## 3 Results
### Pupillometry Data  
In this study, pupil size was parameterized by the pupil diameter estimates returned by the eye tracker. Raw pupil diameter data were recorded throughout the experiment and had to be processed in several steps before final visualization and analysis. The recorded time stamps for all stimuli were normalized first so that the starting point of each sentence was at 0 \(s\). Given the nature of the experiment, eye blinks, or changes due to factors other than the listening task, were potential confounds that needed to be identified. Pupil tracks with shorter duration than the playback stimulus were discarded, as they signify loss of synchronous pupil data. Quick blinks (<125 milliseconds) were identified, removed, and interpolated (linear interpolation began roughly 50 ms before the blink and end at least 150 ms after the blink) without changing the overall pattern of the tracking sequence. Finally, the tracks were smoothed by a 11-point moving average filter. This pre-processing of pupil tracks resulted in the exclusion of approximately 13% of the tracks due to dropouts, too many variations, or long blinks. This process was required to eliminate the risk of data distortion. The validated and pre-processed pupil diameter tracks associated with each talker stimulus were averaged across all listeners and the test-retest sessions. These averaged pupil responses are plotted in Figures 3 and 4 to provide a visual representation of the time course of pupil dilation during the presentation of the talker stimuli. Figure 3a depicts the speech waveform of the Talker 1 stimulus presented to all listeners, while 3b displays the averaged pupil track elicited while listening to this stimulus. The shaded region in Figure 3b represents the 95% confidence interval in the pupil track. As described earlier, the first three seconds of the waveform included the auditory prompt, and the last second of this prompt was designated as the baseline period. Prior to averaging, all listeners' individual pupil tracks were normalized by subtracting the track mean during the baseline period from pupil values at each time point. Figure 3: (**a**) Waveform associated with the Talker #1 stimulus. The first three seconds comprise the auditory prompt “please listen to the following stimulus”, while the following segment is the sentence “the rainbow is a division of white light into many beautiful colors” spoken by Talker #1. (**b**) The time course of the pupil diameter in response to the above stimulus, averaged across listeners and test sessions after baseline normalization. The shaded region represents the 95% confidence interval around the averaged pupil track. Note that the pupil diameter is in arbitrary units set by the EyeLink 1000 system. In the current study, we focused on the peak pupil dilation (PPD) as a dependent measure. From each baseline-normalized average pupil track, the PPD was determined as the maximum pupil diamter during the presentation time of the talker speech sample following the baseline period. It can be observed from Figure 3b that the PPD for Talker 1 speech sample is located at a latency of approximately 3000 ms from the end of the baseline period (i.e., the playback of the Talker 1 stimulus). Figure 4a displays the averaged pupil tracks for all talkers' post-baseline normalization. Salient features from Figure 4a include the differences in the temporal pattern of the pupil tracks and the PPD value for different talker stimuli, and the location of PPD between 2000 and 3500 ms after the initiation of the talker stimulus. To further illustrate how talkers who induced high and low PPD results appear relative to each other, the pupil tracks of 4 talkers were isolated and plotted separately in Figure 4b. Two of the tracks in Figure 4b were elicited while listening to talker stimuli that were judged to exhibit the highest vocal strain and required the most listening effort (Talkers 1 and 18).