This has the advantage that our findings are likely to generalise to other sentence stimuli, but it can also raise a possible concern: The N400 effect may not be due to surprisal itself, but to an unknown confounding variable that was not included in the regression analysis.
this seems unlikely because of two additional findings that only follow naturally if surprisal is indeed the relevant predictor: Significant results only appeared where they were most expected a priori (i.e., on N400 but not on other components) and there was a nearly monotonic relation between the models' word-prediction accuracy and their ability to account for N400 size.