Hearing Impairment and Perceived Clarity of Predictable Speech

Objectives: The precision of stimulus-driven information is less critical for comprehension when accurate knowledge-based predictions of the upcoming stimulus can be generated. A recent study in listeners without hearing impairment (HI) has shown that form- and meaning-based predictability independently and cumulatively enhance perceived clarity of degraded speech. In the present study, we investigated whether form- and meaning-based predictability enhanced the perceptual clarity of degraded speech for individuals with moderate to severe sensorineural HI, a group for whom such enhancement may be particularly important. Design: Spoken sentences with high or low semantic coherence were degraded by noise-vocoding and preceded by matching or nonmatching text primes. Matching text primes allowed generation of form-based predictions while semantic coherence allowed generation of meaning-based predictions. Results: The results showed that both form- and meaning-based predictions make degraded speech seem clearer to individuals with HI. The benefit of form-based predictions was seen across levels of speech quality and was greater for individuals with HI in the present study than for individuals without HI in our previous study. However, for individuals with HI, the benefit of meaning-based predictions was only apparent when the speech was slightly degraded. When it was more severely degraded, the benefit of meaning-based predictions was only seen when matching text primes preceded the degraded speech. The benefit in terms of perceptual clarity of meaning-based predictions was positively related to verbal fluency but not working memory performance. Conclusions: Taken together, these results demonstrate that, for individuals with HI, form-based predictability has a robust effect on perceptual clarity that is greater than the effect previously shown for individuals without HI. However, when speech quality is moderately or severely degraded, meaning-based predictability is contingent on form-based predictability. Further, the ability to mobilize the lexicon seems to contribute to the strength of meaning-based predictions. Whereas individuals without HI may be able to devote explicit working memory capacity for storing meaning-based predictions, individuals with HI may already be using all available explicit capacity to process the degraded speech and thus become reliant on explicit skills such as their verbal fluency to generate useful meaning-based predictions.