How do numbers shift spatial attention? Both processing depth and counting habits matter.

There have been inconsistent reports about whether seeing small versus large numbers (e.g., 1 or 2 vs. 8 or 9) automatically shifts an observer's attention into left versus right hemispace. We report four visual detection experiments (N = 162) where centrally presented uninformative number cues were followed by lateralized targets that required go/no-go responses. Processing depth was manipulated by requiring observers to either distinguish numbers from other symbols (Experiment 1) or to classify numbers by either parity (Experiment 2) or magnitude (Experiments 3 and 4). Attention shifts occurred only after magnitude processing. Importantly, their direction depended on observers’ directional preferences for object counting, which was separately assessed in all experiments. These results clarify the mechanism by which abstract concepts activate their inherent spatial associations and lead to spatial attention shifts. They also demonstrate the feasibility of attentional probing to study mechanisms of symbol comprehension in various contexts, ranging from mental arithmetic to language processing.