MANY-VALUED LOGIC AND EVENT-RELATED POTENTIALS

In previous experiments on event-related potentials (ERPs) during ling uistic judgements, the subjects' decisions have been categorical (e.g. , true vs false). In this experiment, more realistic variations in tru th value and subject certainty were used. Thirty-eight naive undergrad uates read a story about a fictional murder. ERPs were recorded as the subjects rated the strength of their beliefs about statements relatin g to suspects in the crime. Because no subject was sure which of the s uspects was guilty of committing the crime, binary (true-false) catego ry judgments were inappropriate. Three components of the ERP waveforms were affected by the experimental manipulations. An early positive co mponent was largest to sentences concerning the suspect considered mos t likely to have committed the crime. A subsequent broad posterior pos itivity (LPC) also showed significant sentence-type differences, but i t was larger to sentences considered probable-whether they were true o r false-than to more ambiguous sentences. A third ERP component (N400) was negative at midline electrode sites and peaked at approximately 4 20 ms. Subjects' truth-value judgments had no effect on the N400. N400 was, however, affected by the subject's task. It was more negative wh en subjects made graded judgments about truth value than when they mad e binary true-false judgments. Overall, naturalistic judgments of sent ence validity produced a variety of brain responses that reflected dif ferent aspects of linguistic decision making, (C) 1998 Academic Press.