Posted on behalf of Tiffany O’Callaghan
It’s not everyday that researchers try to scare their study subjects in the name of science. Yet in attempting to determine how a destroyed amygdala impacts the experience of fear, University of Iowa neuropsychologist Justin Feinstein and colleagues pulled out all of the stops — from scary movies to a haunted house, spiders and snakes.
They found that even in the face of all of these scary stimuli, a woman whose amygdala was destroyed by a rare condition known as Urbach-Wiethe disease felt no fear. What’s more, when reviewing her personal history, they noted that, despite experiencing many horrific events in her lifetime — including having a knife held against her throat and nearly losing her life to domestic violence — she showed no signs of lingering fear or trauma related to these events. Her last self-reported experience of fear was at age 10, they note, likely prior to when the disease destroyed her amygdala.
The role of the amygdala, the almond-shaped structure located deep in the medial temporal lobe of the brain, has long been linked to fear in both humans and animals. Several studies have shown that rats or monkeys with damaged or destroyed amygdala don’t exhibit the same response to potentially threatening stimuli as their peers, and research in humans with damaged amygdala finds a similar trend in behavior. In addition to behavioral response, researchers have found that humans with amygdala damage also tend to report less subjective experience of fear.
The case study of SM, as the woman is identified, appears in Current Biology and is an extension of these findings: the damage to her amygdala is not associated with merely a decrease in the experience of fear, but the absence of fear altogether. But when Feinstein and colleagues used film clips to induce anger, sadness, happiness, disgust or surprise, they found that SM showed a wide range of emotion, suggesting that the crippling effect of the amygdala damage is limited to her experience of fear.
Not only did SM not experience fear in the real-world scenarios — walking through a haunted house, being exposed to snakes and spiders — or in the lab setting when watching scary movies, instead her reaction to scary stimuli was a notable and even overwhelming curiosity. At the haunted house, for example, while others shrieked with fright when ghouls jumped out from behind corners, SM laughed and struck up conversations. And in spite of declaring that she hated snakes and spiders, when the researchers took her to an exotic animal shop, she showed no reluctance in picking up a snake, and actually had to be prevented from poking a dangerous tarantula.
While these scenarios were all designed to inspire fear without any harmful consequences, in the broader world SM’s immunity to fear increases her exposure to danger. Feinstein and colleagues conclude:
“The unique case of patient SM provides a rare glimpse into the adverse consequences of living life without the amygdala. For SM, the consequences have been severe. Her behavior, time and time again, leads her back to the very situations she should be avoiding, highlighting the indispensable role that the amygdala plays in promoting survival by compelling the organism away from danger. Indeed, it appears that without the amygdala, the evolutionary value of fear is lost.”