What color is H? Is 4 brighter than 9?

Those questions don't make sense to many people but for people with grapheme-color synesthesia, they have real answers.

Grapheme-color synesthesia is a harmless, alternative form of perception caused by subtle differences in the brain, possibly, stronger connections between centers for language and color, that give letters and numbers their phantom colors. It is passed down from parent to child in around 1% of the population. 

In the first long-term childhood study on grapheme-color synesthesia, a pair of psychologists followed 80 children to determine when and how associations between graphemes and colors develop. The group of synesthete children was tested three times between 6 and 10 years old. Each child was presented with 36 graphemes – the letters A to Z and digits 0 to 9 – and asked to choose the 'best' of 13 colors for each. 

Percentage of consistent colors selected by the three participant groups (see legend) in their immediate consistency of 36 graphemes within Sessions 1, 2 and 3 (ages 6/7, 7/8, 10/11 respectively). Intervening years have no data because no testing took place at these ages. 
doi: 10.3389/fnhum.2013.00603

Children with grapheme-color synesthesia had already developed strong associations for around 30% of graphemes at 6 years old. At 7 years old, the same children had associations for around 50% of graphemes, and this increased to 70% of graphemes at 10 years old. The synesthete children were consistent in their choices over this 4-year period. Three children who were synesthetes at ages 6 to 7 were no longer so at 10 years old, indicating that the condition spontaneously disappears in some children as they grow older.

"This repeated testing of child synesthetes in real time allowed us to see for the first time that synesthetic colours emerge slowly during childhood, building up an incremental inventory of colorful letters and numbers," says Dr.  Julia Simner, a cognitive neuropsychologist who specializes in synesthesia at the University of Edinburgh.

The researchers' next challenge is to determine how changes in the intensity of synesthesia - as strengthening or loss with increasing age - can be explained from changes in the organization of the brain.