Sensory deprivation: visual experience alters the mental number line.

Early studies on numeric cognition reported that numbers are spatially organised according to a left-to-right small-to-large 'number line'. We investigated whether this spatial-number organisation is dictated by visual experience. We tested congenitally and late blind, and blindfolded sighted participants in a random number generation task where in one block their heads were alternately turned left or right before uttering the number. We found that the 'random' number generation was biased according to the side where the head was turned to. Consistent with the standard number line, participants with visual experience generated smaller numbers for left turns, and larger numbers for right turns. In contrast, participants without any visual experience showed the opposite pattern of results. These results suggest a role for visual experience in the development of spatial and numerical representations, which is supported by cultural differences in number representation, and provide converging evidence for visually driven organisation of the parietal cortex.