Task-Induced Mental Fatigue and Motivation Influence Listening Effort as Measured by the Pupil Dilation in a Speech-in-Noise Task

Objectives Listening effort and fatigue are common complaints among individuals with hearing impairment (HI); however, the underlying mechanisms, and relationships between listening effort and fatigue are not well understood. Recent quantitative research suggests that the peak pupil dilation (PPD), which is commonly measured concurrent to the performance of a speech-in-noise task as an index of listening effort, may be informative of daily-life fatigue, but it remains unknown whether the same is true for task-induce fatigue. As fatigue effects are known to manifest differently depending on motivation, the main aim of the present study was to experimentally investigate the interactive effects of task-induced fatigue and motivation on the PPD. Design In a pre-/post-fatigue within-subject design, 18 participants with normal hearing (NH) engaged in a 98-trial-long speech-in-noise task (the ‘load sequence’, approximately 40 min. long), which either excluded or included additional memory demands (light vs. heavy load sequence). Before and after the load sequence, baseline pupil diameter (BPD) and PPD were measured during shorter ‘probe’ blocks of speech-in-noise tasks. In these probe blocks, if participants correctly repeated more than 60% of the keywords, they could win vouchers of either 20 or 160 Danish krones worth (low incentive vs. high incentive). After each probe block, participants reported their invested effort, tendency for quitting, and perceived performance. Results The BPD in anticipation of listening declined from pre-to post-load sequence, suggesting an overall decrease in arousal, but the decline did not scale with the magnitude of the load sequence, nor with the amount of monetary incentive. Overall, there was larger pre-to post-load sequence decline in PPD when the load sequence was heavy and when the monetary incentives were low. Post-hoc analyses showed that the decline in PPD was only significant in the heavy-load sequence-low reward condition. The speech-in-noise task performance, self-reported effort, and self-reported tendency to quit listening did not change with the experimental conditions. Conclusions This is the first study to investigate the influence of task-induced fatigue on BPD and PPD. Whereas BPD was not sensitive to the magnitude of previous load sequence and monetary incentives, the decline in PPD from pre-to post-load sequence was significant after the heavy load sequence when the offered monetary incentives were low. This result supports the understanding that fatigue and motivation interactively influence listening effort.