Numerically induced attentional biases in horizontal, vertical, and two-dimensional shapes

Previous studies have demonstrated that numerical magnitudes can induce attentional biases mainly in a horizontal space. The present study aimed to clarify spatial-numerical association in horizontal, vertical, and two-dimensional square stimuli composed of visually aligned strings of relatively smaller Arabic numerals (i.e., 1 or 2) or larger numerals (i.e., 8 or 9). Neurologically and psychiatrically healthy participants, all of whom were Japanese speakers, were instructed to identify the veridical center of the stimuli. The results indicated that, with horizontal stimuli, participants placed their subjective midpoint farther left when smaller numbers were presented compared to larger numbers, consistent with the predicted left-to-right mental number line. With vertical stimuli, however, smaller numbers induced biases upwards, inconsistent with the predicted bottom-to-top representation. For the square, the stimuli with number strings elicited biases with a stronger vertical component, whereas the stimuli without number strings elicited biases with a stronger horizontal component. The upward biases for the square stimuli with number strings may reflect the activation of object-based processing rather than the cognitive function of numerical processing. These findings suggest that distinct mechanisms may predominate over spatial-numerical associations with regard to the biases in the two-dimensional plane.