Early occipito-parietal activity in a word recognition task: an EEG and MEG study

Objective: We investigated early brain activity funder 200 ms after the sti mulus onset) related to the encoding and the retrieval of verbal informatio n. Methods: First, we compared ERPs produced by words which were encoded to ER Ps produced by words from following test phases (correctly identified repet itions and correctly classified new words) in two different experiments. Ex periment 1 consisted of an intentional learning paradigm and experiment 2 c onsisted of an incidental learning paradigm. In addition, we conducted a co ntrol experiment (experiment 3), which was a continuous recognition task wi th two different repetition intervals. Secondly, we conducted a magnetoence phalographic (MEG) study to further investigate early brain activity (exper iment 4). The same intentional learning paradigm as in experiment I was use d. Results: We found that ERPs elicited by correctly classified test words (re peated words and new words) of both experiment 1 and experiment 2 were sign ificantly more negative going than the ERPs elicited by the study words. Th is effect was apparent between 100 ms and 200 ms after the stimulus onset a nd was distributed over occipital and parietal scalp locations. In the cont rol task (experiment 3), these early potential differences were missing (fo r both repetition intervals). Early event-related fields (ERFs) were also f ound to depend on the situation of the study phase and the test phase. This activity difference peaked at 120 ms after the stimulus onset. The distrib utions of the difference magnetic fields were occipito-parietal and thus co nsistent with the findings of experiment 1 (EEG-experiment). Conclusion: Whether the effect we defined in the present study is due to an increase of activity during the test situation or due to a decrease of act ivity during the study situation remains unclear. However, it might reflect attentional processes within a word recognition task depending on whether a word is encoded or an effort of word retrieval has to be made. (C) 1999 E lsevier Science Ireland Ltd. All rights reserved.