| The N400 event-related potential (ERP) provides a sensitive measure of semantic expectancy. Words that are incongruous with the meaning of a sentence produce a large negative deflection compared to semantically appropriate words. A person's mood has been shown to influence semantic processing, facilitating the encoding and retrieval of mood-congruent information. In recent studies in our laboratory, the N400 ERP has been used to index mood-congruent expectancy.;In methodological research, N400 has been shown to depend on the latency of the Late Positive Component (LPC); semantic violations produce a delayed positivity (thus creating the relative negativity of the N400) compared to semantically appropriate words. The current study measures semantic expectancy, mood-related expectancy, and subjective ratings of word expectancy with sensitive single-trial measures of LPC latency.;Forty-two University subjects read a series of brief stories about daily life events. One-third of the stories ended positively, one-third ended negatively, and the remaining third ended with a semantic anomaly. While reading these stories, subjects assumed either an optimistic or pessimistic mood-state. Subjects rated each ending as personally expected or unexpected.;Using a single-trial adaptive filter, peak latencies of the LPC were estimated for each trial. Mean and median peak latencies were calculated by end-word type (Positive, Negative, Incongruous) and expectancy ratings (Expected, Unexpected). A series of repeated-measures ANOVAs supported the three major predictions of the study: (1) Semantically congruous words produced shorter peak latencies than semantically incongruous words. (2) Mood-congruent endings generated shorter peak latencies than mood-incongruent endings. Thus, for optimists, positive words produced shorter peak latencies than negative words. For pessimists, negative words produced shorter peak latencies than positive words. (3) Expected endings resulted in shorter peak latencies than unexpected endings. These results suggest that improved methods of analyzing discrete events in human brain electrical activity will provide important tools for studying emotional and motivational influences on the cognitive process. |
