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For people with synaesthesia, hearing music may make them see a colour or feel a texture. The condition has now been reported for the first time in someone who was born blind, showing that vision isn’t necessary for synaesthesia to develop even though sight is involved in many cases of synaesthesia.

at the University of Trento in Italy and his colleagues studied a 40-year-old man who was born blind and who described his synaesthesia to them.

Bottini found the man by chance. “He was a common friend of a friend,” he says. “He started telling me about this beautiful and very complex synaesthesia that he had in his head.”

The man feels textures in his index fingers in relation to different numbers, months and days of the week. He also associates each number with an imaginary cube precisely located in his 3D mental space.

To learn more about the man’s synaesthesia, the researchers attached 40 squares of textured material to a board and gave him a few minutes to feel them. They asked him to pick out tiles that best matched what certain numbers, months or days of the week feel like to him. For example, the number 3 feels like velvet to him, whereas April feels plastic.

They also carried out the same experiment with 10 people without synaesthesia, who were blindfolded. A month later, the man and these 10 people repeated the test with the materials randomly rearranged on the board to see if the relationship between textures and numbers, months or days remained the same.

For numbers, the two sets of answers given by the blindfolded participants had an average match of about 7 per cent, whereas the man scored 75 per cent.

Previous studies have shown that people with visual synaesthesia score above in similar tests. The researchers didn’t test the synaesthesia involving seeing numbers as cubes in mental space.

The vast majority of synaesthesia cases involve vision in some way, but this finding shows that vision isn’t necessary for the condition to develop, says Bottini. He is still perplexed about why no other people who are blind have reported this condition before. “There should be more people, just statistically,” he says. “Perhaps blindness makes the development of synaesthesia more rare?”

“These interesting findings make it unlikely that vision is necessary for developing synaesthesia, at odds with some previous claims,” says at Birkbeck, University of London.

“Vision may predominate in much of perception and synaesthesia, but not because the brain is processing sights in a wholly different fashion from sounds or touches,” she says. “These additional internally generated cues seen in synaesthesia could even help the blind individual to recall information, effectively ‘imagining’ textures that accompany heard numbers.”