7.1.1 Determinants of Central Auditory Processing
We assessed to what extent hearing sensitivity and different cognitive functions influ-ence central auditory processing ability across the adult life span (Chapter 4). We showed that hearing sensitivity is most important for central auditory processing. Fur-thermore, crystallized intelligence and executive functions showed effects on speech understanding in noise, as opposed to memory functions, which seemed less important.
Thus the ability to perceive speech signals might play a major role in speech under-standing in noise, while cognitive functions are supposedly beneficial at a later speech processing stage, with different involvement of different cognitive functions. Better verbal intelligence and lexical knowledge should help in recognizing single words and complet-ing the sentence even if the signal is disrupted through noise. Accurate control of atten-tion and inhibiatten-tion and better temporal processing also facilitate speech percepatten-tion (in noise). The working memory capacity of most individuals might be sufficient for the pur-pose of keeping in mind and recalling single sentences in speech in noise perception.
Long-term memory storage is not needed for speech understanding in noise.
Previous studies found an association between hearing sensitivity and central auditory processing. Those studies were small and based on samples with restricted age-ranges.32,216 We extend previous findings by showing that the association exists over the entire adult life span and across different hearing ability levels. Moreover, studies linking central auditory processing to cognition had heterogeneous samples with different age ranges, levels of hearing sensitivity loss, and had different inclusion criteria regarding hearing aid use.72 These previous studies did not control for hearing thresholds and/or did not assess different cognitive domains in one model. It is important to assess the relative effect of different cognitive functions to find out underlying mechanisms. When only one cognitive measure is used, this does not only reflect the specific cognitive func-tion that the test is considered to capture but it is also heavily influenced by the general cognitive ability of the person. Our study was able to extend previous results and deter-mine the relative contribution of different cognitive functions for central auditory pro-cessing.
This facilitates our understanding of possible functions and mechanisms involved in cen-tral auditory processing which might inform future research on possible training meth-ods.
7.1.2 Beneficial Effect of Auditory-motor Synchronization for Hearing Function
We investigated a beneficial effect of motor synchronization on auditory performance with meaningful auditory material in a complex task. We further assessed how previous musical training and cortical thickness of specific brain regions relate to different aspects of auditory-motor synchronization (Chapter 5). Although melody discrimination improved with better tapping synchrony, it was overall worse in the tapping than in the listening only condition. Longer previous musical training and thicker Heschl’s gyrus were associ-ated with better melody discrimination and better tapping synchrony. The thickness of the frontopolar cortex could explain who showed and who did not show benefits in the tapping condition. Our results support the simultaneous involvement of two different mechanisms – the alignment of movement with sensory input and the alignment of tem-poral attention – in the effect of motor synchronization on auditory function.116 People can benefit from tapping, when they tap accurately enough to align their motor pro-cessing with the auditory input. Additionally, they need to align their attention to benefit from this sensory enhancement. According to our findings, Heschl’s gyrus might be im-portant for auditory and motor processing and one of the neural correlates of the atten-tional process might be the frontal pole. However, also other more widespread cortical regions are likely involved in this.
Previous research on enhanced processing through motor synchronization has been limited to easy tasks with simple stimulus material.113–116 We extended these findings by showing a beneficial effect of sensory-motor synchronization for auditory perception of complex auditory stimuli. Moreover, we determined how prior musical experience and structural brain differences influence the extent to which an individual can benefit from motor synchronization in complex listening.
This could inform future research directed at development of (personalized) training pro-grams to improve hearing ability or counterbalance hearing loss.
7.1.3 Temporal Association of Hearing and Cognition
We investigated the complex interplay of decline in different hearing functions and cog-nition through determining the longitudinal associations of (1) hearing sensitivity and central auditory processing, (2) central auditory processing and cognition, and (3) hear-ing sensitivity and cognition, in middle-aged adults (Chapter 6). We found that hearhear-ing sensitivity more strongly affected order central auditory processing than higher-order central auditory processing affected hearing sensitivity. The associations between hearing and cognition were bidirectional and weak. This suggests that loss of hearing ability and cognitive decline may share a common cause rather than impairment in one function preceding and triggering impairment in the other.
Previous prospective studies which assessed the association of hearing and cognition15,16 had limitations in that they (1) included older study participants only, (2) assessed hearing only with audiometry, and (3) neglected temporality of events and comparisons of the strengths of effects going from hearing to cognition and from cogni-tion to hearing. We addressed each of the limitacogni-tions, and therefore extended the re-search to different aspects of hearing, middle-aged adults, and the assessment of tem-porality of effects.
Our findings inform future research directed at prevention and treatment applications.
Specifically, we conclude that hearing improvement may have only a limited benefit for prevention or delay of cognitive decline.