Elsevier

Cognition

Volume 136, March 2015, Pages 135-149
Cognition

Effects of prediction and contextual support on lexical processing: Prediction takes precedence

https://doi.org/10.1016/j.cognition.2014.10.017Get rights and content

Highlights

  • Participants read medium (50%) and low-cloze (>1%) words in discourse contexts.

  • Neural activity was analyzed separately for predicted and unpredicted targets.

  • Accurate predictions resulted in early differences on the N250 and N400.

  • Effects of contextual support were delayed by approximately 100 ms.

  • Prediction and discourse plausibility both influenced a late, frontal positivity.

Abstract

Readers may use contextual information to anticipate and pre-activate specific lexical items during reading. However, prior studies have not clearly dissociated the effects of accurate lexical prediction from other forms of contextual facilitation such as plausibility or semantic priming. In this study, we measured electrophysiological responses to predicted and unpredicted target words in passages providing varying levels of contextual support. This method was used to isolate the neural effects of prediction from other potential contextual influences on lexical processing. While both prediction and discourse context influenced ERP amplitudes within the time range of the N400, the effects of prediction occurred much more rapidly, preceding contextual facilitation by approximately 100 ms. In addition, a frontal, post-N400 positivity (PNP) was modulated by both prediction accuracy and the overall plausibility of the preceding passage. These results suggest a unique temporal primacy for prediction in facilitating lexical access. They also suggest that the frontal PNP may index the costs of revising discourse representations following an incorrect lexical prediction.

Introduction

Contemporary approaches to language processing emphasize forward looking, anticipatory processes (Altmann and Mirković, 2009, Elman, 1990, Kuperberg, 2013). Rather than viewing comprehenders as passive processors of incoming linguistic input, these approaches view comprehenders as active constructors of meaning. Part of this meaning construction activity entails using prior knowledge and experience to generate expectations about how a discourse will unfold in the near future. The outcome of the anticipatory process, its success or failure, is viewed as a major factor contributing to the processing load that “bottom-up” perceptual information imposes in a given context. Anticipatory, likelihood-driven processes are believed to contribute to lexical, syntactic, and discourse processing in a variety of theoretical frameworks (e.g., Gibson et al., 2013, Hale, 2001, Jaeger and Snider, 2013, Levy, 2008). Despite this strong emphasis on prediction and anticipation, there is very little direct evidence that successful vs. unsuccessful prediction per se influences language processes as they unfold. Instead, the chain of inference is usually less direct. Contexts are constructed that make successful prediction easier or more difficult, and differences in processing difficulty across these contexts are then attributed to successful prediction. The current study takes a different approach that allows more direct assessment of the consequences of successful prediction on lexical access.

During natural language processing, words are typically embedded in a discourse which provides a broader context for interpreting meaning. Discourse context has been shown to facilitate lexical retrieval as well as memory for congruent words (e.g., Bransford & Johnson, 1972). Most broadly, prior linguistic information can activate networks of related concepts or event schemas which can facilitate processing (Schank, 1975). In more highly constraining contexts, comprehenders can also make predictions about specific lexical items that are likely to appear in the upcoming discourse.1 For example, when listening to the sentence “I could tell he was mad by the tone of his …” the upcoming word ‘voice’ will be processed more quickly (Schwanenflugel and Shoben, 1985, Traxler and Foss, 2000) and with higher perceptual accuracy (Jordan and Thomas, 2002, Miller et al., 1951) than if this word were presented in a less predictive context. A number of studies have shown that lexical prediction may influence comprehension processes before the critical word itself has even appeared. For example, words embedded in highly predictable contexts are skipped more frequently during natural reading (Rayner, Slattery, Drieghe, & Liversedge, 2011). While listening to continuous speech, participants will strategically fixate objects in a visual scene that are likely to be mentioned in the near future (Kamide, Altmann, & Haywood, 2003).

Electrophysiological studies of language processing have shown that a word’s cloze probability, which is defined as the proportion of participants producing this word during an offline sentence completion task, correlates with the amplitude of the N400 component during reading comprehension (Kutas & Hillyard, 1984). Modulation of the amplitude of the N400 has been linked both to the facilitation of lexical integration processes (van Berkum, Hagoort, & Brown, 1999), as well as lexical access and semantic memory retrieval (Kutas & Federmeier, 2000). More recently, it has been suggested that amplitude of this component is attenuated when features of a word have been pre-activated by the preceding context (for a review see Swaab, Ledoux, Camblin, & Boudewyn, 2012).

In constraining contexts, the generation of specific lexical predictions could influence multiple levels of word processing via the pre-activation of phonological, morphological or semantic properties of the predicted word. Evidence for pre-activation comes from a variety of electrophysiological studies using sentence or discourse contexts. In these paradigms a critical probe word is introduced that is either congruent or incongruent with the upcoming, anticipated lexical item. For example, DeLong, Urbach, and Kutas (2005) found it was easier for readers to process the function word ‘an’ while anticipating a phonologically congruent continuation like ‘airplane’ as opposed to a phonologically incongruent one like ‘kite’. Other studies using this technique have found evidence for the pre-activation of animacy and semantic category information, as well as grammatical features like gender (Szewczyk & Schriefers, 2013; Boudewyn, et al., submitted; van Berkum et al., 2005, Wicha et al., 2004).

While accurate predictions appear to facilitate lexical access and integration, there is also evidence for costs when a lexical prediction is disconfirmed. Behaviorally, these costs can result in longer reaction times for unexpected words appearing in highly constraining contexts (Schwanenflugel & Shoben, 1985). In the electrophysiological literature, it has been hypothesized that the costs of incorrect prediction are reflected in a late, post-N400 positivity (PNP). This positivity, which appears over frontal and left hemisphere electrode sites, is larger for unpredictable words in context (Delong, Urbach, Groppe, & Kutas, 2011), and this relationship appears to be modulated by sentence constraint (Federmeier, Wlotko, De Ochoa-Dewald, & Kutas, 2007; but see Thornhill & Van Petten, 2012 for conflicting results). Additionally, this frontal component appears to be distinct from other, posterior positivities which occur in response to anomalous or ungrammatical sentence continuations (DeLong, Quante, Kutas, & plausibility, in press). While little is known about the exact mechanisms underlying this frontal positivity, it has been hypothesized to reflect either the detection or resolution of disconfirmed predictions (see Van Petten & Luka, 2012 for a review).

While these studies provide some evidence for both costs and benefits of predictive processing, there is a methodological limitation across this set of published studies which remains unresolved. Specifically, prior studies have not provided direct evidence on a trial-by-trial basis whether participants actually predicted the target stimulus. As a result, these studies have been unable to fully dissociate the effects of specific lexical pre-activation, from other sources of contextual support (e.g. semantic association or discourse plausibility). While some of the previously mentioned studies have side-stepped this issue by looking for the consequences of prediction earlier in the sentence, these studies have been unable to evaluate the effects of lexical pre-activation on the predicted words themselves. To address this issue, the current experiment used a paradigm that isolates the effects of lexical prediction from other sources of contextual facilitation.

In the experiment reported here, participants read moderately constraining (50% cloze) two-sentence passages, while trying to actively predict the final word of each. After each passage was complete, participants responded by button press whether their prediction was correct. By separately averaging ERP trials for predicted and unpredicted targets, we isolated processing differences at the final critical word that were uniquely driven by prediction accuracy.2 In addition to these 50% cloze passages (whose final words should appear equally often as predicted and unpredicted targets) we also constructed passages that rendered these same critical words highly unpredictable (<1% cloze). This control condition allowed us to also compare unpredicted target words in low-cloze contexts to unpredicted targets in more supportive, medium-cloze contexts. Any differential ERP activity between these two conditions should index the amount of semantic or discourse-level facilitation provided by the preceding context, independent of lexical prediction. For an overview of the experimental paradigm, see Fig. 1.

With this paradigm we tested three main hypotheses. If the generation of specific lexical predictions is an important mechanism by which contextual constraint facilitates lexical access, then self-reported prediction accuracy should have a clear influence on N400 amplitudes at the final critical word. In contrast, if we see a large influence of cloze probability on the N400 but fail to observe an effect of prediction accuracy, this would call into question the importance of lexical anticipation during sentence processing.

Secondly, if we do observe prediction-related ERP differences, the relative time-course of this effect would provide important constraints on models of lexical processing. Specific lexical predictions differ from other forms of contextual facilitation in that they entail the pre-activation of specific word forms. Unlike more general forms of semantic anticipation, lexical pre-activation may facilitate early stages of visual and orthographic processing (Lau, Holcomb, & Kuperberg, 2013). To test this hypothesis, we investigated the effects of prediction accuracy and contextual constraint on the amplitude of the N250, an ERP component thought to reflect processing of visual word forms (Holcomb & Grainger, 2006).

Finally, this paradigm allowed us to more directly assess the processing costs which may be incurred following a disconfirmed lexical prediction. Based on current evidence (Van Petten & Luka, 2012), the frontal PNP may index: (1) the detection of prediction errors, (2) the inhibition of incorrectly pre-activated lexical items, or (3) a revision of the preceding discourse in light of new, unanticipated information. By separately examining the influence of prediction accuracy and contextual support on the PNP, we may have a better basis to determine which of these processing mechanisms it represents.

Section snippets

Participants

Twenty-four undergraduates (16 females) from the University of California, Davis participated in the ERP study after providing informed consent. They were compensated with course credit. The mean age of participants was 20.6 years (range: 18–37, SD = 4.2). All were monolingual English speakers with no history of neurological or reading impairments. None had previously participated in any cloze testing or plausibility norming procedures for these experimental materials. All participants had normal

Behavioral results

In the medium-cloze condition, participants correctly predicted the final critical word for 48.1% of the passages (SD = 6.6%). This value was quite similar to the off-line cloze values previously obtained for these items: 50.7%. For the low-cloze condition, participants reported a correct prediction for 2.4% of the sentence-final words (SD = 2.2%), which was slightly higher than the off-line cloze value of 0.9%. Overall, prediction accuracy differed substantially between the medium-cloze and

General discussion

This study sought to disentangle the influence of lexical prediction from other forms of contextual facilitation during reading. Using participant self-reports, we compared electrophysiological responses to predicted and unpredicted target words appearing in supportive and non-supportive discourse passages. This approach revealed temporally distinct effects of prediction accuracy and discourse context on lexical processing. Prediction showed the earliest influence on ERP amplitudes, with

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