The N400 and the P300 are not all that independent

This study assessed whether two ERP components that are elicited by unexpected events interact. The conditions that are known to elicit the N400 and the P300 ERP components were applied separately and in combination to terminal‐words in sentences. Each sentence ended with a terminal‐word that was highly expected, semantically unexpected, physically deviant, or both semantically unexpected and physically deviant. In addition, we varied the level of semantic relatedness between the unexpected terminal‐words and the expected exemplars. Physically deviant words elicited a P300, whereas semantically unexpected words elicited an N400, whose amplitude was sensitive to the level of semantic relatedness. Words that were both semantically unexpected and physically deviant elicited both an N400 with enhanced amplitude, and a P300 with reduced amplitude. These results suggest an interaction between the processes manifested by the two components.