Supplementary material: vocabulary skills and grapheme-colour synaesthesia from Learning in colour: children with grapheme-colour synaesthesia show cognitive benefits in vocabulary and self-evaluated reading

Cognitive benefits associated with grapheme-colour synaesthesia in adults are well documented, but far less is known about whether such benefits might arise in synaesthetes as children. One previous study on a very small group of randomly sampled child synaesthetes found cognitive benefits in short-term memory and processing speed (the ability to quickly scan an array of images and discriminate between them), but was inconclusive for a test of receptive vocabulary. Using a stratified population sample (Growing Up in Scotland Project, Edinburgh, UK: Scottish Executive, 2007), we investigated the performance of a large cohort of child grapheme-colour synaesthetes using four literacy measures taken at age 10 years. These were three verbal comprehension measures (expressive vocabulary, receptive vocabulary and sentence comprehension), and one measure of academic self-concept in reading (plus one measure of academic self-concept in numeracy as a comparison). After controlling for demographic differences between groups, synaesthetes showed significantly enhanced performance for expressive and receptive vocabulary compared to their peers, but no benefits in sentence comprehension. Child synaesthetes also reported significantly higher academic self-concept for reading, but not for numeracy. Finally, we found that synaesthetes made significantly more progress than controls across the primary school years, although they began school with no a priori advantage. Our study provides powerful new evidence that children with grapheme-colour synaesthesia show vocabulary and literacy differences, which we contextualize within a theory of synaesthetic development.This article is part of the discussion meeting issue ‘Bridging senses: new developments in synaesthesia’.