Listening Effort and Memory Effort in Cochlear Implant Users: A Pupillometry Study

Following a conversation involves numerous cognitive processes. Speech needs to be processed, retained, encoded and an answer needs to be prepared. Hearing impairment leads to degraded speech signal, which results in the need for additional cognitive resources to processing and encoding. This is particularly true for cochlear implant (CI) users, where the auditory nerve is stimulated electrically. The present study examined the cognitive effort required for listening (listening effort) and retaining sentences in memory (memory effort) among CI users by means of pupil dilation. The aim of the present study was to evaluate how CI users allocate their cognitive resources in an auditory dual task and whether changes in pupil size can be indicative of this allocation. Understanding how cognitive effort is allocated during speech comprehension might be helpful in rehabilitation following a CI implantation.

Nineteen German-speaking CI users performed a sentence final word identification and recall test while their pupil dilation was recorded. Participants listened to a sentence in four-talker babble background noise and repeated the final word of each sentence. Sentences were presented in lists of either three or six sentences, and at the end of each list, participants were asked to repeat back as many final words as possible and were evaluated in four conditions with varying list length (three versus six sentences) and the necessity of a recall (recall versus no recall). The pupil size was analyzed before each sentence (sentence baseline) and in terms of the mean pupil dilation relative to the sentence baseline (sentence dilation). The conditions were statistically compared by fitting a line to the pupil measures across groups of three sentences and contrasting the intercepts and slopes, respectively: First, the three sentence conditions were compared with the first three sentences (#1 to 3) of the six-sentence conditions with the factors recall and list length. Second, within the six-sentence conditions, the first half (sentences #1 to 3) was compared with the second half (sentences #4 to 6) with the factors recall and halves.

Over the course of the lists, sentence baseline diverged between three and six sentences, which manifested in the slope of sentence baseline being significantly higher in three than six sentences. The slope of sentence baseline also differed significantly between the first compared with second halve. The slope within the second halve is more positive compared with the first halve. The recall-related divergence was most prominent in the second half of the six-sentence lists, which manifested in a significant interaction between recall and halves in the intercept of the sentence baseline. Sentence dilation did not significantly change across the list presentation.

CI users appeared to allocate their memory effort in accordance with task demands, while their listening efforts remained unchanged. Our results indicated that effort allocation for listening and memory processing can be assessed in CI users using pupil dilation during the sentence final word identification and recall test.