Best Practices and Advice for Using Pupillometry to Measure Listening Effort: An Introduction for Those Who Want to Get Started

Abstract

Within the field of hearing science, pupillometry is a widely used method for quantifying listening effort. Its use in research is growing exponentially, and many labs are (considering) applying pupillometry for the first time. Hence, there is a growing need for a methods paper on pupillometry covering topics spanning from experiment logistics and timing to data cleaning and what parameters to analyze. This article contains the basic information and considerations needed to plan, set up, and interpret a pupillometry experiment, as well as commentary about how to interpret the response. Included are practicalities like minimal system requirements for recording a pupil response and specifications for peripheral, equipment, experiment logistics and constraints, and different kinds of data processing. Additional details include participant inclusion and exclusion criteria and some methodological considerations that might not be necessary in other auditory experiments. We discuss what data should be recorded and how to monitor the data quality during recording in order to minimize artifacts. Data processing and analysis are considered as well. Finally, we share insights from the collective experience of the authors and discuss some of the challenges that still lie ahead.

Keywords: listening effort; methods; pupillometry.

Figures

Figure 1.

Events in a basic pupillometry experiment for measuring listening effort. There are other experimental paradigms that are possible, this illustrates a commonly used sequence of events.

Figure 2.

Pupillary hippus, or small ongoing fluctuations in pupil size that are unrelated to an external stimulus.

Figure 3.

Sequential steps of data processing. Raw data (black, marked no. 1) contain blinks that appear as transient changes in pupil dilation separated by a blank stretch of missing data. De-blinked data (no. 2, in red) expands the gap of missing data to remove the transient excursions. The gaps are interpolated (no. 3 in blue, interpolations in dashed lines). Finally, the data are low-pass filtered (no. 4, green).

Figure 4.

Different baseline intervals end at the onset of the stimulus and extend backwards by variable durations (highlighted in each panel by a shaded vertical area). Comparison of the resulting baseline-corrected data is shown on the far-right panel, revealing negligible differences across baseline durations.

Figure 5.

Illustration of baseline correction and proportionalization of data for two hypothetical individuals each participating in two testing conditions indicated by line color. Raw pupil size is shown on the upper panels, absolute change (mm) in pupil size is shown in the middle panels, and proportional change in shown in the lower panels. Baseline intervals consisted of the 1-s of data prior to stimulus onset indicated at time 0.

Figure 6.

Different amounts of change in pupil dilation for two hypothetical individuals who have different dynamic range of pupil size. In each panel, the difference in peak pupil dilation occupies the same proportion of the overall dynamic range.

Figure 7.

Sixteen individual trials of pupil data, with baseline period marked as thin gray vertical bar, and retention interval marked as thick gray vertical bar. The stimulus is played between these two bars. Baseline level for each trial is marked as the horizontal blue line. Lines plotted in color are low-pass filtered data overlaid on gray raw data that include transient vertical displacements that indicate blinks. Data in red are marked to be dropped from the data set due to excessive data loss or contamination (excessive nontask-evoked dilation) during baseline interval.

Figure 8.

Aggregation of trials displayed in Figure 7, excluding “dropped” trials. The left and right panels display data aligned to stimulus onset and offset, respectively. The thin gray bar (to the left in each panel) corresponds to the baseline interval, and the thicker gray bar (to the right) corresponds to the retention interval. Because stimuli were of variable duration, average values were used for the offset time for onset-aligned data, and for onset in offset-aligned data.

Figure 9.

Temporal precision of pupil dilation morphology is related to difficulty of a task. Listeners with cochlear implants (for whom auditory perception can be quite challenging) show individuated dilations to slowly spoken words that are time locked across trials. Affecting the predictability of the second word in the sequence causes a reduced response shortly after the corresponding word. Such patterns do not emerge clearly for listeners with normal hearing, for whom the task is very easy.