Word, Pseudoword, and Nonword Processing: A Multitask Comparison Using Event-Related Brain Potentials

Johannes C. Ziegler, MireUe Besson, Arthur M. Jacobs, and Tatjana A. Nazir Center for Research in Cognitive Neuroscience, Marseille, France Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021

Thomas H. Carr Michigan State University, East Lansing

Abstract Event-related brain potentials (ERPs) to words, pseudo- to nonwords, suggesting a primary role of orthographic and words, and nonwords were recorded in three different tasks. A phonological processing in the delayed letter search task. To letter search task was used in Experiment 1. Performance was increase semantic processing, a categorization task was used in affected by whether the target letter occurred in a word, a Experiment 3. Early differences between ERPs to words and pseudoword, or a random nonword. ERP results corroborated pseudowords at left posterior and anterior locations suggested the behavioral results, showing small but reliable ERP differ- a rapid activation of lexico-semantic information. These find- ences between the three stimulus types. Words and pseudo- ings suggest that the use of ERPs in a multiple task design words differed from nonwords at posterior sites, whereas makes it possible to track the time course and the activation words differed from pseudowords and nonwords at anterior of multiple sources of linguistic information when processing sites. Since deciding whether the target letter was present or words, pseudowords, and nonwords. The task-dependent na- absent co-occurred with stimulus processing in Experiment 1, ture of the effects suggests that the language system can use a delayed letter search task was used in Experiment 2. ERPs to multiple sources of linguistic information in flexible and adap- words and pseudowords were similar and differed from ERPs tive ways. W

INTRODUCTION spect to its spatial resolution. Therefore, our experimen- tal design tried to take advantage of results from other, Over the past 15 years, the ERP methodology has been more recently developed brain imaging techniques widely and successfully applied to investigate various whose capabilities complement those of ERPs. Research aspects of language processing (e.g., Bentin, McCarthy, & on the biological foundations of language has benefited Wood, 1985;Besson, Kutas, & Van Petten, 1992;Brown & greatly from the development of blood-flow-based imag- Hagoort, 1993; Fischler & Raney, 1991; Garnsey, 1985; ing techniques with good spatial resolution, such as Kutas & Hillyard, 1980,1984, 1989;Kutas & Van Petten, positron emission tomography (PET) and functional 1988;Rugg,1990;Van Petten & Kutas, 1987). In particular, magnetic resonance imaging (fMRI). Results of a number because of its excellent temporal resolution, this tech- of PET experiments have demonstrated that different nique provides a powerful tool for studying the time cortical areas are activated by the visual or auditory course of mental operations and the order in which such presentation of words or wordlike stimuli (Demonet et processing operations are performed (Coles, Gratton, al., 1992;Friston, Frith, Liddle, & Frackowiak, 1991;Frith, Bashore, Eriksen, & Donchin, 1985;Meyer, Osman, Irwin, Friston, Liddle, & Frackowiak, 1991;Mazoyer et al., 1993; & Yantis, 1988;Miller & Hackley, 1992). In the present Petersen, Fox, Posner, Mintun, & Raichle, 1988, 1989; experiments, we used the ERP methodology to investi- Petersen, Fox, Snyder, & Raichle, 1990;Posner, Petersen, gate the time course and the activation of 1-inguistic Fox, & Raichle, 1988). Specifically,for visual presentation, information during the processing of words, pseudo- areas of the left medial prestriate visual cortex have been words, and nonwords in three different tasks. shown to be activated by both words and pseudowords While the ERP technique is known for its fine tempo- but not by random nonwords or false fonts. It has been ral resolution, it provides weaker constraints with re- proposed that these left medial regions are involved in

0 1997 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 9.6, pp. 758- 775

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 computing “visual word forms”-representations of or- These results were interpreted as “somewhat consistent thographic structure-from visual features (Petersen et with the kinds of results described in the PET study” al., 1988; 1990;Shallice, 1988). Furthermore, activation of (Compton et al., p. 309). the left inferior prefrontal cortex has been demonstrated A similarly motivated application of ERP methodology for words but not for pseudowords. This activation has to determining the time course of brain activation iden- been interpreted as reflecting computations linked with tified via PET comes from Snyder, Abdullaev, Posner, and lexically specific semantic processing (Petersen et al., Raichle (1995). These investigators compared a target 1988,1990) and particularly with selection of task-appro- task requiring subjects to generate an appropriate verb priate semantic information (Posner & Carr, 1992;Posner in response to each of a series of visually presented & Raichle, 1994). nouns to baseline task requiring subjects to read each

a Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 Recently, Carr and Posner (1 995) have integrated noun aloud (in the “generate verbs” task the subject these neuroimaging results in a larger framework based might see “hammer” and say “hit”or “pound,”whereas on the “primary gateway” hypothesis. According to this in the “read aloud”or “naming”task the subject who saw hypothesis, the prestriate visual word form system “hammer”would say “hammer”).PET and fMRI studies (pVWFS) is responsible for the elaboration of ortho- (McCarthy, Blamire, Rothman, Gruetter, & Shulman, 1993; graphic information into a code representing an organ- Petersen et al., 1990;Raichle et al., 1994) have found the ized sequence of letter identities. This code is left inferior prefrontal cortex, as well as the anterior transmitted forward to temporal and prefrontal areas for cingulate cortex and Wernicke’s area, to be more highly further lexical and semantic analysis. Unfortunately,how- activated in the generate verbs task than in the read ever, PET’S poor time resolution has not allowed this aloud task. Activity in these frontal and temporoparietal hypothesis about time course to be tested. Since an regions has been attributed to the semantic processing average of 40-sec scanning time is required to construct that is specific to generating a verb that fits with a a PET image in the most commonly used Oi5 tracer particular noun. Following up on these PET and fMRI method, images represent the composite activity of the results, Snyder et al. (1 995) computed difference waves brain, averaged over many trials in which a particular from ERPs measured in the two tasks. Two significant type of judgment is made about a particular type of effects were found, one earlier in the ERP waveform and letter string. Under these conditions, if several brain one later. First, compared to reading the nouns, generat- regions participate in the processing being imaged, it is ing verbs produced greater positivity at frontal elec- not possible to determine which region becomes acti- trodes around 200 to 250 msec after onset of the noun. vated first on any given trial and whether the order in This difference was largest at the left prefrontal record- which various regions become activated changes with ing sites. Source analysis identified two generators under- stimulus or task parameters. lying this positivity, one in the left inferior prefrontal Compton, Grossenbacher, Posner, and Tucker (1 99 1) cortex and the other in the anterior cingulate. Second, attempted to use the good time resolution of ERPs to generating verbs produced greater positivity in the left address this problem. They compared ERPs across three temporoparietal sites accompanied by greater negativity different tasks that varied with respect to the type of at the right temporal sites around 600 to 700 msec after processing they required (visual/orthographic versus onset of the noun. The best-fitting generator for this lexico-semantic). In the first task, participants passively effect lay in Wernicke’s area. viewed words and random nonwords. In the second task, The findings of Compton et al. (1991) and Snyder et they were asked to search for a thickened line segment al. (1 995) suggest that measures of blood flow and meas- or a lowercase letter within words and nonwords. In the ures of event-related electrical activity show conver- third task, they performed lexical decisions on words gence in the brain regions they identlfy as differentially and nonwords. Results showed that the earliest ERP active in particular linguistic performances. Such conver- difference between words and nonwords in the passive gence allows conclusions to be drawn from the ERP data viewing task was found over posterior temporal and about the timing of the activation of these regions and parietal sites of both hemispheres, although it was left encourages further attempts to use ERPs as a source of lateralized when the Hjorth (1982) transform was ap- information about time course and task dependence.The plied to the ERP data. This difference, with the ERPs to basic logic is as follows: If two or more linguistic opera- words being more positive than the ERPs to nonwords, tions can be attributed to fairly well-defined brain loci started at about the 125-msec poststimulus onset over using evidence from neuroimaging, and if those brain the left posterior temporal sites. In contrast, results in loci are sufficiently separated in brain space for their the lexical decision task showed that ERPs to words activity to be differentially detectable at different points were more negative than the ERPs to nonwords in the in the scalp topography of an ERP recording array, the first 200 msec at most scalp sites. However, starting at relative time course of ERPs measured at those points in about 200 msec, the ERPs to words were again more the scalp topography can be translated into conclusions positive than the ERPs to nonwords at left frontal sites. about the relative time course of the involvement of the

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 linguistic operations in the flow of information from category defined by a prime word. Such a task unambi- stimulus to response.’ guously requires semantic processing, at least to dis- In the present series of experiments, we recorded criminate category members from pseudowords and ERPs in three tasks that differed with respect to the words that do not belong to the primed category. Non- amount of letter-specific versus word-specific (lexico-se- words could conceivably be rejected on the basis of an mantic) processing required to perform the task (two orthographic analysis that shows they could not be variants of a letter search task, which in principle re- words (Carr, Posner, Pollatsek, & Snyder, 1979; Shulman quired no specifically lexical or semantic processing, & Davison, 1977). For similar purposes, Compton et al. versus a semantic categorization task that could not be used a lexical decision task but failed to find clear-cut

performed without semantic processing). Moreover, results. Although ERPs to words and nonwords differed Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 three types of stimuli (words, pseudowords, and non- over both posterior and frontal locations, there were no words) were presented that differed with respect to the clear differences in the onset of these differences. Since amount of word-specific information they carried. Based it has been argued that performance in the lexical deci- on the ERP and PET results described above, we ex- sion task might not necessarily require the computation pected to find differences among stimulus types over of word meanings (Coltheart, Davelaar,Jonasson, & Bes- posterior regions in all tasks. In addition, we expected to ner, 1977; Henderson, 1982; Jacobs & Grainger, 1992; find differences among stimulus types over anterior re- Seidenberg & McClelland, 1989), we tried to obtain gions when the task clearly required semantic process- clearer results regarding the time course of these effects ing. Gaining further understanding of the time course by using an unambiguously semantic task. We predicted and task sensitivity of these differences was of primary that the main ERP differences would be found between interest. words, on the one hand, and pseudowords and non- We used an approach similar to Compton et al.’s words, on the other hand, because only words have (1991), but we introduced several changes relative to meaning. Based on the PET findings, we expected these their study. First, in addition to words and nonwords, differences to be larger over the left anterior sites than pseudowords were included to further track the proc- over the posterior sites. essing operations related to the elaboration of ortho- graphic information. Pseudowords are orthographically EXPERIMENT 1 legal and pronounceable letter strings without meaning. Nonwords, in contrast, are orthographically and phono- The aim of Experiment 1 was twofold. The first aim was logically illegal random letter strings. We predicted that to determine the onset latency of the ERP differences the earliest ERP differences should be found between between three types of stimuli that differ with respect nonwords, on the one hand, and words and pseudo- to the linguistic information they may possibly activate. words, on the other hand, because nonwords can be Orthographically illegal nonwords should not activate discriminated from words or pseudowords on the basis orthographic,phonological, or semantic sources of infor- of visual/orthographic structure (and/or pronounceabil- mation to any significant extent. Pseudowords should ity) of the letter string. Based on Compton et al.’ s (1991) activate orthographic and phonological sources of infor- and PET findings, we expected these differences to be mation but not (or only partially) lexical information. larger over the left posterior sites than over the anterior Only words should maximally activate all these sources sites. of information. In the letter search task, the availability Second, to investigate the task dependency of the of these multiple sources of information seems to processing operations performed on the stimulus, we influence performance. Previous results have shown that used tasks that clearly required different levels of word participants are faster and more accurate when detect- processing for their decisions. In both Experiments 1 ing target letters in words and pseudowords than in and 2, we used a letter search task in which participants nonwords (Baron & Thurstone, 1973; Krueger, 1975, were asked to decide whether or not a target letter 1992; Ziegler & Jacobs, 1995). This phenomenon is re- presented on each trial was present in the stimulus. ferred to as a variant of the word superiority effect Similar to the thickened segment or lowercase tasks (Reicher, 1969; Wheeler, 1970). While the letter search used by Compton et al. (1991), this task does not, in task could, in principle, be performed by matching the principle, require lexical or semantic processing of the visual features of the target letter with the visual features stimuli. However, unlike the thickened segment task, it of the letter string, the word superiority effect seems to clearly requires letter-specific shape processing and reflect the possibility that words and pseudowords hence should engage at least the earliest stages of the benefit from multiple sources of linguistic information orthographic system per se. In Experiment 3, we used a that are not available for nonwords (Carr, 1986; Carr & semantic categorization task in order to increase the Pollatsek, 1985; Ziegler 81 Jacobs, 1995). It was thus of need for lexico-semantic processing. Participants were interest to use ERPs to determine whether words, asked to decide whether or not the stimulus (word, pseudowords, and nonwords activate different sources pseudoword, or nonword) belonged to the semantic of linguistic information when the task does not require

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 any type of linguistic information to be activated. Fur- words (Yes: F(1, 24) = 33.1,p < 0.001; No: F(1, 24) = thermore, it was of interest to track down the time 24.6; p < 0.001). Analysis of the error rate showed a course of these potential sources of activation. significant main effect of stimulus type (F(2,48)= 4.1, The second aim of this experiment was to further p c 0.02) and a significant stimulus by response type investigate the primary gateway hypothesis (Carr & Pos- interaction (F(2, 48) = 7.6,p c O.OOl), reflecting the ner, 1995; Posner & Carr, 1992). As mentioned above, significantly higher error rate in the target-present non- regions of the left medial prestriate visual cortex may be word condition than in the other conditions. involved in computing visual word forms from visual features. The pVWFS could function as a primary gate-

way that elaborates the visual and orthographic form of ERP Results Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 a letter string. This orthographic information would then As can be seen in Figures 1 and 2, an Nl-P2 complex be transmitted to temporal and prefrontal areas for fur- was elicited by the presentation of the fixation bars, the ther lexical and semantic analysis. Analyses of the time target letter, and the stimulus. Following the N1-P2 to the course of the differences between words, pseudowords, stimulus, a negative component, peaking around 350 and nonwords at posterior versus anterior recording msec was elicited by the three types of stimuli and was sites were performed to examine this hypothesis. larger for words and pseudowords than for nonwords. A Experiment 1 consisted of a behavioral and an elec- late positive component (LPC) was then generated that trophysiological part. In the first part, we tested whether appeared to be slightly larger for nonwords than for our stimulus material would generate the standard word words or pseudowords. superiority effect in the letter search task using reaction In order to investigate the differences between stim- time (RT) as the dependent variable. In the second part, ulus types with respect to the amplitude of the nega- we recorded ERPs using the same stimuli and partici- tive component, analyses were performed in the 300 to pants. 500-msec poststimulus latency band. Results of three- way ANOVAs, including stimulus type (words, pseudo- RESULTS words, and nonwords), response type (target present vs. target absent), and electrode (seven levels) as factors, Behavioral Results revealed main effects of stimulus type (F(2,48)= 4.60, RTs for correct responses and error rates as a function MSe = 27.27,epsilon = 0.90,p< 0.01) and response type of stimulus type (words, pseudowords, and nonwords) (F(1, 24) = 16.25, MSe = 34.81,p < 0.001).In contrast, are presented in Table 1. They were averaged across the stimulus by response type interaction did not reach target position and stimulus length. significance (F(2,48)= 2.30,MSe = 15.41,p = O.ll).The Two-way repeated measures ANOVAs were performed ERP in this latency band was more positive for nonwords with participants and items as the random factor. The and words (3.86 yV and 3.61 yV, respectively) than for analyses revealed clear main effects of stimulus type and pseudowords (2.72 yV) and for target present (4.13 pv) response type (F1(2,48) = 89.7,p < 0.0001;F2(2, 88) = than for target absent (2.66 yv) trials. The main effect of 37.4,p < 0.0001 and Fl(1, 24) = 46.6;~< 0.0001, Fz(1, electrode location was significant, but the stimulus by 44) = 97.5,p < 0.0001, respectively). The stimulus by electrode interaction failed to reach significance (F(6, response type interaction was not significant either by 144) = 7.34,MSe = 114, epsilon = 0.43,p < 0.001 and items or by participants (F < 1). F < 1, respectively). Planned comparisons revealed that RTs for both Yes For target present trials, we predicted differences in and No responses were reliably shorter for words than the time course of the effects of stimulus type depend- for pseudowords (Yes: F(1, 24) = 6.5,p < 0.01;No: F(1, ing upon electrode location. Since the 300 to 500-msec 24) = l0.3,p < 0.002,respectively) and nonwords (Yes: latency band was too broad to closely examine these F(1,24) = 69.1,p< O.OOl;No:F(l,24) = 66.9;~< 0.001). potential differences, fine-grained analyses of successive RTs were also shorter for pseudowords than for non- 25-msec epochs of the ERPs, from 100 to 600 msec, were

Table 1. Mean Reaction Times for Correct Responses and Mean Percentage of Errors as a Function of Stimulus Type and Response vpe in Experiment 1

TaQet Present Trials Target Absent Trials Stimulus Type RT(msecj Errors ph) RT(msecj Errorsphj Words 557 5.24 602 6.58 Pseudow o r d s 573 5.42 623 5.07 Nonwords 61 1 9.33 655 5.51

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 1Fixationban 1 Targetbttw lsnmuius

FRONTAL FRONTAL

CENTRAL CENTRAL Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021

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1 -a PV I. I I1.1. I I I, 11 D IwIIw12wlW2wo ma - Words - Words ____.Pseudo-Words ____.Pseudo-Words __ Non-Words - Non-Words

Figure 1. Grand average ERPs (N = 25) for words, pseudowords, Figure 2. Grand average ERPs (N = 25) for words, pseudowords, and nonwords on target present trials in the letter search task of Ex- and nonwords on target absent trials in the letter search task of Ex- periment 1 (Yes responses only; number of trials contributing to the periment 1 (No responses; number of trials contributing to the aver- averages, n: for words, n = 959; for pseudowords, n = 951; for non- ages, n: for words, n = 939; for pseudowords, n = 959;for nonwords, words, n = 942). In this and subsequent figures, ERP recordings start n = 954). 200 msec before the presentation of the fixation bars, indicated by the vertical line. Traces corresponding to each recording site are pre- sented, and negative is up.

type nor the effect of response type reached significance = MSe = > and F(1, 24) = performed for each electrode location. As expected, re- (F(2,48) 3.06, 23.73,p 0.05 3.48,MSe = > 0.05,respectively). Furthermore, sults showed different patterns of activation between 30.98,p the interaction between stimulus type and electrode anterior and posterior locations. At the left posterior location was not significant < 1). location, starting at 250 msec, the ERPs to nonwords (F were more positive than the ERE% to words and pseudowords, which did not differ from one another. DISCUSSION Similar results were obtained at the right posterior loca- tion, but the onset latency of these differences was A letter search task was used in the present experiment. somewhat later, at 375 msec, than on the left hemi- Participants had to decide whether a target letter was sphere. In contrast,at the left and right anterior location, present or absent in letter strings that were either words, starting at 275 msec, the ERPs to words were more pseudowords, or nonwords. Analysis of the behavioral positive than the ERPs to pseudowords and nonwords, results revealed reliable word and pseudoword supe- which did not differ from one another. riority effects: Letter search performance was best (fast- Analyses of the LPC in the 500 to 900-msec latency est RTs and fewest errors) for words, intermediate for band revealed that neither the main effect of stimulus pseudowords, and worst for nonwords. These results

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 replicate previous findings of word and pseudoword This overall pattern of results is compatible with the superiority effects in perceptual identification tasks primary gateway hypothesis (Carr & Posner, 1995). First, (Baron & Thurstone, 1973; Carr, Davidson, & Hawkins, ERPs to words and pseudowords did not dissociate at 1978;Krueger, 1975, 1992;Reicher, 1969;Wheeler, 1970; the left posterior location but dissociated later at the left Ziegler & Jacobs, 1995; Ziegler, Van Orden, & Jacobs, anterior location. Second, ERPs to words and pseudo- 1997). words were different from ERPs to nonwords at the left Note that the letter search task only requires a shallow posterior location, whereas ERPs to words were different processing of the letter strings.However, if this task were from ERPs to pseudowords and nonwords at the left simply done by matching visual features of the target anterior location. In terms of the primary gateway hy-

letter against visual features of the letter string, we pothesis, this is consistent with the proposition that, at Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 should not have obtained differences between stimulus structures proximal to the left posterior recording loca- types in letter search performance or in the ERP pat- tion, orthographic and phonological information provide terns. This is especially true since participants were al- a relatively early constraint on the processing of letter ways searching for identical target letters at the same strings and may function as a gateway to deeper linguis- serial position across all three stimulus types (cf. Ziegler tic processing at more anterior locations. & Jacobs, 1995). Our finding of ERP and behavioral An alternative interpretation for the differences be- differences between words, pseudowords, and non- tween words, pseudowords, and nonwords should be words, however, suggests that some form of linguistic considered. Instead of reflecting intrinsic differences in information (e.g., orthographic, phonological, or seman- the processing of words, pseudowords, and nonwords, tic) was extracted and influenced letter search perfor- the effects reported here may be specific to the task mance. performed on the stimulus. Results of numerous ERP This evidence for extraction of linguistic information experiments have shown that decision-relatedprocesses enables us to pursue Carr and Posner’s (1995) primary are associated with increased late positivities (such as gateway hypothesis. Consistent with that idea, further the P300 component; Bentin et al., 1985; Boddy, 1986; analysis of the ERP results revealed differences between Hillyard, Squires, Bauer, & Lindsay, 1971; Polich, 1989; stimulus types that were dependent upon electrode lo- Sutton, Ruchkin, Munson, Kietzman, & Hammer, 1982). cation. Starting at the 250-msec poststimulus onset, the Insofar as participants were asked to decide as quickly ERPs to words and pseudowords differed from the ERPs as possible whether or not the target letter was present to nonwords at the left posterior location. In contrast, in the stimulus,differences between stimulus types may starting at 275 msec, the ERPs to words differed from have been contaminated by decision-relatedpositivities. both the ERPs to pseudowords and nonwords at the left This interpretation is also corroborated by the observa- anterior location. These results are in line with previous tion that the differences in the ERPs to words, pseudo- ERP (Compton et al., 1991) and PET results (Petersen et words, and nonwords were modulated by the pres- al., 1988, 1989, 1990; Posner et al., 1988). At the left ence or absence of the target letter (compare Figures 1 posterior location, that is, at a location close to what has and 2). been called the Visual Word Form System, the ERPs to illegal letter strings (nonwords) were significantly more positive than the ERPs to legal letter strings (words and EXPERIMENT 2 pseudowords), which did not differ from one another. This difference is in the same direction as reported by With respect to the ERP differences as a function of Compton et al. using a lexical decision task but is oppo- stimulus type (word, pseudoword, or nonword), two site to the pattern found using a passive viewing task interpretations were proposed above. First, they could (where the ERPs to words were more positive than the reflect genuine encoding differences between words, ERPs to nonwords). Furthermore, the onset latency of pseudowords, and nonwords because those stimulus the nonword versus word/pseudoword differences oc- types activate different sources of linguistic information. curred somewhat later in our experiment than in Comp- Second, they could result from task-specific decision-re- ton et a1.k experiment (where it started at 125 msec, in lated factors that occur postencoding. To disentangle the passive viewing task). These differences are probably these two interpretations, a delayed letter search task because different tasks were used in Compton et al.’s was used in Experiment 2. Thus, rather than presenting and our study (see also Snyder et al., 1995). At the left the target letter first, the letter string (word, pseudoword, anterior location, starting at 275 msec, the ERPs to words or nonword) was presented first and was then followed differed from the ERPs to pseudowords and nonwords, by the target letter. This modification allowed us to which did not differ from one another. These findings record ERPs to the letter strings when a decision about again are in line with the results of the PET experiments the target cannot be made since the target letter to be reported above, revealing the involvement of left ante- searched for has not yet been presented. ERPs to words, rior regions when stimulus discrimination is based on pseudowords, and nonwords are thus uncontaminated semantic features. by any decision-relatedeffects (see also Kutas & Hillyard,

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 1989). Except for this modification, the stimuli were that since no decision could be made before the target identical to those of Experiment 1. letter was presented, ERPs were collapsed across re- sponse type (target present vs. absent trials). Results in the 300 to 500 msec latency band poststimulus presen- RESULTS tation revealed a main effect of stimulus type (F(2,30) = As can be seen in Figure 3, an Nl-P2 complex was 13.05, MSe = 8.42, epsilon = 0.96,p < 0.001):The ERP elicited by the presentation of the fixation bars, the in this latency band was more positive to nonwords stimulus, and the target letter. As in the previous experi- (5.62 pV) than to words (3.70 pV) and pseudowords ment, a negative component developed around the 350- (4.23 pV), which did not statistically differ from one msec poststimulus onset that was larger for words and another. The main effect of electrode location was sig- Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 pseudowords than for nonwords. An LPC was then gen- nificant (F(6, 90) = 6.62,MSe = 24.34, epsilon = 0.37, erated that was larger for nonwords than for words or p < O.OOl), as was the stimulus by electrode interaction pseudowords. A negative component was also generated (F(12,lSO)= 6.61,M.e= 0.87,epsilon = 0.40,p< 0.001). in response to the target letter that was larger when the Topographical analysis showed that the ERP in the 300 preceding stimulus was a nonword than when it was a to 500-msec latency band was more positive at parietal word or a pseudoword. (6.26 pV) than frontal and central locations (4.69 pV and In order to compare results, analyses similar to Experi- 3.86 py respectively;F(2, 30) = 5.20,MSe = 13.78, epsi- ment 1 were performed in Experiment 2.Note, however, lon = 0.68,p < 0.03). It was also more positive over anterior (6.09 pV) than posterior lateral locations (2.32 pV; F(1, 15) = 9.82, MSe = 69.71,p < 0.006) and more positive over the right (4.80 pV) than left hemisphere Fixation stimulus Target (3.61 pV;F(l,15) = 8.05,MSe = 8.56,~< 0.01). 1bars 1 1lener In order to track down the time course of the effects of stimulus type depending upon electrode location, FRONTAL fine-grained analyses of successive 25-msec ERP epochs were performed as in Experiment 1.Starting at 225 msec poststimulus onset, the ERPs to nonwords were more CENTRAL positive than the ERPs to both words and pseudowords at the left posterior location. This difference reached significance at the right posterior location at 300 and at

PARIETAL L. A 400 msec poststimulus, at the left anterior location. Fi- nally, differences in the same direction were found at the right anterior location, starting at 575 msec poststimulus 1 L. ANTERIOR Nonwords onset. The difference between words and pseudowords TEMPORAL was not statistically significant. Analyses of the LPC in the 500 to 900-msec latency R. ANTERIOR band revealed a main effect of stimulus type (F(2,30)= TEMPORAL 20.66, MSe = 11.44, epsilon = 0.99,p< 0.001):The LPC I was larger for nonwords (6.75 pV) than for words (4.15 or pseudowords (4.33 the latter two did not ” pv pV); a statistically differ from one another. Results of topog- L.POSTERIOR raphical analyses showed a significant stimulus by elec- trode interaction:While the LPC for nonwords was larger over the parietal than over the centro-frontal locations, R.POSTERIOR TEMPORAL the distribution of the LPC for words and pseudowords was more equipotential across midline sites (F(4,60) = 1 13.70,MSe = 0.69,epsilon = 0.44,p < 0.001).

I,I, 1, I,I. I I I1 0 4 Bw 12w 16W ZMO rns Turning to the ERPs elicited by the target letter, results showed that the negativity that developed in the 300 to - Words ____.Pseudo-Words 600-msec latency band postletter onset was larger for - Non-Words nonwords (3.23 pV; i.e., the mean amplitude in this latency band was less positive) than for words (4.73 pV) or pseudowords (4.64 pV; (F(2,30)= 5.82,MSe = 13.65, Figure 3. Grand average ERPs (N = 16) for words, pseudowords, epsilon = 0.98,p < 0.007).The main effect of electrode and nonwords in the delayed letter search task of Experiment 2. ERPs are averaged across target present and target absent trials (num- location was significant (F(6,90) = 12.20, MSe = 33.16, ber of trials contributing to the averages, n:for words, n = 1232; for epsilon = 0.49,p < 0.001),as was the stimulus by elec- pseudowords, n = 1234; for nonwords, n = 1192). trode interaction (F(12,lSO)= 4.06,MSe = 1.04,epsilon

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 = 0.41, p c 0.002). Results of topographical analyses had to keep the target letter in working memory in order showed that the difference between words and to decide whether or not it was present within the letter pseudowords versus nonwords was largest frontally string. In Experiment 2, participants had to keep the (3.05 pV), intermediate centrally (2.12 pV), and smallest entire letter strings in working memory until they knew parietally (1.11 pV). No difference was found in the what the target letter was. Thus, the memory load was 600 to 900-msec latency band following target letter greater in Experiment 2 than in Experiment 1. How can onset. participants efficiently keep words, pseudowords, and nonwords in working memory? Results of many experi- ments in the literature suggest that phonological recod- DISCUSSION

ing plays a primary role for maintaining pronounceable Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 The main aim of this experiment was to study the ERPs stimuli in working memory (e.g.,phonological loop, Bad- elicited by words, pseudowords, and nonwords in the deley, 1986; Carr, 1986; Ziegler & Jacobs, 1995). Clearly, stimulusencoding phase of the letter search task uncon- only words and pseudowords, not nonwords, can acti- taminated by decision-related factors. In order to achieve vate phonological sources of information and, thus, this aim, the presentation of the target letter followed benefit from phonological recoding (Carr & Pollatsek, the presentation of the letter strings so that, upon pres- 1985). Therefore, our results suggest that similar strate- entation of the letter strings, participants could not de- gies and processes may be used for the encoding of both cide whether the target letter was present or not. Results words and pseudowords, whereas different strategies showed that, starting around 225 to 300 msec poststimu- may be used for the encoding of nonwords. The use of lus onset, the ERPs to nonwords were more positive than specific strategies to maintain nonwords in working the ERPs to both words or pseudowords, which did not memory may also explain why the amplitude of the LPC differ from one another. Thus, these results replicate the to nonwords was larger in Experiment 2 than in Experi- finding of Experiment 1 that nonwords were associated ment 1. In other words, the differences between non- with more positive ERPs than were words or pseudo- words, on the one hand, and words and pseudowords, words. This increased positivity to nonwords was pre- on the other hand, may reflect differences in the proc- sent at all lateral recording sites (although it occurred essing operations (in particular, phonological recoding) with different onset latencies) and lasted until the target that are performed on the letter strings. letter was presented. An alternative interpretation of the differences found Note that the positivity associated with nonwords was between words, pseudowords, and nonwords should be larger in Experiment 2 than in Experiment 1. Insofar as considered,however. Although words, pseudowords, and the presentation of the target letter followed the presen- nonwords had the same objective probability of occur- tation of the letters strings in Experiment 2, this larger rence (33%) in both Experiments 1 and 2, nonwords are positivity to nonwords than to words or pseudowords highly unfamiliar letter strings because they are ortho- cannot reflect decision-related processes. One may thus graphically illegal and not pronounceable. In contrast, conclude that the negative components that develop in words are highly familiar letter strings. Pseudowords the 300- to 500-msec latency bands are specifically asso- could also be considered familiar because they contain ciated with the processing of words and pseudowords. familiar letter sequences that are orthographically legal Nonwords, in contrast, are processed differently and are and pronounceable. Consequently, the probability of en- associated with large positive components. Note that countering unfamiliar letters strings, consisting of only this pattern of data is fairly similar to the one obtained consonants (nonwords: 33%),was lower than the prob- in a study by Smith and Halgren (1987) using a lexical ability of seeing familiar letters strings (words and decision task: ERPs to nonwords were more positive- pseudowords: 66%).Numerous results have shown that going and lacked an N400 relative to either pseudo- infrequent, unfamiliar, and surprising events are associ- words or words. Consequently, these ERP differences ated with large late positive components of the P300- may reflect intrinsic differences in the processing of family (see Donchin, 1981; Donchin & Coles, 1988; stimuli that (1) may activate orthographic,phonological, Verleger, 1988, for reviews). The centro-parietal distribu- and lexical sources of information (i.e., words), (2) may tion of the positive components elicited by nonwords in activate orthographic and phonological sources of infor- the present experiment corresponds to the typical dis- mation but only partially activate lexical information tribution of the P300 component. In this respect, note (i.e., pseudowords), and (3) cannot significantly or sys that while the LPC was larger parietally for nonwords, it tematically activate any of those multiple sources of was equipotential across midline sites for words and linguistic information (i.e., nonwords). pseudowords. This again seems to indicate that different In contrast to the results of Experiment 1, no differ- underlying generators may have been activated by the ence was found between words and pseudowords. While three stimulus types. Therefore, the differences found the task to be performed on the stimulus was the same between words and pseudowords, on one side, and non- in both experiments, the underlying processes called words, on the other side, may reflect differences in their into play may be different. In Experiment 1, participants relative probability of occurrence. This, however, would

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 be the case whenever nonwords are manipulated within an experimental design. To summarize, our results show that negative compo- FRONTAL nents may be elicited when linguistic information has

been activated, as in the case of both words and A I pseudowords. Positive components, in contrast, would be elicited when stimuli cannot activate any specific CENTRAL linguistic information, as in the case of nonwords. The finding of early differences between nonwords,

on one side, and words and pseudowords, on the other Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 side, over posterior locations, is in line with the primary PARIETAL gateway hypothesis. These differences may reflect the differential activation of the visual word form system Pseudowords when stimuli contain or do not contain linguistic prop- L. ANTERIOR erties. In contrast to what was predicted on the basis of TEMPORAL v( this hypothesis, however, no differences were found be- tween words and pseudowords over anterior sites. To R. ANTERIOR I some extent, this is not surprising insofar as the letter TEMPORAL search task used here did not necessarily require lexi- cal/semantic processing of the letter strings. Instead, the similarity of the ERPs to words and pseudowords strongly suggests that similar processing operations were L.POSTERIOR performed for both stimulus types (e.g., phonological TEMPORAL recoding). In order to clearly provoke semantic process- ing, which should differentiate words from pseudo- R.POSTERIOR TEMPORAL words, a semantic categorization task was used in Experiment 3. - I I I I I I I I I I I I I] 0 #OWalW2YJU ms EXPERIMENT 3 - Words ____.Pseudo-Words Results of Experiments 1 and 2 showed reliable ERP __ Non-Words differences between words or pseudowords, on the one hand, and nonwords, on the other. In contrast, no clear differences emerged between words and pseudowords. Figure 4. Grand average ERPs (N = 24) for words, pseudowords, To some extent, this could be expected insofar as the and nonwords in the semantic categorization task of Experiment 3 only difference between these stimulus types is that for all trials in which the stimulus did not belong to the semantic two category defined by the prime words (No responses; number of words have meaning while pseudowords do not. Since trials contributing to the averages, n: for words, n = 600;for pseudo- computation of word meaning was not necessary to words, n = 620; for nonwords, n = 602). perform the letter search task, this task might not be sensitive enough to generate differences between pseudowords and words. In order to increase the differ- ences in the ERPs for words and pseudowords, a seman- tic categorization task was used in Experiment 3. in response to both words and pseudowords.Nonwords, Deciding whether a letter string belongs to a semantic in contrast, were associated with a large LPC that lasted category defined by a prime word clearly requires se- from 300 msec to the end of the recording period. mantic processing. Comparison Between Words, Pseudowords, and RESULTS Nonwords for No Responses As can be seen in Figure 4,an Nl-P2 complex was Insofar as pseudowords and nonwords cannot, by defini- elicited by the presentation of the fixation bars, the tion, belong to any semantic category and are therefore prime word, and the stimulus. Early differences in the always associated with No responses, only the ERPs to Nl-P2 complex emerged as a function of stimulus type. correct No responses were compared across the three Specifically,the amplitude of the P2 component seemed stimulus types. Furthermore,in order to compare results larger for words and nonwords than for pseudowords. between experiments, analyses were performed in the Following the Nl-P2 to the stimulus, a negative compo- same latency bands. Results in the 300- to 500-msec nent, peaking around 400 msec, was clearly identifiable posttarget latency band revealed significant differences,

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 with larger negative components to pseudowords (-1.37 Comparison Between Yes and No Responses for pV) than words (1.09 pV) and no such negativity to Words nonwords (4.64 pV; F(2, 46) = 60.75, MSe = 25.39, As can be seen in Figure 5, a very localized effect of epsilon = 0.83,p< 0.001).The main effect of electrode priming is present between 300 and 600 msec. Results location and the stimulus by electrode interaction were of two-way ANOVAs, including priming (primed vs. un- significant (F(6, 138) = 18.52, MSe = 48.10, epsilon = primed) and electrodes (seven levels) as factors, showed 0.41, p < 0.001 and F(12, 276) = 9.49, MSe = 3.92, that larger negative components were associated with epsilon = 0.35, p < 0.001). Topographical analyses words that did not (1.85 pV) than words that did (3.76 showed that the negative component to words and pV) belong to the prime category (F(1, 23) = 15.51, pseudowords was largest at the central location (-1.74 Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 MSe = 19.88,p < 0.001).The main effect of electrodes pV and -4.72 respectively), intermediate at the parie- pV, was significant as was the response by electrodes inter- tal location (-1.88 pV and 0.60 respectively), and pV, action (F(6, 138) = 17.28, MSe = 29.12, epsilon = 0.40, smallest at the frontal location (2.01 pV and -1.30 pv p < 0.001 and F(6, 138) = 7.78,MSe = 2.30, epsilon = respectively; context by electrodes interaction: F(4, 0.38,p< 0.001,respectively). The priming effect (i.e., the 92) = 3.49,MSe = 1.72, epsilon = 0.69,p < 0.02).This difference between words that did or did not belong to component was also larger over posterior lateral loca- the semantic category defined by the prime word) was tions (1.44 pV) than over anterior ones (7.59 pV; F(1, 23) = 44.10,MSe = 61.77,p < 0.001).Overall, the ERP in this latency band was more positive over the right (5.45 pV) than the left hemisphere (3.59 pV; F(1, 23) = 4.20,MSe = 59.39,p < 0.05). In order to analyze the time course of these effects, fine-grained analyses of successive 25-msec ERP epochs were performed between 0 and 400 msec. Results FRONTAL showed that nonwords were significantly more positive than pseudowords as soon as 25 to 50 msec poststimu- lus onset at the left posterior location. This difference CENTRAL c was significant somewhat later (75 to 100 msec) at the left anterior location as well as at the right anterior and right posterior locations (200 to 225 msec and 275 to 300 msec, respectively). PUnprimed The ERPs to pseudowords start to differ from the ERPs to words at the left anterior and posterior locations (75 to 100 msec). This difference reached significance later L. ANTERIOR at the right anterior and posterior locations (200 to 225 TEMPORAL msec and 275 to 300 msec, respectively). The ERPs to words differed from the ERPs to nonwords at the left R. ANTERIOR and right posterior locations between 375 and 400 msec TEMPORAL and between 400 and 425 msec at the left and right anterior locations. Finally, analyses in the 500- to 900-msec latency band showed that the LPC was larger to nonwords (8.61 pV) L.POSTERIOR than to words (2.97 pV) or pseudowords (2.59 pV; F(2, TEMPORAL +m 46) = 46.73,MSe = 40.81,epsilon = 0.81,p< O.OOl).The main effect of electrode location and the stimulus by R.POSTERIOR electrode interaction were significant (F(6,138)= 14.30, TEMPORAL * MSe = 40.40, epsilon = 0.50,p< 0.001 and F(l2,276) = 1 -a w 6.61,MSe= 4.55,epsilon = 0.43,p< 0.001,respectively). I, I,I I I I I I1I II The LPC was largest at the parietal location (6.44 pV), OIW100tawl6W~ms intermediate at the frontal location (4.98 pV), and small- - Primed est at the central location (3.57 pV; F(2, 46) = 4.95, ____.Unprined MSe = 29.87,epsilon = 0.62,p< 0.03).The LPC was also larger over anterior lateral locations (5.71 pV) than over posterior ones (-2.32 pV;F(l,23) = 45.14,MSe= 103.18, Figure 5. Grand average ERPs (N = 24) in the semantic categoriza- tion task of Experiment 3 for words that belonged to the semantic p < 0.001) and larger over the right (2.54 pV) than the category defined by the prime words (primed) and words that did left hemisphere (0.85 pV F(1, 23) = 4.85, MSe = 42.30, not belong to this category (unprimed number of trials contributing p < 0.03). to the averages, n:for primed, n = 621 and unprimed, n = 600).

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 larger centro-parietally than frontally (F(2, 46) = 4.76, similar to real words, they are not as easily categorized MSe = 24.60, epsilon = 0.57,p c 0.03) and was slightly as the other two stimulus types. In other words, the larger over the right (-1.64 pV) than the left hemisphere overall increased negativity to pseudowords may reflect (-1.02 pV:F(l, 23) = 3.69,MSe = l.00,p c 0.06). increased processing difficulties. While pseudowords have generally been compared to words in ERPs and language experiments (e.g., Bentin et al., 1985), it is DISCUSSION specifically interesting to also include nonwords because In order to increase the degree of semantic processing no lexical search or semantic analysis of the consonant required to perform the task and to differentiate the letter string need to be performed for a semantic deci-

ERPs to words and pseudowords, a semantic categoriza- sion to be made. Therefore, nonwords can quickly be Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 tion task was used in Experiment 3. This aim was excluded from further analysis, which may be reflected achieved. For the first time across the three experiments by the early onset of the LPC, around 350 to 400 msec. reported here, results clearly showed differences in the For both words and pseudowords, further operations to Ems to words and pseudowords. Furthermore, the dif- compute a letter string’s meaning are required; they are ferences between stimulus types were much larger and possibly reflected by the negative components peaking had a shorter onset latency in this experiment than in around 300 to 500 msec. Thus, the decision that a par- the previous letter search experiments. Therefore, these ticular word or any pseudoword does not belong to the results strongly suggest that differences between words, semantic category may occur later, around 550 to 600 pseudowords, and nonwords are intimately linked with msec, as suggested by the late LPC onset. the type of processing performed on the stimulus. Since This argument about ERP patterns is supported by RT this conclusion is largely based on a comparison be- data from Carr et al. (1979, Exp. 4). Subjects performed tween experiments, the implications of these findings various tasks on a series of letter strings consisting of will be considered further in the general discussion. words, pseudowords, nonwords, and meaningful but or- The finding of larger negative components to pseudo- thographically and phonologically irregular acronyms, words than words in the 300- to 500-msec latency band such as FBI (Federal Bureau of Investigation), CIA (Cen- replicates several findings in the literature using a lexical tral Intelligence Agency), and JFK aohn E Kennedy). One decision task (Bentin et al., 1985; Boddy, 1986; Boddy & of the tasks was lexical decision; in this task, words Weinberg, 1981; Smith & Halgren, 1987). Bentin et al. required a Yes response and the other three stimulus interpreted this result as reflecting the amount of lexical types required a No response. The results showed that search required by the stimulus. In a somewhat similar subjects rejected both the nonwords and the irregular vein, Smith and Halgren attributed this pattern of results acronyms very rapidly. Yes responses to words were to the extended effort to activate stimulus meaning significantly slower, and No responses to pseudowords whenever uncertainty about the appropriate classifica- were the slowest of all. Carr et al. argued that an early tion of items exists. In our experiment, however, the spelling check evaluated the letter strings’ orthographic differences between pseudowords and words start ear- structure. Ill-formed strings that could not possibly be lier than 300 msec and as early as 75 to 100 msec words were rejected on the basis of this initial analysis. depending upon electrode location. Thus, it seems that Well-formed strings that might be words were submitted the processing operations required to identlfy words and to further processing. pseudowords differ during early stages of processing. Our results also replicate a well-established ERP prim- Following this logic, it is unclear, however, why the ERPs ing effect: Negative components in the 300- to 600-msec to words and nonwords that clearly differ on the basis latency band were larger to target words that did not of orthographic structure do not differentiate before 375 belong to the semantic category defined by the first to 400 msec poststimulus onset. One way to possibly word (prime) than to target words that did belong to account for these seemingly incompatible results is to this category (Bentin, 1987; Bentin et al., 1985; Besson, consider that both words and nonwords are categorized Fischler, Boaz, & Raney, 1992; Boddy, 1986; Boddy & as such very early on. This early categorization process Weinberg, 1981; Heinze, Munte & Kutas, in press; Hol- (e.g., spelling check) would be reflected by the in- comb, 1988; Holcomb Sr Neville, 1990; Kounios & Hol- creased positivity to words and nonwords compared to comb, 1992, 1994; Kutas & Hillyard, 1989; Polich, 1985). pseudowords, in the P200 component latency band (200 This component is referred to as the N400 component. to 300 msec poststimulus onset) but would not, how- Results of numerous experiments have shown that this ever, reflect the final decision of whether or not the component is a good index of semantic expectancy (see stimulus belongs to the semantic category defined by Fischler & Raney, 1991; Kutas & Van Petten, 1988, for the prime words. It would mainly allow nonwords to be reviews). Results in the semantic categorization task add excluded from further analysis. support to this interpretation. Words that were expected The processing operations performed on pseudo- on the basis of the prime words elicited smaller N400 words, in contrast, are different. To the extent that components than words that were unexpected. Further- pseudowords are orthographically and phonologically more, the priming effect (i.e., the N400 effect) showed

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 a centro-parietaldistribution and was slightly larger over much larger and had shorter onset latencies in this the right than left hemisphere, which corresponds to experiment than in the previous letter search experi- what has been typically reported in the literature (e.g., ments. Thus, large differences between stimulus types Kutas, Van Petten, & Besson, 1988). emerged when a semantic analysis was necessary in In regard to the primary gateway hypothesis, the early order to decide whether the stimulus belonged to the difference in the ERPs to pseudowords and nonwords at semantic category defined by the prime. the left posterior location between 25 and 50 msec is Interestingly, results also point to differences in the consistent with this hypothesis.This early difference may time course of word, pseudoword, and nonword proc- reflect the gateway mechanism that excludes nonwords essing between experiments and electrode locations. from further linguistic analysis since nonwords do not First, at the left posterior location, significant differences Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 contain sufficient linguistic information. According to between nonwords and pseudowords were found as this hypothesis, the later difference in the ERPs to words early as 25 to 50 msec in the semantic categorization and pseudowords at the anterior location would reflect task, at 225 msec in the delayed letter search task, and lexico-semantic processes that are available for words at 250 msec in the letter search task. The early onset of but not for pseudowords. these differences in the semantic categorization task may reflect a spelling check, which quickly excludes non- GENERAL DISCUSSION words from further analysis since they are orthographi- cally and phonologically illegal and, therefore, cannot The present series of experiments was designed to ex- belong to any semantic category. In the letter search amine the time course of the ERP differences to words, tasks, however, it would be harmful to exclude non- pseudowords, and nonwords as a function of the level words from further processing because they may con- of linguistic processing required by the task. In Ekperi- tain the critical target letter that has to be searched for. ment 1, a letter search task was performed that only Our finding that differences between the stimulus types required a shallow level of processing. Results showed occur later in the letter search tasks is consistent with small, albeit reliable, differences between stimulus type this hypothesis. Second, significant differences between that differed according to electrode location.At posterior words and pseudowords were found at the left anterior locations, the ERPs to nonwords were overall more posi- location between 75 and 100 msec in the semantic tive than the ERPs to words that did not differ from the categorization task and at 275 msec in the letter search ERPs to pseudowords. At anterior locations, in contrast, task (no difference was found between words and the ERPs to words were overall more positive than the pseudowords in the delayed letter search task). Again, ERPs to both pseudowords and nonwords. These dif- the earlier onset of the differences in the semantic cate- ferences, however, were affected by the presence or gorization task than in the letter search tasks point to absence of the target letter in the stimulus, which sug- the greater importance of lexico-semantic information, gested an influence of decision-related processes on the the basic difference between words and pseudowords, observed stimulus differences. In order to examine these when the task requires semantic information to be proc- differences uncontaminated by decision-related proc- essed. Together, these results suggest that (1) the differ- esses, a delayed letter search task was used in Experi- ences between words, pseudowords, and nonwords ment 2. In this experiment, the stimulus was presented always had shorter onset latencies in the semantic cate- first so that participants could not immediately decide gorization than in the letter search tasks, and (2) these whether the target letter, which was presented later, was differences occurred first at the left posterior locations. present or absent in the stimulus. This manipulation These results are in line with the PET results described clearly increased the differences between nonwords, on in the “Introduction,”showing activation of the left pos- the one hand, and words and pseudowords, on the other terior and anterior areas after the presentation of words hand, with the ERPs to nonwords being more positive, or wordlike stimuli. The present results indicate that across electrode sites, than the ERPs to both words and activation occurred earlier at posterior areas than ante- pseudowords, which did not differ. Therefore, when the rior ones. task required a shallow level of processing and ERPs Since the earliest studies of reading, it has been argued were not contaminated by decision-related effects, non- that words are processed differently from letter strings words seem to be processed differently than words and that do not form words (Cattell, 1886a, 1886b; Erdmann pseudowords. In contrast, similar processing operations & Dodge, 1898). Whenever readers encounter words, seem to be involved to detect a target letter with-h both they cannot help but read them (Kahneman & Chajczyk, words and pseudowords. In order to differentiate the 1983; Stroop, 1935). Furthermore, words activate differ- ERPs to words and pseudowords, a semantic categoriza- ent sources of linguistic information (orthographic, tion task was used in Experiment 3. This aim was phonological, and semantic). However, the extent to achieved since the ERPs to pseudowords were overall which the activation of this information is automatic and more negative than the ERPs to words. Furthermore, stimulus-driven or task-dependent and strategically con- these differences as a function of stimulus type were trollable is a topic of much controversy (see Carr, 1992;

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 Cohen, Dunbar, & McClelland, 1990; Friedrich, Henik, & all but three were right-handed according to self-report; Tzelgov, 1991; Neely, 1991; Posner, Sandson, Dhawan, & two of the right-handed participants had a left-handed Shulman, 1989, for reviews). The results reported here relative in his or her immediate family. All participants suggest that differences between words, pseudowords, had normal or corrected-to-normal vision. and nonwords are strongly dependent upon the type of processing required to perform the task. Focusing atten- Materials tion on lexico-semantic processing led to larger and earlieremerging stimulus differences than focusing at- Five-hundred-and-forty stimuli were generated for this tention on orthography and letter content. The language experiment, of which one-third were words, one-third

system seems to be highly flexible and adaptive: If opti- pseudowords, and one third nonwords. Of the stimuli, Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 mal task performance can benefit from multiple sources 240 were four letters long and 300 were five letters long. of linguistic information, there is no doubt that this The words were familiar French words; their word fre- information will be activated and used. It remains the quency ranged between 17 and 4177 occurrences per case, however, that there are still limits to cognitive million with an average frequency of 339 occurrences control. Documenting and explaining the balancing act per million (SE = 54.2) according to a frequency count that goes on between flexibility and control is a formi- for the French language (Tmbs, 1971). Pseudowords were dable challenge; the present study may give promising constructed by changing one letter from a real word and directions for further research on this issue by extending were orthographically and phonologically legal. Non- the two major principles applied in the present ap- words were composed of a string of consonants and proach: multitask and multistimulus comparisons.As con- were neither orthographically nor phonologically legal. cerns the multitask approach, it may be useful to Ten consonant target letters were used in this experi- compare performance across tasks that share basic op- ment (B, C, D, G, L, N, P, R, S, and T). The target letter was erations and differ in theoretically defined task-specific present in half of the stimuli and occurred equally often operations (e.g.,Grainger &Jacobs, 1996;Jacobs & Grain- in each position (in one-quarter, in positions 1, 2, 3, and ger, 1994; Snyder et al., 1995). For example, future re- 4, respectively, for 4-letter stimuli, and in one-fifth, in search could compare ERPs in the phoneme monitoring positions 1,2,3,4,and 5, respectively,for 5-letter stimuli). task to ERPs in the letter search task to investigate the primary role of phonological recoding as suggested in Design Experiment 2. As concerns the multistimulus approach, linguistic properties of stimuli could be manipulated in The experiment consisted of two parts, an RT session more “fine-grained”steps than in the present experi- and an ERP session, the order of which was balanced ments. For example, pseudohomophones (e.g., brane- across participants. Two lists of 270 stimuli each were brain) could be used as stimuli because they have a constructed; one list was used in the RT session and the word’s phonology but a different spelling (orthography). other list in the ERP session so that none of the stimuli Nonhomophonic homographs (e.g., lead-lead) could be used in one session was repeated in the other. Each list used as stimuli because they have identical spelling but contained 90 words, 90 pseudowords,and 90 nonwords; different phonology and meaning. Homophonic homo- the target letter was present in half of the trials. Within graphs (bank-bank) could be used as stimuli because each list, the order of presentation was pseudoran- they have identical spelling and phonology but different domized. Each session consisted of three blocks of 90 meanings. Finally, it would be interesting to use inconsis- trials each. A pause of 10 min was provided between tent stimuli (i.e., words with multiple mappings between each block. spelling and phonology: pint-mint) for which the degree of inconsistency could be manipulated according to Procedure large-scale statistical analyses of a language’s structure (e.g., Ziegler,Jacobs, & Stone, 1996). A trial consisted of the following sequence of events. First, fixation bars were presented for 500 msec in the center of a CRT screen. The target letter was then pre- METHODS sented for 800 msec. After a blank interval of 200 msec, Experiment 1: Letter Search Task the stimulus was presented for 200 msec centered at the fixation point. The intertrial interval was 2 sec. Upper- Participants case target letters and letter strings were presented and Twenty-eight native French speakers were paid for their the viewing distance was 60 cm. The experiment was participation in the experiment that lasted about 3 h. The controlled by a Compaq 486 personal computer. data from three participants were discarded due to tech- At the beginning of the experiment, participants were nical problems. Of the 25 remaining participants (age informed that they would participate in two sessions range 19 to 34, mean = 23.2 years; 10 women, 15 men) successively,an RT and an ERP session. The sequence of

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 events within a trial was described to them. In the RT Experiment 2: Delayed Letter Search Task session, they were asked to indicate by pressing a re- Participants sponse key whether the target letter was present (right key) or absent (left key) in the stimuli. Both speed and Sixteen native French speakers (age range 19 to 32, accuracy were stressed. Trials in which no response was mean = 22.2 years; 9 women, 7 men) were paid for their given within 1200 msec following stimulus onset and participation in the experiment that lasted about 3 h. All trials in which RTs were shorter than 250 msec were but one were right-handed according to self-report;five discarded from subsequent analysis. An auditory feed- participants had a left-handed relative in his or her im- back indicated wrong responses. In the ERPs session, the mediate family. All participants reported normal or cor- rected-to-normal vision. None of them had participated timing within each trial was identical, but the response Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 was delayed (i.e., after a blank interval of 800 msec a in the previous experiment. row of XIcXxs served as a response cue). As soon as the XXXXs were presented, participants were required to Materials and Design press one of two keys depending upon whether or not the target letter was present in the stimulus. Participants The same stimuli were used as in Experiment 1. Like- were asked to avoid eye blinks between the onset of wise, the same two stimulus lists, composed of 270 fixation bars and the presentation of the XXXXs on the stimuli each, were used. One list was presented to half screen; they were trained to blink between trials. After of the participants, and the other list, to the other half. the instructions were given, the participants were given Within each list, the order of stimulus presentation was 12 practice trials consisting of four trials of each stimulus pseudorandomized. type, the target letter being present in half of the trials. The experimental trials were then presented. Procedure The sequence of events was similar to the ERP session Recordings of the previous experiment, except that the order in An EEG was recorded via Ag/AgCl electrodes from seven which the stimulus and target letter were presented was scalp sites: three along the midline at Fz, Cz, and Pz and reversed. First, fixation bars were presented for 500 two lateral pairs over anterior-temporal regions (F7 and msec in the center of a CRT screen. Then, the stimulus F8: 30% of the distance between FPz and Oz through T3 (word, pseudoword, or nonword) was presented for 800 and T4, respectively) and over posterior-temporal re- msec. After an interstimulus interval of 200 msec, the gions (T5 and T62:70% of the distance between FPz and target letter was presented for 200 msec centered at the Oz through T3 and T4, respectively), each referred to the fixation point. The intertrial interval was 2 sec. Each left mastoid. Eye movements and blinks were monitored stimulus and target letter was presented in uppercase. via an electrode on the lower orbital ridge referred to The experiment was controlled by a Compaq 486 per- the left mastoid. sonal computer. The instructions were the same as in The EEG was amplified by Grass P5 RPS107 amplifiers Experiment 1. with a 0.01 to 30-Hz (half-amplitude cutoff) bandpass. The sampling rate was 250 Hz.Approximately 10%of the Recordings and Data Analysis trials were contaminated with eye movements or muscle artifacts; these were rejected off-line. Electrode imped- The EEG was recorded from the same locations using ances never exceeded 3 k0. the same montage as in Experiment 1. Furthermore, the recording parameters, the constraints for artifact rejec- tion, and data analysis were the same as in Experi- Data Analysis ment 1. The ERPs were averaged off-line for a 2700-msec epoch, within each condition for each subject and time-locked Experiment 3: Semantic Categorization Task to the onset of the fixation bars. ERP data were analyzed by computing the mean amplitude in selected latency Participants windows relative to a 200-msec prefixation bars base- Twenty-four native French speakers were paid for their line. Repeated measures analyses of variance (ANOVAs) participation in the experiment that lasted about 3 h. Of were carried out with the Greenhouse-Geisser correc- the 24 participants (age range 19 to 33, mean = 24.7 tion for inhomogeneity of variance applied where appro- years; 12 women, 10 men), all but two were right-handed priate; reported are the uncorrected degrees of freedom, according to self-report;eight of the right-handed partici- the epsilon value, and probability level following correc- pants had a left-handed relative in his or her immediate tion. Unless specified, Tukey (HSD) tests were used to family. All participants reported normal or corrected-to- test the significance of posthoc comparisons. normal vision. Eight of the participants also took part in

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/jocn.1997.9.6.758 by guest on 29 September 2021 Experiment 1 and 16 of the participants took part in response and the other half, a No response. On No trials, Experiment 2. one-third were words, one-third,pseudowords, and one- third, nonwords. The experiment then started with the first block of trials. The experiment was controlled by a Materials Compaq 486 personal computer. One-hundred-and-eightystimuli were generated for this experiment. This stimulus set consisted of (1) 30 words that were the three most typical exemplars of 10 differ- Recordings and Data Analysis ent semantic categories according to French category The EEG was recorded from the same locations using

norms (Dubois, 1982; fruit, animal, vehicle, drink, family the same montage as in the previous experiments. Fur- Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/9/6/758/1754817/jocn.1997.9.6.758.pdf by guest on 18 May 2021 member, color, part of the body, furniture, music instru- thermore, the recording parameters, the constraints for ment, building), (2) 30 words that did not belong to the artifact rejection, and the data analysis were the same as category specified by the first word but were exemplars in the previous experiments. of other semantic categories,(3) 30 orthographically and phonologically legal pseudowords that were constructed by changing one letter from a real word, (4) 30 ortho- Acknowledgments graphically and phonologically illegal nonwords, com- This research was supported by a grant of the Ministere de la posed of a string of consonants, and finally, (5) 60 word Recherche et de la Technologie (92.C.0420) to M. Besson, fillers that belonged to the prime category but were not J. Pynte, F? Courrieu, C. Frenck-Mestre, and A. Jacobs, a German typical exemplars. Thus, half of the stimuli presented in Academic Exchange Service Grant (DAAD-Doktorandensti- the experiment required a Yes response and the other pendium aus Mitteln des zweiten Hochschulsonderpro- gramms) to J. Ziegler, and a CNRS Visiting Scientist Grant to half required a No response. As in the previous experi- T. Carr. Part of this research was done while Tom Carr was a ments, the words were familiar French words; their word research associate at the Center for Research in Cognitive frequency ranged between 14 and 946 occurrences per Neuroscience, Marseille, France. million with an average frequency of 142 occurrences We are indebted to Arnaud Rey for running parts of the per million (SE = 26.2) according to a frequency count experiments and to Monique Chiambretto, Raymond Fayolle, and Guy Reynard for electronic and software assistance. for the French language (Imbs, 1971). Reprint requests should be sent to Dr. Johannes Ziegler, Centre Design de Recherches en Neurosciences Cognitives, Centre National de la Recherche Scientifique, 31, ch. Joseph Aiguier, 13402 A list of 180 stimuli was used and stimuli were presented Marseille CEDEX 20, FRANCE, or via e-mail: ziegler@LNE in pseudorandomized order. A session comprised two cnrs-rnrS.fr. blocks of 90 trials each. A 10-min pause was provided between each block. Notes 1. Perhaps the ideal solution given current technologies is to Procedure collect ERP and PET results from the same subjects performing The sequence of events within a trial was similar to the the same tasks. Although combining PET and ERP methods within the same subjects has been successfully applied in one used in the previous experiments.First, fixation bars research on visual selective attention (Heinze, Mangun, were presented for 500 msec in the center of a CRT Burchert, Hinrichs, Scholz, Munte, GOs, Scherg, Johannes, screen placed 60 cm in front of the subject. The name Hundeshagen, Gazzaniga, & Hillyard, 1994), it requires special- of a semantic category (prime word) was then presented ized laboratory facilities and, to our knowledge, has rarely been applied to psycholinguistic research (but see Nenov, Halgren, for 800 msec. After a blank interval of 200 msec, the Smith, Badier, Ropchan, Blahd, & Mandelkern, 1991). stimulus was presented for 200 msec centered at the 2. These lateral recording sites were chosen based on Comp- fixation point. All stimuli were presented in uppercase. ton et a1.k (1991) results. Participants were asked to indicate by pressing one of two response keys whether the target stimulus did (right key) or did not (left key) belong to the prime category. REFERENCES Their response was delayed until a row of XXXXs ap- Baddeley, A. D. (1986). Working memo?y. Oxford, UK: Claren- peared on the screen, 800 msec after target offset. The don Press. intertrial interval was 2 sec. Participants were asked to Baron, J., & Thurstone, I. (1973). An analysis of the word supe- avoid blinking from the onset of fixation bars until the riority effect. Cognitive Psycbolow, 4, 207-228. ECXXs appeared on the screen; they were trained to Bentin, S. (1987). Event-related potentials, semantic proc- esses, and expectancy factors in word recognition. Bruin blink during the intertrial interval. C Language, 31, 308-327. Following the instructions, the participants first saw a Bentin, S., McCarthy, G., & Wood, C. C. (1985). Event-related practice set of 12 trials, half of them requiring a Yes potentials, lexical decision and semantic priming. Elec-

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