The amygdala, fear, and carbon dioxide

12 July 2013 by Pete Etchells, posted in Brain stuff

Like many other disappointed bloggers, I didn't get shortlisted for the Wellcome Trust Science Writing Prize this year. So here's the piece I wrote, for your viewing pleasure. The shortlisted entries are always brilliant, and I'm really looking forward to reading this year's crop. If you've been shortlisted, best of luck to you!


Just imagine them in their pants. I know it’s supposed to help you get over your nerves, but it doesn’t really help when the idea just keeps going around in your head, causing a dread feeling of panic moments before you give a talk. It especially didn’t help me when I could see my PhD supervisor in the audience.

Anxiety is a double-edged sword. In some situations, it can be incredibly useful. In a landmark paper in 1994, Isaac Marks and Randy Nesse argued that anxiety is an emotion that has been moulded by evolution; when you’re in a potentially dangerous situation, it’s perfectly reasonable to feel ‘on edge’, because it makes you more aware of your surroundings and more able to deal with any threats. All well and good when that threat was something giant, scary and hungry. But nowadays, what we see as threats tend to be much more benign.

Say we get an unexpected bill through the post. We might feel scared or worried about it, and anxiety, our natural defence against threat, kicks in. The trouble is, it comes in the form of a ‘fight or flight response’; adrenaline is released into our system, which increases our heart rate, makes us sweat, breathe more rapidly, and feel nervous. These are all useful actions if we need to run away or fight something, but they’re not very helpful when we have to deal with that unexpected bill – running away after ringing the billing company generally doesn’t help matters (neither does fighting the phone). This forms the basis of anxiety disorders – when our anxiety response kicks in too often, in inappropriate situations, it becomes very difficult to function.

Psychologists are increasingly trying to understand what happens to our bodies and our behaviour when we become anxious, so that we might better understand how and why things go wrong in anxiety disorders. An ideal experiment would be to grab a load of participants, divide them into two groups that have similar characteristics, and run some sort of study while one of the groups is anxious, and the other group isn’t, to see what differs between them. The trouble is, it’s quite hard to make people anxious – some studies get participants to give a talk in front of a crowd, while others have tried setting people really hard anagram puzzles so that they deliberately fail. Ingenious ideas, but they come with problems. Not everyone gets nervous talking in front of people, and some people don’t really care about puzzles. In both cases, chances are that people will be anxious during the tasks, and you’re only able to test them in your experiment afterwards. What you need, ideally, is for people to be anxious during testing.

Over the past few years, researchers have developed a technique that deals with this problem – all it needs is a little tinkering with what you’re breathing. Normal air is about 78% nitrogen, 21% oxygen, 0.03% carbon dioxide, plus various tiny amounts of other gases. However, in the mid-2000s Jayne Bailey and colleagues at the University of Bristol found that if people inhaled air in which some of the nitrogen was replaced with carbon dioxide, they became more anxious. That is, they showed an increase in heart rate and blood pressure, along with increases in fear and tension. The advantage of this is that you’re not suffocating the participants, as the oxygen content remains the same, but upping the carbon dioxide concentration to 7.5% results in a state of anxiety that can be sustained up to about 20 minutes, with rapid recovery afterwards. More than enough time to run an experiment.

Why does this happen? It turns out that it’s all the fault of an area of the brain called the amygdala, which plays an important role in generating fearful behaviours. However, the amygdala also acts as a carbon dioxide monitor – it watches the acidity level of the bloodstream. If you breathe in more carbon dioxide, the pH level of your blood drops (becoming more acidic). The amygdala is sensitive to this, and because more carbon dioxide might be a result of suffocation, it kick-starts a behavioural fight or flight response to try and resolve that situation – in other words, it makes you anxious.

So by taking advantage of this little quirk of the brain, it’s much easier for psychologists to run experiments that help us to understand how we operate when we’re anxious, which in turn might help us in understanding how things go wrong in anxiety disorders. So in a few years, we might be able to help people overcome their fears of otherwise normal situations, like talking to a big audience. Until then though, we’ll just have to carry on imagining the crowd in their pants.

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