EXPRESS: Task-Irrelevant Lateral Placeholders Support Horizontal Spatial-Numerical Associations

Horizontal Spatial-Numerical Associations (SNAs) are often interpreted as evidence that numerical magnitude automatically activates left-right spatial codes. Here, we tested whether horizontal spatial-numerical mapping instead depends on whether the task environment provides usable horizontal structure. Participants performed a go/no-go magnitude classification task in which stimulus location and response execution were held constant while the availability of horizontal spatial structure was manipulated. In Experiment 1, both digit presentation and response execution were central, with participants using a single hand in each block. This created a spatially neutral task context in which no magnitude-response hand compatibility effect emerged. In Experiment 2, the task was identical except for the addition of two noninformative lateral placeholders, which introduced a minimal left-right spatial frame. Under these conditions, the data indicated a modest magnitude-response hand compatibility pattern, although the evidence was not fully conclusive. These findings are consistent with task-dependent and joint-use accounts of SNAs, suggesting that horizontal spatial-numerical mappings depend on the joint availability of magnitude and spatial codes rather than arising obligatorily from numerical magnitude alone. More broadly, the findings suggest that even minimal, task-irrelevant perceptual structure can contribute to the spatial scaffolding needed for horizontal SNAs to emerge.