Neural temporal processing in the aging auditory system: disentangling effects of aging and peripheral hearing loss

Good speech perception skills are important for communication and social integration. Many persons, however, suffer from impaired speech perception in everyday, noisy listening situations, which causes them to avoid or miss out on social events, thereby affecting their quality of life. There is general consensus that difficulties in speech perception result from hearing impairment, age, and cognitive problems. Nevertheless, it remains unclear to what extent each of these three factors act upon speech perception. This lack of knowledge can be attributed to the interrelatedness between hearing impairment, age, and cognitive declines, which makes it difficult to investigate their differential contributions to difficulties in speech perception. Yet, such knowledge is needed to develop effective intervention strategies for remediating these highly prevalent difficulties.

The aim of the present research was to unravel the differential impact of hearing impairment and age on speech perception. Therefore, a specific research design was applied. We recruited persons of three age categories, i.e., young (20-30 years), middle-aged (50-60 years), and older (70-80 years), who were either normal-hearing (NH) or hearing-impaired (HI). All participants passed a sensitive cognitive screening, indicating that they had normal cognitive capacities. In this way, we minimized confounding effects of cognition. By comparing outcomes among the three NH age groups, on the one hand, and between NH and HI similar-aged adults, on the other hand, we could investigate effects of age and hearing impairment, respectively.

We performed behavioral speech tests and electrophysiological measurements to gain insight into the extent as well as the nature of the declining effects of age and hearing impairment on speech perception. Speech perception performance was evaluated in different masking noises to verify whether the impact of age and/or hearing impairment depends on the type of background noise. As for the electrophysiological measurements, we investigated neural temporal processing in the auditory brain, and neural envelope encoding in particular. The slowly varying temporal envelope of speech is known to drive speech perception and the envelope modulations are encoded by synchronized neural oscillatory activity along the central auditory pathway. In order to look into this neural envelope encoding, we recorded auditory steady-state responses (ASSRs) to speech-related acoustic modulations. The degree of neural envelope encoding was examined all along the auditory pathway, from the brainstem up to the cortex. The functional hemispheric asymmetry, i.e., asymmetrical processing of specific modulation frequencies towards the left or the right hemisphere, which is an intrinsic aspect of speech processing, was investigated as well.

The present research demonstrates that speech perception in noise declines by middle age and decreases further on to older age, independent of hearing sensitivity. We also show that the declining effect of hearing impairment is the same at a young, middle, and older age. The declining effects of age and hearing impairment are most pronounced in cognitively demanding background noises (informational masking). Moreover, in such background noises, both hearing impairment and age account for a significant part of the speech perception difficulties of HI aging persons, whereas hearing impairment is the predominant degrading factor in background noises that merely cause energetic masking.

Our electrophysiological measurements reveal that age as well as hearing impairment are characterized by changes in the neural encoding of the temporal envelope in both subcortical and cortical auditory regions. As for the factor age, cortical neural envelope encoding increases and the subcortical encoding decreases with advancing age, irrespective of peripheral hearing sensitivity. Furthermore, at an older age, there is a reduction in functional hemispheric asymmetry. As for hearing impairment, we demonstrate that it is inherently related to enhanced neural envelope encoding from the brainstem up to the cortex – most saliently when audibility is corrected for – at a young and middle age. No such neural enhancement is apparent in HI older adults. Yet, hearing impairment at an older age (presbycusis) is characterized by a functional hemispheric asymmetry that is not apparent in older adults who have normal peripheral hearing sensitivity.

Finally, we performed regression analyses to determine whether the age- and hearing-related changes in speech perception performance can be explained by the observed changes in neural temporal processing. Significant correlations were, indeed, identified. We show that enhanced neural envelope encoding in the auditory cortex is predictive of poor speech perception in NH adults. In HI adults, enhanced neural envelope encoding in the brainstem is associated with inferior speech perception.

This doctoral research clarifies the differential contributions of age and hearing impairment to difficulties in speech perception and provides a detailed characterization of the impact of age and hearing impairment on neural envelope encoding in the auditory brain. Moreover, the observed correspondences between neural envelope encoding and impaired speech perception yield promising directions for developing advanced intervention strategies to improve speech perception skills, and, in turn, quality of life.