Generalized event knowledge activation during online sentence comprehension

Ross Metusalem; Marta Kutas; Thomas P Urbach; Mary Hare; Ken McRae; Jeffrey L Elman

doi:10.1016/j.jml.2012.01.001

. Author manuscript; available in PMC: 2013 May 1.

Published in final edited form as: J Mem Lang. 2012 May 1;66(4):545–567. doi: 10.1016/j.jml.2012.01.001

Generalized event knowledge activation during online sentence comprehension

Ross Metusalem ^a, Marta Kutas ^a, Thomas P Urbach ^a, Mary Hare ^b, Ken McRae ^c, Jeffrey L Elman ^a

PMCID: PMC3375826 NIHMSID: NIHMS351551 PMID: 22711976

Abstract

Recent research has demonstrated that knowledge of real-world eventsplays an important role inguiding online language comprehension. The present study addresses the scope of event knowledge activation during the course of comprehension, specifically investigating whether activation is limited to those knowledge elements that align with the local linguistic context.The present study addresses this issue by analyzing event-related brain potentials (ERPs) recorded as participants read brief scenariosdescribing typical real-world events. Experiment 1 demonstratesthat a contextually anomalous word elicits a reduced N400 if it is generally related to the described event, even when controlling for the degree of association of this word with individual words in the preceding context and with the expected continuation. Experiment 2 shows that this effect disappears when the discourse context is removed.These findings demonstrate that during the course of incremental comprehension, comprehenders activate general knowledge about the described event, even at points at which this knowledge would constitute an anomalous continuation of the linguistic stream. Generalized event knowledge activationcontributes to mental representations of described events, is immediately available to influence language processing, and likely drives linguistic expectancy generation.

Keywords: sentence processing, psycholinguistics, language comprehension, event knowledge, event-related potentials, expectancy generation

Introduction

Despite its great complexity, the world is a highly structured environment. Events in the world are not random, but instead exhibit regularities that people learn throughout their lives. Knowledge of these regularities, for example that police arrest criminals or that glassoften breaks when dropped, supports numerous cognitive capacities, including language comprehension.The importance of event knowledge to language comprehension has been appreciated for some time. Work dating back several decades has unequivocally demonstrated that successful comprehension involvesengaging real-world knowledge stored in long-term memory(Bransford & Johnson, 1972;Sanford & Garrod, 1981; Schank, 1980; van Dijk & Kintsch, 1983).Some theories of language comprehension invokethe mapping between linguistic input and relevant event knowledge as entirely fundamental to what it is to understand language (Altmann & Mirkovic, 2009; Elman, 2009; Sanford & Garrod, 1998; van Dijk & Kintsch, 1983). Although much early workon discourse comprehension supports this notion, these studies generally do not provide an account of how event knowledge is engaged in real time, as a sentence is comprehended incrementally.

Determining the time-course of event knowledge activation and use is a central issue in developing any theory of online language processing, and this is the central goal of this study. Some theories propose that the early stages of online comprehension are largely informationally encapsulated, with only syntactic and basic lexical information being immediately accessed as a word is processed in real-time (Bornkessel & Schlesewsky, 2006; Frazier, 1978, 1990, 1995; Frazier & Rayner, 1982; Rayner, Carlson, & Frazier, 1983). However, in recent years, accumulating evidence indicates that event knowledge exerts a rapid influence on incrementalcomprehension processes, suggesting that the earliest stages of comprehension engage the wealth of world knowledge a comprehender brings to the task of understanding language.

At the level of individual words, lexical priming studies suggest that the processing of isolated words immediately activates knowledge of events of which the words are components. Agent, patient, instrument, and location nouns prime verbs denoting events in which these elements participate (McRae, Hare, Elman, & Ferretti, 2005), and nouns denoting events prime people and objects typically found at those events (Hare, Jones, Thomson, Kelly, & McRae, 2009). Instrument nouns prime objects on which those instruments are typically used, and location nouns prime typical people and objects found at those locations (Hare, Jones, et al., 2009). Chwilla and Kolk (2005) found priming in a word triplet paradigm when the combination of two primes denote an event to which the target word is related (e.g., director and bribe together prime dismissal). Ferretti, Kutas, and McRae (2007) found that priming of event locations by event verbs is modulated by verb aspect: imperfective verbs prime event locations while perfective verbs do not (e.g., was cooking primes kitchen, but had cooked does not), demonstrating that isolated verb processing results in immediate accessibility of likely event locations only when the verb denotes the event as ongoing. This notion is consistent with other experimental work showing that imperfective verbs highlight the internal structure of an event, while perfective verbs promote a representation of the event as a completed whole (Madden & Zwaan, 2003). It is worth noting that these priming results are unlikely to be due to undifferentiated associations between individual lexical items. In Chwilla and Kolk(2005), the individual prime words were not normatively associated with each other or the target word, meaning that the priming effect most likely resulted from rapid integration of the meanings of the two prime words through mapping onto an event knowledge structure. Likewise, in Ferretti et al. (2007), direct lexical associations cannot account for the obtained effect, as priming of event locations by a single verb depends upon the grammatical form of that verb. If direct associations between lexical items alone were at play, verb aspect should not modulate the priming effect. It thus appears that even in the absence of a sentential or discourse context, language comprehension involves rapid mapping of linguistic input onto event knowledge, resulting in activation of other event-relevant information. Such a finding suggests that mapping linguistic input to event knowledge is a fundamental characteristic of the language comprehension system.

From this conclusion, it follows that event knowledge should exert an immediate influence during online sentence and discourse comprehension. Sentences and discourse provide not only individual content words capable of evoking event knowledge, but also cues as to the relations between these words, providing rich information to be used in mapping linguistic input onto event knowledge.Along these lines, Altmann and Kamide (1999) demonstrated that the thematic fit between a verb and possible patient objects, in combination with concurrent visual input, can drive expectations for postverbal arguments. Participants viewed a scene depicting, for example, a boy, a cake, and several toys. Participants were faster to fixate the target (cake) when they heard The boy will eat the cake than when they heard The boy will move the cake, with fixation patterns suggesting that participants were able to anticipate the target following the verb eat but not following the less restrictive verb move. This finding shows that upon encountering the verb, comprehenders were able to rapidly apply their world knowledge (i.e., that a boy will likely eat something that is edible), in conjunction with visual information (i.e., that the cake is the only edible object in the scene), to anticipate an upcoming referent. Expanding upon this finding, Kamide, Altmann, and Haywood (2003) found that comprehenders launch anticipatory saccades based on the event implied by the integration of an agent noun and following verb - for example, participants were more likely to launch anticipatory saccades to a picture of a motorbike upon hearing The man will ride… than upon hearingThe girl will ride….This finding indicates that comprehenders can integrate information provided at multiple points in a sentence to engage event knowledge, which then,in conjunction with information provided by a visual display, guides anticipatory processing.

Additional evidence for the rapid influence of event knowledge has been obtained in reading time studies. Postverbal patient nouns are read faster when they are plausible given the combination of a preceding agent and verb (Bicknell, Elman, Hare, McRae, & Kutas, 2010) or preceding instrument and verb (Matsuki, Chow, Hare, Elman, Scheepers, & McRae,2011). These results, like those of Kamide et al. (2003), demonstrate that the online assessment of thematic fit of a post-verbal argument is not determined by the verb alone, but rather by information regarding the real-world event denoted by the verb in combination with its preceding arguments. Verbs therefore appear to encode thematic roles in an event-specific fashion, a conclusion that has major consequences for theories of lexical knowledge (Elman, 2009).

Reading time studies have shown event knowledge to exert a rapid influence in the syntactic domain, as well. Expectations for a reduced relative clause are affected by the status of the grammatical subject as a typical agent (The crook arrested by the detective was guilty) or patient (The cop arrested…)of the initial verb(McRae, Spivey-Knowlton, & Tanenhaus, 1998).Similarly, postverbal regions of intransitive sentences are read faster when the subject is a plausible patient of the verb versus a plausible agent (e.g., into tiny bits is read faster following The glass shattered than following The brick shattered), and the converse holds true for transitive sentences (Hare, Elman, Tabaczynski, & McRae, 2009).This finding and that reported by McRae et al. (1998) show that event knowledge exerts an immediate influence on syntactic expectations during incremental comprehension.

Researchusing the event-related brain potential (ERP) technique has demonstrated that the compatibility of an eliciting word with the described event modulates the amplitude of the N400 component.The N400 is a negative going deflection in the ERP waveform peaking at approximately 400ms following the onset of a word. Its amplitude is inversely related to the eliciting word’s degree of contextual fit, which is often captured offline by the word’s cloze probability (the proportion of people who provide the word as a continuation of the sentence at that point). N400 amplitude is generally viewed as an index of online meaning processing via semantic memory activation (Kutas & Federmeier, 2000, 2011; Kutas & Hillyard, 1980, 1984). The previously discussed studies by Ferretti et al. (2007), Chwilla and Kolk (2005), and Bicknell et al. (2010) included ERP experiments that showed their obtained reaction time and reading time effects are reflected in N400 amplitude modulations. Hagoort and colleagues have shown that violations of world knowledge that are nevertheless semantically appropriate elicit N400s of similar amplitude to lexico-semantic violations (Hagoort, Hald, Bastiaansen, & Petersson, 2004). Given that Dutch trains are typically yellow, the target word white in The Dutch trains are white and very crowded elicits an N400 equal in amplitude to that elicited by sour in The Dutch trains are sour and very crowded. This amplitude similarity, in addition to similarity in topography, onset, and peak latency of the N400 for the two violation types, suggests that both world knowledge and lexical semantic knowledge influence online meaning processing in a similar fashion, a conclusion consistent with the notion that world knowledge and lexical semantic knowledge are not entirely dissociable (Elman, 2009). These and other ERP studies (e.g., Camblin, Gordon, & Swaab, 2007; Hald, Steenbeek-Planting, & Hagoort, 2007; Otten & Van Berkum, 2007) strongly suggest that general event knowledge engaged during discourse comprehension serves to guide online semantic processing, and that N400 amplitudecan serve as an accurate index of this process.

The Scope of Real-time Event Knowledge Activation

The research discussed abovehas conclusively demonstratedthrough a variety of techniques that event knowledge is an important source of information used to guide language comprehension in real time. When we comprehend language about events, we activate knowledge regarding those events and use this knowledge to facilitate onlinelinguistic processing. It is important to note,however,that previous work described above demonstrates facilitated processing exclusively for words that are semantically congruent with the linguistic context.With respect to the scope of event knowledge activation during the course of incremental comprehension, such research allows us to infer that activation includes elements that are congruent with both the event being described and (crucially) the linguistic context itself.The question we ask here is whether realtime event knowledge activation extends to elements that are anomalous within the local linguistic context (i.e., are not predicted by the context) but which are nonetheless consistent with the global event being described. That is, despite the fact that the reviewed lexical priming results demonstrate that processing isolated words engages a range of event knowledge elements, it is possible that during online sentence comprehension, the cues provided by the unfolding linguistic context constrain event knowledge activation to only those elements that would constitute semantically congruent or expected continuations of the sentence at that point. Alternatively, as we hypothesize, additional event-relevant but contextually anomalous entities might become activated in comprehenders’ minds even when they violate the linguistic context.

To illustrate, consider the passage in (1):

(1)A huge blizzard ripped through town last night. My kids ended up getting the day off from school. They spent the whole day outside building a big snowman in the front yard.

Given previous results indicating that comprehenders utilize event knowledge to guide linguistic expectancy generation, it is likely that upon processing building a big, a comprehender will activate objects often built by children playing in the snow, such as a snowman or igloo. However, people know more about “playing in the snow” events than snowmen and igloos, such as the likelihood that the children are bundled up with hats, mittens, and jackets, but none of these items satisfy the semantic constraints imposed by the verb building. Therefore while these items are salient components of “playing in the snow” knowledge, the linguistic context suggests that these items should not appear in the sentence at this point. If specific cues provided by the linguistic context interact with activation of event knowledge to yield activation of only those elements that would constitute congruent or expected continuations of the current sentence at that point, it would suggest that event knowledge activation, as it occurs as a sentence is comprehended incrementally, serves mainly to facilitate processing of the linguistic input that the comprehender is likely to receive. On the other hand, the ability of comprehenders to make causal or bridging inferences clearly indicates that language comprehension must engage general event knowledge that is not explicitly stated in the discourse. The issue we address, thus, is how activation of particular elements of event knowledge is constrained in real time by the specific cues present in an unfolding sentential context.

This specific issue speaks directly to the more fundamental question regarding the nature of the mental representations that are dynamically constructedduring incremental sentence comprehension. Mental representations of described events are not static, but rather are cotinuously modified during the course of comprehension. If event knowledge is itself not merely used to guide incremental sentence processing, but is itself a target of the comprehension process, then we would expect to find facilitated processing for salient event knowledge elements even at specific times at which the elementsdo not align with the particular cues provided by the local linguistic context.

This is the hypothesis we pursue here. This hypothesis generally aligns with previous research on inferencing. This research has demonstrated that discourse comprehension often involves instantiation of relevant, unstated information accessed from long-term memory (see Graesser, Singer, & Trabasso (1994) and McKoon& Ratcliff (1992) for reviews of inferencingresearch and theories of inference generation)¹.While inferencingresearch has established that discourse comprehension can engageunstated but relevant background knowledge at least in some circumstances, we do not know how such knowledge activation interacts with linguistic contextual cues as a sentence is comprehended incrementally. Research in this area typically employs paradigms in which a probe word or sentence is presented following comprehension of a preceding sentence or discourse. Such paradigms allow one to assess activation of unstated event knowledge after comprehending a discourse, but they do not provide insight into how activation of this knowledge interacts with explicit linguisticcues unfolding in real time.Facilitated processing of a probe following discourse comprehension may show that this knowledge is activated at some point following offset of discourse presentation, but it is an open question whether contextual cues encountered as a sentence is processed incrementally dynamically limit activation of event-relevant information to only that which aligns with the linguistic context.

In fact, there are experimental results suggesting that unfolding contextual cues may limitthe scope of event knowledge activation. Traxler, Foss, Seely, Kaup, and Morris (2000) monitored participants’ eye movements during reading of sentences such as The [lumberjack/young man] chopped the axe early in the morning, and found no difference in first fixation or gaze duration times for the target word (axe) following the different agents (lumberjack vs. young man). The authors concluded that schematic knowledge does not drive general activation of words describing event participants during online comprehension. If it did, axe should have been read faster following lumberjack chopped, because an axe is a prototypical instrument associated with a“lumberjacking” schema. In an earlier study, Kintsch and Mross (1985) found similar results using a cross-modal lexical decision task. Participants listened to brief stories describing common events (e.g., catching a flight) and made a lexical decision to a visually presented word mid-discourse. The authors found only a marginal priming effect for event-associated versus -unassociated words (e.g., gate vs. stack in the airport scenario), and in a version of the experiment employing all-visual presentation, this marginal priming effect disappeared completely. This result, like that found by Traxler et al., suggests that online event knowledge activation does not extend to elements that do not fit the local linguistic context. However, Traxler et al. used only a few sentence-initial words to establish a situation, and Mross and Kintsch tested for activation of ensuing actions. Thus, we believe that these results should not be taken to indicate conclusively that linguistic cues dynamically limit event knowledge activation. The relationship between these studies and the present studyisdiscussedin more detail in the General Discussion.

The Present Study

In two experiments, we investigated the real-time activation of contextuallyanomalousevent knowledgeelements during incremental language comprehension. We recorded participants’ EEG as they read brief scenarios such as the “playing in the snow” example in (1) above. In each scenario, two sentences established an event context, and a third and final sentence included one of three target word types. In one condition, participants read a highly expected word (e.g., building a snowman; the expected target). In another condition, participants reada contextuallyanomalous word that was unrelated to the event being described (e.g., building a towel; the event-unrelated target), and in a third, theyread ananomalous word that crucially was related to the described event (e.g., building a jacket; the event-relatedtarget). ERPs elicited by these three target types were analyzed, with special attention paid to the amplitude of the N400 component.

Given previous N400 results, we predicted that the expected targets should elicit an N400 of very small amplitude (perhaps even a positivity in the 200-500ms N400 latency range), as they arecontextually congruous and on average are highly expected in context. Furthermore, the event-unrelated targets should elicit a large N400 because they are contextually anomalous and completely unexpected. The major issue hinges on the amplitude of the N400 elicited by the event-related but contextually anomalous targets. If they elicit an N400 equal to the event-unrelated targets, it would suggest that fit with the local linguistic context is necessary to observe facilitated processing of event-related concepts, and that linguistic cues greatly constrain event knowledge activation in real time. However, if event knowledge activationextends beyond contextually congruent information to include other prominent elements of event knowledge, then the event-related targets should elicit a reduced N400 compared to the event-unrelated targets.

There are a number of previous ERP studies demonstrating N400 reduction for unexpected contextually inappropriate words based on various relationships between those words and the preceding context or most expected word. Unexpected words elicit a reduced N400 when semantically related to the most expected word (Kutas & Hillyard, 1984), with context-independent semantic categorystructurelikely supportingthis effect (Federmeier & Kutas, 1999). Along similar lines, incorrect reinstatement of a categorical anaphor elicits a reduced N400 when the reinstatement is from the same category as the correct antecedent (e.g., seatreferring to the antecedent stool, but being reinstated in a subsequent sentence as couch; Ditman, Holcomb, & Kuperberg, 2007). Other workhasshown that semantic illusion stimuli elicit N400s of similar amplitude to the most expected word (e.g., In a recent trial, a ten-year sentence was given to the victim; Sanford, Leuthold, Bohan, & Sanford, 2010). Thiseffect holds even for words that violate animacy restrictions of a verb when the violating word has been stated previously and constitutes the theme of the discourse (Nieuwland & Van Berkum, 2005). In addition, words that violate real-world plausibility can elicit smaller N400s than generally plausible words when the discourse explicitly highlights the implausible situation as acceptable (Nieuwland & Van Berkum, 2006). Discourse cues do not completely trumpprior knowledge, though; words that align with world knowledge but not the specific discourse messageelicit smaller N400s than words that align with neither (Hald et al., 2007; Otten & Van Berkum, 2007).

These findings highlight several important issues with respect to the present study. First, because studies have shown N400 reduction due to semantic relatedness between an unexpected target word and the most expected word, we controlled this relation in our study. Second, because Nieuwland & Van Berkum (2005) found that a contextually anomalous word can elicit a reduced N400 when explicitly mentioned as the theme of the discourse, we ensured that our event-related and event-unrelated targetswerenotpreviously mentioned in their discourse contexts.Finally, the latter studies underscore the specifictheoretical contribution of the present study.Previous research has established that event knowledge can exert an influence on N400 amplitude independently of information provided by the discourse message. For example,when the discourse describes traffic issues and road layout in Venice, canalsnonetheless elicits a similar N400 to roundabouts following The city of Venice has many… (Hald et al., 2007). However, no study has investigated how association with a described event affects the N400 elicited by a word that is anomalous at a particular point in the linguistic stream. In doing so here, this study seeks to determine whether event knowledge elements that violate the local linguistic context are active during the course of incremental comprehension. Reduction of the N400 elicited by a contextually anomalous but event-related word would indicate that incremental comprehension involves activation of event knowledge elements beyond those supported by the local linguistic context. Uncoveringsuch an extendedscope of event knowledge activation would add to our knowledge of how information in long-term memory is engaged during the course of comprehensionand would inform all theoriesregarding how people comprehend language about real-world events.

Experiment 1

In Experiment 1, participants’ EEG was recorded while they read three-sentence scenarios describing typical events. The dependent measure was the amplitude of the N400 elicited by three types of target words: a highly expected word, a contextually anomalous word that was related to the described event, and an equally anomalous word that was unrelated to the event. Based on numerous studies in which cloze probability of targets has been manipulated, we anticipated that the expected targets would elicit the lowest amplitude N400 response. Because both the event-related and event-unrelated targets were zero-cloze and generally nonsensical in context, one possibility is that their N400 responses would not differ from one another, but would besignificantly greater than the expected targets. However, if activation of event knowledge extends to elements that violate the linguistic context, we would observe a three-way contrast: expected targets would elicit the lowest amplitude N400, event-unrelated targets would elicit the largest, and event-related targets would elicitan N400 intermediate to these two extremes.

Methods

Participants

Thirty undergraduates (22 women; ages 18 to 22 years) at the University of California, San Diego completed the ERP experiment for course credit. All were right-handed, native monolingual English speakers, and none reported any history of learning or reading disabilities or neurological or psychiatric disorders.

Materials

The experimental materials consisted of 72 scenarios similar to that presented in (1) above. Each scenario consisted of three sentences. The first two sentences were designed to establish the event context. The third sentence contained one of three possible target words: the most expected word (the expected target), a contextually anomalous word that was related to the event described (the event-related target), or a contextually anomalous word that was unrelated to the event (the event-unrelated target).

We began by creating 72 three-sentence scenarios in which the first two sentences were complete, and the third sentence terminated at a point at which we believed that a specific noun would be deemed highly likely to come next. In a cloze task, participants read the scenarios and provided the word they believed would be most likely to continue each story. Thirty UCSD undergraduates (23 women; all native English speakers), none of whom participated in the ERP experiment, completed the cloze task through an online form. Cloze probability was calculated as the proportion of participants who provided a particular response for a given scenario. For each scenario, the word with the highest cloze probability was chosen as the expected target. Across the 72 scenarios, the mean cloze probability of the expected target was 0.81 (range = 0.367 to 1.00; SD = 0.17).

Following the cloze task, we used the scenarios with the expected targets filled in to probe comprehenders’ knowledge of people or objects that were unstated but highly likely to be components of the described events. Responses were used to select the event-related targets.Participants were instructed to read each scenario and to “paint a mental picture” of the event being described. They were told that these mental pictures would probably include various people and objects that were present at the event but not explicitly mentioned in the text itself. They were asked to list up to five of these people/objects for each scenario. Forty-five UCSD undergraduates (26 women; all native English speakers), none of whom participated in the ERP experiment or previous cloze task, completed the task through an online form.Responses for each scenario were given weighted scores based on order of mention (5 points for the first response, 4 for the second, etc.). The highest scoring response that was not provided as a response in the cloze study (i.e., was zero-cloze) was chosen as the event-related target. In a few instances, the highest scoring event related target was deemed by the authors to constitute a sensible continuation of the sentence despite not being provided as a response in the cloze task. In these cases, the next highest scoring zero-cloze response was chosen. This was to ensure that each event-related target, although highly likely to be part of the described event, would be truly anomalousin context. Across the 72 scenarios, the mean score for the event-related targets was 92.4 (range = 38 to 171; SD = 35.3). The maximum possible score for a given item, had every participant provided the same item as their first choice for that scenario, was 225 (45 subjects x score of 5).

We then constructed event-unrelated targets by shuffling the event-related targets across scenarios. This ensured that the event-related and event-unrelated targets consisted of the same lexical items, thereby controlling lexical factors across the two conditions. The 72 experimental items were split into three rotation groups of 24 scenarios each, allowing for three lists to be constructed by rotating each group through the three conditions. Each scenario thus appeared once in each list and once in each condition across the three lists. To minimize variability across the lists, the rotation groups were matched on the following factors: mean cloze probability, log frequency, and orthographic length of the expected targets; mean event-relatedness score, log frequency, and orthographic length of the event-related targets. Event-related targets were then shuffled across the scenarios within each rotation group to obtain the event-unrelated targets. Event-unrelated targets were shuffled ensuring that each was zero-cloze. In all but two of the 72 scenarios, event-unrelated targets had event-relatedness scores of zero². In addition, the shuffling was done in such a way as to match the event-related and event-unrelated targets within each scenario for animacy and concreteness. The norming results are presented in Table 1. Following these norming procedures, additional material was added to the final sentence of each scenario to make each target word sentence-medial. All experimental items are presented in the Appendix.

Table 1.

Norming results for the three rotation groups and the stimuli set overall.⁷

		Group 1	Group 2	Group 3	Overall
Expected targets	Cloze probability	0.81	0.80	0.80	0.81
	Log frequency	6.95	7.01	6.88	6.95
	Orthographic length	5.58	5.71	5.75	5.68

Event-related targets	Cloze probability	0.00	0.00	0.00	0.00
	Log frequency	6.89	6.91	6.76	6.86
	Orthographic length	5.96	5.96	5.71	5.87
	Event-relatedness score	91.17	89.71	96.21	92.36

Event-unrelated targets	Cloze probability	0.00	0.00	0.00	0.00
Event-unrelated targets	Event-relatedness score	0.00	0.13	0.04	0.06

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One final issueconcerned ensuring that the two anomalous target types were not associated with the expected targets.Previous studies have shown that a contextually inappropriate word can elicit a reduced N400 when closely associated with the most expected word (Federmeier & Kutas, 1999). We quantified the degree of association between the expected target and the two anomalous target types using the University of South Florida Free Association Norms (Nelson, McEvoy, & Schreiber, 2004).From Nelson et al.’s norms, we looked for a forward association from the expected target (the cue) to either the event-related or event-unrelated target (the response).Of the 72 experimental scenarios, 65 contained expected targets that appeared in the Nelson norms; mean association scores for the event-related and event-unrelated targets were calculated across these 65 items; mean associative strength was calculated across these items. The mean associative strengthwas 0.0005 for the event-related targets, and 0.0001 for the event-unrelated targets. Therefore, associative strengths for event-related and event-unrelated targets correspond to one response per two thousand participants and one response per ten thousand participants, respectively. These extremely low scores, andthe small, statistically nonsignificant difference between them were deemed acceptable for the purposes of the study.

Each scenario was accompanied by a simple Yes/No comprehension question (also provided in the Appendix). These were included to ensure that participants read each scenario for comprehension. In addition to the 72 experimental items, 24 filler items were included. Like the experimental scenarios, each of these items consisted of three sentences describing a real-world event. None contained any contextually anomalous words. These fillers were included to achieve an even number of trials containing anomalous and non-anomalous words.

Procedure

Participants sat in a soundproofed, electromagnetically shielded chamber and read each scenario from a computer monitor. They were instructed to read the scenarios carefully, as they would be required to answer a comprehension question following each scenario. The first two sentences of each scenario were presented in paragraph format. Once participants understood them, they pushed a button to advance to the final sentence. A small red cross then appeared at the center of the screen to cue participants to avoid behaviors such as blinking, eye movements, and muscle tension, which could introduce artifacts into the EEG signal. The final sentence was presented directly above the cross one word at a time (rapid serial visual presentation, RSVP) with a 350ms stimulus onset asynchrony (SOA), divided into a 200ms stimulus duration and a 150ms inter-stimulus interval (ISI). After the offset of the final word, participants answered a yes/no comprehension question by pushing a button with their left or right hand (e.g., left for “No”, right for “Yes”). Yes/No hand was counterbalanced across participants.

The experiment was divided into five blocks, with the first block containing 20 trials and the remaining four blocks containing 19 trials each. Participants were able to request a short break within or between blocks. The entire experimental session, including preparation of EEG recording, lasted approximately two hours. Ten participants were randomly assigned to each of the three lists. Stimulus presentation order was fully randomized for each participant.

Immediately following the experiment, each participant completed an Author Recognition Test and the Magazine Recognition Test. These tests were local adaptations of the earlier tests described in Cunningham (1990) and Stanovich and Cunningham (1992). ART/MRT questionnaires consisted of 80 author names/magazine titles, 40 real and 40 fake. Participants were told to mark each author/magazine they knew to be real and to avoid guessing. Scores were calculated as number of correct IDs minus the number of false positives across both tests. Stanovich and Cunningham (1992) present these tests as measures of print exposure, and show that performance on these tests correlates with measures of analytic intelligence, vocabulary, verbal fluency, reading comprehension, and history and literature knowledge. We hypothesized that performance on these tests might also reflect individual differences inparticipants’ knowledge of real-world events, which aligns with Stanovich and Cunningham’s proposal that increased print exposure contributes toa richer world knowledge basethan that supported by individualexperience alone. With ART/MRT performance likely capturingindividual differences in linguistic proficiency and world knowledge, we hypothesized that varying performance on these tests might coincide with individual differences in N400 responses to the event-related and event-unrelated target words.

EEG recording and processing

EEG was recorded from 26 tin electrodes distributed evenly across the scalp, referenced online to the left mastoid and re-referenced offline to the average of the left and right mastoids. Electrodes were placed on the outer canthus and infraorbital ridge of each eye to monitor eye movements and blinks. All electrode impedances were kept below 5 K Ω. EEG was amplified with Nicolet amplifiers with a bandpass of 0.016 to 100 Hz and digitized at a rate of 250 Hz. A diagram of the scalp electrodes is provided in Figure 1.

Layout of the 26 electrodes across the scalp. The embedded table summarizes the electrode groupings for the distributional analyses in Experiments 1 and 2. Filled-in electrodes were included in the distributional analyses.

To obtain ERPs for the three target word types, each participant’s EEG was time-locked to target word onset and averaged within each condition across a 2044ms epoch, relative to a 500ms pre-stimulus baseline. Trials contaminated by blinks, muscle tension (EMG), channel drift, and/or amplifier blocking were discarded before averaging. Approximately 9% of target word epochs were rejected due to such artifacts, with losses distributed approximately evenly across the three conditions.Individual participant averages were then averaged together to obtain a grand average ERP for each condition. Each participant’s performance on the comprehension questions was assessed before entering the participant’s data into the analysis. All participants scored at least 89% correct on the 72 target comprehension questions, indicating that they read the scenarios for comprehension as instructed, and therefore no participant’s data were excluded.

Results

N400 amplitude

Figure 2 displays the grand average target word ERPs for the 26 scalp electrodes from 500ms pre-stimulus to 1000ms post-stimulus, arranged according to the distribution of electrodes across the scalp presented in Figure 1. The N400 can be seen as a negative-going deflection in the ERP waveform peaking at approximately 400ms post-stimulus. To analyze N400 amplitude differences across the three conditions, mean ERP amplitudes from 200-500ms were entered into a repeated measures ANOVA with three levels of Condition and 26 levels of Electrode. A main effect of Condition was obtained, F(2,58) = 38.33, ε_GG = 0.7756, p<.001, as was a Condition-by-Electrode interaction,F(50,1450) = 7.26,ε_GG = 0.1075,p<.001.³

Grand average ERPs elicited by the target words in the three conditionsfrom Experiment 1. Ticks on the abscissa mark 200ms intervals from target word onset. Negative voltage is plotted up.

Planned comparisons revealedthe critical pattern of results in which the event-related condition lay statistically between the expected and event-unrelated conditions. N400 amplitude in the event-unrelated condition was be significantly greater (i.e., more negative) than the event-related condition(F(1,29) = 13.00, p<.01; no interaction with Electrode), which in turn was greater than the expected condition (F(1,29) = 35.44, p<.001; Condition-by-Electrode interaction: F (25,725) = 7.77, ε_GG = 0.097, p<.001). This graded N400 effect can be seen clearly in the waveforms, beginning at approximately 200ms post-stimulus and peaking around 400ms. (See Figure 3 for a close-up of the midline parietal electrode site.)In summary, the event-related targets elicit a reduced N400 compared to the event-unrelated targets, with both types of anomalous targets eliciting larger N400s than the expected targets. In other words, the N400 effect for the event-unrelated targets (i.e., the N400 amplitude difference between the event-unrelated and expected targets) is larger than that for the event-related targets (i.e., the difference between the event-related and expected targets), as illustrated by the difference waves presented in Figure 4.

Grand average ERPs at the midline parietal electrode (MiPa) in Experiment 1.

Difference waves reflecting the size of N400 effectsin the event-related and event-unrelated conditions in Experiment 1.

Because we argue that the differences in N400 amplitudein the three conditions reflect the activation of event knowledge during online comprehension, and with ART/MRT performance providing an indirect measure of both linguistic proficiency and general world knowledge, we analyzedthe relationship betweenART/MRT performanceandN400 amplitude. Participants were assigned to two groups according to a median split on ART/MRT performance. The high scoring group consisted of the top 15 scorers (M = 24.5, SD = 5.5); these were separated from the bottom 15 scorers (M = 12.7, SD= 4.4). In an omnibus ANOVA with within-subjects factors of Electrode (26 levels) and Condition (3 levels) and a between-subjects factor of ART/MRT Group (2 levels), Group did not interact with Condition, F(2,56) = 0.13, ε_GG = 0.7711,p>.8, and there was no three-way interaction between Group, Condition, and Electrode, F(50,1400)=1.36,ε_GG = 0.1071, p>.2.

However, a separate analysis of each ART/MRT group suggests that individual differences may be present.The bottom 15 participants show an effect of Condition in the omnibus ANOVA, F(2,28) = 16.51,ε_GG = 0.7234,p< .001, and a Condition-by-Electrode interaction,F(50,700) = 6.52, ε_GG = 0.0850,p< .001, but the pairwise comparison between the event-related and event-unrelated conditions shows only a marginal effect of Condition, F(1,14) = 3.81, p = .071, and no Condition-by-Electrode interaction, F(25,350) = 1.15,ε_GG = 0.1821,p> .3. The numerical difference between event-related and event-unrelated N400s was 0.891μV. The top 15 participants show an effect of Condition in the omnibus ANOVA, F(2,28) = 21.37,ε_GG = 0.8152,p < .001, and a marginal Condition-by-Electrode interaction, F(50,700) = 2.26,ε_GG = 0.0928,p =.059. Importantly, the pairwise comparison between the event-related and event-unrelated conditions shows a significant effect of Condition, F(1,14) = 10.06, p< .01, and no Condition-by-Electrode interaction, F(25,350) = 1.02,ε_GG = 0.1176,p>.3. The numerical difference between event-related and event-unrelated targetN400s was 1.297μV. These analyses therefore are suggestive of ART/MRT-based individual differences, though the current design and data do not allow us to make definitive conclusions about the nature of such possible differences at this time. We present this analysis simply to highlight the potential for individual differences in activation of event knowledge during online language comprehension. Such individual differences are worthy of future investigation, but will not be discussed further.

Scalp distribution

The graded N400 effect is generally widespread across the scalp but appears most prominently at medial posterior sites. Figure 4 contains ERP difference waves (event-related minus expected and event-unrelated minus expected) that portray the size of the N400 effect for each anomalous target type at each electrode. Figure 5 presents scalp topographies of mean amplitude in the 200-500ms window for the two N400 effects. The scalp distributions of the two N400 effects were assessed by analyzing mean amplitudes from 200 to 500ms of the difference between each anomalous target type and the expected targets. Mean amplitudes were entered into a repeated measures ANOVA with two levels of Difference (event-related minus expected vs. event-unrelated minus expected), two levels of Hemisphere (right vs. left), two levels of Laterality (lateral vs. medial), and four levels of Anteriority (prefrontal vs. frontal vs. parietal vs. occipital).⁴

Scalp topographies of the N400 effects in the event-related and event-unrelated conditions from Experiment 1. The left plot reflects the N400 effect for the event-related targets, and the right the event-unrelated targets. Values correspond to mean amplitude 200-500ms post-stimulus onset.

The results show a main effect of Difference, F(1,29) = 12.71, p<.01, again showing that the N400 effect is larger for the event-unrelated condition than the event-related condition.The analysis also shows main effects of Hemisphere, F(1,29) = 25.78, p<.001, Laterality,F(1,29) = 23.52, p<.001, and Anteriority,F(3, 87) = 7.01,ε_GG = 0.3824,p<.05. The main effect of Hemisphere indicates that N400 effects were larger over the right than left hemisphere, and the main effect of Laterality indicates that N400 effects were larger over medial than lateral sites. The main effect of Anteriority results from larger N400 effectsat frontal, t(29) = 3.54, p<.01, and parietal, t(29) = 3.29, p<.01, sites than at prefrontal sites.⁵A Hemisphere-by-Laterality interaction, F(1,29) = 14.61, p< .001, indicates that while right medial sites show larger N400 effects than left medial sites,t(29) = 3.83, p< .001, the drop-off from medial to lateral sites is larger in the left, t(29) = 5.98, p<.001 than right,t(29) = 3.21, p< .01, hemisphere.A Laterality-by-Anteriority interaction,F(3,87) = 10.32, ε_GG = 0.7602, p<.001,reveals that the effect of Anteriorityis reliable at medial sites (prefrontal vs. frontal: t(29) = 4.71, p<.001; prefrontal vs. parietal: t(29) = 4.64, p<.001)butnot lateral sites (all t(29) <1.89, p>.07). However, a three-way interaction between Hemisphere, Laterality, and Anteriority, F(3,87) = 3.15,ε_GG = 0.7191,p< .05, suggests that this Laterality-by-Anteriority interaction holds only in the left hemisphere. Right lateral sites show an effects of Anteriority that become statistically nonsignificant after controlling for multiple comparisons (prefrontal vs. frontal: t(29) = 2.05, p<.05; prefrontal vs. parietal: t(29) = 2.45, p<.05), while left lateral sites do not show even marginal effects of Anteriority (all t(29) < 1.31, p>.2).

On the whole, these results indicate that the N400 effects are largest at right centro-parietal sites and decrease in size moving laterally and toward the front of the scalp. This right-lateralized,centro-parietal distributionis typical of that found in many N400 studies. In this analysis, none of the distributional factors interacted with Difference, indicating that the distribution of the N400 effects in the event-related and event-unrelated conditions were statistically equivalent.

Late window analysis

Although we generated no predictionsregarding effects outside of the N400 time window, we conducted an analysis of the 500-900ms post-stimulus window in which late positive effects typically arise.We entered mean amplitudes from 500-900msinto a repeated measuresANOVA with three levels of Condition and 26 levels of Electrode. The results show no main effect of Condition, F(2,58) = 0.59, ε_GG = 0.785, p > .5, but do show a reliable Condition-by-Electrode interaction, F(50,1450) = 7.06, ε_GG = 0.1009, p<.001. Looking at the ERPs in Figure 2,it generally appears that at anterior electrodes, the event-unrelated targets elicit a greater negativity than the other two target types, with this difference being slightly more prominent over the left hemisphere. At posterior electrodes, the event-unrelated targets elicit a greater positivity than the other two target types until approximately 800ms, at which point the event-related targets begin to exhibit a positive shift, as well.The presence of effects in the late window is worth noting, but becausethis experiment was not designed to investigate late positivity effects, we conduct no further analyses in this time window. We return to this late effect briefly in the General Discussion.

Discussion

The N400 elicited by a locally anomalous word is reduced if that word denotes a prominent component of the event being described. Previous research has demonstrated that event knowledge exerts a strong, rapid influence during incremental language comprehension, and the present experiment demonstrates that event knowledge activation during comprehension interacts with, but is not fully constrained by, local linguistic cues.Unstated event-relevant information is active even at points at which contextual cues indicate that it would not make a sensible continuation of the linguistic stream, leading to the facilitated processing we observe in the form of N400 reduction. The timing of the effect relative to the presentation of the word - namely, that it appears in the canonical N400 time window - highlightsthat this activated event knowledge influences processing within several hundred milliseconds of word onset.

The present results also show that a contextually anomalous event-related word does elicit a larger N400 than a highly expected word. This finding indicates that local contextual cues do interact with event knowledge activation to yield the greatest facilitation for words that are compatible with both the described event and local contextual cues. Words that satisfy both contextual and event knowledge constraints elicit the lowest amplitude N400s, words that satisfy neither elicit the largest, and words that satisfy event knowledge but not contextual constraints elicit N400s of intermediate amplitude. Thus, while comprehension involves activation of event knowledge elements beyond those that are contextually congruent, linguistic input dynamically modulates the activation levels of these elements such that contextually congruent elements receive greater activation than incongruent elements.

This result may be interpreted in at least two ways. It may be the case that N400 reduction for event-related targets arisesthrough construction of a relatively detailed, unified mental representation of the described event that combines both explicitly stated information and additional knowledge of the comprehender.Under this account,the observed effect is driven by discourse comprehension processes and the mental representations they serve to construct. Yet, recall that previous studies have shown direct lexical priming between elements of event knowledge, even in the absence of normative association (Hare, Jones, et al., 2009).Presumably, processing an isolated word does not give rise to a detailed mental representation of an event along the lines of that constructed through comprehension of a full discourse, but yet it can prime event components either through direct association or through some mediating knowledge structure. An alternative explanation for the results of Experiment 1 is that this type of priming is at play. Under this account, the mental representation constructed through processing the full discourse context is not necessary to observe N400 reduction for event-related targets. Instead, only the presence of associated words in the preceding context is required. This contrasts with the former interpretation of the results, which argues that a full, coherent mental representation of the described event drives the effect. To illustrate, consider the “playing in the snow” example in (1). Under the discourse processing account, jacket elicits a reduced N400 because it refers to additional knowledge activated to enrich the representation of the specific event described by the full discourse. Under the priming account, jacket elicits a reduced N400 simply because it is preceded by one or more content words that prime it directly, with the mental representation of the event described by the discourse as a whole playing no role.

Contrary to the lexical primingaccount, previous research suggests that effects of lexical priming during discourse comprehension are generally weak or nonexistent. Camblin, Gordon, and Swaab (2007) report a series of experiments addressing the role of lexical associative priming and higher-level discourse information in online sentence comprehension. Using eye-tracking and ERP measures, the authors examined responses to contextually congruent words that either did or did not align with the general discourse context and either were or were not preceded by a direct lexical associate. Across all experiments, they generally found thateffects of discourse context were immediate and strong, while effects of an immediately preceding lexical associate weredelayed and weak, if present at all.Reliable effects of lexical association arose only when single sentences were presented in isolation or when texts were made incoherent through scrambling of words. Camblin et al. conclude that“automatic spread of activation, as a function of associative relations between words, does not contribute to processing of words in sentences that are part of a larger discourse” (p. 126). With the scenarios in the present experiment providing relatively rich discourse contexts before target presentation, the findings presented by Camblin et al. strongly suggest that lexical priming is not at play here.

Despite this evidence suggesting that lexicalassociative priming does not affect the processing of words embedded in full discourses, we conducted an analysis addressing this alternative account directly. First, we sought to quantify the level of association of our event-related and event-unrelated targets with their preceding context words. If the event-related targets were more strongly associated with the individual context words than the event-unrelated targets, we would have evidence suggesting that the results of Experiment 1 could potentially be driven by direct priming of the event-related targets through activation of the semantic representations of one or more key content words (as opposed to construction of a mental representation of the described event as a whole). To quantify lexical semantic associations, we utilized Latent Semantic Analysis (Landauer, Foltz, &Laham, 1998), which capturessemantic association between individual words or groups of words through co-occurrence statistics computed over large-scale corpora. In investigating lexical priming between event associates (e.g., hospital and doctor), Hare, Jones, et al. (2009) demonstrated that LSA captures lexical priming between associated elements of event knowledge that do not exhibit association in a standard free-association task. Hare, Jones, et al. argue that LSA captures priming between event associates because words associated with a common eventshould often co-occur together in text describing that type of event. Because it is plausible that our event-related targets were preceded by context words denoting event associates more often than the event-unrelated targets, the present analysis required a measure capable of capturing priming between event associates. We therefore chose LSA over free association norms due to its abilityto capture such associations.⁶

We obtained LSA scores using the one-to-many application on the LSA website at lsa.colorado.edu. We computed the cosine between each target word and its entire preceding context, using the General Reading - Up to First Year of College topic space with 300 factors. We found a statistically significant difference between the event-related and event-unrelated targets: the mean association score for the event-related targets was 0.27, and the mean score for the event-unrelated targets was 0.22,t(71) = 3.032, p<.01.The LSA association scores therefore provide support for the lexical priminginterpretation of the results of Experiment 1.

To determine whether the N400 reduction found in Experiment 1 might be due to priming of the event-related targets by particular context words, we identified a subset of the 12 experimental items that showed the largest differences between LSA association scores for the event-related and event-unrelated targets. We then reanalyzed the data from Experiment 1 excluding these 12 items. In the remaining 60 experimental items, the new mean LSA score for the event-related targets was 0.24, and for the event-unrelated targets was 0.23, t(59) = 0.5216, p = .60. An analysisof the ERPs identical to that describedaboveagain showed that the event-related targets elicit smaller N400s than do the event-unrelated targets, F(1,29) = 13.7, p<.001. This result, combined with the findingspresented by Camblin et al. (2007) suggesting that lexical priming exerts at most a weak influence during discourse comprehension, strongly suggests that the N400 reduction observed in Experiment 1 is a discourse-level phenomenon, resulting from online activation of generalized event knowledge during the construction of a coherent mental representation of the described event. This interpretation is further tested in Experiment 2.

Experiment 2

Experiment 1 demonstrated that a contextually anomalous word elicits a reduced N400 if that word refers to a salient component of the comprehender’s general knowledge regarding the described event, and that the effect results from engagement of generalized event knowledge during construction of a mental representation of a described event. The interpretation of the N400 reduction for event-related targets as arising through discourse comprehension processes leads to the prediction that if the discourse context was stripped from the experimental scenarios, the observed N400 reduction for event-related targets should disappear. Without the first two context sentences, the scenarios would be devoid of the discourse context necessary to drive activation of generalized event knowledge. Experiment 2 investigated this hypothesis by comparing the ERPs elicited by the three target types when only the final sentence of each scenario from Experiment 1 was presented.