Comparing methods of analysis in pupillometry: application to the assessment of listening effort in hearing-impaired patients

Numerous studies showed that task-evoked pupil dilation is an objective marker of cognitive activity and listening effort. However, these studies differ in their experimental and analysis methods. Whereas most studies focus on a single method, the present study sought to compare different pupil-dilation data analysis methods, including different normalization techniques, baseline periods, and baseline durations, in order to assess their influence on the outcomes of pupillometry results obtained in an auditory task. To that purpose, we used pupillometry data recorded in response to words in noise in hearing-impaired individuals. The start-time of the baseline relative to stimulus timing turned out to have a significant influence on conclusions. In particular, a significant interaction in the effects of signal-to-noise ratio and hearing-aid use on pupil dilation was observed when the baseline period used started early relative to the word—an effect likely related to anticipatory, pre-stimulus cognitive processes, such as attention mobilization. This was the case even with only correct-response trials included in analyses, so that any confounding effect of performance in the word-repetition task was eliminated. Different normalization methods and baseline durations showed similar results, however the use of z-score transformation homogenized variability across conditions without affecting the qualitative aspect of the results. The consistency of results regardless of normalization methods, and the fact that differences in pupil dilation and subjective measures of listening effort could be observed despite perfect performance in the task, underlines the relevance of pupillometry as an objective measure of listening effort.