Stage-Specific Processing in Numerosity Working Memory: ERP Evidence for Load and Mismatch Effects in a Delayed Match-to-Sample Task

Numerosity can be represented in symbolic formats and non-symbolic dot arrays. How numerosity load unfolds across WM encoding/maintenance and test-stage comparison within a single paradigm remains unclear, especially within the tested 4–6 range. We used a delayed match-to-sample task manipulating numerosity (4–6) and match status, with two test blocks (dot–digit and dot–dot). Behaviorally, a higher numerosity reduced accuracy and increased RTs in both blocks, with larger costs in dot–dot; the mismatch reliably slowed RTs. At sample onset, occipital P1 and N1 amplitudes decreased with increasing numerosity, consistent with greater perceptual/processing demands at higher load, with the strongest differences at the high end of the range. During the delay, numerosity modulation was temporally specific, emerging in the 450–650 ms posterior window and remaining significant after FDR correction across the four consecutive delay windows. At the test, the mismatch elicited a more negative N2 in both blocks (larger in dot–dot), while numerosity also modulated N2 only in dot–dot, showing a monotonic increase in negativity with load. Controlling for condition-mean logRT did not eliminate these N2 effects. P3 showed no reliable modulation, whereas a later positive component was enhanced by mismatch selectively in dot–dot. Together, these results indicate stage-differentiated effects: numerosity load impacts early encoding and a circumscribed maintenance interval, whereas mismatch effects arise primarily during the test-stage comparison, with additional late evaluative activity when formats are aligned.