Horizontal Trumps Vertical in the Spatial Organization of Numerical Magnitude

The vast majority of research on the spatial nature of numerical representation has focused on the horizontal axis, highlighting the left-to-right orientation of increasing values (i.e., the mental number line). Recent evidence points to spatial organization along the vertical (bottom-to-top) axis as well. We argue, however, that these findings are better characterized as generalized magnitude mappings between number and other magnitude dimensions (e.g., near-far spatial extent), rather than genuine vertical orientation. Here we replicate generalized magnitude mappings for number (Exp. 1A) and show that they take precedence over left-to-right orientation (Exp. 1B), likely because of the direct mapping between dimensions. In contrast, we find no evidence of spontaneous organization along the true vertical axis (Exp. 2A), and left-to-right orientation trumped bottom-to-top orientation when the two were inconsistent with each other (Exp. 2B). Reliable bottom-to-top orientation was evident only after priming of magnitude relations among numbers (Exp. 3). Together, these findings demonstrate that number is more strongly represented horizontally than vertically. We suggest that experience with cultural tools may drive this asymmetry, and we highlight other cognitive and environmental factors that may influence the mental organization of magnitude dimensions beyond number.