Individual Differences in Reading Speed are Linked to Variability in the Processing of Lexical and Contextual Information: Evidence from Single-trial Event-related Brain Potentials

Abstract

In the current paper, we examined the effects of lexical (e.g. word frequency, orthographic neighborhood density) and contextual (e.g. word predictability in the form of cloze probability) features on single-trial event-related brain potentials in a self-paced reading paradigm. Critically, we examined whether individual differences in reading speed modulated single-trial effects on the N400, an ERP component linked to semantic memory access. Consistent with past work, we found that word frequency effects on the N400 were attenuated with increasing predictability. However, effects of orthographic neighborhood density were robust across all levels of predictability. Importantly, individual differences in reading speed moderated the influence of both frequency and predictability (but not orthographic neighborhood density) on the N400, such that slower readers showed reduced effects compared to faster readers. These data show that different lexical factors influence word processing through dissociable mechanisms. Our findings support a dynamic semantic-memory access model of the N400, in which information at multiple levels (lexical, sentential, individual) simultaneously contributes to the unfolding neural dynamics of comprehension.

Keywords: EEG; Lexical processing; event-related brain potential; reading; semantics.

Figure 1.

(a) Grand-averaged event-related brain potential waveforms as a representative midline parietal electrode for sentence-final words, as a function of cloze probability (low cloze probability: <5%, moderate cloze probability: ~25%, and high cloze probability: ~85%). The N400 shows the widely-replicated graded inverse response as a function of cloze probability. Note that negative is plotted up. The N400 single-trial measurement window is illustrated in gray. (b) Scalp topography of the canonical central-parietal distribution of the N400 context effect (low cloze – high cloze) between 300 and 500 ms. The highlighted black electrode marks the midline parietal electrode presented in Figure 1a.

Figure 2.

Modulation of lexical features by contextual expectancy. The top figure plots the model-estimated partial-effects plot of the (log) Word frequency X Cloze probability interaction. The bottom figure plots the model-estimated partial-effects plot of the OLD20 X Cloze probability interaction. Words that were more frequent (larger values) showed lower N400 amplitudes, and words that had larger orthographic neighborhoods (smaller OLD20 scores) showed greater N400 amplitudes. Effects of word frequency were attenuated with increasing cloze probability, whereas effects of orthographic neighborhood were invariant to cloze.

Figure 3.

Modulation of word frequency and cloze effects by individual differences in reading speed. The top figure plots the model-estimated partial-effects plot of the (log) Word frequency X Reading speed interaction. The bottom figure plots the model-estimated partial-effects plot of the Cloze probability X Reading speed interaction. Faster readers showed larger effects of both word frequency and cloze probability on the N400 compared to slower readers.