< Terug naar vorige pagina

Publicatie

Neural tracking of the speech envelope: unraveling the effects of listening effort, age and hearing impairment

Boek - Dissertatie

Partly due to the rapid pace of aging of the world population, it is expected that by 2050 more than 900 million people will experience hearing loss. Since adequate hearing is a prerequisite for daily life communication, hearing impairment increases the risk of social isolation and poorer physical functioning, which in turn negatively affects quality of life. Currently, hearing aids are the most used and well-known treatment for hearing impairment. Although these devices restore hearing sensitivity, adequate speech perception in noisy environments is often not achieved. Research has suggested that, next to hearing loss, age-related cognitive decline and temporal processing deficits also contribute to the speech-in-noise difficulties experienced by older adults. To provide rehabilitation strategies that overcome these difficulties, there is a need for a better understanding of the neural mechanisms underlying speech-in-noise difficulties. When we listen to natural speech, our neural activity tracks the low amplitude modulations of speech, also called the speech envelope. Recent studies have demonstrated the potential of neural envelope tracking to objectively measure speech understanding. This could provide additional information to current behavioral speech tests and improve the fitting of hearing aids as neural envelope tracking does not require active cooperation of the patient. This could be particularly useful for difficult-to-test populations such as young children, intellectually disabled persons and older adults with severe cognitive impairment such as dementia. Although this seems promising, previous studies mainly measured neural envelope tracking in a specific well-controlled population, i.e. young, normal-hearing adults. The aim of this doctoral thesis was to investigate the effects of three individual-related factors on neural envelope tracking: listening effort, age and hearing impairment. As daily life speech understanding can be challenging for both normal-hearing and hearing impaired adults, individuals can differ in the amount of allocated neural resources, also called listening effort, that they need to expend to achieve a particular level of speech understanding. This could, however, result in a confound when using neural envelope tracking to objectively measure speech understanding. In view of this, we investigated the effect of listening effort, on neural envelope tracking in young, normal-hearing listeners. Five measures were included to quantify listening effort. Our results demonstrated that different measures can reflect different aspects of effort, e.g. perceived effort versus processing load. Listening effort was not found to substantially modulate neural envelope tracking. Nevertheless, participants showed increases in envelope tracking with increasing speech understanding, suggesting that neural envelope tracking can be used as a reliable objective measure for speech understanding. With advancing age, hearing loss becomes more prevalent. To disentangle these two closely intertwined factors, we designed two studies in which we investigated neural envelope tracking in normal-hearing adults across the adult lifespan and compared the results with those of age-matched hearing impaired adults. For both normal-hearing and hearing impaired adults, neural envelope tracking was measured to sentences and a story masked by different levels of a stationary noise or competing talker. A competing talker was included to investigate the effects of hearing impairment on neural segregation of different talkers. Participants also completed two cognitive tests, measuring verbal working memory and inhibition, to investigate the interplay between cognition, age, hearing impairment and neural envelope tracking. Our results reveal that aging and hearing impairment both result in major alterations of neural envelope tracking. More specifically, envelope tracking was found to increase supralinearly with advancing age, resulting in an enhanced envelope tracking for older normal-hearing adults. This enhancement is likely to underlie the speech-in-noise difficulties experienced by older normal-hearing adults as we found that worse cognitive scores were associated with this enhancement. Hearing impaired adults showed additional enhanced envelope tracking compared to their age-matched normal-hearing peers. As we only observed this for the attended, target talker, our results suggest that in order to neurally segregate different talkers, persons with a disabling hearing loss need enhanced cortical envelope tracking to compensate for peripheral deficits. Furthermore, enhanced envelope tracking in hearing impaired adults may be caused by different neural mechanisms than those related to age since no significant relation with cognitive skills was observed. Finally, middle-aged and older normal-hearing and hearing impaired adults showed a significant increase in neural envelope tracking with increasing speech understanding similar to their young normal-hearing counterparts, highlighting the potential of neural envelope tracking to objectively measure speech understanding. In conclusion, this doctoral thesis demonstrates substantial effects of age and hearing impairment on neural envelope tracking which contribute to the current understanding of the mechanisms underlying impaired speech understanding. In addition, observing the link between speech understanding and neural envelope tracking in different populations, supports the value of neural envelope tracking in diagnostic tests, rehabilitation strategies and self-fitting hearing aids.
Jaar van publicatie:2019
Toegankelijkheid:Open