Even in those with relatively good audiograms, age-related neural degeneration may manifest in declines in coding the monaural and binaural temporal properties of dynamic and complex sounds that are important for listening in everyday life. Experiments conducted in controlled laboratory conditions have shown that there are age-related differences on a range of psychoacoustic measures of suprathreshold auditory processing, especially measures of temporal processing (Fitzgibbons & Gordon-Salant, 2010; Füllgrabe, Moore, & Stone, 2014) and binaural processing (Eddins, Ozmeral, & Eddins, 2018). Laboratory experiments indicate that many older adults are not as able as younger adults to detect differences in the fundamental frequency of two talkers’ voices when they are presented monaurally, and declines in this auditory ability seem to be important in helping listeners to segregate a target voice from a competing voice. Furthermore, age-related declines in binaural processing may explain why it is more difficult for older adults to take advantage of the spatial separation between different sound sources in the complex auditory scenes encountered in everyday life (Pichora-Fuller, Alain, & Schneider, 2017). Indeed, using the Listening in Spatialized Noise-Sentence Test (LiSN-S) (Cameron, Glyde, & Dillon, 2011), older adults did not gain as much advantage as younger adults in using voice and spatial separation cues to understand speech in noise, and these age-related differences were attributable to both auditory and cognitive abilities (Besser, Festen, Goverts, Kramer, & Pichora-Fuller, 2015).

For most adults with cochlear pathology, the analytic approach to rehabilitation has been transformed over the last decade because advanced signal processing algorithms in hearing aids have increasingly improved the audibility of speech in quiet, and even in some noisy situations. Nevertheless, once audibility has been restored by sufficient amplification, older individuals vary in their ability to understand speech because of individual differences in auditory temporal and cognitive processing (Füllgrabe et al., 2014; Pichora-Fuller, 2003; Humes, 2007). These factors may also explain some of the variation in how well individual listeners learn to understand speech in noise when it is processed by hearing aids (Gatehouse, Naylor, & Elberling, 2006; Humes, 2003; Lunner & Sundewall-Thorén, 2007; Rudner, Foo, Rönnberg, & Lunner, 2009).

Research in cognitive science has provided new insights into how speech understanding can be altered when listening is challenged by a mismatch between the reduced abilities internal to an individual (e.g., ARHL) and the demands imposed by external conditions, such as environmental noise or multiple voices in a crowd (Mattys, Davis, Bradlow, & Scott, 2012). Perhaps even more importantly, progress has been made in realizing that the motivation, goals and strategies of individuals influence when and how they harness their residual auditory abilities and complement them with other sensory, cognitive, and social abilities to achieve participation in everyday life. When listening becomes effortful in challenging situations, an individual who is motivated to persist in participating (rather than giving up) may draw on a combination of internal auditory and cognitive resources to meet the demands imposed by demanding external listening conditions. An international expert group, following classical ideas from cognitive psychology (Kahneman, 1973), defined effort as the deliberate allocation of mental resources to overcome obstacles in goal pursuit when carrying out a task, with listening effort applying more specifically when tasks involve listening, as illustrated in the Framework for Understanding Effortful Listening (FUEL) (Pichora-Fuller et al., 2016). A framework like the FUEL provides a new way for rehabilitative audiologists to understanding how and when auditory processing and cognitive processing combine to affect listening in everyday situations that vary in the demands they impose on a listener with particular auditory and cognitive abilities. A better understanding of the aspects of central auditory and cognitive processing involved in speech understanding in adverse conditions should enable new rehabilitation interventions to be designed that are more tailored to meet the needs and goals of individual older adults and their listening lifestyles. Using this framework, audiologists will be poised to enter a new era of rehabilitation that complements amplification and signal-based solutions with a more synthetic cognitive approach that pivots on individuals’ personal and social goals. In the long term, this combined analytic-synthetic approach is expected to support compensatory brain reorganization, optimize social participation, and promote aging well.

Cognitive Aging in Healthy Older Adults

In everyday activities, these three aspects of cognitive processing—working memory, attention, and speed of processing—may be highly interrelated. Working memory is a limited capacity system for temporarily storing and processing the information required to carry out complex cognitive tasks such as learning, reasoning, and comprehension. An individual’s working memory capacity, or span, is measured in terms of ability to simultaneously store and process information. For example, in the well-known working memory span task (Daneman & Carpenter, 1980; Pichora-Fuller, Schneider, & Daneman, 1995), after listening to or reading a set of sentences and performing some kind of judgment task after each sentence to ensure it has been comprehended (i.e., processed), the individual is asked to recall the sentence-final words for all of the sentences in the set (i.e., what has been stored). The number of sentences in the sets is increased and the working memory span is determined to be the largest set size for which the person can correctly recall a minimum specified proportion of the sentence-final words. The assumption is that, as the processing demands increase, there will be a corresponding decrease in how much can be stored in this limited capacity system; the working memory span is used to gauge the trade-off in the allocation of the limited capacity of working memory to processing versus storage. Working memory span can be used to compare different individuals when the task is held constant or to evaluate how the efficiency of processing shifts with variations in the task for a given individual (Pichora-Fuller, 2007).

It has been argued that there is an interface between working memory and attention insofar as complex working memory tasks reflect differences in how well individuals can (1) control attention to activate and keep the information relevant to the current goal or task available and (2) dampen or inhibit irrelevant information under conditions where there is risk of interference or distraction (Barrett, Tugade, & Engle, 2004; Engle, 2010). In general, speed of processing or the amount of time required to process information is assumed to reflect the amount of processing demanded by a task. If a task is easy and can be done very automatically without needing much control or effort, then it will impose a very low processing demand and take little time. In contrast, if a task is difficult and requires attentional control or effort, then the processing demand will be high and it will take more time to complete the task. Slowing of information processing is a hallmark of cognitive aging, but more specifically, it seems that perceptual processing speed accounts for much of the age-related variance in working memory capacity (Redick, Unsworth, Kelly, & Engle, 2012). In challenging conditions, the processing demand imposed by a task will require more attentional effort and performance will be slowed. These conditions include situations involving listening to speech in noise. It often takes more effort for an older person to listen in conditions with levels of noise that do not require similar effort on the part of younger listeners (Anderson, Gosselin, & Gagné, 2011). Of course, two of the most common complaints of older listeners or of people who are hard-of-hearing are that even if they can understand what has been said, it takes more effort and time for them when they have to follow ongoing speech.

Interactions Between Auditory and Cognitive Aging in Healthy Older Adults

In contrast to the compounding of auditory and cognitive declines, there is also the possibility that there could be compensatory benefits from interactions between auditory and cognitive processing. Listeners with better hearing may be less vulnerable to cognitive overload and listeners with better cognition may be more successful than peers with lower cognition when it comes to attending in complex listening situations (Heyl & Wahl, 2012; Sörqvist, Stenfelt, & Rönnberg, 2012) or even when learning how to use fast-acting signal processing in hearing aids (Gatehouse et al., 2006; Humes, 2003; Lunner & Sundewall-Thorén, 2007; Rudner et al., 2009).

The implication of this research in cognitive neuroscience is that new approaches to AR could facilitate compensatory brain reorganization as older listeners with hearing loss are engaged in relearning how to listen and comprehend speech using signals delivered by hearing technologies. Evidence of brain plasticity is very encouraging for rehabilitation professionals because it suggests that older adults can compensate by finding new ways to perform complex tasks successfully, such as listening to speech in noise (Peelle, Troiani, Wingfield, & Grossman, 2010). A recent systematic review and meta-analysis examined the results of nine studies in which cognitive outcomes were measured following auditory and cognitive training in adults with hearing loss; the overall certainty of the evidence of benefit based on these few preliminary studies was low for auditory training and very low for cognitive training, but a combined auditory-cognitive approach seemed to be the best (Lawrence et al., 2018). Insofar as a goal of new auditory-cognitive interventions would be to enhance cognitive compensation so that it can be deployed to overcome demands in a wider range of challenging listening situations, more research on combined auditory-cognitive interventions should be undertaken and evaluated in terms of a wider range of outcome measures to assess auditory, cognitive, communication and participation functioning in everyday life. Furthermore, combined auditory-cognitive interventions will likely be of even greater importance when older adults must contend with hearing loss in combination with clinically significant cognitive impairment.

Combined Auditory and Cognitive Impairments

Impairments in hearing and cognition both increase markedly with age, such that the majority of those over 75 years of age have hearing loss (Bainbridge & Wallhagen, 2014) and about 20% have mild cognitive impairment (MCI), with the rate of progression from MCI to Alzheimer’s disease (AD) being roughly 10% per year (Petersen et al., 2009). Since hearing impairment and cognitive impairment are both highly prevalent in older adults, dual impairments should be common.

Compensatory benefit from vision may modulate auditory-cognitive links in aging (Wettstein, Wahl, & Heyl, 2018). However, audiovisual processing can be compromised in people with hearing loss (Musacchia, Arum, Nicol, Garstecki, & Kraus, 2009). Notably, a 2015 report on the Global Burden of Disease estimated that hearing loss and vision loss, respectively, were the second and third most common impairments worldwide (Vos et al., 2016), and dual sensory (hearing and vision) loss affects about 20% of people over 80 years of age (Smith, Bennett, & Wilson, 2008). Compared to hearing loss alone, dual sensory loss is associated with an even greater likelihood for cognitive decline, functional decline, and even mortality (Brenowitz, Kaup, Lin, & Yaffe, 2019; Laforge, Spector, & Sternberg, 1992; Lin et al., 2004). At an exploratory workshop on “Sensory and Motor Dysfunctions in Aging and Alzheimer’s Disease” convened by the National Institute on Aging in the USA, experts concluded that there was a meaningful interface among sensory, motor, and cognitive dysfunctions that warranted more research (Albers et al., 2015). The possibility that AR could modify the risk of dementia is a topic of active research (Deal et al., 2017). AR designed to address sensory-cognitive links in aging adults will need to consider vision as a potentially compensatory sensory modality or a potentially comorbid sensory loss that could increase risk of cognitive decline.

The explanation for the correlations between hearing and cognitive impairments remains to be determined (for a discussion, see Lin et al., 2013; Uchida et al., 2019). It is possible that central presbycusis and executive dysfunction may result from similar neurodegenerative processes (Gates et al., 2010) or even that age-related sensory declines drive cognitive declines (Humes, Busey, Craig, & Kewley-Port, 2013). Alternatively, the association between hearing impairment and cognitive impairment may be so strong because individuals with better hearing simply exhibit less apparent cognitive decline, even though their brains are changing in a way similar to those who have hearing loss. Or maybe those with better hearing experience slower cognitive decline because they are better able to maintain healthy active lifestyles that are cognitively stimulating, thereby slowing cognitive decline in line with the “use it or lose it” perspective. In any case, it seems clear that it is time for audiologists to become more involved in the diagnosis and management of cases in which hearing and cognitive health may both be in question. Some individuals may begin AR with clinically normal cognition and later develop cognitive impairments while others may already have cognitive impairments when AR begins.

New diagnostic guidelines for MCI and dementia were released in 2011 (Albert et al., 2011; McKhann et al., 2011), with an emphasis on the diagnostic reality that there is a continuum from normal age-related changes in cognition to advanced dementia. As new approaches to diagnosing and managing dementia emerge, there is an opportunity to develop best practices for how hearing should be factored in when cognitive tests are interpreted and recommendations for care are made by other health professions. For example, the Montreal Cognitive Assessment (Nassreddine et al., 2005) is a cognitive screening test that is often used in primary care, but its accuracy may be reduced by hearing or vision impairments (Al-Yawer, Pichora-Fuller, & Phillips, 2019; Dupuis et al., 2015) or noisy test environments (Dupuis, Marchuk, & Pichora-Fuller, 2016). Within audiologic practice, in addition to the audiogram, it seems that there is value to incorporating central auditory testing (e.g., with dichotic digits or sentences) into assessment (Gates et al., 2011; Rosenhall, Hederstierna, & Idrizbegovic, 2011). New practice guidelines are also needed to inform audiologists as to the possible need to screen cognition to determine if referral to other health professionals is indicated. New partnerships with neuropsychologists and psychogeriatricians will be critical as audiologists develop more effective approaches to the rehabilitation of older adults with comorbid sensory and cognitive impairments (Dupuis, Reed, Bachmann, Lemke, & Pichora-Fuller, 2019). In particular, older adults with combined sensory and cognitive impairments who were receiving home care or residing in long-term care had greater functional difficulties in activities of daily living, less social engagement, increased loneliness, and reduced independence compared to those with only cognitive impairment (Guthrie et al., 2018). In general, AR should be a key component in health planning to promote aging well and to prevent older adults from becoming less active and socially isolated. With the anticipated rapid increase in cases of AD, there is great interest in promoting active lifestyles to stave off or slow down dementia (Fratiglioni, Paillard-Borg, & Winblad, 2004), but the role of ARHL in this endeavor remains to be determined (Pichora-Fuller, 2010).

The traditional basic audiometric evaluation is used as the first step in developing a rehabilitative plan and is most successful in defining the degree of hearing loss, rather than differentiating peripheral from central pathology. Individuals with audiometric hearing loss are approached as having primarily peripheral pathology; their rehabilitation usually focuses on hearing aid fitting. The prevalence of clinically significant hearing loss increases markedly with age (ISO, 2017). Using the screening criterion proposed recently in the United Kingdom (Davis et al., 2007), people whose pure-tone threshold at 3 kHz is in excess of 35 dB HL would be considered potential candidates for hearing aids. Accordingly, the majority of those 75 years of age or older would be referred for a hearing aid evaluation. Using the World Health Organization (WHO) grades of hearing impairment based on puretone averages (Stevens et al., 2013), about one-third of adults who are 65 years of age or older would be categorized as having disabling hearing loss defined as a better-ear four-frequency (0.5, 1, 2, and 4 kHz) pure-tone average exceeding 35 dB HL (WHO, 2013). With the aging of the population and the baby boom generation reaching retirement age, there will be a steadily increasing number of older adults who will seek help for hearing problems. Indeed, initial help seeking for age-related hearing problems occurs earlier now than it did a decade ago, with a shift in the average age of a first-time hearing aid user from about 70 to 63 years (Abrams & Kihm, 2015). The inaudibility of speech due to pure-tone hearing loss fully explains the speech-communication problems of about half of older adults, suggesting that amplification by hearing aids would be a good solution for their problems; however, other solutions would be needed to address the problems of the other half of the older adult population for whom audibility does not fully explain their speech-communication difficulties (Humes, 2019; Humes et al., 2019). Apart from audibility, the speech-communication difficulties of the other half of the older adult population may be attributable to age-related changes in auditory processing and/or cognition (Humes, Kidd, & Lenz, 2013). It is easy to overlook the more central auditory or cognitive factors that may contribute to the complaints of older listeners. Whether a person has peripheral auditory pathology or not, it is possible that there could be neural degeneration of a sort that would be considered to involve primarily CAP. To date, it is uncommon for older adults with good audiometric thresholds who have complaints about auditory functioning to be assessed for CAP disorders and, if any rehabilitation is provided, it usually involves technologies other than conventional hearing aids (e.g., FM), environmental modifications, listening training, and/or behavior change to manage challenging listening situations.

Recently, the American Academy of Audiology Task Force on Central Presbycusis published a report and in defining central presbycusis, the task force used “an historical narrow structural definition . . . focused on modality-specific changes to auditory portions of the central nervous system from above the cochlear nucleus to the auditory cortex” (Humes et al., 2012, p. 637). Using that definition, it was concluded that there was

insufficient evidence to confirm the existence of central presbycusis as an isolated entity. On the other hand, recent evidence has been accumulating in support of the existence of central presbycusis as a multifactorial condition that involves age- and/or disease-related changes in the auditory system and in the brain. (Humes et al., 2012, p. 636)

In contrast to the current view of cognitive neuroscience, the traditional audiologic view of CAP has relied on an anatomical framework and a perspective that has viewed an array of auditory functions as modular processes which could be delineated and assessed independently from each other (Pichora-Fuller & Singh, 2006). Crucially, the task force did point out that the definition they used was “in contrast to a broad functional definition of central presbycusis, one that might encompass any age-related changes in the central nervous system beyond the auditory periphery that might impact communication, including cognitive changes” (Humes et al., 2012, p. 637).