How synaesthesia grows in childhood, and dies out:

Synaesthesia is well studied in adults and is thought to be a result of unusual connections created during brain development, but it has been hardly studied in children - until now.

A new study published online in Brain searched for letter-colour synaesthetes in 6-8 year old children and found not only are they relatively common, but that the condition changes as the children grow.

Synaesthesia is where the senses are crossed, so perceiving something in one sense triggers a perception in one of the other senses. The type targeted by this study was letter-colour synaesthesia where people perceive colours when they see certain letters.

Synaesthesia is known to be partly inherited and there is brain imaging evidence that people with the letter-colour type have greater number of white matter connections between brain areas known to be involved in word and colour perception.

A popular theory is that synaesthesia results from an unusual form of brain development where certain connections in the brain are not 'cut' or 'pruned' during the early months of life.

However, letter-colour synaesthesia requires that the person can read and understand letters, which usually doesn't happen until much latter, so it is likely that there is something going on throughout the critical learning period when children begin to learn to read.

This new study, led by psychologist Julia Simner, tested over six hundred six to seven year-old children with a computerised test that showed them letters and numbers and asked them to select a colour which best fitted the character on screen.

After 10 seconds, the test was repeated. One of the hallmarks of people with letter-colour synaesthesia is that their associations remain constant, so this helped pick out who was the most consistent.

Children who did better than average on this were tested again with a surprise test at 12 months, and those who were more consistent at 12 months than the average child at 10 seconds were classified as having synaesthesia.