Early neurophysiological signatures of multi-digit number length encoding

The Arabic number system links number length with magnitude: longer numbers represent larger values. In numerical comparisons, interference arises when physical and numerical dimensions are misaligned, and processing improves with greater numerical distance. While ERP studies have dissociated numerical processing effects, evidence for early encoding of numerical information has been limited due to confounding visual properties. In two ERP experiments, we examined whether number length influences early multi-digit number processing, independent of visual size, by using scribbled line patterns to control physical length. Experiment 1 (N = 27) compared digit values in tie numbers (numbers with repeated digits) to standard "555", ignoring number length. Experiment 2 (N = 27) judged number length, ignoring digit identity. Targets varied in number length and numerical distance, creating congruent and incongruent conditions. ERP results revealed enhanced parieto-occipital N1 negativity (~ 120-150 ms) for longer numbers, indicating early number length encoding independent of overall visual size. Central P2p (~ 150-190 ms) reflected refined numerical distance processing. Centro-parietal P3 (~ 300-360 ms) showed congruity effects only when numerical magnitude was task-relevant. These findings reveal three distinct processing stages: early encoding (N1), refined magnitude comparison (P2p), and conflict resolution (P3). This novel neurophysiological evidence showcases the unique influence of number length and numerical syntax on magnitude evaluation.