What’s different about the brains of the minority of us who feel other people’s physical pain?

If a friend sees you suffering and tells you "I feel your pain", it may be more than an expression of empathy. For about a quarter of people, it could be literally true. A recent study, led by Thomas Grice-Jackson at the University of Sussex, found that 27 per cent of participants experienced so-called "mirror pain" – watching someone falling off a bicycle or receiving an injection, for instance, caused them to experience physical pain of their own.

Now in a paper in Frontiers in Human Neuroscience, the same team of researchers has explored the neurological underpinnings of mirror pain. When some people have this experience, they don't just show more activity in the so-called "pain matrix" (the network of brain regions linked to the experience of pain), they also show unusual patterns of neural activity that suggest they struggle to distinguish other people's experience from their own.

The researchers studied 44 participants, 18 men and 26 women, who had completed a survey that involved watching 16 movies depicting people experiencing injections and sports injuries, and reporting whether they experienced any pain themselves, and if so, what it was like.

Twenty-one of the participants had said they didn't experience any pain sensations; 13 others were "Sensory/Localiser responders" – they experienced a discrete pain in the same body locations as the injured people in the videos; and the rest were "Affective/General responders" – they described a more whole-body response. For the last group, watching someone getting hurt may trigger a general "nauseating" kind of pain, for example.

The researchers used fMRI to scan all these participants' brains while they looked at a series of 16 images depicting hands and feet suffering a variety of everyday painful incidents (like fingers being caught in a car door) as well as similar but pain-free images (like a hand closing a car door). After each image, they were asked to rate how much pain they'd experienced, if any.

The researchers expected the Sensory/Localiser types to show more activity in the somatosensory cortices (neural areas involved in processing touch) whereas the Affective/General volunteers would show more activity in affective, motivational brain regions, like the anterior insula. In fact, they didn't find this. When witnessing pain, both types of mirror pain volunteers showed greater activity in all these regions than the non-responders.

However, differences between the mirror-pain subgroups emerged in the way that different parts of their brains communicated with each other. When the Sensory/Localiser group witnessed painful incidents, there was greater communication between the anterior insula and the right temporal-parietal junction (rTPJ) regions of their brains, for example. There is evidence from other studies that the rTPJ is involved in representations of the self and "other". "Our explanation of the Sensory/Localised group is that they systematically fail to attribute shared bodily representations to others," the researchers wrote. There were hints of distinct patterns of neural communication for the other subgroup, but the researchers admitted: "The explanation for the Affective/General group is presently lacking."

The research highlights how there is great variety in our experience of seeing other people in pain. But it's clear we are only just beginning to understand the brain basis of these different ways of empathising.