Hearing loss or background noise will increase the cognitive resources required for decoding auditory inputs, leading to effortful listening. [31]. AS children with hearing loss may often experience effortful listening, which could seriously affects the quality of learning and work. In the present study, we acquired the pupil dilation response (BPD, PPD and MPD) during a listening-answer task in two conditions (SNR= -2 dB and SNR = + 15 dB) for NH and AS children. We showed the influence of SNR and spectral distribution of the test materials on listening effort (as indexed by pupil response) in school-aged children with Alport syndrome. Additionally, the present results also corroborate a close link between cognitive demands imposed by a listen-answer task and task-evoked pupil dilation, which was consistent with research in adults. This provides further evidence of the negative effects of long-term hearing loss on the auditory cognitive ability.
Our findings extend prior works in two ways. First, we provide evidence that AS children with mild to moderate U-shape hearing loss need more cognitive resources mainly in the stage of thinking not just listening compared to NH children under current experimental conditions. This is the first study to evaluate listening effort on school-aged children with mild to moderate U-shape hearing loss by pupillometry. This is a special group because according to traditional audiometry battery, this group has audiograms that don’t suggest hearing loss in speech-related information, and speech communication is successful. Second, our longer-term speech material and listening-answer task format reflect better ecologically realistic scenarios, which regularly experienced by school-aged children in everyday life. We have also shown that the task could elicit task-evoked pupillary response reliably in HI and NH children. Therefore, this task could reveal the process of cognitive resource allocation in real situation.
Our hypothesis was to explore the influence of SNR and spectrum distribution on listening effort, however, we found significant effect of SNR instead of MER13 on accuracy or pupil response. There was a trend that AS children may have worse performance when listening to auditory materials with more energy in 1k-3k Hz. Note that speech materials used were not processed in the spectral aspect, therefore the difference in spectral concentration between high and low group is natural and small. Although this allows us to use pre-validated speech materials (especially important when this task format is new), it might explain the insignificant trend observed in the effect of MER13. We assumed that AS children might have significantly larger pupil size and worse performance if manipulate the spectral features of the speech materials in a more causal experimental design.
In our study, we used two kinds of time window to analyze pupil responses. In the time window containing only listening to dialogue, no matter SNR = -2 dB or SNR = + 15 dB, there was no significant difference of pupil size between AS children and NH children. The results of the mixed effect model also suggested that SNR, hearing status and MER13 had no significant effects on PPD and MPD, indicating that AS children might not need to provide much extra cognitive resources for the expenditure of intense effort at this stage. This is consistent with the results from clinical audiograms and speech recognition tasks that AS children are generally successful in speech communication.
However, when looking at the time window including listening dialogues, questions and thinking, larger pupil size were observed in AS children at -2 dB SNR compared to their NH peers, suggesting that extra effort might appear especially at the stage of thinking for AS children when the task is difficult. In this time window, the subjects need to re-extract the dialogue content they heard, connect it with the questions and make decisions after reasoning or calculating at this stage. The results show significant main effect of SNR and hearing status on MPD, which is more robust compared to PPD in longer stimuli designs[32]. MPD gives further information for the sustained cognitive load in a given time window[25]. Larger MPD in this time window represents constant allocation of cognitive resources in AS children. Increasing SNR will result in reduction of MPD, which reminds us that intervention might release effortful listening.
The framework of ELU (Ease of Language Understanding Model) may propose an explanation to our results[33]. In time window including listening dialogues, questions and thinking, the listeners needed to store the sentence and reconstruct information that were either missed or distorted. When there is a mismatch between the input signal and the long-term memory, explicit processing needs to be initiated, which depends on cognitive abilities such as working memory capacity and efficiency[34]. This explicit processing requires extra cognitive resources from the limited pool of resources that would otherwise be used for other mental tasks (i.e., calculation, deduction etc). Therefore we see a lower score of AS children in answer correct at difficult SNR conditions because they could not invest as many cognitive resources to perform the mental processing to get the correct answer AS children also need to allocate more cognitive resources in general, which is reflected by larger pupil size.
Note that without utilizing this new behavioral task that involves both listening and associated mental processing, we might not observe this difference between AS and NH children in both behavioral and physiological responses. Ecologically, it is rare to find a speech communication scenario where only passive listening and repetitions are required, as in some previous listening effort studies. Typically, listeners need to decode the speech inputs, process the information and prepare a response accordingly to either extract more information or giving more information. All of these require cognitive resources from a limited pool or resources. Extra resource needs at the stage of speech comprehension would impair the accuracy and efficiency of later stages, leading to poorer social communication, learning and development. This highlights the importance to investigate ecological speech communication scenarios in order to identify groups at higher risk of cognitive development.
It is widely accepted that pupil response have a close relationship with cognitive function[17], but the exact relationship between the ability to allocate cognitive resources and cognitive function remains unclear. According to the hypothesis proposed by Van der[35], our results are more consistent with the second hypothesis, namely, people with better cognitive function utilize cognitive resources more efficiently, so they only need to invest relatively a few cognitive resources when executing a task, it is suggested that long-term hearing loss lead a negative effect on the efficiency of utilizing cognitive resource. Therefore, AS children need to devote more cognitive resources to cope with the current task, and show larger mean pupil dilation due to low efficiency of utilizing cognitive resources.
As indicated, some researchers considered that BPD can reflect the arousal state of the cognitive system[36]. Gilzenrat believed that larger BPD before the experiment represents an over-arousal state of the cognitive system, indicating that the subjects are anxiety or stressful[37]. Moreover, Alhanbali found that there was a significant correlation between BPD and fatigue scales[38], smaller BPD represents low arousal state of the cognitive system, indicating great fatigue of the subjects. According to the Yerkes-Dodson Law, both over arousal and low arousal are associated with poor performance of cognitive task, while arousal levels between these two extremes are expected to have the best performance[39] [40]. Animal studies have also revealed an inverted U-shaped relationship between task performance and arousal levels. McGinley found that optimal signal detection behavior and sound-evoked responses of mice occurred only at intermediate arousal when membrane potentials were stably hyperpolarized, instead of low arousal or over arousal [41]. In our study, BPD of AS children are smaller in all listening conditions compared to NH children, suggesting they suffered from auditory fatigue when processing auditory cognitive. The negative impact of long-term hearing loss was their cognitive system are often in a state of low arousal. Recent findings indicated that long-term hearing loss of AS children affects cognition in two aspects, one is efficiency of utilizing cognitive resource, the other is arousal state, and both of them further affects listening effort (Fig-9).
There are several limitations of the present study that need to be mentioned. First, we only measured objective listening effort but no specialized test for cognitive abilities such as working memory capacity and efficiency. We might have gained more insight into the associations between listening effort and cognitive function if we had also included variables controlling for individual differences in cognitive abilities. Second, we did not manipulate experimentally the spectral distribution of the speech materials but rather choose to use pre-validated and more natural stimuli. The results of the study only proved that the MER13 had an influence on the behavioral test, but no significant results were found. Strictly modulating of spectral distribution might be improved in future studies.