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Question: How does a stimulus provoke a fight-or-flight response in the brain? 

Joseph LeDoux: Well so, whenever you encounter some sudden danger out there in the world, the information from that stimulus, let’s say it’s a snake on the path would go into your brain through your visual system, if it’s a visual stimulus. And then will rise through the visual system through the standard pathways. So, every sensory system has this very well organized set of circuits that ultimately leads to a stop in the part of the brain called the thalamus en route to reaching the sensory cortex. So each sensory system has an area in the cortex. The cortex is that wrinkled part of the brain that you see whenever you see a picture of the brain. And that’s where we have our perceptions and thoughts and all of that. 

So, in order to have a visual perception, information has to be transmitted from the eye, from the retina, through the optic nerve, into the visual thalamus, and from the visual thalamus to the visual cortex, where the processing continues and you can ultimately have the perception. So, the visual cortex connects directly with the amygdala, and so that one route by which the information can get in; retina, thalamus, cortex, amygdala. But one of the first things that I discovered when I started studying fear was that that pathway, the usual pathway that we think about for sensory processing was not the way that fear was elicited, or not the only way. 

What we found was that if the cortical pathway was blocked completely; rats could still form a memory about a sound. We were studying sounds and shocks. But we’ve also done it with a visual stimulus. So, what we found was the sound had to go up to the level of the thalamus, but then it didn’t need to go to the auditory cortex or the visual cortex as if it were a visual stimulus. Instead, it made an exit from the sensory system and went directly into the amygdala, below the level of the cortex. That was really important because we generally think that the cortex is required for any kind of conscious experience. So, this is a way that information was being sent through the brain and triggering emotions unconsciously. So, the psychoanalysts love this because it vindicated the idea that you could have this unconscious fear that the cortex has no understanding of. 

So, the idea was elaborated a bit into this concept where it’s possible that the cortical and the thalamic inputs to the amygdala could become dissociated in people for one reason or another such that the stimulated, or triggering amygdala from the thalamus wouldn’t necessarily match those that the cortex is attending to. So, in order, again to consciously attend to the stimulus, you need the cortex. 

So, let’s say we were having lunch one day and there’s a red-and-white checkered table cloth, and we have this argument. And the next day I see somebody coming down the street and I say, I have this gut feeling about this guy, he’s an SOB and I don’t like him. And maybe what’s going on there is that he’s got a red-and-white checkered necktie on. Consciously, I’m saying it’s my gut feeling because I don’t like the way he looks, but what’s happened is that the necktie has triggered the activation of the amygdala through the thalamus, the so-called low road, triggered a fear response In me, which I now consciously interpret as this gut feeling about not liking the guy. But in fact, it’s being triggered by external stimuli that I’m not processing consciously. 

So, this is important because a lot of people have fears and phobias and anxieties about things they don’t understand. They don’t know why they’re afraid or anxious on a particular time. It may be through various kinds of experiences, the low road gets potentiated in a way that it’s activating fears and phobias outside of conscious awareness and that doesn’t make sense in terms of what the conscious brain is looking at in the world, or hearing in the world because they’ve been separately parsed out. 

So, the subcortical pathway... we’ve been able to time all of this very precisely in the rat brain and, in order for a sound to get to the amygdala from the sub cortical pathway takes about 10 or 12 milliseconds. So, take a second and divide into 1,000 parts, and after 12 of those little parts, the amygdala was already getting the sound. Consciously, for you to be consciously aware of the stimulus, it takes 250-300 milliseconds. So, the amygdala is being triggered much, much faster than consciousness is processing. 

So, the brain ticks in milliseconds, the neurons process information on the level of milliseconds, but the mind is processing things on the order of seconds and half-seconds here. So, if you have a fear response that is being triggered very rapidly like that, consciously you’re going to be interpreting what’s going on, but it’s not going to necessarily match what’s really going on. 

Recorded on May 5, 2010
Interviewed by Austin Allen

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