How to make “number lines” stable in the mind’s eye

Humans are prone to mentally organise the ascending series of integers according to reading habits, so that in western cultures small numbers are positioned to the left of larger ones on a mental number line (MNL, Dehaene et al., 1993). Despite 140 years since seminal observations made by Sir Francis Galton in human observers that experienced vivid MNLs (Galton, 1880a,b), the neural and functional mechanisms that give rise to MNLs remain elusive. Here, in a series of three experiments we show that spatially organised MNLs are not \"all or none\" phenomena and that MNLs get progressively less noisy and more stable in the minds eye, the more an observer fully combines left/right spatial codes with small/large magnitude codes to operate on numbers. These results show that conceptualising the ascending series of integers in spatial terms depends on contextual clues rather than being inherent to the semantic representation of numbers.