A longitudinal study of grapheme-color synesthesia in childhood: 6/7 years to 10/11 years

Grapheme-color synesthesia is a condition characterized by enduring and consistent associations between letter/digits and colors. This study is the continuation of longitudinal research begun by Simner et al. (2009) which aimed to explore the development of this condition in real time within a childhood population. In that earlier study we randomly sampled over 600 children and tested them aged 6/7 and 7/8 years. We identified the child synesthetes within that cohort and measured their development over 1 year, in comparison to a group of non-synesthetic children with both average and superior memories. We were able to show the beginnings of a developmental progression in which synesthetic associations (e.g., A = red) mature over time from relatively chaotic pairings into a system of fixed consistent associations. In the current study we return to this same population three years later when participants are now 10/11 years. We used the same paired-association memory task to determine the synesthetic status of our participants and to also establish synesthetes' inventories of grapheme-color associations. We compared their inventories to those from age 6/7 and 7/8 years to examine how synesthesia matures over time. Together with earlier findings, our study shows that grapheme-color synesthesia emerges with a protracted trajectory, with 34% of letters/digits fixed at age 6/7 years, 48% fixed at 7/8 years and 71% fixed at 10/11 years. We also show several cases where synesthesia is not developing in the same time-frame as peers, either because it has died out at an older age, or because it was slower to develop than other cases. Our study paints the first picture of the emergence of synesthesia in real-time over four years within a randomly sampled population of child synesthetes.