A comparison of sentence- and discourse-level semantic processing: An ERP study

Brain and Language 83 (2002) 367–383 www.academicpress.com

A comparison of sentence- and discourse-level semantic processing: An ERP study Nirit Salmon* and Hillel Pratt Evoked Potentials Laboratory, Behavioral Biology, Technion-Israel Institute of Technology, Haifa 32000, Israel Accepted 26 February 2002

Abstract Eighteen subjects listened to sentences ending with semantically congruent or incongruent words. Each congruent sentence was embedded at the end of a short story so that the final word was semantically acceptable at the sentence level but incongruent in terms of discourse context. The same stories were also presented with congruent endings. Auditory ERPs elicited by the final words in these sentences and stories were compared. Both sentence- and discourse-level semantic integration were associated with N400 and Late Positive Component (LPC) effects in addition to a new component, P550. Local and global semantic processing, although evoking the same components, were characterized by differential effects on ERP amplitudes according to the amount of text integrated and its congruence. These results indicate similar cognitive processes of context build up, underlying sentence- and discourse-semantic processing. Ó 2002 Elsevier Science (USA). All rights reserved. Keywords: Event-related potentials; Semantic processing; Discourse; Sentence; N400; P550; LPC

1. Introduction In the past three decades many studies have sought to examine the ways in which semantic information is used in language comprehension. Language comprehension is conceived as a continuous process of evaluation and updating of hypotheses regarding the words, which may appear in the unfolding text. Words that are predicted in a certain context, are identified faster and more accurately than either isolated words or words appearing in an anomalous semantic context, suggesting a role for context build up in word perception. Semantic effects of previous contexts have been studied using two main experimental conditions: word lists and sentences. Only few studies examined semantic effects in the wider and more natural context of discourse (St. George, Mannes, & Hoffman,. 1994, 1997; Till, Mross, & Kintsch, 1988; Van *

Corresponding author. Fax: +972-4-8226783. E-mail addresses: [email protected] (N. Salmon), [email protected] (H. Pratt).

0093-934X/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII: S 0 0 9 3 - 9 3 4 X ( 0 2 ) 0 0 5 0 7 - 2

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Berkum, Hagoort, & Brown, 1999). This study aimed at a more complete understanding of natural semantic processes, comparing semantics in the sentence and discourse contexts. Some earlier studies used word or sentential natural speech stimuli (Holcomb & Neville, 1990, 1991; McCallum, Farmer, & Pocock, 1984; Osterhout & Holcomb, 1993). However, the standard method in most ERP studies has been visual presentation of sentences one word at a time. This procedure is evidently an artificial condition rather than a natural setting for comprehension. The auditory modality is the primary common natural channel for communication. To the best of our knowledge, semantic processing in discourse, using natural speech stimuli has not been reported previously. Ever since the pioneering experiments of Kutas and Hillyard (1980a, 1980b, 1980c, 1982), ERP language studies have tended to measure different aspects of comprehension using the N400 component. N400 is a negative monophasic potential shift beginning at about 250 ms post-stimulus, peaking around 400 ms with a centroparietal prominence, and lasting till 500–600 ms after word onset. Targets of both visual and auditory modalities produce N400 components, that are similarly affected by semantic conditions. Still, the N400 is larger, begins earlier and lasts longer in the auditory than in the visual modality (Holcomb & Neville, 1990). N400 was first described as a difference waveform between ERPs to semantically congruent final words in sentences and their anomalous counterparts. Further examination revealed that it is also evoked by incongruent intermediate words in sentences (Kutas & Hillyard, 1983). Moreover, semantically acceptable intermediate and final words also elicit an N400, the amplitude of which is inversely affected by the expectancy level produced by the preceding sentential context (Kutas & Hillyard, 1984; Van Petten & Kutas, 1990). Based on these findings, N400 was assumed to reflect semantic processing modifications with the progression of context construction. Readers and listeners process a sentence meaning very quickly as it is being developed and clarified. The extent to which the sequential process of constructing local semantics in a sentence either resembles or differs from constructing wider semantics in discourse and the way these processes are reflected by ERPs are not yet clear. Later studies demonstrated that semantic manipulations in a wider context of discourse also evoke and influence the N400 component (St. George et al., 1994, 1997; Van Berkum et al., 1999). The goal of this study was to further study these effects and to elucidate the way the semantic system processes information under natural conditions using sentence and discourse linguistic levels. Semantic contextual effects have been related to post-lexical integrative processing and to analysis of semantic compatibility (Osterhout & Holcomb, 1992; Rugg, 1990; Rugg, Furda, & Lorist, 1988). The N400 amplitude is inversely affected by the semantic compatibility of a given word and its context. The more cognitive effort is involved in integrating a word into an ongoing context the larger the N400 amplitude elicited by that word. Since discourse may provide a richer context, it may require less effort in integrating a final wordÕs meaning compared to a sentence, and hence may elicit a smaller N400. Such effects on N400 may support language models proposing similar processing of local- and global-semantic word integration. These models suggest that language comprehension always goes beyond sentence level semantics. A single sentence is understood in relation to the background knowledge of the listener and also in relation to the complete text when presented within discourse. In a recent study, visual ERPs in response to sentence and discourse stimuli were compared (Van Berkum et al., 1999). It was shown that discourse level stimuli elicited similar N400 as sentence level stimuli, with no significant latency differences. Similar findings may be expected in response to auditory stimuli.

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Other psycholinguistic models describe language comprehension as an ongoing process, which goes through several stages with increasing complexity. According to these models semantic processing includes at least two main distinct stages: processing of local semantics in a sentence, followed by relating this sentence to the wider context of the preceding discourse (Garrod & Sanford, 1994; Marslen-Wilson & Tyler, 1980). Multi-stage language processing is a common notion. In word recognition models three stages of processing are commonly distinguished (Frauenfelder & Tyler, 1987; Kintsch & Mross, 1985; Till et al., 1988; Zwitserlood, 1989). The first is the context-independent sense activation, which results in the activation of a subset of lexical elements (Seidenberg, Tanenhaus, Leiman, & Bienkowski, 1982; Tanenhaus, Leiman, & Seidenberg, 1979; Till et al., 1988). The second is the sense selection, in which the element that best matches the discourse context is selected. The third stage is the sense elaboration, which integrates the element into the entire discourse. The complete meaning of a word is probably not encoded in lexicon but is constructed in a specific discourse context, enabling considerable flexibility and creativity of language (see Till et al., 1988). The deactivation of irrelevant word meanings is made by 350–400 ms following the text ending. Establishing a complete contextual meaning requires thematic inference, which takes 500–1000 ms. Apparently such inferences are not made immediately but appear to be part of the context wrap-up processing (Kintsch & Mross, 1985; Seidenberg et al., 1982; Tanenhaus et al., 1979; Till et al., 1988). Based on the multi-stage comprehension models, one might expect that relating a sentence to earlier discourse would be a slower process than the process of sense integration at the local sentence level (Fodor, Ni, Crain, & Shankweiler, 1996; Till et al., 1988). If this is so, semantically congruent and incongruent final words may take longer to be integrated in discourse than in a single sentence, and may therefore elicit a delayed N400. Moreover, embedding a sentence ending with a semantically congruent word in discourse in which that word is semantically unacceptable may evoke a small N400, similar to that evoked by the isolated sentence, and possibly, in addition, later components reflecting the discourse semantic abnormality of the global context. In this study we conducted an experiment, which allowed us to examine brain activity evoked while processing congruent and incongruent words integrated in a sentence context. The congruent sentences were embedded in short stories forming semantically incompatible endings. Final words of the semantically incongruent stories were replaced with compatible alternatives. Brain activities evoked by these conditions were compared. Since the N400 amplitude was found sensitive to expectation level of words, based on sentential context and word associations, we predicted that final words in coherent stories would elicit a smaller N400 than final words in coherent sentences. That is because discourse context, leading to the target word, is richer and may elicit higher expectations for certain words to appear than sentential context. Possible late effects of discourse context, associated with late shifts following the N400 were also examined.

2. Methods 2.1. Subjects Eighteen paid subjects (10 female, 8 male) ages 18–30 (mean 21.6) participated in the study. All subjects were native Hebrew speakers, right handed with no known language or hearing impairment or any neurological disorder.

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2.2. Stimuli The natural speech stimuli consisted of one list of 90 randomized sentences and a second list of 90 randomized stories, all digitally recorded in a young male voice. The list of sentences included 45 semantically congruent sentences. The same sentences were also presented with a final alternative word, which was semantically anomalous. The length of all sentences ranged from 3 to 8 words (mean 5.13). The list of stories included 3 sentence long texts. Each of the 45 semantically congruent sentences from the sentence list, was embedded at the end of a short story. The final word was thus semantically appropriate in the context of the last sentence of the story but did not semantically match the global discourse context. Each of the 45 semantically incongruent stories had a semantically congruent story counterpart, which was terminated by an appropriate final word. The contents chosen for the stories included mostly narratives, familiar daily situations and informative texts. The average story length was 17.75 words. See Appendix A for example of stimuli in each of the 4 semantic conditions. 2.3. Event-related potentials Silver disc electrodes were placed according to the international 10–20 system at nine scalp locations: Fz, Cz, Pz, F4, C4, P4, F3, C3, and P3. Three additional electrodes were used: a chin reference, an arm ground electrode, and a third electrode placed just below the outer canthus of the left eye. This third electrode along with the Fz electrode monitored eye movements. Impedance of all electrodes was kept below 5 kX. The EEG was amplified by 100,000 at a band pass of 0.1–100 Hz, for all electrodes except the EOG channel which was amplified by 20,000. Signals were digitized on-line with a sampling frequency of 256 Hz. The subjects listened to the speech stimuli at a comfortable intensity level through headphones (Sony MDRCD770). 2.4. Procedure All procedures were approved by the institutional review board for human subjects (Helsinki Committee) and all subjects were paid for their participation. Subjects were seated on a reclining chair in an electrically shielded sound attenuated chamber. Initially, subjects listened to stories and were asked to decide whether the last word of the story was logically appropriate or not by pressing one of two buttons. Subjects were instructed to avoid blinking except after their button press, while waiting for the next story. Stories with semantically congruent and incongruent last words were randomly presented with equal probability, and a total of 45 stories of each type were presented in the session. The second semantic decision task presented subjects with sentences. Subjects were instructed to make a semantic decision regarding the final words of the sentences as in the first task. Sentences with semantically congruent and incongruent last words were randomly presented with equal probability, and a total of 45 sentences of each type were presented. Reaction time and performance accuracy (% of correct responses) were measured alongside the electrophysiological signals. 2.5. Data analysis Prior to statistical analysis, EEG data were segmented into separate epochs beginning 100 ms before stimulus onset and ending 1000 ms after stimulus onset.

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Fig. 1. Grand averaged ERP waveforms recorded at Cz to each of the experimental semantic conditions. Note that both congruent sentences and stories yielded a smaller N400 than their incongruent counterparts. N400 was also smaller in response to stories than to sentences. P5 was larger to semantically congruent stimuli and to discourse level integration and consequently unnoticeable in the grand averaged waveform to incongruent sentences. LPC was larger to incongruent than congruent stimuli and to sentences than to stories.

Epochs were then corrected for eye movement artifacts using the eye movement correction program described by Attias, Urbach, Gold, and Shemesh (1993). Trials contaminated by noise of EOG and excessive signal amplitudes were excluded from further analysis. Corrected EEG epochs in response to final words of each of the four experimental conditions were averaged separately and filtered using a 13–15 Hz low pass filter (Fig. 1). Peak amplitudes and latencies were measured and a Principal Component Analysis (PCA) was conducted as well. PCA was conducted across averaged waveforms of each subject, at each scalp locations, in each experimental condition, for each stimulus type. Component peak latencies and amplitudes and the eigenvectorsÕ coefficients from PCA were analyzed by a repeated-measures analysis of variance (ANOVA) for the factors of subject, electrode site (maximal activity, midline and laterality) and stimulus type (semantic congruency, context level). Probability levels on the within-subjects F tests were corrected based on the Greenhouse–Geisser adjustments, using Box epsilon probability level. A subsequent Post hoc analysis, Bonferonni Multiple-Comparison Test, was conducted in order to assess significance of contrasts.

3. Results 3.1. Behavioral measures Performance accuracy (% of correct responses) was above 95% for all subjects for all experimental conditions. Reaction times were faster in response to semantically congruent stimuli than to incongruent stimuli, as expected. Reaction times were also found faster in response to sentence- than to discourse-level semantic integration.

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Table 1 Mean peak measures in the 4 semantic conditions

N1 N4a N4b P5 LPC

Amplitude (lv) Latency (ms) Amplitude (lv) Latency (ms) Amplitude (lv) Latency (ms) Amplitude (lv) Latency (ms) Amplitude (lv) Latency (ms)

Congruent story

Incongruent story

Congruent sentence

Incongruent sentence

)3.09 103 )2.76 389 )0.97 479 1.64 535 3.55 809

)3.04 103 )3.69 400 )2.20 491 0.22 549 4.62 854

)2.78 99 )3.15 393 )2.66 488 0.34 543 4.5 850

)2.46 96 )4.87 408 )4.92 485 )1.53 551 5.87 847

3.2. ERP measures ERPs elicited in all experimental conditions (Fig. 1) included two major negative deflections: an early component peaking about 100 ms post-stimulus onset (N1) and a later double peaked component that was evoked at 300–500 ms and was marked as N400a and N400b. In addition, two positive deflections were revealed, an unexpected robust component that peaked at 535–550 ms post-stimulus onset (P5) and a late positive component (LPC) which peaked at approximately 850 ms post-stimulus onset. Table 1 lists the mean componentsÕ measures for each of the semantic conditions. 3.2.1. N1 Repeated measures ANOVA showed a significant electrode effect on N1 amplitude [F ð8; 136Þ ¼ 3:3; p ¼ :02; e ¼ 0:7]. Amplitudes were found higher over parietal sites than over central and frontal sites [F ð2; 34Þ ¼ 7:13; p ¼ :008; e ¼ 0:81]. No significant linguistic effects on N1 measures were found. 3.2.2. N400a In general, most effects on amplitude were found significant, while no significant effects on latency were found. A significant electrode effect on amplitude was found [F ð8; 120Þ ¼ 14:7; p < :00001; e ¼ 0:99]. Maximal amplitude was measured over the centro-parietal site (Pz) and it was significantly different from amplitudes measured at all central and frontal electrodes. N400a was progressively smaller from the back of the scalp (mean ¼ 4:39 lV) along the center (mean ¼ 3:59 lV) to the frontal scalp (mean ¼ 2:88 lV) [F ð2; 30Þ ¼ 22:9; p ¼ :00004; e ¼ 0:99]. It was also slightly larger at sites over the right hemisphere (mean ¼ 3:79 lV) than over the left (mean ¼ 3:39 lV) [F ð2; 30Þ ¼ 6:22; p ¼ :006; e ¼ 0:84]. In addition, peak amplitudes for semantically incongruent conditions (mean ¼ 4:28 lV) were larger than for semantically congruent conditions (mean ¼ 2:96 lV) [F ð1; 15Þ ¼ 12:44; p ¼ :003; e ¼ 0:91]. An examination of the context level factor showed a marginally significant effect with larger amplitudes in the sentence- than in the discourse-level conditions [F ð1; 15Þ ¼ 4:41; p ¼ :053; e ¼ 0:5]. 3.2.3. N400b The effect of electrode site on peak amplitudes showed a tendency towards more negative amplitudes at parietal sites, largest at P4 [F ð8; 112Þ ¼ 2:41; p ¼ :08;

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e ¼ 0:52. Regular prob level showed significance, p ¼ :01]. A significant effect of scalp laterality on N400b amplitude was found with slightly larger amplitudes over the right side of the scalp (mean ¼ 2:94 lV) than over the left (mean ¼ 2:52 lV) [F ð2; 28Þ ¼ 5:24; p ¼ :01; e ¼ 0:74]. The experimental conditions had a main effect on peak amplitude [F ð3; 42Þ ¼ 18:65; p < :00001; e ¼ 0:99], but not on peak latency. A tendency for interaction between experimental condition and scalp laterality was found [F ð6; 84Þ ¼ 3:4; p ¼ :07; e ¼ 0:46]. Congruent stories yielded shorter latencies (mean ¼ 479 ms) than congruent sentences (mean ¼ 494 ms) at left scalp locations. Peak amplitudes in sentence level conditions were larger (mean ¼ 3:79 lV) than in discourse level (mean ¼ 1:58 lV) conditions [F ð1; 14Þ ¼ 25:1; p ¼ :0001; e ¼ 0:99]. The effect of semantic congruency on peak amplitude was significant [F ð1; 14Þ ¼ 32:13; p < :0001; e ¼ 0:99], and interacted with electrode frontality [F ð2; 28Þ ¼ 5:8; p ¼ :03; e ¼ 0:57]. N400b amplitude was larger in response to semantically incongruent conditions (mean ¼ 3:56 lV) than to semantically congruent conditions (mean ¼ 1:81 lV) at all sites. Incongruent sentences and stories elicited a larger N400b than their congruent counterparts, as shown by the Newman– Keuls multiple comparison test. 3.2.4. P5 Repeated measures ANOVA showed a main effect of electrode site on P5 amplitude [F ð8; 136Þ ¼ 4:84; p < :001; e ¼ 0:96]. The most positive amplitude was measured over the midline parietal site (Pz). Strong effects of electrode frontality [F ð2; 34Þ ¼ 3:63; p ¼ :054; e ¼ 0:53], and laterality [F ð2; 34Þ ¼ 13:4; p < :0001; e ¼ 0:99], were found. Post hoc analyses revealed that peak amplitude was larger over parietal than frontal sites. It was also found largest over midline sites, smaller over the left and smallest over the right scalp. Main effects of experimental conditions on peak amplitude were found as well [F ð3; 51Þ ¼ 17:15; p < :00001; e ¼ 0:99]. The largest P5 amplitude was measured in response to congruent stories and the smallest in response to incongruent sentences; both were significantly different from peak amplitudes measured in each of the other three semantic conditions. Main effects of semantic congruency [F ð1; 17Þ ¼ 20:6; p ¼ :0002; e ¼ 0:99], which interacted with electrode site [F ð8; 136Þ ¼ 4:04; p ¼ :05; e ¼ 0:52], and context level [F ð1; 17Þ ¼ 32:5; p < :0001; e ¼ 0:99], were indicated. Post hoc analyses demonstrated larger peak amplitudes in response to semantically congruent (mean ¼ 0:99 lV) than to incongruent conditions (mean ¼ 0:65 lV) at all sites. Peak amplitudes were larger for discourse(mean ¼ 0:93 lV) than for sentence-level conditions (mean ¼ 0:59 lV). None of the factors examined had main effects on P5 latency. 3.2.5. Late positive component (LPC) Repeated measures ANOVA showed a main effect of electrode site on LPC amplitude [F ð8; 136Þ ¼ 18:84; p < :00001; e ¼ 1:0]. The most positive amplitude was measured over the centro-parietal site (Pz) that was found significantly different from each of the central and frontal sites. These analyses also revealed that peak amplitude was largest over parietal sites with a graded decrement toward frontal sites (P > C > F ) [F ð2; 34Þ ¼ 46:48; p < :00001; e ¼ 1:0]. A significant effect of electrode laterality on amplitude was found [F ð2; 34Þ ¼ 7:11; p ¼ :003; e ¼ 0:89]. Peak amplitude was largest over midline sites, and smallest over the left scalp. Main effects of experimental conditions on peak latency [F ð3; 51Þ ¼ 3:42; p ¼ :02; e ¼ 0:7], and peak amplitude [F ð3; 51Þ ¼ 4:62; p ¼ :01; e ¼ 0:77], were found as well. The largest LPC amplitude was measured in the incongruent sentence condition and was significantly different from that measured in the congruent story condition. The shortest

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LPC latency was measured in the congruent story condition and it was significantly different from that measured in the incongruent story condition. A main effect of context level on peak amplitude [F ð1; 17Þ ¼ 8:16; p ¼ :01; e ¼ 0:77], but not on peak latency, was found. LPC amplitude was found to be larger in sentence- than in discourse-level conditions. Peak latency tended to be longer in response to sentences than to stories. Semantic congruency was found to significantly affect peak amplitude [F ð1; 17Þ ¼ 4:83; p ¼ :04; e ¼ 0:54], and it interacted with the electrode frontality factor [F ð2; 34Þ ¼ 8:43; p ¼ :02; e ¼ 0:6]. Larger amplitudes were measured in response to incongruent- than to congruent-conditions at parietal, central and frontal sites. 3.3. Principal component analysis PCA across each of the experimental conditions yielded three similar looking eigenvectors (Fig. 2). The third explained only 4.5–6.5% of the variance among waveforms and was therefore excluded from further analysis.

Fig. 2. Eigenvectors resulting from PCA of ERPs across subjects at the various discourse level conditions. Note the similarity of eigenvector 1 to LPC, eigenvector 2 to N400 or to P5 when inverted (negative coefficients). The percentages next to each eigenvector denote the percentage of variance in the waveforms accounted for by the eigenvector. Amplitudes are in arbitrary units.

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3.3.1. Sentence level conditions Eigenvector 1, which accounted for 38.6% of the variance among waveforms, had a parietal [F ð2; 34Þ ¼ 62:76; p < :00001; e ¼ 1:0], and midline scalp distribution [F ð2; 34Þ ¼ 5:6; p ¼ :01; e ¼ 0:76], and was largest at Pz. It was significantly affected by semantic congruency [F ð1; 17Þ ¼ 7:0; p ¼ :01; e ¼ 0:7], which interacted with electrode site [F ð8; 136Þ ¼ 6:85; p ¼ :01; e ¼ 0:72]. Coefficients were larger to incongruent than to congruent sentence conditions at all sites. These results are all compatible with the LPC findings obtained using peak measurements. Eigenvector 2 (accounting for 23.3% of variance) was significantly affected by semantic congruency, with larger coefficients to incongruent than to congruent conditions [F ð1; 17Þ ¼ 15:9; p ¼ :0009; e ¼ 0:96]. This eigenvector resembled the N400 component in morphology and characteristics. An examination of the eigenvectorÕs negative coefficients showed smaller coefficients for congruent than for incongruent sentences. Repeated measures ANOVA showed a tendency for parietal scalp prominence, with smaller coefficients for midline sites. Its morphology (when inverted) and characteristics are compatible with the P5 peak measurements. 3.3.2. Discourse level conditions Coefficients of eigenvector 1 (accounting for 31.23% of variance) were largest at parietal sites and significantly different from those found at central and frontal sites [F ð2; 34Þ ¼ 37:63; p < :00001; e ¼ 1:0]. An interaction between semantic congruency and electrode frontality was found [F ð2; 34Þ ¼ 10; p ¼ :02; e ¼ 0:64]. Coefficients were larger for congruent than for incongruent stories in parietal and central locations. The opposite effect was found in frontal locations, where coefficients were larger to incongruent than congruent stories. Coefficients of eigenvector 2 were largest at parietal sites and significantly different from those at frontal sites [F ð2; 34Þ ¼ 13:1; p ¼ :001; e ¼ 0:95]. They were also larger at right scalp electrodes than on the left [F ð2; 34Þ ¼ 8:6; p ¼ :001; e ¼ 0:94]. Eigenvector 2 (accounting for 20.55% of variance) was significantly affected by semantic congruency [F ð1; 17Þ ¼ 6:3; p ¼ :02; e ¼ 0:66], which interacted with electrode site [F ð8; 136Þ ¼ 5:62; p ¼ :01; e ¼ 0:7]. Coefficients were larger to incongruent than to congruent stories at all sites. These characteristics as well as the waveform resemble those found using N400 peak measures. An examination of the eigenvectorÕs negative coefficients revealed a left dominant, frontally distributed eigenvector. Coefficients were found smaller for congruent than for incongruent stories. 3.3.3. Semantically incongruent conditions Eigenvector 1 (accounting for 37.0% of variance) had the largest coefficients at parietal sites, was smaller at central electrodes and smallest at frontal locations [F ð2; 34Þ ¼ 78:17; p < :00001; e ¼ 1:0]. It had a tendency towards a midline distribution as opposed to the left [F ð2; 34Þ ¼ 3:1; p ¼ :06; e ¼ 0:5]. An interaction between electrode frontality and context level was found [F ð2; 34Þ ¼ 37:53; p ¼ :0006; e ¼ 0:99]. Coefficients were larger to sentence- than to discourse-level conditions at centro-parietal sites. The opposite effect was found for frontal electrodes, where coefficients were larger to discourse- than to sentence-level conditions. Eigenvector 2 accounted for 25.7% of the variance among waveforms. Coefficients were larger at parietal compared to frontal sites [F ð2; 34Þ ¼ 7:42; p ¼ :01; e ¼ 0:74], and at the right electrodes compared to the left [F ð2; 34Þ ¼ 6:42; p ¼ :007; e ¼ 0:82]. A main effect of context level was found [F ð1; 17Þ ¼ 57:15; p < :00001; e ¼ 1:0], which interacted with electrode frontality [F ð2; 34Þ ¼ 6:6; p ¼ :03; e ¼ 0:6]. Coefficients were larger to sentence- than to discourse-level conditions at all sites.

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Eigenvector 2 resembled the N400 component. Examination of the eigenvectorÕs negative coefficients revealed a frontally distributed eigenvector. Coefficients were smaller at the left compared to the right electrodes. A main effect of context level indicated smaller coefficients in the discourse- than in the sentence-level conditions. 3.3.4. Semantically congruent conditions Eigenvector 1 (accounting for 28.14% of variance) was found to have parietal [F ð2; 34Þ ¼ 47:7; p < :00001; e ¼ 1:0], and right scalp dominance [F ð2; 34Þ ¼ 4:5; p ¼ :01; e ¼ 0:73]. Eigenvector 2, (explaining 23.3% of variance) showed main effects of context level, with larger coefficients to sentence- than to discourse-level conditions [F ð1; 17Þ ¼ 13:46; p ¼ :001; e ¼ 0:93]. Analyzing ERPs to congruent conditions showed no further significant effects.

4. Discussion This study was designed to examine language processing of semantic information at different linguistic levels. Natural language was simulated as best as possible by stimuli consisting of sentential and discourse contexts presented aurally. Local semantic integration at a sentence level was compared to a wider semantic integration at discourse level by recording ERPs in response to final congruent and incongruent words in two semantic decision tasks. Language processing is a parallel, multi-stage process. Semantic processing has been attributed to two primary theoretical cognitive models. Both models agree on the early stages of language processing, sensory encoding of acoustic–phonetic information and lexical access, but disagree on late post-lexical processes. The ERP components associated with the early stage of acoustic–phonetic processing are N1 and P2, attributed to the auditory cortex. As expected, no significant linguistic effects on N1 measures were found, suggesting its insensitivity to semantic processing factors, which probably occur later. The ERP components examined in the present study, associated with the later stages of language processing and found related to semantic processing, are N400, P550, and LPC. 4.1. N400 Examination of the averaged waveforms revealed a double peaked N400a and N400b. The PCA indicated that N400b was in fact a summation of N400 and the early part of the LPC. This summation can account for the N400b latency difference between eigenvector 2 and the averaged waveforms. N400a may be a summation of small constituents, which were not emphasized by the experimental conditions, and therefore did not appear distinctly in PCA. Both N400a and N400b in the present study remarkably resembled the typical N400 described in the literature. N400a peaked at about 400 ms and N400b at 485 ms post-stimulus with a centro-parietal and parietal prominence, respectively. Both were slightly larger over the right hemisphere than over the left. Both congruent sentence and discourse contexts yielded a decrease in N400 amplitude, in comparison to their incongruent counterparts, i.e., both had a similar effect. The discourse N400 and the sentential N400 seem to be similar. N400b amplitude in response to congruent sentences was larger than to congruent stories (a similar marginally significant effect was found for the N400a amplitude). This finding is corroborated by the results of PCA conducted across waveforms to semantically congruent conditions. Coefficients of the N400-like vector (eigenvector

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2) were larger to sentence- than to discourse-level conditions. These findings can be explained as reflecting a quantitative difference between the long stories and the short single sentences. To verify this proposition one may suggest a control condition: recording ERPs to final words in sentences presented after two irrelevant sentences (that replicate the length of the discourse stimuli, but do not comprise a discourse). However, the quantity of the verbal stimuli is difficult to control in this situation. A sentence following other verbal material, is no longer single and might be processed as an incompatible ending for its previous context. One must take into an account that such a condition would probably elicit a larger N400 and a smaller P5 than the congruent discourse condition. Although similar in length, this control condition contains an additional incongruity factor confounding the interpretation of differences between the two conditions. Another possible explanation is that richer context and stronger semantic-associative priming-like effects may have characterized the discourse text in comparison to the isolated sentence, thereby evoking a smaller N400. This explanation is consistent with earlier studies, that examined ERPs elicited in response to congruous intermediate words as a function of their ordinal position in the sentence (Kutas, Van Petten, & Besson, 1988; Van Petten & Kutas, 1990). Readers or listeners have little background knowledge when presented with isolated sentences, and must therefore build an integrated mental representation of the sentence content as it proceeds. Component amplitude is assumed to reflect the accumulation of the semantic context or the degree to which a word is expected within the context. Words that were positioned near the end of a sentence, benefit from a more extensive semantic context, derived from the earlier part of the sentence. In contrast, words positioned in the beginning of a sentence are preceded by poor semantic context, thereby eliciting a larger N400. Additional studies demonstrated the sensitivity of the N400 amplitude to expectancy level of words (Kutas & Hillyard, 1984; Kutas et al., 1988). N400 amplitude grows smaller as the congruent word close probability is higher, suggesting that N400 may indicate the level of a wordÕs semantic ‘‘priming’’ or activation by the context. Similar mechanisms may explain findings in the present study. In comparison to final words in sentences, words positioned at the end of a story benefit from semantic information derived from previous words at the local sentence level as well as by previous sentences at the global level of the story. Such wider semantic integration yields a higher semantic expectation and results in a smaller N400. Van Petten and Kutas (1991) found that semantically congruent conditions yield a decrease in N400 amplitude in response to content words as the sentence progresses. On the other hand, in sentences that were grammatical acceptable but semantically incoherent, word position had no effect on N400 amplitude, and syntactic and random words elicited equivalent N400 amplitudes, indicating that the word position effect is due to semantic aspects of sentence comprehension. This effect was absent when these sentences were embedded in an appropriately connected text (see Van Petten, 1995). The explanation suggested by the author was that readers build conceptual representations of the contents being read and apply them when processing sentences within a discourse context. Sentences in discourse may benefit from a stronger semantic context from their beginning, equally decreasing the N400 amplitude across the sentence. Based on the above, it is reasonable to conclude that the N400 component reflects the conceptual representation of preceding context with no distinction of sentence boundaries. This conclusion is consistent with a study conducted by Van Berkum et al. (1999). In their study short stories were presented auditorily and the last sentence was presented visually, word by word, and contained semantically anomalous or appropriate words. A second group of subjects were

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presented with the same critical sentences only. A smaller N400 effect in response to coherent sentences in isolation than when embedded at the end of stories forming semantically anomalous discourse was found. This finding suggests that at least a part of the effect was derived from relating target words to the wider discourse. St. George et al. (1994) also demonstrated an effect of global coherence on N400, using noncoherent paragraphs, which were comprehended only with a presentation of an informative title. N400 was larger in response to words of the untitled paragraphs than of the titled paragraphs, indicating that global contextual constrains can affect N400 beyond any syntactic and lexical association contributions. The present study showed that stories elicited a standard N400 effect, which was similar to that elicited by sentences. The PCA results supported those obtained using peak measures. The N400 primary component (eigenvector 2) was found significant in both sentences and stories, explaining approximately the same degree of variance among waveforms. The similarity of these N400 effects suggests an activity of largely overlapping neural generators. On the basis of the present study along with the above quoted findings, it would be reasonable to conclude that N400 reflects comprehension processes at a conceptual level of representation rather than sentential or discourse level. Therefore, the distinction between sentence- and discourse-level semantic integration may be unwarranted. Some psycholinguistic models suggest that the language comprehension system uses integration processes in order to merge different kinds of information such as lexical, semantic, syntactic, pragmatic, and general knowledge shared by the speaker and listener into an ongoing discourse representation (Kintsch, 1988; MarslenWilson, 1987). This integration forms the context by which each utterance is interpreted. In these models the more difficult it is to integrate information into an ongoing representation, the larger the N400 elicited by that information (Rugg, 1990; Rugg et al., 1988). A comparison of sentential–discourse context shows a primary difference—the amount of text integrated. The sources of linguistic and general knowledge, which are activated by a certain sentence, would be the same whether the sentence is in isolation or embedded in text. But when presented in text, previous sentences would activate additional contextual sources to be integrated, thus forming a richer context. In the present study, comprehension processes in sentences and stories evoked the same components but with different amplitudes, according to the amount of text integrated and its clarity and congruence. The latency of N400a and N400b was not affected by any of the stimulus characteristics, indicating similar time course for semantic processing of sentences and stories. Moreover, it was shown that at left scalp locations congruent stories yielded shorter latencies than congruent sentences. Therefore, the hypothesis suggesting that word integration in discourse is a slower and more complex process than word integration in sentences is not supported. These results are compatible with the findings of a study that compared the onset of the two N400 effects in anomalous sentences and stories (Van Berkum et al., 1999). Both effects appeared at about the same time, within 200–300 ms after onset of the anomalous word. Since N400 in the present study was found smaller in response to stories than to sentences but of similar latency, it is evident that global context affects word processing as quickly as sentential context. Each incoming word is related to previous semantic linguistic input, local or global, within 200–300 ms. 4.2. P5 A new and unexpected robust positive monophasic deflection was observed—P5. It began at about 460–490 ms after word onset and peaked at about 535–550 ms, i.e., after

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the appearance of N400, with a centro-parietal prominence. A positive going potential, P560, has already been reported by Kutas and Hillyard (1980a, 1980b, 1980c) in their pioneering study of N400. In that study, semantically acceptable final words in sentences, which were physically aberrant, elicited P560. Obviously, this is not the case with the P5 described here. All speech stimuli used in the present study were physically similar. Moreover, the P5 component was significantly affected by linguistic conditions. Semantically congruent stimuli elicited a larger P5 than semantically incongruent stimuli. P5 peak amplitude was also larger for the discourse- than for the sentence-level conditions. These findings can be explained in terms of global integration. It is reasonable to assume that this component reflects conclusion drawing, which verifies compatibility and congruency of complex semantic integration in text comprehension. None of the factors examined had main effects on P5 latency, indicating similar time course of processing across all conditions at the P5 time window. As described earlier, the latency of the N400 complex was also unaffected by any of the stimulus characteristics. Based on these results, there is no indication for two distinct stages of processing: an earlier processing of local semantics followed by a later stage relating to the wider context. Since P5 was found larger in response to stories than to sentences but within the same time range, it is evident that global context affects word processing at a similar stage as does sentential context. The P5 and N400 amplitude effects reflect at least two additive contributions. First, integration of final words into a more complex and wider text, and second, their integration into a more coherent and clearer text, both of which yield an increase in P5 amplitude and a decreased N4. A possible explanation for the P5 amplitude effects may be its overlap with the N400 complex. These components, N400 and P5, seem to covary as a function of stimulus characteristics. The N400 effect extends up to 500–600 ms following word onset and may yield a subsequent larger overlapping late positivity in semantically congruent discourse conditions in comparison to semantically incongruent sentence conditions. PCA results support the N400-P5 covariance since the procedure could not separate the two components. Both are reflected in eigenvector 2. Still, in spite of the covariance of N400-P5, inspection of the waveforms evoked in response to discourse contexts (Fig. 1) demonstrate a much larger and prominent P5 in response to congruent stories than to incongruent stories, while N400 did not change much. It may be argued that based on P5 scalp distribution, this positivity may be a late manifestation of P300, as proposed in previous studies. A recent study reported a similar effect of N400 and a late P300 that peaked at about 600 ms after a target stimulus (Cansino & Tellez-Alanis, 2000). Subjects were presented with country names followed by city names, and were asked to indicate whether the city belonged to the country or not. Recognition of incongruent factual knowledge yielded an increased N400 whereas the late P300 decreased in amplitude. Latency shift was reported for the N400 alone, so these effects on the two components were not entirely reciprocal. As early as 1986, Neville, Kutas, Chesney, and Schmidt (1986) reported a broad positivity following the N400 component. The authors presented subjects with 4-word sentences, followed by a fifth word, which was either congruent or incongruent. A P650 was evoked in response to the fifth word, regardless of its congruity. A similar component was elicited at about 560ms after the fourth word onset, and was absent in the responses to the preceding words in the sentence. Salustri, Chapman, Chapman, and Mccrary (1993) claimed that the fourth word in Neville et al.Õs experiment was always the keyword in the sentence, which dictated its essential meaning. Therefore, they conducted a visual experiment, which manipulated the position of meaningful keywords in short phrases, while other words remained unchanged. A positivity was evoked around 500 ms post-onset in occipital and

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anterior temporal regions. It had longer duration in response to keywords than to the unchanged words especially in the occipital region but these findings were not statistically significant. The authors suggested that this positivity may in fact be a late P300, even though its scalp distribution was atypical. A similar component was also addressed by Bentin, McCarthy, and Wood (1985). They examined ERP differences between primed and unprimed words. ERPs were shown to differ significantly diverging 200–250 ms after stimulus onset, reaching the N400 peak. An additional late positivity peaking between 550 and 650 ms, provisionally referred to as P300, was reported. The component was found to have the shortest latencies and largest amplitudes in response to target words compared to unprimed words. Having fast RTs, the N400 target–prime difference was still present, but the tentatively P300 latency difference was not significant. Obviously, apparent experimental effects on this component must be distinguished from effects on earlier overlapping portions of the ERP waveforms. We believe that it is unlikely that P550 described in our study is a delayed manifestation of the P300 component. Some previous studies found no semantic effects on P300. Chwilla, Brown, and Hagoort (1995) presented their subjects with physical and lexical decision tasks. Each consisted of low and high probability, related and unrelated semantic pairs. P300 amplitude was modulated as a function of the probability of the word pairs, with larger amplitudes for either related or unrelated word pairs that occurred with lower probability. In the present study the late positive shift cannot be explained in terms of probability since the occurrence of a semantically congruent or anomalous word was equal. Moreover, direct modulations of P5 by semantic factors in the present study cannot be ruled out. P300 reflects a confirmation of the subjectÕs expectation. A surprising event would elicit an increased P300 while in this study it elicited a decreased P550. Conversely, an expected situation elicited the opposite effects. It is clear that semantic deviations activate a different constellation of brain activity than physical deviations (Kutas & Hillyard, 1980a, 1980b, 1980c). This physiological dichotomy between P300 and N400 may stand for the P300 and P550 as well, indicating different modes of cognitive reactions to different expected or unexpected events. It still remains to be examined whether the P5 component is specific to semantically congruent stimuli or whether it accompanies other linguistic and nonlinguistic expectancies. 4.3. Late positive component (LPC) LPC peaked at approximately 850 ms with a centro-parietal prominence. An earlier study demonstrated that the LPC was modulated by semantic context in adults using semantically congruous and incongruous final words in spoken sentences (Juottonen, Revonsuo, & Lang, 1996). A later study found that the amplitudes of both N400 and LPC were larger for low- than for high-frequency words (Fernandez et al., 1998). The present study suggests new aspects of LPC. We showed that LPC was also larger to incongruent final words in stories, that were semantically acceptable at the sentence level, than to congruent final words. Moreover, LPC was modulated not only by semantic congruency but also by a context level effect. Peak amplitudes were found larger and latency tended (p ¼ :08) to be longer in response to sentence- than to discourse-level conditions. This study demonstrated an effect of global coherence on LPC as well as on components preceding it. These results bring further support to the claim of similar processing of local and global integration. Moreover, the associated shorter latencies with discourse level integration contradict linguistic models suggesting a slower processing of words in stories than in sentences. These results are also incompatible with the hypothesis of two distinct stages in

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semantic processing. Comprehension appears to proceed, as the integrated mental representation is being formed based on the contents being heard without taking into account sentence boundaries. We believe that LPC may have a different cognitive role than that of N400. PCA resulted in two distinct eigenvectors indicating two independent components; each affected differently by the experimental conditions. Eigenvectors 1 and 2 exhibited homologies to LPC and N400, respectively, in morphology, scalp distribution and modifications across experimental manipulations. N400 was more negative while LPC was more positive in response to semantic incongruity and to sentence level semantics, with the opposite effect in response to semantic congruency and to discourse level processing. LPC shortest latencies and lowest amplitudes were elicited by congruent story conditions, whereas incongruent story conditions elicited the longest latencies and high amplitudes. The highest amplitudes were evoked by the semantically incongruent sentences where the evoking words were most anomalous. Based on these results it is reasonable to assume that LPC may reflect a completion of the learning of a new semantic representation, during which a fulfillment of expectancies and an evaluation of congruency occurs. 4.4. Principal component analysis PCA results supported those obtained using peak measurements regarding morphology, scalp distribution and modulations across experimental conditions, demonstrating a resemblance of eigenvector 1 to LPC. Eigenvector 2 resembled N400 and P550 (inverted in polarity). In addition to confirming peak measures, PCA contributed additional information. Analyzing waveforms to final words within discourse showed a resemblance to LPC only in frontal regions. Eigenvector 1 coefficients were larger with incongruent stories than with congruent stories, as expected in frontal locations, but the opposite effect was shown in parietal and central locations. In addition, an examination of the negative coefficients of eigenvector 2 revealed a different distribution than that of P5. It was frontally distributed and more dominant over the left scalp. Still, similar to the P5 component, it was more dominant in response to congruent than to incongruent stories. On the other hand, analyzing waveforms to final words in sentences revealed a perfect match of eigenvector 1, eigenvector 2 and eigenvector 2 inverted in polarity, with LPC, N400, and P550, respectively. Analyzing ERPs to incongruent final words showed that coefficients of eigenvector 1 were larger to sentence- than to discourse-level conditions in centro-parietal sites. The opposite effect was shown in frontal regions, where coefficients were larger to discourse- than to sentence-level conditions. An examination of the negative coefficients of eigenvector 2 revealed, again, a frontally distributed component, which was more dominant over the left scalp. As expected, it was more dominant in response to stories than to sentences. On the basis of these PCA results, it seems that semantic processing of discourse is associated with a more frontal scalp distribution whereas semantic processing of sentences is related to a more parietal distribution. These results indicate activation of multiple sources in the P5-LPC (500–1000 ms) time frame, of which at least two: a parietal- and a frontal source, are sensitive to different aspects of semantic processing.

5. Summary This study showed that both sentences and stories evoked the same components: N400, P5, and LPC, and affected them similarly. These components have similar

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characteristics (morphology, scalp distribution and effects of experimental manipulation) when evoked in sentences or stories. Moreover, no latency differences were detected between sentence and story conditions. Therefore, there was no support for the suggestion that processing discourse texts is slower and more complex (consisting of at least 2 stages) than processing sentences. Findings showed that processing of discourse stimuli is just as fast as processing sentences. The similarity suggests an activity of largely overlapping neural generators, with different sub-populations as indicated by PCA, underlying the processing of either sentences or stories. These processes, in spite the similarity, are characterized by differential effects on amplitudes according to the amount of text integrated and its congruency. It would be reasonable to conclude that these components reflect comprehension processes at a conceptual level of representation rather than sentential or discourse level. The two conditions, sentences vs. stories, consisted of a common process of context build-up as the information unfolds with no distinction of sentence- and discourse-level boundaries. Appendix A. Examples of semantic stimuli Congruent discourse

Incongruent discourse

Congruent sentence Incongruent sentence

My computer system suddenly broke down IÕm glad I had back-up disks Fortunately, I didnÕt lose any files My computer system suddenly broke down IÕm glad I had back-up disks Fortunately, I didnÕt lose any friends Fortunately, I didnÕt lose any friends Fortunately, I didnÕt lose any rainbows

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