The Current Study: Goals, Rationale, andWorking Hypotheses

The above review suggests that different components ofscalp-recorded ERP, which are generated in differentbrain structures, may be differently sensitive to the levelat which words are processed. Previous research of lev-els-of-processing effects on ERPs focused primarily onthe N400, providing inconclusive results. On the onehand, several studies reported that the N400 was notelicited or not modulated under shallow-processing con-ditions (Bentin, Kutas, & Hillyard, 1993; Chwilla, Brown,& Hagoort, 1995; Deacon, Breton, Ritter, & Vaughan,1991). Other studies, however, reported N400 primingeffects with shallow-processing tasks (Besson, Fischler,Boaz, & Raney, 1992; Kutas & Hillyard, 1989). Therefore,a more systematic, within-subject manipulation of levelsof processing is required, in which the task effects ondifferent ERP components are assessed. To the best ofour knowledge, no such studies have been published. Amajor goal of the present study was to bridge this gap.In particular we sought (1) to investigate the neurophysi-ological manifestations of processing words at differentlevels (2) to assess the time course of processing withineach of those levels, and (3) to test the hypothesis thatlexical processes can be temporally and functionallydissociated from semantic processes.To achieve our goals we have asked participants toperform several tasks, each designed to promote activityat each of the levels of processing implied by the word-recognition model we adopted. The activity associatedwith the visual/orthographic analysis of the stimulus wasassessed comparing the ERPs elicited by letter strings tothose elicited by strings of alphanumeric symbols andnonorthographic ASCII forms, in a “font-size” discrimina-tion task. We hypothesized that orthographic analysis isautomatically induced by letter strings but not by nonor-thographic stimuli. The possibility that phonological orsemantic activity would account for the differences be-tween ERPs elicited by orthographic versus nonortho-graphic stimuli was controlled by comparing words,pseudowords, and unpronounceable strings of con-sonants (hereafter labeled “nonwords”). Words andpseudowords are distinct from nonwords by beingphonologically legal and differ from each other in theirsemantic value. We assumed that the onset of ortho-graphic processing would precede the onset of anyother activity related to the recognition of printed stim-uli. The second and third levels would be phonologi-cal/phonetic and phonological/lexical. We had no apriori predictions regarding the relative timing of thesetwo levels. Phonetic processing was promoted by arhyme-detection task, whereas the lexical processingwas induced using a series of lexical decision tasks.Phonology is probably involved in both rhyme detectionand lexical decision for letter strings. However, in theformer task it mediates the activation of phonetic struc-tures that are necessary for detecting the rhyme,whereas in the latter we presumed that phonetic struc-tures are not needed and probably not generated. There-fore, the phonology in the lexical decision task leads toword recognition and may entail other linguistic proc-esses than the “shallower” rhyme-detection task. Finally,the fourth level of processing words was semantic. Se-mantic processing was induced by asking the partici-pants to distinguish abstract from concrete words. It isimportant to realize that none of these tasks could sepa-rately provide evidence for a particular level (or kind) ofprocessing. Obviously, words can be (and probably were)processed at all levels, regardless of task. We hoped,however, that the demand characteristics of each taskwould intensify the activity at the respective levels andthat across-task comparisons in the timing and scalpdistribution of the ERPs might help disentangle oneprocess from another.To avoid speeded response-related processes, we didnot measure RTs. Rather, we used an “oddball” paradigmin which the distinction among the targets and thedistractors was based on processing the words at theabove described levels. Thus, in the font-size task thesubjects were instructed to keep a silent count of “tar-gets” that were characterized as being twice the size ofthe “distractors.” The type of stimulus (words, pseudo-words, illegal nonwords, alphanumeric symbols, orforms) was irrelevant to the task. In the rhyme task,subjects were instructed to keep a silent count of stimuli(words and pseudowords) that rhymed with a predesig-nated French word, while disregarding other words,pseudowords, and nonwords. In the lexical decisiontasks, subjects were instructed to keep a silent count ofwords either presented among nonwords (a relativelyshallow discrimination) or among pseudowords deeper discrimination). In a third lexical decision condi-tion, the subjects were instructed to keep a silent countof pseudowords interspersed among words. Finally, inthe semantic decision task, subjects were instructed tokeep a silent count of abstract words, disregarding con-crete words, pseudowords, or nonwords that were pre-sent in the same list. Table 1 describes the experimentalparadigm. Note that, both within and across lists, ourrelevant comparisons were among the distractors. Thetargets were expected to elicit a late positive component(P300), whose latency and amplitude were presumed toreflect the different levels of discrimination difficultybetween targets and distractors in each task.