What Makes You Feel Fear?
by SARAH ZIELINSKI
February 04, 2013 2:08 PM
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Movies like The Shining frighten most of us, but some brain-damaged people feel no fear when they
watch a scary film. However, an unseen threat — air with a high level of carbon dioxide — produces
a surprising result.
Warner Bros./Photofest
In shorthand often used to describe the brain, fear is controlled by a small, almondshaped structure called the amygdala.
But it's not quite that simple, as a study published Sunday in Nature
Neurosciencedemonstrates.
One of the best ways to figure out how parts of the brain work is to study people who
have damage in those specific areas. Take a patient known as SM, a middle-aged
woman with a rare condition called Urbach-Wiethe disease that has damaged the
amygdala on both sides of her brain.
A few years ago, SM told Justin Feinstein, then a graduate student at the University of
Iowa in Iowa City, that she'd never felt fear, even when she'd been confronted by a knifewielding assailant.
Feinstein put her claim to the test. His group had SM handle a snake, took her on a tour
of a haunted house and showed her scenes from scary movies like The Shining. SM
was unfazed.
But another University of Iowa researcher, neuroscientist John Wemmie, had a
different test for SM: breathing carbon dioxide. In people without brain damage, exposure
to high levels of carbon dioxide evokes fear and a hunger for air. In some, it can even set
off a panic attack.
"The body is not so good at detecting low levels of oxygen but is really good at detecting
high levels of carbon dioxide," Wemmie says.
And so Feinstein, Wemmie and their colleagues administered a new test to SM, along
with a pair of twin sisters with similar brain lesions from the same rare disease. All three
volunteers were exposed to high concentrations of the gas.
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In these brain scans, amygdala damage can be seen in three patients (known as SM, AM and BG)
with Urbach-Wiethe disease. See the dark spots within the areas circled in red. A healthy person is
shown (left) for comparison.
Corey Feinstein/Iowa Neurological Patient Registry, University of Iowa, Courtesy of Nature
The researchers thought that the patients would be insensitive to the test. "We were
pretty shocked when exactly the opposite happened," Wemmie says.
A few seconds after inhaling the gas, SM started to wave her hand towards the mask
covering her face, crying "help me!" to a nearby experimenter. After the mask was
removed, SM's skin was flushed, her nostrils flared and her eyes were wide open.
When asked about the emotions the experience caused, SM replied: "Panic mostly,
'cause I didn't know what the hell was going on."
The other two patients had similar panic attacks, saying they felt like they were
suffocating. One even named her feeling as "fear."
"All the patients found the experience highly aversive, and none of them found it
pleasurable in a way that they'd want to do it again," says Feinstein, now a postdoctoral
researcher at the California Institute of Technology.
"This is interesting because it shows that the loss of the amygdala does not prevent
[these patients] from experiencing fear altogether," says Caltech neurobiologist and
Howard Hughes Medical Institute investigator David Anderson, who wasn't involved in
the study.
However, he notes, other studies have found that some patients with a damaged
amygdala are capable of being afraid. "There is more to fear than just the amygdala," he
says.
Feinstein thinks that the fear induced internally by carbon dioxide may be processed
differently in the brain than the fear caused by an outside source like a scary movie. It
may be that learned fears and innate ones are handled differently.
"We're still learning a lot about how the brain processes fear," he says, "and what this
finding suggests is we still have a long way to go."