Freeze or Flee: How Our Brains Control the Fear Response
by Rosemarie Foster, freelance science writer and vice president
of Foster Medical Communications, NY
Posted June 7, 2002
How many of us can't gaze at a commercial airliner soaring high overhead
without remembering that fateful day in downtown Manhattan last fall?
The feelings of fear, insecurity, and sadness that we felt on September
11th may come rushing back, triggered merely by the site of a plane
in flight whose destination we do not know. Why do we respond this
way? And what is going on in our brains to make such emotions suddenly
resurface?
Joseph LeDoux and his research team are trying to answer just those
sorts of questions. As Director of the Center for Neural Science at
New York University, LeDoux has pioneered the study of emotions on
the biological level, deep within the recesses of the brain. The rat
brain, to be exact.
"How do our brains make us who we are?" asked LeDoux at
a talk he gave at the Academy on April 8th. This year, LeDoux's latest
book, Synaptic Self: How Our Brains Become Who We Are, was published
-- joining his previous work, The Emotional Brain: The Mysterious
Underpinnings of Emotional Life, published in 1996.
LeDoux described his two decades of investigation of the biology
of fear, and the responses of the laboratory rats that helped him
draw some very provocative conclusions. His findings are relevant
to the management of anxiety disorders, which account for about half
of the mental health problems reported in the U.S. and which can result
from malfunctions in the way we deal with fear.
LeDoux began by giving a primer on the wiring of the brain, and the
importance of neurotransmitter exchange between its billions of neurons
across gaps called synapses. Such synaptic wiring regulates all brain
functions, such as perception, emotion, motivation, thinking, and
memory. "But the trick is to understand how we as people can
emerge out of all of this," said LeDoux, whose group is focusing
on the study of traumatic memories and the physiological responses
they can incite.
Who we become and what personalities we develop are a combination
of nature -- the influence of genes -- and nurture, the experiences
we encounter throughout our lives. "The relationship between
genes and personality is not a simple one, but does contribute,"
asserted LeDoux. "But experiences are also very important."
Specifically, our experiences help us to learn, through an intricate
system of memory processing employed by our brains.
The brain does not process all of our memories the same way. Rather,
LeDoux explained, there are multiple memory systems, each devoted
to different kinds of functions. For traumatic memories, two systems
interact: one conscious, one unconscious. For example, if you were
in a car accident and you returned to the accident scene, you might
remember what happened, who was with you at the time, and other objective
details of the event -- so-called conscious memories. But your blood
pressure and heart rate might escalate, you could begin to sweat,
and your muscles might tense -- all unconscious memories that cause
your body to respond in a particular way as a result of the past experience.
Moreover, neuroanatomists have learned that these memory systems
are mediated by two structures residing in the brain's temporal lobe:
the hippocampus, which regulates conscious memories, and the amygdala,
an almond-shaped area of tissue controlling unconscious memories.
LeDoux and his colleagues have focused their inquiry on the latter
structure, which he calls "the emotional processing system of
fear."
According to LeDoux, nature installed an amygdala into most organisms
as a survival mechanism. Early on, he noted, evolution hit upon a
way of wiring the brain to produce responses likely to keep the organism
alive in dangerous situations. The solution was so effective that
it has not been altered much over centuries, and works essentially
the same way in rats and in people -- as well as many, if not all
other vertebrate animals.
"Evolution seems to have gone with an 'if it ain't broke, don't
fix it' rule when it comes to the fear system of the brain,"
said LeDoux. "The things that make rats and people afraid are
very different, but the way the brain deals with danger appears to
be similar. We can, as a result, learn quite a lot about how emotional
situations are detected and responded to by the human brain through
studies of other animals."
The LeDoux lab conducts "fear conditioning" in rats to
study the function of the amygdala, its connections with other parts
of the brain -- such as the cortex, which is responsible for thought
-- and what happens in the brain when the amygdala is damaged. At
the heart of their studies is a tone-shock system: They condition
a rat by sounding a tone and delivering a minor shock, and they measure
the rat's physiological responses.
Thereafter, whenever the rat hears the tone, it may freeze in its
tracks or respond with an increase in blood pressure and heart rate,
even when no shock is delivered. Just the anticipation of a shock
is enough to trigger a physical reaction in the animal. LeDoux's group
is now altering synapses in the rats' brains -- particularly those
in the lateral nucleus of the amygdala, the gateway into the system
-- and studying the resulting responses.
But the amygdala doesn't work alone. Through a series of complex
connections between it and other parts of the brain -- some links
stronger than others -- we learn how to respond to fearful situations.
One key interaction in this system is the one that exists between
the amygdala, the thalamus, and the sensory cortex.
When we see or hear something frightening, such as an unanticipated
loud noise, we may freeze, jump, or turn to see what caused it. That
reaction can be traced to the connection between the thalamus and
the amygdala, between which signals travel quite quickly but not so
precisely. LeDoux called this route the "low road." But
the same signal is processed differently between the thalamus and
the sensory cortex -- at a speed several milliseconds slower than
the low road, but in a way that allows us to more perceptively assess
the situation. LeDoux termed this route the "high road."
Our responses to fearful stimuli are a combination of both the low
and high roads, noted LeDoux. The low road is nature's self-defense
mechanism, making us stop in our tracks or jump out of the way when
danger is near -- it is a passive reaction. The high road then allows
us to determine what's going on, and how we can come to grips with
the situation -- that is, to take action.
But it's possible that in some people with persistent fear and anxiety,
the low road may be stronger than the high road. The result: a rapid
firing of signals to the amygdala, but an impaired ability to figure
out what is causing the reaction and how to deal with it. Indeed,
all of the modern psychotropic drugs used to treat anxiety and fear
(the serotonin reuptake inhibitors) work by inhibiting cells in the
amygdala.
"The fear system is one of the many implicit systems in the
brain," concluded LeDoux. "But it is not a memory system.
Learning and memory are features of such implicit systems as the fear
system." The results of his investigations may shed significant
light on the physiology of anxiety disorders, and lead to better means
of managing them.
Fear is just one emotion that contributes to the total being we call
self. To address the connection between the brain and other aspects
of the self, the New York Academy of Sciences is hosting a conference
called The Self: From Soul to Brain, September 26-28, 2002, at the
Academy's headquarters in New York City. The meeting will be organized
and chaired by Joseph LeDoux, New York University, Author, Synaptic
Self: How Our Brains Become Who We Are, 2002, Viking.
