From the latest New Scientist

Synaesthesia is a baffling phenomenon. Some synaesthetes experience colours when they read words, others hear sounds when smelling certain odours. But little is known beyond the notion that it involves a mixing of the senses.

Now, the first big study of synaesthesia is starting to change that. David Eagleman of Baylor College of Medicine in Texas has looked at more than 1000 synaesthetes who see colours when viewing some letters and numbers. He found that frequently used letters are most likely to evoke colours, while letters such as Q and X are less likely to do so.

However, Eagleman spotted two frequently used letters that bucked this trend: I and O. He also noticed that the numbers 1 and 0 are often not coloured. Eagleman thinks this may be because these characters are made up from natural shapes that we learn to recognise before mastering the alphabet.

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