Five plus two equals yellow.

Abstract

Mental arithmetic in people with synaesthesia is not coloured by visual experience. In synaesthesia, ordinary stimuli elicit extraordinary experiences. For example when C., who is a digit–colour synaesthete, views black digits, each number elicits a photism — a visual experience of a specific colour. It has been proposed that synaesthetic experiences differ from imagery in their consistency1, automaticity2,3 and reliance on external stimuli to induce them4. Here we demonstrate that C.'s photisms are both consistent and automatic, but we find that an externally presented inducing stimulus is not necessary to trigger a photism and that simply activating the concept of a digit is sufficient.