The N400 inhibition hypothesis can predict LPP amplitudes

The semantic processing generating the N400 event-related potential (ERP) could be an inhibition preventing information from being placed into working memory (WM). This predicts that the amplitude of the late positive potential that follows, the LPP, should decrease. Conversely, when N400s are small, less inhibition would occur, LPPs should be larger and the WM content richer. This was observed in Sinha et al. (2023): when N400s to equivocal story endings were as small as those for univocal endings, LPPs to equivocals were larger and their ambiguity, detected. Here, to further test such LPP predictions, the half of their 101 participants who had similar or smaller N400s to equivocals than to univocals (the SN4s) were separated from the half having larger such N400s (the LN4s). As predicted, LPPs of SN4s were larger for equivocals than for univocals, while LN4s had smaller such additional LPPs. These LPP differences were distinct from N400 ones: equivocality interacted with time-window and the subtractions of the ERPs to univocals from those to equivocals revealed two peaks. Complementarily, in Sinha et al. (2025a), N400s were found a bit larger for equivocals than for univocals. There, LPPs were only a bit larger for the former than for the latter. Here, the same separation of 50 SN4s from 50 LN4s and the same subtractions led to similar results: two peaks having distinct scalp distributions. We thus report further support for the N400 inhibition hypothesis framework and a method for detecting the semantic processing strategy occurring in each participant.