The SNARC effect is usually defined as the compatibility between Response hand and Numerical magnitude (R/SNA), with faster left-hand responses to small numbers and faster right-hand responses to large numbers. By contrast, the compatibility between stimulus Spatial position and Numerical magnitude (S/SNA) has received little attention. We disentangled R/SNA and S/SNA contributions for peripheral target numbers, with their magnitude being either task-irrelevant or task-relevant, while also manipulating the Spatial position-Response hand compatibility (S/R). Across two Posner exogenous cueing experiments, we orthogonally varied target spatial position (left/right), cue validity (valid/neutral/invalid), and the S/R compatibility (S/R-compatible/incompatible). Experiment 1 employed a localization task, and Experiment 2 a magnitude classification task. We applied a chronometric framework to decompose response times into three components: reflexive attention (cue-driven), voluntary attention (target-driven), and response planning. In Experiment 1, S/SNA emerged in the invalid cues condition only, suggesting that it was driven by the voluntary attention component. In Experiment 2, S/SNA emerged at the response-selection stage and was progressively suppressed by voluntary attention, being strong in the valid cue condition and weak in the invalid one. R/SNA followed a similar but weaker pattern. Notably, the S/R compatibility effect was driven by voluntary attention in the magnitude task but inhibited by it in the localization task. These findings show that spatial-numerical associations rely on different mechanisms depending on whether number magnitude is task-relevant, and that voluntary attention can either drive or inhibit spatial compatibility effects.
