Listening to speech-in-noise can require substantial mental effort, even among younger normal-hearing adults, as individuals must maintain attention to the target speaker, process the linguistic content, and comprehend the intended message. Numerous studies have demonstrated that the task-evoked pupil response (TEPR) tracks the increased effort exerted to recognize words or sentences in increasing noise. However, few studies have examined the trajectory of listening effort across longer stretches of speech that might be more representative of real-world listening. Furthermore, there is a paucity of research investigating the extent to which expectations about upcoming listening difficulty, also common in real-world situations, modulate the TEPR. Pupil size during a baseline period prior to stimulus onset has been argued to reflect anticipatory arousal and attention mobilization to meet upcoming task demands. Research in human and animal models suggests that the largest cortical evoked responses are observed at intermediate baseline pupil sizes, when individuals are optimally engaged: neither too inattentive nor too distractible. Using a sustained listening task, the present study sought to understand the dynamic relationship between attention mobilization as reflected in trial-level changes in baseline pupil size, and listening effort, as reflected in the TEPR. 

Thirteen younger normal-hearing adults listened to 60-s-long audiobook passages while pupil size was recorded. Participants were instructed to listen to one of two competing speakers, with the target speaker presented at either the same level (0 dB SNR) or at a quieter level (-6 dB SNR) than the non-target speaker. Passages were blocked by SNR (signal-to-noise ratio), and each passage was repeated three times within an SNR block. Generalized additive mixed models (GAMMs) were used to analyze the time course of the TEPR for each trial as a non-linear function of baseline pupil size—the median pupil size during a 2-s neutral period before the onset of each passage. 

The results revealed a significant three-way interaction between SNR, trial repetition, and baseline pupil size that affected the deployment of effort during sustained listening. At lower baseline pupil sizes, reflecting lower attention mobilization, TEPRs were more sustained across time in the harder –6 dB SNR condition than the 0 dB SNR condition, particularly when attention mobilization remained low by the third repetition. At intermediate baseline pupil sizes, differences between conditions were largely absent, suggesting these young adult listeners had optimally readied their attention for both SNRs. Lastly, at higher baseline pupil sizes, potentially indicative of over-mobilization of attention, the effect of SNR was reversed in the first 30 s of story listening, such that participants initially appeared to be overwhelmed by the harder SNR condition, resulting in reduced TEPRs in the –6 dB SNR condition that recovered in the second half of the story. However, listeners who had still under-mobilized their attention by the third repetition exhibited rapidly decreasing TEPRs in both SNRs. Together, these findings suggest that changes in the way task-evoked listening effort unfolds over time depend on the extent to which individuals successfully mobilize their attention in anticipation of difficult listening conditions.