Numbers around Descartes: A preregistered study on the three-dimensional SNARC effect

The Spatial-Numerical Association of Response Codes (SNARC) effect suggests that numbers are represented along a horizontal left-to-right oriented, mental number line, with small numbers on the left and large numbers on the right. Much less evidence exists for vertical (down-to-up) and sagittal (near-to-far) SNARC effects. This might be due to the employment of different experimental paradigms among studies and to the, sometimes, inexact definition of the vertical and sagittal axes. We investigated for the first time the SNARC effect along the horizontal, vertical, and sagittal axes, by means of a classic SNARC task. Our results suggest the presence of three equally-strong SNARC effects. Our findings can be considered as evidence in favor of a three-dimensional, mental representation of numbers, in the form of a mental number space, defined by Cartesian coordinates.