ERP manifestations of processing printed words at different psycholinguistic levels: Time course and scalp distribution

The aim of the present study was to examine the time course and scalp distr ibution of electrophysiological manifestations of the visual word recogniti on mechanism. Event-related potentials (ERPs) elicited by visually presente d lists of words were recorded while subjects were involved in a series of oddball tasks. The distinction between the designated target and nontarget stimuli was manipulated to induce a different level of processing in each s ession (visual, phonological/phonetic, phonological/lexical, and semantic). The ERPs of main interest in this study were those elicited by nontarget s timuli. In the visual task the targets were twice as big as the nontargets. Words, pseudowords, strings of consonants, strings of alphanumeric symbols , and strings of forms elicited a sharp negative peak at 170 msec (N170); t heir distribution was limited to the occipito-temporal sites. For the left hemisphere electrode sites, the N170 was larger for orthographic than for n onorthographic stimuli and vice versa for the right hemisphere. The ERPs el icited by all orthographic stimuli formed a clearly distinct cluster that w as different from the ERPs elicited by nonorthographic stimuli. In the phon ological/phonetic decision task the targets were words and pseudowords rhym ing with the French word vitrail, whereas the nontargets were words, pseudo words, and strings of consonants that did not rhyme with vitrail. The most conspicuous potential was a negative peak at 320 msec, which was similarly elicited by pronounceable stimuli but not by nonpronounceable stimuli. The N320 was bilaterally distributed over the middle temporal lobe and was sign ificantly larger over the left than over the right hemisphere. In the phono logical/lexical processing task we compared the ERPs elicited by strings of consonants (among which words were selected), pseudowords (among which wor ds were selected), and by words (among which pseudowords were selected). Th e most conspicuous potential in these tasks was a negative potential peakin g at 350 msec (N350) elicited by phonologically legal but not by phonologic ally illegal stimuli. The distribution of the N350 was similar to that of t he N320, but it was broader and including temporo-parietal areas that were not activated in the "rhyme" task. Finally, in the semantic task the target s were abstract words, and the nontargets were concrete words, pseudowords, and strings of consonants. The negative potential in this task peaked at 4 50 msec. Unlike the lexical decision, the negative peak in this task signif icantly distinguished not only between phonologically legal and illegal wor ds but also between meaningful (words) and meaningless (pseudowords) phonol ogically legal structures. The distribution of the N450 included the areas activated in the lexical decision task but also areas in the fronto-central regions. The present data corroborated the functional neuroanatomy of word recognition systems suggested by other neuroimaging methods and described their timecourse, supporting a cascade-type process that involves different but interconnected neural modules, each responsible for a different level of processing word-related information.