A woman with a rare condition found herself laughing uncontrollably at the sight of other people being tickled.
The University of California recently released their findings into the woman, known only as TC. She complained of a bizarre phenomenon: whenever she saw someone else being tickled, she could feel the sensation on her own skin.
After a series of experiments, researchers concluded she suffered from mirror-touch synaesthesia, an uncommon condition believed to be caused by the brain's mirror neurons being unable to differentiate between observing something and physically feeling it.
The neurons activate when we see someone else being touched, but the brain tends to block the response in order to separate ourselves from others.
For between 1.6 percent and 2.5 percent of people, those blocking signals are weaker, causing them to feel the same sensation they witness someone else having.
When TC watched others being tickled, she would laugh hysterically and rub her own armpits in an attempt to stop the feeling.
Researchers tested TC's reactions to watching tickling in various circumstances. She seemed to feel the sensation most strongly when she could see the facial expressions of the people being tickled, if she knew the person being tickled or if they looked like her.
She had the strongest reaction of all while watching videos of herself being tickled.
They also observed her reactions to jokes and funny videos, concluding that she had a 'normal' sense of humour and ruling out the possibility that she simply found other people being tickled hilarious.
When TC watched other people touch different surfaces and materials such as velvet and silk, she reported being able to feel the same textures on her own hands.
Watching others lower their hands into icy water, she said she could feel the wetness but not the cold.
Researchers say that by observing those with mirror-touch synaesthesia, they can learn more about how humans feel empathy toward others.