n This paper presents evidence of the disputed existence of an electrophysiological marker for the lexical-categorical distinction between open-and closed-class words. Event-related brain potentials were recorded from the scalp while subjects read a story. Separate waveforms were computed for open-and closed-class words. Two aspects of the waveforms could be reliably related to vocabulary class. The first was an early negativity in the 230-to 350-msec epoch, with a bilateral anterior predominance. This negativity was elicited by open-and closed-class words alike, was not affected by word frequency or word length, and had an earlier peak latency for closed-class words.
The second was a frontal slow negative shift in the 350-to 500-msec epoch, largest over the left side of the scalp. This late negativity was only elicited by closed-class words. Although the early negativity cannot serve as a qualitative marker of the open-and closed-class distinction, it does reflect the earliest electrophysiological manifestation of the availability of categorical information from the mental lexicon. These results suggest that the brain honors the distinction between open-and closed-class words, in relation to the different roles that they play in on-line sentence processing.