Speech comprehension in the presence of background masking sound requires a listener to invest more cognitively, for example, relying more on attention and memory, which makes listening effortful (Eckert et al., 2016; Herrmann and Johnsrude, 2020a; Peelle, 2018; Pichora-Fuller et al., 2016). Assessing listening effort is increasingly relevant in the study of speech-comprehension challenges in older adulthood, because older listeners may experience listening effort in noisy situations long before they are diagnosed with having a hearing loss (Helfer and Jesse, 2021; Pichora-Fuller and Levitt, 2012). Listening effort may thus be a potentially early diagnostic marker of hearing loss.

The most common objective tool to assess listening effort is pupillometry (van der Wel and van Steenbergen, 2018; Winn et al., 2018; Zekveld et al., 2018). The pupil dilates when listening effort increases due to masking of speech (Cui and Herrmann, 2023; Herrmann and Ryan, 2024; Koelewijn et al., 2018, 2019; Zekveld et al., 2010), degradation of speech (Kılıç et al., 2024; Zekveld Adriana et al., 2023), or linguistic speech-comprehension challenges (Ayasse and Wingfield, 2018; Kadem et al., 2020; Wendt et al., 2016). However, measuring the pupil also has limitations because its size is sensitive to changes in light (Knapen et al., 2016; Suzuki et al., 2019; Thurman et al., 2021) and the angle of the eye relative to the eye-tracking camera, among other factors (Brisson et al., 2013; Fink et al., 2024; Hayes and Petrov, 2016). To account for the latter, participants are typically instructed to fixate on a point on a computer screen (Farahani et al., 2020; Ohlenforst et al., 2017; Winn and Teece, 2021; Zekveld et al., 2018), but this might reduce external validity because it can reduce memory and mental imagery (Johansson et al., 2012) and reduce behavioral effects of speech comprehension (Cui and Herrmann, 2023). Further, pupillometry is most commonly used in trial-by-trial sentence-listening paradigms (Ayasse and Wingfield, 2018; Borghini and Hazan, 2018; Kadem et al., 2020; Wendt et al., 2016; Winn and Teece, 2021; Winn et al., 2015; Zekveld et al., 2010, 2019), whereas an increasing number of works leverage continuous story-listening paradigms to mitigate the less naturalistic nature of short, disconnected sentences (Brodbeck and Simon, 2020; Broderick et al., 2018; Ding and Simon, 2014; Lalor and Foxe, 2010; Panela et al., 2024). The extent to which pupillometry is sensitive to listening challenges under such continuous conditions is a topic of increasing interest (Cui and Herrmann, 2023; Fiedler et al., 2021; Seifi Ala et al., 2020; Widmann et al., 2025; Zhao et al., 2019).

A recent line of studies suggests that eye movements may provide an alternative objective measure to assess challenges during listening. Eye movements decrease when listening becomes difficult, for example, due to background noise that masks speech (Contadini-Wright et al., 2023; Cui and Herrmann, 2023; He et al., 2024; Herrmann and Ryan, 2024). Eye movements also decrease during periods of high compared to low memory load (Dalmaso et al., 2017; Kosch et al., 2018; Walter and Bex, 2021) and due to visual-task difficulty (Nakayama et al., 2002), suggesting domain-general reductions in eye movements under high cognitive load. This suggests that eye-movement systems in cortical and subcortical brain structures are sensitive to task challenges, possibly to reduce the impact of distracting visual information.

Eye movements appear to be sensitive to speech masking even during continuous story listening (Cui and Herrmann, 2023). However, in many of the previous works, including the study using continuous story listening (Cui and Herrmann, 2023), some visual stimulus was presented on the computer screen (e.g., fixation point, one moving dot, several moving dots). This visual dot stimulation was used to increase the likelihood of eye movements, allowing for a better investigation of their reduction due to speech masking (Cui and Herrmann, 2023; Herrmann and Ryan, 2024). It is thus unclear whether visual stimulation is advantageous for assessments of listening challenges through eye movements or whether free viewing on a blank screen is as effective. Finally, the relationship between eye movements and effortful listening has thus far mainly been investigated in younger, normal-hearing adults (Contadini-Wright et al., 2023; Cui and Herrmann, 2023). In a recent sentence-listening study, eye movements were also sensitive to increased speech masking in older adults (Herrmann and Ryan, 2024), but whether this generalizes to continuous story listening is unclear.

In the current study, younger and older adults listened to continuous stories masked by background babble at different signal-to-noise ratios (SNRs) either while a blank screen or a moving-dots display was presented. The aim was to investigate whether pupil size and eye movements are sensitive to speech masking (as a manipulation that induces listening effort) during naturalistic speech listening, and whether the sensitivity to speech masking differs between viewing conditions and/or age groups.