Exploring N400 Predictability Effects During Sustained Speech Comprehension: From Listening-Related Fatigue to Speech...

This study investigated the predictability effect on the N400 as an objective measure of listening-related fatigue during speech comprehension by: (1) examining how its characteristics (amplitude, latency, and topographic distribution) changed over time under clear versus noisy conditions to assess its utility as a marker for listening-related fatigue, and (2) evaluating whether these N400 parameters could assess the effectiveness of speech enhancement systems.

Two event-related potential experiments were conducted on 140 young adults (aged 20 to 30) assigned to four age-matched groups. Using a between-subjects design for listening conditions, participants comprehended spoken sentences ending in high- or low-predictability words while their brain activity was recorded using electroencephalography. Experiment 1 compared the predictability effect on the N400 in clear and noise-masked conditions, while experiment 2 examined this effect under two enhanced conditions (denoised using the Transformer- and minimum mean square error-based speech enhancement models). Electroencephalography data were divided into two blocks to analyze the changes in the predictability effect on the N400 over time, including amplitude, latency, and topographic distributions.

Experiment 1 compared N400 effects across blocks under different clarity conditions. Clear speech in block 2 elicited a more anteriorly distributed N400 effect without reduction or delay compared with block 1. Noisy speech in block 2 showed a reduced, delayed, and posteriorly distributed effect compared with block 1. Experiment 2 examined N400 effects during enhanced speech processing. Transformer-enhanced speech in block 1 demonstrated significantly increased N400 effect amplitude compared to noisy speech. However, both enhancement methods showed delayed N400 effects in block 2.

This study suggests that temporal changes in the N400 predictability effect might serve as objective markers of sustained speech processing under different clarity conditions. During clear speech comprehension, listeners appear to maintain efficient semantic processing through additional resource recruitment over time, while noisy speech leads to reduced processing efficiency. When applied to enhanced speech, these N400 patterns reveal both the immediate benefits of speech enhancement for semantic processing and potential limitations in supporting sustained listening. These findings demonstrate the potential utility of the N400 predictability effect for understanding sustained listening demands and evaluating speech enhancement effectiveness.