n synaesthesia, ordinary stimuli elicit extraordinary experiences. For example when C., who is a digit–colour syn-aesthete, 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 consistency 1 , automaticity 2,3 and reliance on external stimuli to induce them 4. 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. As a measure of consistency, we asked C. to name the colour of her photisms elicited by the digits 1 to 9 shown in random order, 10 times each. C.'s pairings between digits and colour names were 100% consistent across repetitions. To assess automaticity, we used a Stroop task. Colour-naming reaction times were recorded for coloured squares (baseline condition) or for digits displayed in colours that were either congruent or incongruent with C.'s photisms (Fig. 1). Her mean reaction times for each condition were: congru-and Tukey post-hoc analyses revealed that incongruent trial reaction times were significantly slower than baseline (P < 0.001) and congruent trial reaction times (P < 0.001) — the last two conditions did not differ. C.'s large significant difference in congruent/incongruent reaction times (245 ms) contrasts with the small, non-significant , congruent/incongruent differences (ranging from ǁ11 to 17 ms) of eight non-synaesthetes tested using identical stimuli. Figure 1a shows the Stroop performance of C. and of the non-synaesthete most similar to C. in terms of baseline-trial colour-naming reaction times. C.'s performance indicates that the digits automatically elicited photisms that interfered with her colour-naming on incongruent trials. For C., viewing the digit 7 automatically induces a yellow photism. To assess whether C.'s photisms could be elicited without physically showing her an inducing stimulus , we presented, in sequence, a digit (for example, 4), an operator (for example, +), a second digit (for example, 3), and a colour patch. C. had to name the colour patch as quickly as possible, and then report the solution to the arithmetic problem. On each trial, the colour patch was either congruent or incongruent with the photism associated with the arithmetic solution. For example, 5 + 2 was followed either by a yellow patch, which is congruent with C.'s photism colour for 7, or a red patch, …