A woman – known only as TC – bursts into uncontrollable laughter when she watches someone being tickled. And no, she is not laughing at them as they squirm around in discomfort but because she has an unusual condition called mirror-touch synaesthesia.
This means she can watch a person being touched (or tickled) and feel the exact same sensation on her own body. In this case, she might not be the one being tickled but she feels as though she is.
Researchers from the University of California, San Diego, conducted a series of experiments on TC to explore this bizarre phenomenon further – for example, watching her reaction to seeing other volunteers touch different surfaces, douse their hand in ice cold water, and be tickled. To make sure her response to the tickling was a genuine example of mirror-touch synaesthesia and not because she found the situation funny, the team also tested her reactions to jokes and funny video clips. The results have been published in the journal Neurocase.
When TC watched other people being tickled, she reacted as if she was experiencing it firsthand. That is with intense laughter. She even attempted to neutralize the sensation by rubbing her own armpit. The researchers point out her reaction was not as extreme or as frequent when she was shown the video clips or told a joke.
The reaction to second-hand tickling was even more acute when the volunteer was someone she knew well or who looked like her – or when she could watch their facial expressions. The most intense responses occurred when she watched a video clip of herself being tickled.
It wasn't just tickling, however. TC reported sensations of touching velvet, brushes, silk, and table surfaces just from seeing someone else touch different surfaces. When she saw a volunteer dunk their hand in icy water, she sensed the wetness but not the temperature.
So, what is going on here?
It comes down to a group of cells in the brain called mirror neurons. These are activated whenever we or someone we see is being touched. But, for most of us, signals from other parts of the brain "block" the response when it is someone other than ourselves, helping us separate the "self" from other people. For people like TC with mirror-touch synaesthesia, however, these signals are weaker. Hence, they feel others' sensations as if they were their own.
The researchers believe this could help us understand how empathy works. As Claudia Sellers, one of the researchers involved in the study, told New Scientist, “We all sit on a spectrum of empathy,” – TC's story is just an extreme example.