Objective correlates of an unusual subjective experience: A single-case study of number-form synaesthesia

There is a universal and often unconscious tendency to mentally associate the number sequence with a spatial continuum (the mental number line). Here we study one individual who reports a strong and vivid sense of space when processing numbers. For him, the number sequence has a precise spatial form: a curvilinear right-to-left oriented line. We used various tasks to demonstrate that this numerical-spatial association is not a mere figment of his imagination, but a constrained experiential phenomenon consistent across sessions and automatically triggered by the visual presentation of numbers. We also show that this idiosyncratic representation can coexist with another implicit association, the SNARC effect (Spatial-Numerical Association of Response Codes, where small numbers are associated with the left side of space). This effect is present in individuals without explicit number forms and is not affected in the present subject in spite of his reversed subjective representation.