Three
women with an extremely rare type of brain damage had never felt fear
in their adult lives. Snakes and scary movies didn't do it for them. In
fact, they couldn't even recognize a fearful expression on someone
else's face. But when given a hit of carbon dioxide that made them feel
like they couldn't breathe, the women experienced something surprising
and novel: They were panicked.
The
amygdalae, a pair of almond-shaped structures buried deep inside the
brain, are thought to be the mind's storage center for fears. Damage
across both of these nuggets of gray matter
is
uncommon, but the three women in this case study all suffer from
Urbach-Wiethe disease, which has wasted away this part of their brains.
is
uncommon, but the three women in this case study all suffer from
Urbach-Wiethe disease, which has wasted away this part of their brains.
One of these patients, known only as SM,
had been extensively studied before, and scientists had marveled at her
lack of a response to frightening external stimuli in experiments. The
woman, who is in her 40s, had also been in life-threatening and
traumatic situations outside the lab. She was held up at knife point and
at gun point, and she was nearly killed in an act of domestic violence,
but none of these experiences induced fear.
,
had been extensively studied before, and scientists had marveled at her
lack of a response to frightening external stimuli in experiments. The
woman, who is in her 40s, had also been in life-threatening and
traumatic situations outside the lab. She was held up at knife point and
at gun point, and she was nearly killed in an act of domestic violence,
but none of these experiences induced fear.
One
scary stimulus that the scientists hadn't tested in their experiments
with SM was carbon dioxide. Inhaling the gas, also referred to as CO2,
can make you feel like you're starved for air, and it's been known to
trigger panic attacks, especially in people with panic disorder. For the
new study, a research team led by scientists at the University of Iowa,
tested how SM and a set of twin sisters with Urbach-Wiethe disease
reacted to CO2. [What Really Scares People: Top 10 Phobias]
In
two trials, not only did all three report feeling fear, but they also
all had panic attacks, the researchers said. Meanwhile, just three out
of 12 in a control group of people with no brain damage panicked after
inhaling CO2.
But
if fear had been foreign to the women, how could the scientists know
that's what they were feeling? There apparently were some clear signs
observed in all three.
"First,
all of the patients found the feelings induced by the CO2 to be novel
and described the experience as 'panic,'" the team wrote. "Second, all
of the patients displayed similar behavioral responses to CO2, including
gasping for air, distressed facial expressions and escape behavior (for
example, ripping off the inhalation mask)."
The
researchers were surprised by the results. They said the higher rate of
panic attacks among the Urbach-Wiethe patients suggests that the loss
of amygdala function might actually spur the development of panic
disorder.
The results also indicate that there could be other pathways for fear in the brain that
skirt the amygdalae. While external scary stimuli are processed through
visual and auditory pathways that fire off signals to the amygdalae,
CO2 might trigger a response in another part of the brain, such as the brain stem or insular cortex.
that
skirt the amygdalae. While external scary stimuli are processed through
visual and auditory pathways that fire off signals to the amygdalae,
CO2 might trigger a response in another part of the brain, such as the brain stem or insular cortex.
"Thus,
CO2 may directly activate extra-amygdalar brain structures that
underlie fear and panic," the researchers wrote last week in the journal
Nature Neuroscience.
