For the first time, psychologists have documented the prevalence of a form of synaesthesia – the condition that leads to a mixing of the senses – in a large sample of children. Over a twelve month period, Julia Simner and colleagues tested 615 children aged six to seven years at 21 UK schools and conservatively estimated that 1.3 per cent of them had grapheme-colour synaesthesia, in which letters and numbers involuntarily trigger the sensation of different colours.
“[This] implicates over 170,000 children age 0–17 in the UK alone, and over 930,000 in the USA,” the researchers said, “and suggests that the average primary school in England and Scotland (n = 168 pupils) contains 2.2 grapheme-colour synaesthetes at any time, while the average-sized US primary school (n = 396 pupils) contains 5.1.” Inevitably, the prevalence for synaesthesia as a whole, considering all the sub-types, would be even higher.
A hall-mark of grapheme-colour synaesthesia is that the colour triggered by a given letter or number is always the same – a fact the researchers exploited to identify the condition in school children.
Indeed, when asked to associate letters with colours, the children identified as synaesthetes showed more consistency over a 12-month-period than the other children did over a ten second period!
The study also showed how synaesthetic associations develop over time. The children with synaesthesia had an average of 10.5 reliable grapheme-colour associations when first tested aged six to seven, compared with 16.9 when tested a year later.
“It is not known whether the developmental pattern shown by our synaesthetes (i.e. 6.4 new coloured graphemes per year) represents a linear acquisition, or whether greater gains are made in later years,” the researchers said, “…our lab is currently tracking the development of this group to follow their transition into adult-like consistency.”
J. Simner, J. Harrold, H. Creed, L. Monro, L. Foulkes (2008). Early detection of markers for synaesthesia in childhood populations. Brain, 132 (1), 57-64 DOI: 10.1093/brain/awn292