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.