For the writer Vladimir Nabokov, “aa” was the colour of polished ebony and “ee” was yellow. Nabokov had synaesthesia: his sensory perceptions mingled with one another. In his case, he saw colours when hearing certain vowels, but many forms of synaesthesia are possible. Only 1 in 25 people have synaesthesia, but this new research shows that certain intuitions about “sound colours” shared by many more people than this.

In this study, over 1,000 people took part in an online test where they chose colours for 16 spoken vowels. A large majority felt that “aa” was more red than green, and “ee” more light than dark, whether they had synaesthesia or not. According to Mark Dingemanse, one of the researchers, “There seems to be a logic to how we link sound and colour, and the structure of language has an important role in this process.”

Credit: Cuskley

In the above image the vowel-color associations in two non-synesthete subjects (left) and in two synesthetes (right) can be seen. Synesthetes more precisely chose the same color for a particular sound. However, all four participants created groups of sounds that lay close to a particular Dutch vowel, such as “ee” [i:] (upper left), and they all chose lighter colors for “ee” than for “aa” or “oo.” General principles for vowel-color associations exist, whether one has synesthesia or not.

Source (Radboud University Nijmegen. “Associating colors with vowels? Almost all of us do!.” ScienceDaily. ScienceDaily, 4 April 2019.)

Original paper: Cuskley, C., Dingemanse, M., Kirby, S. and Van Leeuwen, T.M., 2019. Cross-modal associations and synesthesia: Categorical perception and structure in vowel–color mappings in a large online sample. Behavior research methods, 51(4), pp.1651-1675.