A woman in the US who is incapable of experiencing fear could lead researchers towards a treatment for post-traumatic stress disorder and other anxiety conditions.
The study, by researchers at the University of Iowa, University of California, Los Angeles, and California Institute of Technology in Pasadena, centred on a 44-year-old woman who is missing a part of her brain essential for processing and acting on fear.
The authors believe their research, published today in Current Biology, offers valuable new insights into the connection between the brain and behaviour in situations that would normally evoke fear, as well as the conscious experience of fear.
Lead author Justin Feinstein, a University of Iowa PhD student, says fear is an essential survival mechanism.
"Normally, the amygdala is constantly sorting through all the information coming into our brain through the different senses in order to rapidly detect anything that might impact our survival," he says.
"Once it detects danger, the amygdala orchestrates a rapid full-body response that compels us to stay away from the threat, thereby improving our chances for survival."
Scientists have known for more than thirty years that the amygdala, a tiny almond-shaped part of the brain, plays an important role in multiple aspects of fear processing. Work with rats and monkeys also demonstrated the amygdala was responsible for fear conditioning, fear recognition and fear-related behaviours.
But little was known about the amygdala's role in the conscious experience of fear, as animal subjects aren't able to report on their internal subjective experience.
That's where the patient referred to as SM comes into the picture. She has a rare condition, which has left lesions on both bilateral amygdala, essentially disconnecting the part of her brain that recognises and processes fear.
The researchers say SM performs within the normal range on standardised IQ tests, memory, language, and perception, and her emotional responses only seem to be limited when it comes to fear.
The authors say by studying SM, they can confirm for the first time that the amygdala is also required for triggering the feeling of fear in humans.
Snakes, spiders and scary movies
SM was exposed to a series of situations which the researchers believed would have made most people feel fear, including handling snakes, getting close to spiders and watching scary movies. She was asked at different times to rate her fear on a scale of 0 (no fear at all) to 10 (extreme fear).
She also kept an electronic diary over a three-month period. In all situations which should have induced fear she recorded zero or near zero.
Feinstein says the new findings suggest that methods designed to safely and non-invasively turn off the amygdala might hold promise for those suffering from post-traumatic stress disorder (PTSD).
"By understanding how the brain processes fear through cases like SM, we may one day be able to create treatments that selectively target the brain areas that allow fear to take over our lives," she says.
Professor Fred Westbrook at the University of New South Wales School of Psychology says the Iowa team's research confirms data collected in previous animal-based research and studies on normal human subjects. He says it could have important consequences for neuroscience and psychology.
"Understanding what the amygdala's doing is almost certainly going to be critical for things like anxiety disorders. For example, post traumatic stress disorder," he says.
"A characteristic of PTSD is that whenever you encounter the cues that were present at the disorder, these cues will trigger flashbacks, autonomic responses, panic attacks."
"There's a lot of interest at the moment with regards to understanding the amygdala and the other circuits that it's involved in, in order to get a better understanding of normal fear and pathological forms of fear," he says.
Westbrook says the work by Feinstein and colleagues could also lead to ways of predicting who is vulnerable to anxiety disorders like PTSD.