Number Synaesthesia: When Hearing “Four Plus Five” Looks Like Gold

Whilst lexical-colour synaesthesia has been widely studied, the number-colour pairing has received relatively less attention. Since one very strong variable is the structure and "ordinality" within a set of inducers (e.g., letters of the alphabet, days of the week and months of the year), the inherent hierarchy of numbers suggests that number-synaesthesia should be found in most synaesthetes. Three synaesthetes were recruited who showed between 80 and 100% consistency of synaesthetic responses to a set of stimuli across a three month interval. Their synaesthesia for numbers was investigated using an extension of Dixon et al.'s (2000) "2 plus 5 equals yellow" paradigm. Participants saw single digits and operators for an arithmetic process followed by a coloured square which was either congruent or incongruent with their individual colour for the answer; the task was to name the colour followed by stating the solution to the arithmetic sum. To see whether number-processing synaesthesia extends beyond the visual domain into the auditory domain the same procedure was used with the digits and operators being presented aurally. Relative to age-matched controls congruency effects were found for the synaesthetes. These findings add to the suggestion that synaesthesia can occur at a conceptual level where physical experience of an inducer is not necessary. Additionally, the findings showed that for one synaesthete there was a statistical difference in the auditory but not visual condition whilst for the remaining two synaesthetes, the reverse pattern was observed. This variable pattern adds to the body of literature that further illustrates the heterogeneity of synaesthesia. The findings are incorporated into Rich and Mattingley's (2002) neurocognitive model of synaesthesia and observations which cannot yet be explained are highlighted as future directions for research.