She is just one of many people who suffer from a range of disorders that give rise to "extreme empathy". Some of these people, like Barrett, empathise so strongly with others that they experience the same physical feelings - whether it's the tickle of a feather or the cut of a knife. Others, who suffer from a disorder known as echopraxia, just can't help immediately imitating the actions of others, even in inappropriate situations.
Far from being mere curiosities, understanding these conditions could have many pay-offs for neuroscience, such as illuminating conditions like phantom pain. They may even help answer the age-old question of whether empathy really is linked to compassion.
There is a general consensus that empathy-linked conditions arise from abnormalities in the common mechanisms for empathy found in all humans: although few of us experience sensations as powerful as Barrett's, we all wince at a brutal foul on the football field and feel compassion for someone experiencing grief. Many studies have suggested that our capacity for empathy arises from a specific group of neurons, labelled mirror neurons. First discovered in macaque monkeys, they are situated in and around the premotor cortex and parietal lobe - regions that span the top of the brain near the middle of the head. These neurons fire both when you perform an action and when you see someone else perform that action.
Although the challenges inherent in placing electrodes in people's brains have so far made it difficult to prove convincingly that individual neurons also act like this in humans, fMRI scans have supported the idea that certain populations of neurons do seem to behave in this mirroring fashion.