Let's See If You Can Hear: The Effect of Stimulus Type and Intensity to Pupil Diameter Response in Infants and Adults.

Objectives: Pupil dilation can serve as a measure of auditory attention. It has been proposed as an objective measure for adjusting hearing aid configurations, and as a measure of hearing threshold in the pediatric population. Here we explore (1) whether the pupillary dilation response (PDR) to audible sounds can be reliably measured in normally hearing infants within their average attention span, and in normally hearing adults, (2) how accurate within-participant models are in classifying PDR based on the stimulus type at various intensity levels, (3) whether the amount of analyzed data affects the model reliability, and (4) whether we can observe systematic differences in the PDR between speech and nonspeech sounds, and between the discrimination and detection paradigms.

Design: In experiment 1, we measured the PDR to target warble tones at 500 to 4000 Hz compared with a standard tone (250 Hz) using an oddball discrimination test. A group of normally hearing infants was tested in experiment 1a (n = 36, mean [ME] = 21 months), and a group of young adults in experiment 1b (n = 12, ME = 29 years). The test was divided into five intensity blocks (30 to 70 dB SPL). In experiment 2a (n = 11, ME = 24 years), the task from experiment 1 was transformed into a detection task by removing the standard warble tone, and in experiment 2b (n = 12, ME = 29 years), participants listened to linguistic (Ling-6) sounds instead of tones.

Results: In all experiments, the increased PDR was significantly associated with target sound stimuli on a group level. Although we found no overall effect of intensity on the response amplitude, the results were most clearly visible at the highest tested intensity level (70 dB SPL). The nonlinear classification models, run for each participant separately, yielded above-chance classification accuracy (sensitivity, specificity, and positive predictive value above 0.5) in 76% of infants and in 75% of adults. Accuracy further improved when only the first six trials at each intensity level were analyzed. However, accuracy was similar when pupil data were randomly attributed to the target or standard categories, indicating over-sensitivity of the proposed algorithms to the regularities in the PDR at the individual level. No differences in the classification accuracy were found between infants and adults at the group level, nor between the discrimination and detection paradigms (experiment 2a versus 1b), whereas the results in experiment 2b (speech stimuli) outperformed those in experiment 1b (tone stimuli).

Conclusions: The study confirms that PDR is elicited in both infants and adults across different stimulus types and task paradigms and may thus serve as an indicator of auditory attention. However, for the estimation of the hearing (or comfortable listening) threshold at the individual level, the most efficient and time-effective protocol with the most appropriate type and number of stimuli and a reliable signal to noise ratio is yet to be defined. Future research should explore the application of pupillometry in diverse populations to validate its effectiveness as a supplementary or confirmatory measure within the standard audiological evaluation procedures.