The Influence of Speech Material, Intelligibility, and Masker Type on the Pupil Dilation Response in Cochlear Implant Users

Abstract

OBJECTIVES: The pupil dilation response is often used as a physiological measure of listening effort. One way to manipulate listening effort is to manipulate intelligibility. Only a few studies assessed the relationship between intelligibility and the pupil dilation response in cochlear implant (CI) users. Those studies did not consistently replicate the effects of intelligibility on the pupil dilation response evoked by sentence recognition in noise as observed in individuals with typical hearing or hard of hearing. Therefore, this study examined the effects of intelligibility on the pupil dilation response in CI users using several types of speech material. The type of masker is also known to influence listening effort as measured by pupillometry in individuals with typical hearing and hard of hearing. To our knowledge, this is not yet investigated in CI users. Therefore, this was also assessed in the current study. Finally, to account for subjective experiences, ratings of listening effort, difficulty, performance, and tendency of giving up were assessed. DESIGN: Twenty-eight postlingually deafened adult unilateral or bimodal CI users participated. Their ages ranged from 41 to 71 yrs old (mean = 58.4). A 3 × 3 within-subjects design was used with 3 intelligibility conditions (quiet, 50%, and 20% intelligibility in continuous noise) and 3 types of speech material (digit triplets, consonant-vowel-consonant [CVC] words, everyday sentences). Linear mixed model analyses assessed the effects of speech material and intelligibility condition on the mean pupil dilation (MPD), peak pupil dilation (PPD), and baseline pupil size (BPS). A paired sample t test was used to compare the effect of masker type (continuous noise versus a single interfering speaker) on these pupil measures for 50% sentence intelligibility. For all conditions, participants rated their experienced listening effort, difficulty, performance, and tendency of giving up. RESULTS: In the main analysis (N = 23), MPD, PPD, and BPS differed between the speech materials, but these measures did not depend on speech intelligibility. CVC words elicited the lowest MPDs and PPDs, followed by digit triplets and everyday sentences, with the latter showing the most pronounced pupil responses. The BPS was larger for sentences compared with CVC words and digit triplets. CVC words and sentences were rated as the most effortful and difficult, with the lowest self-perceived performance, and evoking the highest tendency to give up. Self-perceived performance was furthermore influenced by intelligibility (quiet > 50% > 20%). The other subjective ratings, such as listening effort, decreased with increasing intelligibility. A separate analysis on the data for the digit triplet material, including all CI users (n = 28), showed an intelligibility effect on the PPD. Finally, the analysis (N = 15) of the effect of masker type showed that PPDs and BPSs were larger for sentences masked with an interfering speaker compared with continuous noise. CONCLUSIONS: The intelligibility conditions in this study did not result in any measurable differences in listening effort, as indicated by the pupil dilation responses of unilateral or bimodal postlingually deafened CI users for everyday sentences or CVC words. A main effect of intelligibility condition on PPD was only found for digit triplets. Speculatively, this may be because the relative simplicity and low cognitive load evoked by this speech material allowed intelligibility effects to become apparent in this specific group of listeners. Subjective ratings indicated greater listening effort, difficulty, and a tendency to give up at lower intelligibility levels. When sentences were masked with an interfering speaker, both PPD and subjective listening effort increased compared with masking with continuous noise. These findings suggest that manipulating masker type can alter the pupil dilation response and perceived listening effort in CI users, even when using everyday sentences.