Armed with tarantulas, snakes, and horror-movie clips, Caltech neuroscientists, together with collaborators at the University of Iowa and the University of Southern California (USC), have studied a woman who is unable to experience the emotion of fear.
The work, described in the December 16 online issue of the journal Current Biology, provides the first in-depth investigation of how the experience of fear depends on a specific brain region called the amygdala and offers new insight into our conscious experience of emotions. The amygdalae, which register rapid emotional reactions, are implicated in depression, anxiety, and autism.
The individual, a woman known as SM who has extensive damage to the amygdala on both sides of her brain—rendering the brain regions essentially non-functional—showed no fear when subjected to numerous situations that normally induce the emotion.
The research team measured fear behavior by videotaping the subject as she walked through a haunted house at Halloween, handled large tarantulas and snakes in an exotic pet store, and viewed frightening film clips from horror movies. Detailed questionnaires and interviews were used to explore her experience of fear in response to all these situations, and during the day-to-day events in her real life.
"The complete absence of any fear behavior, or any report of subjective fear experience, was truly remarkable," says Justin Feinstein, a graduate student in clinical psychology at the University of Iowa and first author on the study. "This was especially striking because SM can feel emotions other than fear normally. This was not a subtle impairment, but something that just hit you in the face."
The other members of the research team were Ralph Adolphs, the Bren Professor of Psychology and Neuroscience at Caltech; Antonio Damasio, Dornsife Professor of Neuroscience and Director of the Brain and Creativity Institute at USC; and Daniel Tranel, Professor of Psychology and Neurology at the University of Iowa. The investigators have a long history of collaboration using an expansive registry of brain lesion patients at the University of Iowa.
According to Adolphs, the findings are especially valuable in light of the large amount of research on the amygdala in animals showing that the brain region is important for fear behaviors and fear learning. "One thing you simply cannot measure definitively in animals is their conscious experience of fear," Adolphs argues. "This study really fills an important open question; we had all assumed that the amygdala would be important for many aspects of fear processing, but demonstrating it also plays a role in feeling fear was not trivial to do."
In addition, Tranel says, "these findings will be of special importance to understanding psychiatric illnesses in which the amygdala is thought to be dysfunctional, such as depression, PTSD, and phobias."
