Sustained attention as measured by RT variability is a strong modulator for the P600, but not the N400

Contier, Friederike; Weymar, Mathias; Wartenburger, Isabell; Rabovsky, Milena · 2021 · Crossref

DOI: 10.1101/2021.11.18.469143

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Summary

This study investigates the functional distinction between two prominent event-related potential (ERP) components in language processing: the N400 and the P600. While the N400 is typically associated with semantic integration and the P600 with syntactic processing or revision, their underlying cognitive mechanisms remain debated. Specifically, researchers have proposed that the N400 reflects automatic processing, whereas the P600 reflects controlled processes requiring executive resources. Since the availability of such resources depends on sustained attention, which fluctuates over time, the authors tested whether P600 and N400 amplitudes are differentially modulated by levels of sustained attention. To address this question, the authors re-analyzed EEG and behavioral data from a previous study by Sassenhagen & Bornkessel-Schlesewsky (2015). Twenty native German speakers performed a sentence processing task where they read sentences phrase-by-phrase and responded as quickly as possible to detect morphosyntactic or semantic violations. Sustained attention was quantified using a moving reaction time coefficient of variation (RT CV) calculated across trials. Lower RT variability indicated periods of high sustained attention, while higher variability indicated low sustained attention. The authors controlled for trial type variability and used linear mixed-effects models to assess the impact of RT variance on ERP amplitudes, analyzing data from a parietal region of interest. The results demonstrated a clear dissociation between the two components. P600 amplitudes were significantly larger during periods of low RT variance (high sustained attention) compared to periods of high RT variance (low sustained attention). This effect remained significant across different window sizes for calculating RT variance and was robust after controlling for reaction time and violation type. In contrast, N400 amplitudes were unaffected by RT variance. The N400 showed the expected sensitivity to semantic violations but did not vary with fluctuations in sustained attention, even when analyses were restricted to semantic violations or adjusted for spatial distribution. These findings provide independent evidence supporting the hypothesis that the P600 reflects controlled, attention-dependent processes, while the N400 reflects more automatic processing. The sensitivity of the P600 to sustained attention aligns with theories positing that it involves executive control or error monitoring mechanisms that require cognitive resources. Conversely, the stability of the N400 amplitude regardless of attentional state supports its characterization as an automatic response to semantic incongruity. This study clarifies the functional significance of these ERP components by linking them to the automaticity-control dimension through the lens of sustained attention.

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• neuro workload indices