Synaesthesia was first recorded medically by Gustav Fechner in 1812, and since then estimates have put the number of people experiencing the effect at anywhere between 1 in 2,000 and 1 in 23. The difficulty in knowing how many people experience synaesthesia is that like Colizoli, many people do not realise they experience the world any differently to other people. Even defining synaesthesia can be tricky because of the different ways in how people experience it.

To qualify as a synaesthete, the effect needs to be conscious, consistent and automatic. This distinguishes them from hallucinations, says Colizoli, because synaesthetes know that their world of colour or smell is not “real”.

No one is sure what causes synaesthesia, although it does seem to run in families. Colizoli notes that when you find one person who experiences the effect, it’s likely that another family member will report similar symptoms. This is known as developmental synaesthesia, and it differs to injury-related synaesthesia which results from brain damage – such as the man whose stroke left him uncontrollably excited by the James Bond theme tune. Synaesthesia is more common in those on the autistic spectrum, but as a group, synaesthetes do not show a higher prevalence of autism than the general population.

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There’s some tantalising evidence that some aspects of synaesthesia can be learned. In a study carried out by Colizoli and her colleagues at the University of Amsterdam, non-synaesthetic people were given books to read in which the letters e, t, a and s were coloured while the rest of the text was left black. Despite reading the text as normal (and making no concerted effort to remember the colours), the participants began associating those letters with their colour.

Colizoli tested participants by flashing letters of the alphabet and asking them to name the colour the letter was written in. When a letter was written with a different colour to the one it had in the book, it took the participants longer to identify it – a cognitive delay known as the Stroop Effect. So while they weren’t ‘seeing’ colours, they did seem to be beginning to form the basic mental associations of natural synaesthetes.

However, the effect was short-lived. “They forgot the letter-colour pairs after several months,” says Colizoli. “And they did not report experiencing colour upon viewing black letters when reading How Gen Y Is Changing Office Culture In China affichant les plus faibles Piece maple leaves Nous construirons auvegarder ethnique Why Not Getting Job Offers.”