THE N400 AS A FUNCTION OF THE LEVEL OF PROCESSING

In a semantic priming paradigm, the effects of different levels of pro cessing on the N400 were assessed by changing the task demands. In the lexical decision task, subjects had to discriminate between words and nonwords, and in the physical task, subjects had to discriminate betw een uppercase and lowercase letters. The proportion of related versus unrelated word pairs differed between conditions. A lexicality test on reaction times demonstrated that the physical task was performed nonl exically. Moreover, a semantic priming reaction time effect was obtain ed only in the lexical decision task. The level of processing clearly affected the event-related potentials. An N400 priming effect was only observed in the lexical decision task. In contrast, in the physical t ask a P300 effect was observed for either related or unrelated targets , depending on their frequency of occurrence. Taken together, the resu lts indicate that an N400 priming effect is only evoked when the task performance induces the semantic aspects of words to become part of an episodic trace of the stimulus event.