Electrocortical distinction of vocabulary types.

Psycholinguistic theories propose that words of the 2 major vocabulary classes, content (open-class) and function (closed-class) words, are computationally distinct and have different neuronal generators. This predicts distinct EEG patterns elicited by words of these 2 classes. To test this prediction, content and function words, together with matched pseudowords, were presented in a lexical decision task (where subjects had to decide whether stimuli were meaningful words or not). Evoked potentials were recorded from 17 electrodes 12 of which were located in close vicinity of the perisylvian cortices. Already 160 msec post stimulus onset, substantial differences in activity patterns distinguish the 2 vocabulary classes. A hemisphere by word class interaction revealed interhemispheric differences for function words but not for content words. Potentials evoked by function words were more negative over the left hemisphere compared to the right. These results evidence that brain mechanisms underlying function and content word processing are different. The following explanation of the data is proposed: content words correspond to neuronal assemblies equally distributed over both hemispheres, while assemblies corresponding to function words are strongly lateralized to the left hemisphere and primarily located in the perisylvian region.