Perceiving numbers influences actions in number-space synesthesia.

Number and space are spontaneously linked in human cognition and have been metaphorically described as a 'mental number line'. In number-space synesthetes (NSS), numbers are visualized in specific spatial arrays, in an idiosyncratic and explicit manner. Here we tested whether the numerical-spatial association that characterizes NSS has the potential to affect individual's actions when the number is not a task-relevant feature. We used a Simon task that implies stimulus-response matching. Task-irrelevant Arabic numerals (1, 4, 6, 9) appeared simultaneously at the same location as a colored target. A group of 15 age-matched controls and 4 NSS completed a 2×2 within-subjects design consisting of Simon (compatible and incompatible) and number-space (congruent and incongruent) trials. Mean reaction time (RT) and RT cumulative distribution functions across Simon and number-space conditions for both NSS and controls were analyzed. Relative to controls, synesthetes' RTs were overall affected by the number-space mapping. Moreover, synesthetes' Simon effects occurred at later response times. These results provide strong evidence that numbers are cognitively represented as action codes and that the strength of the association shapes the impact of the numerical representation on individuals' actions.