AN EVENT-RELATED POTENTIAL STUDY OF SEMANTIC CONGRUITY AND REPETITIONIN A SENTENCE-READING TASK - EFFECTS OF CONTEXT CHANGE

This experiment examined the joint effects of semantic congruity and r epetition on event-related brain potentials (ERPs) elicited in a sente nce priming task. In an initial training phase, subjects were familiar ized with a list of 60 congruous and incongruous sentences. During the second phase, ERPs were recorded as subjects silently read a set of 1 80 unconnected sentences. One third of the sentences were presented ex actly as they had been seen in training (Old sentences), one third wer e presented for the first time (Completely New sentences), and one thi rd involved a re-pairing of the frames and completions of the congruou s and incongruous Old sentences (New Pair sentences). The N400 congrui ty effect was reduced for Old as compared with Completely New and New Pair sentence completions. These results suggest that N400 reflects pr ocesses that are sensitive to both existing semantic associations and representations of previous episodes that include the context of the e liciting stimulus. A late positive component (LPC) involving a sustain ed positive shift in the waveform after 600 ms was largest for incongr uous completions and occurred somewhat earlier for Old sentences. This pattern of results is consistent with the notion that the LPC is an i ndex of episodic retrieval and elaborative processes. The data also su ggest the presence of an early onset slow positive shift that is only evident for New Pair congruous sentences.