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
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.
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.
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.
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.
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