So that's what you meant! Event-related potentials reveal multiple aspects of context use during construction of message-level meaning

Abstract

Factors that modulate the influence of contextual information on semantic processing in language comprehension have been thoroughly investigated with the N400 component of the event-related potential (ERP), a direct measure of initial contact with semantic memory. Although context has a strong and immediate impact on processing, multiple mechanisms contribute to the construction of message-level representations during normal comprehension. Some of these may be engaged after or concurrent with the formation of an initial meaning representation, and can then serve to revise or reshape meaning. In this study, ERPs were recorded while participants read plausible sentences that continuously varied in the amount of contextual constraint for the sentence-final word, defined via extensive norming data including the range of possible alternative completions for the contexts. Consistent with numerous past studies, the amplitude of the N400 was graded with expectancy, as amplitudes decreased with increasing constraint. Additionally, a left-lateralized, broad, slow negativity onsetting around 400-500 ms was largest for sentences with moderately strong constraint. Within this range of constraint, the negativity was larger for sentences with fewer alternative completions compared to those with many different ones. The timing and scalp distribution of the effect resemble brain responses linked to engagement of working memory resources, ambiguity resolution, and comprehension of jokes. Similar to cases of "frame-shifting" in non-literal language, this effect may reflect processing associated with reinterpretation or reconsideration of contextual material when multiple interpretations of a sentence were likely.

Figure 1

Grand-average ERPs for the six 50-item cloze probability bins at 12 electrode sites. Whereas the N400 is graded with cloze probability, the later left fronto-temporal negativity is most enhanced for the 75–90% cloze range. Negative is plotted up.

Figure 2

Topographic maps of correlation coefficients for N400 amplitude vs cloze probability, and for the difference between Unexpected items and the 75–90% cloze bin averaged over 600–900 ms post-stimulus onset.

Figure 3

Scatterplot showing the relationship between item-level negativity (600–900 ms post-stimulus onset at left frontal sites) and the number of unique alternate completions for the item's sentence context, for the 75–90% cloze range. Negative is plotted up.

Figure 4

Topographic map of correlation coefficients for the items analysis of mean amplitudes in the 600–900 ms timewindow and number of alternative completions for each sentence, within the moderately constraining range (75–90%). Like the scalp distribution of the overall ERP effect, the correlation map shows a left-frontal focus.

Figure 5

ERPs for moderately constraining contexts with few and many alternative completions, plotted from 700 ms prior to 1000 ms after the onset of the sentence-final word (penultimate word onsets 500 ms prior to final word). Waveforms for the 50–75%, 90–100%, and Unexpected items are shown for reference. The effect of few versus many alternate completions is similar in timecourse and distribution to the overall effect of the late negativity.