Sweaty palms, fast heartbeat, yes, we’re scared Sean Murphy/Getty

Is fear all in the mind? Experiments on twins who can’t feel fear are suggesting that some emotions are experienced only after we become aware of changes to our body.

Many studies have shown that the amygdalae – two almond-shaped regions near the centre of the brain – are crucial for feeling fear. People who have lost their amygdalae through brain injury or disease also lose the ability to feel fear.

In 2013, Justin Feinstein at the University of Iowa in Iowa City and his colleagues managed to scare three “fearless” people – two female identical twins and a woman known as S.M., none of whom have amygdalae – by getting them to inhale carbon dioxide, making them choke. This was the first time that S.M. had experienced fear since she was a child. It showed that the amygdalae are not essential for all kinds of fear.


Spider logic

One theory of emotion suggests that feelings are generated not by the brain directly, but through an awareness of our body. When we see a spider, for example, the theory says we don’t feel fear because emotional centres of the brain are activated and trigger the perception of fear, instead it is because the brain evaluates the situation and releases hormones that increase our heart rate and give us sweaty palms. It is our awareness of these changes to our body that we interpret as fear.

Now Feinstein’s colleague Sahib Khalsa, then at the University of California, Los Angeles, and his team have given the fearless twins a drug that mimics the action of adrenaline. High levels of this drug can cause temporary breathing difficulties and rapid heart palpitations. The team asked the twins and a group of volunteers with intact amygdalae to rate their experience of these physical changes on a scale from zero to 10.

The bodies of both twins reacted in the same way as the other volunteers – their heart and breathing rates increased, but only one twin said she was aware of both these changes – rating her breathing difficulties as 10 out of 10 on the scale. She also reported a choking sensation and had a panic attack.

All the volunteers also noted breathing difficulties and a quarter had panic attacks. The other twin noted a slight awareness of her heart rate changing, but she rated her physical symptoms close to zero and did not panic.

Panic circuits

The results indicate that there are mechanisms beyond the amygdalae that are involved in fear and anxiety and support the theory that an awareness of the physiological state of one’s body may be necessary to experience some kinds of emotion.

However, the fact that the twins – who both have damaged amygdalae because of a rare genetic condition – had different responses suggests there may be environmental factors at play, too.

“This seems to be more evidence that extreme panic has its own underlying circuitry that is different from normal fear,” says Nick Medford, who studies consciousness and emotion at the Sackler Centre for Consciousness Science, in Brighton, UK.

Further experiments involving scanning the twins’ brains while they experience fear would help us identify new mechanisms of emotion in the brain, says Khalsa.

Such studies could also lead to new targets for the treatment of anxiety disorders. There is some evidence to suggest that people who have abnormal awareness of their bodily functions can be more prone to anxiety and depression.

“If you could pinpoint neural regions responsible for our awareness of our body state and the different mechanisms that generate fearful feelings, you might have new targets to modulate or reduce anxiety,” says Khalsa.

Journal reference: The Journal of Neuroscience, DOI: 10.1523/jneurosci.4109-15.2016