New Scientist: "Why I and O are dull for synaesthetes"

Why I and O are dull for synaesthetes: the researcher 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 or learning to count."

I did wonder!