Can you feel your heart beating softly against your breastbone? Or perhaps you feel hungry, thirsty or in pain? If so, you are perceiving your internal state -- a process called interoception. It's thought that to generate emotions, we first need to interpret our body's internal state of affairs.
So if we see a rabid dog, we only feel fear once we recognise an increase in our heart rate or perceive a sweaty palm. Some people with conditions that involve having poor interoceptive abilities also have trouble interpreting their emotions.
But researchers have also speculated that interoception is important for understanding what other people are thinking, and even guessing what they think a third person might be thinking -- known as theory of mind. The idea is that if we have trouble distinguishing our own emotions, we might also find it hard to interpret the emotions -- and corresponding mental states of others.
To investigate in their study, Geoff Bird, now at the University of Oxford, and his team asked 72 volunteers to count their heartbeats, but without using their fingers to take their pulse -- a measure of interoception.
The participants then watched videos of various social interactions. After each clip, they were asked multiple-choice questions that tested their ability to infer the characters' mental states.
For instance, one scene showed a man called Tom trying to flirt with a girl called Gemma, who was clearly interested in a second, shyer man, Barry.
Some questions required the participants to understand the emotions of a certain character -- for instance, "Is Gemma feeling annoyed?" Participants who were better at counting their own heartbeat performed better on such questions. "They were more empathetic," says Bird.
But there was no link between interoceptive abilities and accuracy on theory of mind questions that didn't involve any emotions, such as "What does Barry think Gemma thinks Tom's intentions are?" This suggests that our ability to interpret signals from our own body only helps us understand the thoughts of others when emotion is a factor.
"Studies like these show nicely that interoceptive abilities are engaged in different ways for different tasks," says Anil Seth at the University of Sussex in Brighton. "But these relations are likely to be highly complex, so it would be interesting to look also at other dimensions of interoception, like breathing."
Bird says that interoceptive difficulties probably play a role in a range of symptoms experienced by some people with conditions such as autism and schizophrenia. For instance, some people with autism find loud noises and bright lights upsetting. These things are linked to interoception, making our hearts beat faster and raising our level of arousal.
"It's purely theoretical for now," says Bird, "but if you're not good at distinguishing the internal signals that arise from loud noises and bright lights from others that are related to pain, say, then maybe those [innocuous] signals could be interpreted as painful."
"It's not yet been shown whether training your interoception also improves your empathy, but it's an experiment we'd like to try," adds Bird. One way to do this is to get people to listen to a tone that beats in time with their heart and gets quieter over time. There's also some evidence that looking in a mirror can improve interoception.
We don't know yet what effect such training might have on our ability to discriminate between our own emotions and those of other people. "Could training better interoceptive awareness make it more difficult for people to disentangle their own feelings from those of others?" asks Lara Maister at Royal Holloway, University of London.