Many-valued logic and event-related potentials

In previous experiments on event-related potentials (ERPs) during linguistic judgments, the subjects' decisions have been categorical (e.g., true vs false). In this experiment, more realistic variations in truth value and subject certainty were used. Thirty-eight naive undergraduates read a story about a fictional murder. ERPs were recorded as the subjects rated the strength of their beliefs about statements relating to suspects in the crime. Because no subject was sure which of the suspects was guilty of committing the crime, binary (true-false) category judgments were inappropriate. Three components of the ERP waveforms were affected by the experimental manipulations. An early positive component was largest to sentences concerning the suspect considered most likely to have committed the crime. A subsequent broad posterior positivity (LPC) also showed significant sentence-type differences, but it was larger to sentences considered probable--whether they were true or false--than to more ambiguous sentences. A third ERP component (N400) was negative at midline electrode sites and peaked at approximately 420 ms. Subjects' truth-value judgments had no effect on the N400. N400 was, however, affected by the subject's task. It was more negative when subjects made graded judgments about truth value than when they made binary true-false judgments. Overall, naturalistic judgments of sentence validity produced a variety of brain responses that reflected different aspects of linguistic decision making.