In the first article he describes how the brain creates "flashbulb memories," which he defines as a neuronal event more than a brain event:
As information go through a brain system, if that information is significant, it will form a lasting trace as a result of the release of the transmitter. The other things that are going on at that same time will form a trace, a connection between the active neurons and that will stay in the brain in the form of what we then experience as memory later.In the second article he gives a brief overview of how his lab has learned to disrupt memory consolidation by injecting a protein synthesis inhibitor directly into the amygdala of a rat. This provides a theoretical pathway for preventing traumatic events from becoming PTSD.
by Joseph LeDoux
December 24, 2013
Whenever we have a memory about some experience, it turns out that there are probably a lot of different systems in the brain that are being activated. Sometimes, as scientists, we talk about memory systems. I think that’s a misnomer because if you think about what memory is, it’s really plasticity in the nervous system. It’s the ability of neurons in the brain to change and neurons in every part of the brain that we’ve looked at have this capability of changing or to become plastic when their experiences change.
From the point of view of the neuron, an experience is the arrival of neurotransmitters being released by another neuron. So, what that does is it changes the way that neuron responds. Across many such events like that in the brain, a memory is formed, or multiple memories are formed. So, it’s really inappropriate to talk about memory systems because almost every system in the brain forms memories.
The way I like to think about it is memory is a feature of neurons rather than a function of brain systems. As information go through a brain system, if that information is significant, it will form a lasting trace as a result of the release of the transmitter. The other things that are going on at that same time will form a trace, a connection between the active neurons and that will stay in the brain in the form of what we then experience as memory later.
Let’s take a situation where you’re driving down the road and you have an accident. You hit your head on the steering wheel and you hit it really hard and the horn gets stuck on and so you hear this loud and annoying noise while you’re bleeding, in pain. It’s really awful and terrible. And then a few days later, you hear the sound of a horn. That sound will go to various parts of your brain simultaneously. When it goes to a part of the brain called the hippocampus, it will remind you of the situation that you’re in, that you were driving, that you had an accident, that you were with John and Peg. But it won’t have the emotional impact unless it also goes to a different part of the brain called the amygdala, which instead of reminding you of the details of the event, will trigger emotional responses in your brain and body. The responses in the body will feed back to the brain, and all of that activity in the brain will give rise to what we call the emotion.
So there are really two different memories, one cognitive and one emotional. They are stored and represented in different brain systems. Again, it’s not that that memory is a function of those systems, but is a feature of the neurons in those systems that allow the system to do its job better.
So, let’s take the case of say, the visual system. It allows us to see the world -- the auditory system allows us to hear the world, the motor system allows us to move in the world. All of these systems are plastic, so plasticity is a feature that allows the visual system to remember what you saw last time so you can see it a little better the next time or the motor system to perform a response better the next time because you’ve done it the previous time. So, again, we want to think of plasticity as a feature of neurons rather than a function that a system is performing.
So, back to the fear example: the sound of the horn goes to one part of the brain, the amygdala, and gives rise to the emotional response into the other part of the brain, the hippocampus, and gives rise to cognitive representation. So, we call this the hippocampal memory, a memory about the emotion, whereas the amygdala memory we call the emotional memory itself. Now these two things happen simultaneously. The amygdale memory is triggered unconsciously. It doesn’t have to be aware of the stimulus in order for that to be triggered. Hippocampal memory is probably triggered unconsciously as well, but you become aware of the memory when it’s triggered because that’s what a hippocampal memory does, it creates a representation of the conscious experience.
But that conscious representation now is going to be amplified by the emotional arousal that is taking place. It’s going to create a new emotional memory, or new memory about emotion that’s going to have that kind of emotional stamp on it. It’s the interaction between cognitive systems and emotion systems in the brain that create what’s called flashbulb memories, which are very vivid strong memories of a particular experience. So, everyone in New York in September 11, 2001, knows what they were doing just as every person my age in November of 1963 remembers the assassination of John Kennedy.
It used to be thought that these flashbulb memories were more accurate than other memories. But new research has shown that these flashbulb memories are not more accurate, they’re just more vivid. So the accuracy is kind of suspect and one of the consequences, or one of the implications of that is that, memories are constructed, or reconstructed when they’re retrieved. And at that point of retrieval, the memory has the opportunity to be changed. And that’s one of the main topics we’ve been working on lately.
In Their Own Words is recorded in Big Think's studio.
Image courtesy of Shutterstock
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by Joseph LeDoux
December 26, 2013
A good example of the way memory changes during the retrieval process is a situation where someone goes to court to testify about a crime that they witnessed. On the day of the crime they gave their summary of what happened to the police, so there’s a police record. And then when they go into the court, they talk about something in a completely different way, which it turns out happens to match what they read about in the newspaper.
When they read the newspaper, what they did was updated their memory about the experience. Then in the future, when you retrieve that memory, it’s hard to distinguish what actually happened and what you’ve incorporated since then through other kinds of experiences.
This is now a well-known phenomenon. Each time a memory is taken out, the opportunity is there for it to be changed. This is an updating process and normally it’s a useful thing. If you meet somebody at a party and he’s a nice guy, but then you find out he’s an axe murderer or something like that, you have to immediately change your memory of that person, so you’ve updated it. But there are other ways the memory gets updated as well. In a lab we conditioned a rat to be afraid of a tone. So, the next day, the rat hears the tone and he freezes, because that’s how rats express their fear of the stimulus. But immediately after presenting that tone, we gave the rat a certain kind of drug - which I’ll explain below - and we tested the rat the next day, and the memory was no longer present, or at least can’t be accessed. So, what’s going on?
It’s been known for a long time that memory formation or memory consolidation requires the synthesis of new proteins in the part of the brain that are forming the memory. Researchers discovered that if you block protein synthesis after retrieval you can also disrupt the stability of the memory later. But that idea got lost in the late ’60’s and didn’t stick around. What stuck around was the idea that memories are consolidated and once they are consolidated, each time that you take it out, you take out that same trace over and over again.
The new research that my lab helped rejuvenate in the year 2000 was about manipulating the memory after the rat experiences the retrieval process. So, we gave the rat the tone, and then we blocked protein synthesis after retrieval, rather than after learning. And when we do that the memory is eliminated just as well after retrieval as it’s prevented from being acquired.
So, the unique feature of our experiment was, we were able to do this in the side of the brain where the memory is being formed and stored, which in the case of fear, memory is the amygdala. So, because we did all of the basic work of figuring out all of those circuits, we could go in and put a tiny amount of a protein synthesis inhibitor in the amygdala. And that’s important because you can also do this experiment by giving the protein synthesis inhibitor systemically to the whole body, like if you take a pill, that goes into your body and reaches your brain and does all the stuff, but it’s going everywhere and that’s why many drugs have side effects. So, if you take an anti-anxiety drug, it not only relieves anxiety but it would make you sleepy, it might alter your sex drive, etc.
What we’re doing here is avoiding one of the bad consequences of protein synthesis inhibitors which is that it makes you nauseous and sick and so forth if you take it systemically. And it is pretty toxic, so you would never give a drug like that to a human. This is only something you can do in an animal experiment. So, the protein synthesis inhibitor in our studies was put directly in the amygdala and we avoid all of those side effects and negative consequences since it’s a tiny amount and it doesn’t affect the rest of the body.
When we do that, the rats the next day don’t freeze to the tone. They don’t remember that the tone is now dangerous. So this has triggered a whole wave of research now on the possibility of using this as a treatment for Post Traumatic Stress Disorder because, theoretically we can have the people come in, remember their trauma, give them a pill and the next time the cues about the trauma come along, they won’t have the emotional response to it.
In Their Own Words is recorded in Big Think's studio.
Image courtesy of Shutterstock