of this because the fraction of all humanity who has ever existed lived on earth who has actually gotten a chance to see their brain is very tiny and you are you now are well to that club.

Katie - Wow. I feel very honored!

Michael - There is your brain looks perfectly lovely. What we're looking at here is a slice right in the midsection of the brain so we can see the right hemisphere. We're looking towards the right hemisphere and the left hemisphere has been stripped away. We can see the prefrontal cortex off here to the left, posterior visual cortices is back here. This is your brainstem.

Katie - Everyone knows what a head looks like. But to slice the ead as it were is a bit of an odd arrangement, so that I can see my skull and the eye sockets. A kind of wiggly fleshy almost like walnut like bit of the brain at the top and that goes from back to front.

Michael - Yes indeed. So those are the gyri and sulci those are the technical terms, the folds of the brain. And there even though they look random they're not, they're actually reasonably consistent across people to the extent that they actually have names. Here what you're looking at is the corpus callosum.

Katie - Below the wiggly walnut bit. It’s more like a band slightly lighter in color from left to right.

Michael - Indeed so everyone's aware of that there is a left hemisphere of the brain and the right hemisphere of the brain, the left side the right side. Well the corpus callosum connects the two halves together allowing the two sides to talk to one another and you're looking around at it right there.

Katie - What was the thing that looks a bit like a cabbage leaf?

Michael - The cabbage leaf here is your cerebellum a critical structure in coordination and fine motor movements. But it's actually also involved in higher level cognition as well. Most people have heard of the brain's grey matter. The grey matter is where your brain cells live. The white matter underneath it, kind of this big bulky area, that's where the projections are going from one region of the brain to another region of the brain, so the axons that allow the brain regions to communicate with one another. So they're white because these axons are encased in a fatty substance called myelin, a myelin sheath and that's significant because it basically increases conductivity. It increases the rate of communication between one region and another region, if there's myelin covering. So your brain is full of that basically.

Katie - Right. But I guess that's a good thing?

Michael - It's a very good thing. You would hate life if it wasn't there.

Katie - Now this was structural MRI. It gave me a picture of my brain. But Michael does functional MRI which means he images the brain whilst it's doing things. By getting people to associate a trigger or reminder image with a particular scene and then putting them into the scanner, Mike tells them to stop the scene coming to mind when they're exposed to that same trigger. And he can see which brain areas are involved in this memory suppression.

Michael - Most people confront a little bit of a challenge initially but if you give them practice at suppressing something over and over and over, eventually the thing doesn't come to mind anymore. And in fact eventually if we test people's memory later on, people actually can't recall it anymore even when they want to recall it, so if they suppressed it often enough it causes forgetting and we call that phenomena suppression induced forgetting.

One of the most pervasive symptoms in psychiatric disorders whether you're talking about OCD, anxiety disorder, pathological worry or rumination in depression or flashbacks in PTSD or intrusive memories, there's kind of a commonality there of the memory delivering things to your mind that you don’t want and difficulties in preventing that from happening. And so if we can document how the brain controls unwanted thoughts and memories when somebody doesn't have a psychiatric condition, and we understand the networks involved then that really deeply, then there is a hope that we can better identify what might be going wrong and people suffering from that intrusive symptomatology and then develop interventions to address them.

Katie - So what actually is going on in the brain when we try to suppress an unwanted memory? In Mike's office, he filled me in.

Michael - It's all about stopping it really. So how do we stop memory from doing what it usually does? And understand this we build on a model of stopping physical actions which were also quite good at. So stopping yourself from reaching and grabbing a hot pot. We know that the prefrontal cortex is critical for this so particularly the right prefrontal cortex. It interacts with motor cortical regions to shut down the action, and we thought maybe the same thing same kind of thing happens except that the prefrontal cortex interacts with the structures involved in memory like the hippocampus and that's indeed what we find. We find that when you put people in that situation, you give them a reminder of something that you ask them to not think about, they engage this right prefrontal region to shut down activity in the hippocampus.

Katie - So now the group know which brain areas are involved in active forgetting in healthy volunteers, the next step is to look at the brains of people suffering with these unwanted memories. The aim being that if they can spot the differences in brain activity this may one day inform potential treatment.

But it's not just unwanted memories that we can suppress. Michael also studies how we suppress distracting ones. And he's recently published a paper on just this.

Michael - Suppose you go to the same supermarket over and over and over, and so you bring your car and you park in a different spot each time. When you can out of the supermarket you ask yourself where did I park my car? There'll be that momentary confusion, did I park over to the left or over to the right? That confusion is generated by the fact that your memory is delivering multiple alternatives to you based on your past experience and eventually you suss it out and you think oh yes that's right I parked over there today. In that little moment of confusion created by an overabundance of answers it’s  sorted out by a process of active forgetting, of suppressing the distracting alternatives and in in making that selection retrieving one thing at the expense of others. That gradually causes you to forget those other things and there's a reason why you don't remember every time you've parked in a parking lot.

Katie - So to remember one thing is to forget something else.

Michael - Yeah more or less yeah.

Katie - And you did this in rats. I'm guessing the parking is a bit of a metaphor, you didn't have rats parking cars. Why is it important to study this in rats?

Michael - Because I think everybody has the kind of question what's actually happening in the brain when you forget? What changes in neurons are happening? We can't get it by studying humans alone because I can’t open up someone's brain just to explore what's happening when they're forgetting something. It’s unethical. So our best approximation of this is to study forgetting in animals. Our friends the rodents engage active forgetting mechanisms in much the same way that we do and that's the subject of our recent paper with we've shown that to be the case.

I think active forgetting solves a problem that is shared across multiple mammalian species and that's the problem of finding the memories we need. Our brains are capable of storing massive amounts of information.

Katie - It's a bit like a search engine right?

Michael - It's like a search engine, it’s the problem of finding the information that we need when we need it. Not so much in storing it. And so if you have too many alternatives that you have to search through, you have to solve that problem of selection. I need to retrieve this thing, not these 10 other things. And that's the problem that rats confront and that we confront. We think there is a common solution which is  to suppress distracting memories and render them less accessible.

Katie - So it's kind of like search engine optimization for the brain?

Michael - I hadn't thought about it that way, but sure, why not!


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