Event-related potentials (ERPs) were recorded from the scalp to investigate the processing of word stimuli. Three tasks were used: (1) a task comparing words that provided an anomalous or normal sentence ending, (2) a word-list task in which different word types were examined, and (3) a word-list task in which semantic priming was examined. ERPs were recorded from a 50-channel montage in an attempt to dissociate overlapping ERP features by their scalp distributions. The focus of these studies was the N400, an ERP previously associated with language processing (Kutas & Hillyard, 1980). The temporal interval typically associated with N400 (250–500 msec) was found to contain overlapping ERP features. Two of these features were common to both sentence and word-list tasks—but one appeared different. Anomalous sentence endings and words with semantic content in lists both showed coincident negative left frontotemporal and midline-anterior ERP foci, peaking at 332 msec for sentences and 316 msec for word lists. The most negative voltage obtained in the sentence task peaked at 386 msec and had a midline-posterior focus. A right frontotemporal focus developed after the midline-posterior focus and outlasted its duration. The most negative voltage for content words in lists was reached at 364 msec. The distribution of this ERP was extensive over the midline and appeared to differ from that observed in the sentence task. Modulation of language-related ERPs by word type and semantic priming was investigated using the word-list tasks, which required category-detection responses. Two novel findings were obtained: (1) The ERP distributions for words serving grammatical function and content words differed substantially in word lists. Even when devoid of any sentence context, function words presented significantly attenuated measures of N400 compared to content words. These findings support hypotheses that suggest a differential processing of content and function words. (2) Semantic priming functionally dissociated two ERP features in the 250–500 msec range. The later and most negative midline ERP feature (peaking at 364 msec) was attenuated by semantic priming. However, the earlier left frontotemporal feature (peaking at 316 msec) was enhanced by semantic priming. The isolation of this novel language-related ERF' that is sensitive to semantic manipulations has important consequences for temporal and mechanistic aspects of theories of language processing.

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