Breaking numbers apart: The role of digit separators in magnitude perception

Digit separators are widely used in Hindu-Arabic numbers to group digits into readable triads, yet their influence on multi-digit number processing has been rarely studied. We examined whether digit separators affect how large numbers (millions or greater) are processed, beyond the effect of number length. In two experiments, participants compared pairs of large multi-digit numbers presented with or without digit separators between groups. The numbers differed in their leftmost digit, numeric scale (i.e., number length), or both, yielding four left-digit-scale compatibility conditions: compatible (e.g., 5,000,000,000 vs. 2,000,000), incompatible (e.g., 2,000,000,000 vs. 5,000,000), same left-digit (e.g., 2,000,000,000 vs. 2,000,000), and same scale (e.g., 5,000,000 vs. 2,000,000). In Experiment 1, participants selected the numerically larger number; in Experiment 2, they judged which number was longer, or whether both were equal in length. In Experiment 1, the presence of digit separators slowed responses, contrary to the expectation that it would facilitate processing. In both experiments, participants showed sensitivity to leftmost digit values: when the leftmost digit and numeric scale pointed to different responses (incompatible condition), responses were slower than when they agreed (compatible condition), producing a left-digit-scale compatibility effect. In Experiment 2, where digit values were task-irrelevant, the left-digit-scale compatibility effect emerged only in the presence of digit separators. These findings demonstrate that digit separators activate semantic processing of digit values, even when visual appearance alone could guide the task, revealing that syntactic formatting actively shapes numerical magnitude perception.