Zero as a semantic boundary: Rethinking the mental number line.

Zero poses a unique challenge for numerical cognition because it denotes absence in cardinal contexts, yet functions as a formal entity in ordinal or interval systems. We examined how adults compare zero to positive single-digit numbers across symbolic, nonsymbolic, and mixed formats. Results replicated classic distance and end effects for positive numbers. However, the cognitive status of zero was found to be strictly format-dependent. In the symbolic format, the distance effect was driven by boundary values (0 and 1) and vanished when they were removed, suggesting symbolic zero functions as a structural anchor that defines the semantic transition from absence to quantity. In contrast, nonsymbolic zero was integrated into a continuous, nonlinear magnitude gradient, following the same psychophysical power-function pattern as other quantities. Furthermore, the [0, 1] pair elicited a unique facilitation in nonsymbolic and mixed formats, but not in the symbolic format. Together, these findings suggest that zero is not merely a point on a scale but a multi-faceted construct: while nonsymbolic zero is processed as a perceptual category of absence, symbolic zero acts as a semantic boundary that redefines the internal structure of the mental number line.