The P600 reflects cost of new information in discourse memory.

Discourse processing depends on long-term semantic memory and discourse memory (i.e. the organization and maintenance of a mental model). Using event-related potentials, the information-processing functions of the activities manifested by a centroparietal negativity (N400) and a posterior positivity (P600) were investigated during the processing of inferences. On the basis of findings from semantic priming, it was predicted that the standard N400 signature would modulate differences in the degree of inferential processing (semantic memory). Instead of N400 modulations, however, the two conditions that depended on a more demanding drawing of inferences elicited a P600, indicating that updating the mental model encumbers discourse memory capacity. The results highlight the functional significance of this positivity for discourse updating, and support the view that the P600 reflects distinct language processes.