For a woman with profound brain damage, SM seems rather unremarkable. Her IQ tests normal; she speaks like an average person, and her memory and perception show no sign of dysfunction. But the 44-year-old woman does have one very specific, very unusual, and for neuroscientists, a very interesting impairment: she has no amygdala, the part of the brain that's the central switching box for analyzing external threats. SM has no fear.
SM's story received a great deal of attention lately thanks to a paper describing her condition that was published in the journal Current Biology. (Neurophilosopy did a particularly incisive and digestible rundown of the paper's findings.) The authors introduced SM, whose amygdalae were destroyed by a genetic condition called Urbach-Wiethe disease, to a variety of situations that a normal person might well find fear-inducing. They took her to an exotic animal shop where she handled snakes and looked at tarantulas; they took her to a haunted house attraction; and showed her clips of movies like "The Blair Witch Project". In each case, she showed no signs of fear, and reported feeling no anxiety. In fact, while scampering through the haunted house she was so delighted and curious that she scared one of the monsters by trying to poke its mask.
For most of us, fear seems like a negative emotion, one that stresses us out and inhibits us from trying things that might make our life more rewarding. But as the Current Biology paper makes clear, SM's fearlessness has cost her a great deal. On the most obvious level, it has left her vulnerable to all kinds of dangers. She lives in a dangerous part of a big city, and several times she has walked obliviously into potentially violent encounters. One time, she was held up at gunpoint; another time, a drug addict accosted her and held a knife to her throat. Intriguingly, though she did not feel scared during those encounters, she did report feeling angry and upset afterward. Her emotional deficit is quite specific.
But in a sense SM's fearlessness is not the worst part of losing her amygdalae. Because, as it turns out, the amygdala does much more than regulate the fear response. Monkeys who have that part of the brain removed exhibit what's known as Kluver-Bucy syndrome, a condition that's characterized not only by fearlessness but also by docility, hypersexuality, and a tendency to explore the world by touching things with the mouth.
These monkeys share with similarly afflicted humans a striking lack of self-control. In SM's case, one of the most pronounced effects of her brain damage was a lack of social restraint. She often behaved inappropriately, acting excessively friendly and making crude sexual remarks.
Recently, Psychology Today blogger Kelly McGonigal wrote about another woman who also lost the functioning of her amygdala. The 24-year-old, who McGonigal calls Lucy, is an epileptic who suffered such severe temporal-lobe seizures that her doctors decided to remove part of her brain, including one of her two amygdalae. In the wake of the operation, her seizures were dramatically reduced. But then, writes McGonigal:
Five years later, Lucy showed up in the emergency room again. She has suffered another generalized seizure. The attending physicians described her as "lethargic and unresponsive, but medically stable." That's when things get weird. According to the original case report in the Journal of Neurology, Neurosurgery, and Psychiatry, "She was left unattended in an examination room. About 30 minutes later, she was found in an adjacent room performing fellatio on an elderly male cardiac patient."
It turns out Lucy had continued to have seizures after the surgery, usually when she forgot to take her medication. For one to two hours after each seizure, she would engage in a wide variety of unusual sexual behaviors, such as masturbating in public and trying to seduce family members and neighbors. She also lost control around food in the post-seizure period. Her family reported extreme binge eating episodes that disappeared between seizures.
What had happened was that the recurring seizures effectively turned off her sole remaining amygdala, leaving her with no function in that region at all.
Together, the cases of SM and Lucy show that the amygdala, while indeed being a crucial junction in the brain's fear circuitry, also plays a surprisingly crucial role in an even more important brain function: self-control. This has traditionally been seen as falling under the purview of the brain's "higher" cortical regions, especially the prefrontal cortex. And indeed, patients with prefrontal lesions often show inappropriate behavior and hypersexuality just like SM and Lucy. But what's clear from studying these amygdala-less women is that the "lower" parts of the brain -- specifically, the limbic region that generates our emotional responses -- is just as key. When it comes to keeping ourselves in check, it seems, emotion is as powerful as thought.
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