Hemispheric asymmetry and the mental number line: comparison of double-digit numbers.

To investigate the spatial representation of numbers and its possible hemispheric organisation, 46 participants were presented with a series of double-digit numbers in a fixed standard number comparison task using 65 (in Experiment 1) and 55 (in Experiment 2) as the standard. Visual field of presentation, distance from the target number and magnitude were manipulated. Reaction times decreased with increasing distance from the target number, in line with the well-established distance effect. There was no main effect of visual field, however the interaction between visual field, distance and magnitude was significant. For large distances, numbers of small magnitude were responded to faster in the left over the right visual field, while there was a right visual field advantage for large magnitude numbers. These results suggest that the representation of numbers has spatial qualities (i.e., smaller numbers are represented to the left of a comparison point, larger numbers to the right).