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

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.

Keywords: Event-related potentials; N250; N400; Prediction; Sentence processing.

Figure 1

A summary of the current experimental paradigm. Participants were instructed to actively predict the final word of each discourse and to respond after each trial whether their prediction was correct. ERPs time-locked to the onset of the final critical word (e.g. “novel”), were averaged offline as a function of prediction accuracy (predicted vs. unpredicted) and contextual support (medium vs. low-cloze).

Figure 2

Averaged event-related potentials for passage-final critical words. All ERP figures were low-pass filtered at 25 Hz for presentation purposes. Separate averages are displayed for unpredicted low-cloze items, unpredicted medium-cloze items, and predicted medium-cloze items.

Figure 3

ERP difference waves representing the effect of prediction accuracy (unpredicted medium-cloze minus predicted medium-cloze) and contextual facilitation (unpredicted low-cloze minus unpredicted medium-cloze) at the final critical word.

Figure 4

Topographic distributions of the two grand average difference waves, plotted over time. Note the different voltage scales for each measurement window and the two, separate time windows used for the parietal negativities.

Figure 5

Scatterplots representing the by-items correlations for unpredicted passage-final words. Values represent mean ERP amplitudes between 430–530ms at central-parietal electrode sites. The top figure shows the relationship between ERP amplitude and passage plausibility (1=makes perfect sense, 7=makes no sense at all). The bottom figure shows the relationship between ERP amplitude and semantic overlap between the critical target and the most-likely alternative completion (calculated using Latent Semantic Analysis). More negative amplitudes were observed for both implausible and semantically unrelated targets.