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Joseph LeDoux is a professor and a member of the Center for Neural Science and Department of Psychology at NYU. His work is focused on the brain mechanisms of emotion[…]

When you have a phobia about something you don’t understand, it could be from an experience you’ve had in the past.

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

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

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

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

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

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

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

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

So,rn the brain ticks in milliseconds, the neurons process information on thern level of milliseconds, but the mind is processing things on the order rnof seconds and half-seconds here. So, if you have a fear response that rnis being triggered very rapidly like that, consciously you’re going to rnbe interpreting what’s going on, but it’s not going to necessarily matchrn what’s really going on. 
Recorded on May 5, 2010
Interviewed by Austin Allen