Issues in Cognition, Audiology, and Amplification

SPECIAL EDITION | January 2021 Hearing Review

Index to Other Articles in this Special Edition:

Can Hearing Aid Use Offset Cognitive Decline? by Samira Anderson, AuD, PhD

The Brain on Hearing Aids: Can Treatment with Hearing Aids Improve Neurocognitive Function in Age-Related Hearing Loss? By Hannah Glick, AuD, PhD, and Anu Sharma, PhD

Deep Neural Networks in Hearing Devices, By Lars Bramsløw, PhD, and Douglas L. Beck, AuD

Issues in Cognition, Audiology, and Amplification

An update on what current science says about cognitive decline and hearing loss

By Douglas L. Beck, AuD, and Michael Harvey, PhD, ABPP

Research into how the brain and a person’s cognitive status is impacted by auditory input, speech, and social engagement continues at a fast pace. In this article, we summarize many of the most important and recent articles examining the relationships between cognition, audition, and amplification.

About this month’s guest-editor: The Hearing Review thanks Douglas L. Beck, AuD, for guest-editing this month’s special edition about hearing aids and cognition. An expert on this topic, Dr Beck previously guest-edited an “Expert Roundtable on Cognition, Audition, and Amplification” in the September 2015 HR. That issue was honored with an EDDIE Award in FOLIO magazine’s B-to-B Healthcare/Medical/Nursing category, which is the rough equivalent of an “Oscar” in the trade publishing world. He also guest-edited a special issue on “Audiology and Neuroscience” in the July 2018 HR. Dr Beck is the VP of Academic Sciences at Oticon Inc, Somerset, NJ, and serves as Senior Editor of HR’s Inside the Research column which focuses on new research-related findings in hearing healthcare. He has previously served as Editor in Chief at AudiologyOnline and as Web Content Editor for the American Academy of Audiology (AAA). Dr Beck is also an Adjunct Clinical Professor of Communication Disorders and Sciences at the State University of New York, Buffalo. — Karl Strom, editor

There are many reports in the scientific literature which offer keen, revealing, and often surprising insights into the vast and multi-tiered relationships between hearing, listening, cognition, dementia, and amplification. The goal of this article is to highlight reports on these topics and offer clinical and academic insight for hearing care professionals (HCPs) and the patients they serve.

Some Basics on Listening, Cognition, and Dementia

It may be best to start with a few basic facts and definitions:

Hearing is simply perceiving sound and is most often reflected by thresholds on an audiogram.¹
Listening is the ability to make sense of sound by assigning meaning to it.^1,2
Listening disorders refer to multiple processes which impair the ability of a person to attend to, focus on, or process the primary spoken message. Examples include: auditory processing disorders, auditory neuropathy spectrum disorder, cochlear synaptopathy, hidden hearing loss, etc, and may result from traumatic brain injury, acoustic trauma, and more.³
Cognition refers to aspects of information processing, such as formation of knowledge, attention, memory, reasoning, and computation.⁴
Cognitive impairment is when a person has difficulty processing information, such as remembering, learning new things, concentrating, or making decisions that affect their everyday life.⁵
Mild Cognitive Impairment (MCI) is the stage between cognitive decline associated with normal aging and the more serious decline of dementia.
Cognitive decline is a normal process which often accompanies aging and may involve decreased speed at which the brain functions.
Dementia is a more serious form of cognitive decline and often includes memory loss, as well as language problems and reduced thinking and problem-solving ability. Dementia may render a person unable to take appropriate care of oneself.

Measuring Cognitive Ability

Regardless of which cognitive attribute or sub-skill is measured (eg, short, long-term and working memory, learning ability, attention, pattern recognition, etc) accurately predicting one’s overall cognitive ability is not tenable.⁶ Moreover, since many measured attributes change over time, we must be careful to distinguish cognitive performance from cognitive decline. Specifically, a low score on a cognitive diagnostic or screening measure may not indicate cognitive decline. Indeed, it may represent a consistent score over time. In other words, a single frame/snapshot from a movie does not tell you the entire story.

Standardized diagnostic tests are important to professionally detect and monitor an individual’s level of cognitive function. The psychometric properties of diagnostic tests must be reliable (ie, must produce similar results under consistent conditions) and valid (ie, must measure what it is supposed to measure). However, when employing screening tests, we must be cautious to not over-read the test. Although there are many useful cognitive screening tests, perhaps the most common are:

Mini-Mental State Examination (MMSE);
The Montreal Cognitive Assessment Scale (MoCA);
The Elderly Cognitive Assessment Questionnaire (ECAQ);
The Abbreviated Mental Test Score;
The Mini-Cog, and
Cognivue.

These screening tests are intended for rapid assessment, and much like a hearing screening, are not likely to solely form the basis of a diagnosis. That is, the typical purpose of most screeners (audiology/psychology/SLP/medicine/etc) is to identify people who should have a formal diagnostic evaluation. Humes⁷ reported the MOCA may be the preferred test for clinicians seeking a quick assessment of cognitive function for older adults.

Formal psychological or neuropsychological testing, assessment, and diagnosis tools include the Wechsler Memory Scale (WMS) and the Wechsler Adult Intelligence Scale (WAIS). When these formal tests are used in tandem with the clinical history, biochemical and neuroimaging findings, each contributes to a comprehensive overview of the level of cognitive functioning and diagnostic accuracy in individuals with cognitive decline (eg, see Mahendran et al, 2015⁸).

Beck et al^9,10 noted “hearing status matters.” Of course, this is particularly true when cognitive and dementia screenings and diagnostic tests are delivered orally to people who may have unknown hearing or listening problems or disorders. As such, hearing care professionals (HCPs) play an important role with regard to candidates for cognitive and dementia evaluation. In their analysis, Beck et al recommended the Mini-Cog dementia screening tool.⁹ Administering the Mini-Cog requires only a few minutes, does not require specialized training, and it is not impacted by education level, socioeconomic status, ethnicity, or language biases.

Of note, a high percentage of people undergoing dementia assessment have undiagnosed hearing loss. Allen et al¹¹ found 87% of those attending dementia assessment sessions had hearing loss, yet 80% of those evaluated were unaware of their hearing loss. Livingston et al,¹² Uchida et al,¹³ Ray et al,¹⁴ and Panza et al¹⁵ concur hearing loss may directly affect the diagnosis and the experience of dementia or dementia-like behaviors. As one may anticipate, hearing loss may exacerbate or mimic symptoms of dementia. As such, the authors suggest hearing assessments should occur prior to formal cognitive and/or dementia assessment, with appropriate amplification/modifications employed, to assure comfortable audibility of the spoken words.

Hearing Loss and Cognitive Decline

Sensory impairments may occasionally be due to the same neuro-degenerative disease process(es) as those which impact cognition. Hearing loss and visual loss (in particular) may accelerate cognitive decline directly or indirectly via social isolation.¹⁶ Beck et al¹⁷ summarized four hypotheses which may explain how hearing loss and listening disorders might be associated with cognitive decline and dementia:

The Common Cause hypotheses indicates hearing loss and cognitive decline may share the same neuro-degenerative or vascular cause. Livingston et al¹² explained hearing loss may occur secondary to attenuated blood flow/cerebral perfusion.
The Cascade via Social Effects hypothesis notes that hearing loss may cascade into social disengagement and loneliness and depression, which may cascade into accelerated brain atrophy, cognitive decline, and dementia.
The Cascade via Auditory Deprivation hypothesis states that auditory deprivation results in impoverished cortical input, which potentially causes neuroplastic changes, cascading into cognitive decline and dementia.
The Cognitive Load hypothesis presumes that hearing loss prevents typical functioning of the superior temporal lobe, and resultantly, cognitive resources may be diverted from memory (or other) functions into auditory processing, thereby increasing cognitive load, and leading to cognitive decline and dementia.

Griffiths and colleagues¹⁸ proposed a new hypothesis/mechanism based on the critical interaction between auditory cognitive processes in the medial temporal lobe (MTL) and dementia pathology. They report aberrant auditory pattern analysis, working memory and object processing may interact in people with hearing loss and dementia.

Hearing Loss, Listening Disorders, and Cognition

It is well known that hearing loss and listening disorders¹⁹ cause communication difficulty, stress, depression, anxiety, and negatively impact quality of life (QOL). The National Institutes of Health²⁰ states the incidence of hearing loss in the United States is 1 of 3 people over age 65 and 2 of 3 people over age 75. As such, the relationship between hearing loss, hearing difficulty, and cognitive issues (ie, cognitive decline, dementia, MCI, etc) and our aging population has been of significant interest and has been (and continues to be) investigated by many researchers across multiple disciplines.

Beck and Clark²¹ reported people with hearing loss often need to dig deep into their cognitive reserve to make sense of a world delivered to them via an attenuated, distorted, and/or compromised auditory system. After the auditory system transmits degraded information (compromised neural code) to the brain, additional cognitive resources are required to make sense of the degraded information. They observed “audition matters more as cognitive ability decreases, and conversely, cognition matters more as auditory ability decreases.”

Lin et al²² reported 1984 older adults without cognitive impairment per the modified MMSE. Participants underwent audiometric testing and were followed for 6 years. Of note, 1162 participants had hearing loss and their annual rates of decline on the modified MMSE was 41% worse (and their Digit Symbol Substitution test was 32% worse) than those individuals with normal hearing. Further, as compared to those with normal thresholds, the people with hearing loss had a 24% increased risk for incident cognitive impairment. Lin and colleagues reported participants with hearing loss had an increased risk for cognitive impairment which was linearly associated with the severity of the hearing loss. That is, as hearing loss increased, so too did the likelihood of cognitive impairment. They concluded that hearing loss is independently associated with accelerated cognitive decline and incident cognitive impairment in community-dwelling older adults.

Livingston et al¹² noted in 2017 that hearing loss is a significant risk factor for dementia. They suggested that approximately two-thirds of dementia risk is genetic, while one-third is likely attributable to 9 behavioral risk factors which are modifiable:

Less education
Mid-life hypertension
Mid-life obesity
Hearing loss
Late-life depression
Diabetes
Physical inactivity
Smoking, and
Social isolation.

In 2020, The Lancet Commission²³ reported up to 40% of dementia was modifiable and they added 3 more risk factors; alcohol consumption, traumatic brain injury, and air pollution. Among the 12 risk factors, hearing loss remains the most significant with a Population Attributable Factor (PAF) of 9%. To address dementia risk, the Commission calls for nine ambitious recommendations to be undertaken by policy makers and by individuals, including “Encourage use of hearing aids for hearing loss and reduce hearing loss by protecting ears from high noise levels.”²³

Mamo and colleagues²⁴ investigated associations between speech-in-noise (SIN) performance and cognitive function as assessed via a neurocognitive battery across 250 participants. They reported older adults (average 77 years old) who demonstrated memory, language, executive functioning, and other cognitive problems had poorer speech-in-noise performance, which was determined to be unrelated to their hearing loss. The authors suggest that for people with more significant hearing loss (more than the study participants) the impact of their hearing loss would be greater on their cognitive abilities. Likewise, Brenowitz et al²⁵ stated the genetic risk for Alzheimer’s Disease (AD) also influences SIN ability, such that SIN difficulty may manifest prior to Alzheimer’s Disease.

As one ages and traverses from excellent hearing thresholds (for example, thresholds of 0 dB HL) to normal hearing thresholds (for example, thresholds of 20 dB), Golub and colleagues²⁶ reported, as hearing ability decreases—although still within “normal” threshold ranges—measurable aspects of cognition decrease, too, in a clinically meaningful way. Gaeta et al²⁷ concluded reduced audibility does, indeed, have a negative effect on MMSE scores in “cognitively intact” participants.

Lee et al²⁸ reported that attending to a primary speaker in multi-talker babble was more taxing for people with hearing loss than for those with thresholds within normal limits. They speculated that perhaps, for those with hearing loss, their perceptual organization may also be impaired.

Deal et al²⁹ reported a 10 dB increase in hearing thresholds may portend a 14% increased risk for dementia. The authors concluded hearing loss is associated with an increased risk of developing dementia in older adults.

Yuan and colleagues³⁰ reported their meta-analysis of 11 cohort studies addressing a probable association between hearing loss and cognitive impairment in adults (mean age of 60 years old or greater at baseline). Their results indicated that for those with average hearing thresholds worse than 25 dB HL, the risk of cognitive impairment was 29% greater than for those with no hearing impairment. For those with moderate-to-severe hearing impairment (average PTA >40 dB HL), their cognitive impairment risk was 29-57% greater than for those with normal hearing. The authors stated that, for every 10 dB increase in hearing loss, there was an estimated 12% increase in the incidence of cognitive impairment.

Jafari et al³¹ emphasized hearing loss is the third-leading cause of chronic disability in older adults. They stated hearing loss has been shown to predispose people toward cognitive impairment and dementia. They report a large body of evidence demonstrates age-related hearing loss is detrimental to physical and mental health, cognition, independence, social interaction, and QOL in the elderly. Further, they report as hearing loss increases, it may manifest significant consequences on verbal communication, as well as social, functional, and psychological well-being. Additionally, they note hearing loss is associated with a multitude of negative health outcomes:

Increased risk of cognitive impairment and dementia
Reduced quality of life
Low level of activity
Frailty
Social isolation, and
Poor general health.

The Impact of Hearing Aids on the Human Brain and Cognition

Hearing aids are the most common tool used to provide audibility for people with hearing loss and to provide an improved signal-to-noise ratio (SNR) for those with listening disorders. As such, it is important to consider whether hearing aids can reduce or slow cognitive decline or dementia associated with age-related hearing loss.

Indeed, sophisticated contemporary hearing aids no longer simply make sounds louder or more audible. Rather, advanced hearing aid technologies allow us to better address the needs of the human brain.³² Sophisticated hearing aids better represent the real-world dynamic acoustic landscape due to a multitude of onboard and digital remote microphone technologies and algorithms, many of which didn’t exist 5 to 10 years ago. The preservation of interaural timing differences (ITDs) and interaural loudness differences (ILDs), improved sound quality, Bluetooth streaming to the hearing aids from almost any sound source (including radio, TV, FM, smart phone, laptops, tablets, computers, dedicated sound systems, etc), an improved signal-to-noise ratio delivered with less listening effort and improved selective attention, all help provide a realistic dynamic acoustic landscape.³³ For example, hearing aids which preserve ILDs are of enormous importance with regard to knowing “where to listen.” Specifically, when ILDs are captured, maintained, and delivered through the hearing aid system, the brain is provided more information from which to detect where sound (such as the primary speaker) originates, facilitating improved and more focused selective attention. Of note, ILDs can be 20 dB or greater from 5000-8000 Hz³⁴ and are of significant importance with regard to understanding speech in noise. Further, it has often been demonstrated that sophisticated hearing aids can reduce steady-state noise from fluorescent lights, refrigerators, computer fans, HVAC systems, and more.³⁵ In essence, sophisticated hearing aids can deliver more of the primary speech signal with dynamic high fidelity, and less background noise, even in very challenging acoustic environments.³⁶ Advanced hearing aid technologies reduce listening effort and provide an improved SNR.³⁷ Improving the SNR has been identified as the single-most important factor associated with improving the ability to understand speech-in-noise.³⁸

Amieva et al³⁹ published a large-scale (n=3670) prospective population-based study assessing the relationship between self-reported hearing loss and cognitive trajectories over a 25-year period for three groups. Her groups included: 1) Those with self-reported hearing loss without hearing aid use (“unaddressed hearing loss”); 2) Those with self-reported hearing loss with hearing aid use, and 3) Those with no self-reported hearing loss. Measures of cognitive performance and cognitive complaints, functional ability, and symptoms of depression were acquired. The authors reported people in Group 1 (unaddressed hearing loss) generally presented with earlier rates of cognitive decline as compared to hearing aid users (Group 2) and those without hearing loss (Group 3). They concluded that addressing one’s hearing loss by use of hearing aids may slow cognitive decline by alleviating communication difficulties and improving mood and social interactions.

Deal et al²⁹ reported on 40 adult participants in a random controlled trial. Participants were each determined to be dementia-free and were between 70-84 years old. All had untreated, adult-onset, bilateral, mild-to-moderate hearing loss. Participants were randomized into one of two groups. Group 1 was “Best-Practices Hearing” (BPH) and received rehabilitation with hearing aids. Group 2 was labeled “Successful Aging” (SA) and received rehabilitation without hearing aids. Both interventions were delivered over the course of 4 one-hour sessions every 1-3 weeks post-randomization. For those participants in Group 1 (BPH group), there was an improvement in the cognitive domain score for memory. For those in Group 2 (SA group), outcomes demonstrated no change or worse function.

Ray et al⁴⁰ assessed hearing versus memory via word recall, and hearing versus executive function via verbal fluency. Their analysis was based on the English Longitudinal Study of Aging, involving 4072 people with a mean age of 67.4 years old. They reported 3056 (41.4%) had mild hearing loss and 755 had severe hearing loss. A total of 834 used hearing aids. Across all participants, there was a negative association between hearing loss and cognitive function, such that untreated hearing loss was linked to a greater decline in cognitive function. Further, they concluded that “Although hearing loss and cognition are linked, untreated hearing loss drives the association…” They stated, “Cognitive decline associated with ARHL is probably preventable by early rehabilitation and increased opportunistic (hearing) screening for the elderly.”⁴⁰

Jafari et al³¹ reported hearing amplification devices partially restore hearing and tend to improve overall cognitive performance in older adults.

Amieva and Ouvrard⁴¹ stated “the available data globally support the hypothesis that hearing aids have a positive impact on long-term cognition in older adults suffering from hearing loss.” Further, they noted important caveats which prevent conclusive causal statements from being declared, including the scarce number of studies, contrasting results, the absence of interventional studies such as random controlled trials, and more.

Glick and Sharma⁴² reported Age Related Hearing Loss (ARHL) is associated with cognitive decline and functional and structural brain changes. The authors determined multiple deficits were improved after 6 months of daily hearing aid use. Specifically, they provided striking evidence of compensatory cortical neuroplasticity secondary to the daily use of hearing aids for 6 months.^42,43 They noted a reversal in cross-modal reorganization and gains in speech perception and cognitive performance. (See Drs Glick and Sharma’s article on p 18 of this special issue.)

Pereira-Jorge and colleagues⁴⁴ reported 14 participants with an average age of 51 years who had bilateral moderate sensorineural hearing loss and 11 people with normal hearing with an average age 47 years. All received fMRIs, audiometric evaluations, and cortical thickness evaluations. The authors reported that after one year of continuous hearing aid use (10 hours per day) participants with hearing loss improved their speech reception thresholds (SRTs) from an average of 46 dB to approximately 37 dB, and they showed increased fMRI activity in the auditory and language cortices and multimodal integration areas, as well as the occipital region. Further, their cortical thickness increased in parts of the pre-frontal cortex, the middle temporal gyrus, and more. The authors report one year of hearing aid use “is related” to anatomical and functional brain changes. They report the importance of multi-modality as a fundamental factor in human brain organization (and re-organization), and they note the idea of sensory inputs processed in specific, unimodal cortices is outdated.

Sarant and colleagues⁴⁵ reported 99 adults, ages 60-84 years, with mild-to-moderate sensorineural hearing loss and no diagnosed or suspected cognitive impairment. All participants received hearing aids fitted to NAL-NL2 and with probe-mic real-ear measures. After 18 months of hearing aid use, the authors reported improvements in SRTs in quiet and noise, and “significant improvement in cognition associated with hearing aid use.” Further, they reported that after 18 months, self-reported overall QOL “was significantly improved.”

Cognitive Stimulation and Training

If appropriately stimulated, the human brain continues to develop throughout the lifespan. When humans access and process appropriate stimulation through vision, audition, taste, smell, and tactile senses, they gain the ability to access new and updated information. Stated differently, when the human brain is stimulated, “synaptic density” increases, thereby facilitating additional synapses in the brain; this in turn promotes positive cortical neuroplasticity and flexibility (ie, the ability to handle stress and respond to novel situations).

Synaptic density might be casually considered analogous to “the vegetation of your brain.” The analogy continues, “Is your brain more like a jungle with thick vegetation or more like a desert island with a single palm tree?” Given a “weed-whacker disease” like Alzheimer’s Disease or other neurocognitive disorders that attack your brain, how quickly will the effects be seen? To a large extent, the answer depends on your synaptic density. With increased synaptic density, your brain is likely to remain healthier longer; likewise, with decreased synaptic density, your brain quickly prunes the fewer (less redundant) available synapses, and damage becomes more evident, more quickly.⁴⁶

For example, the Advanced Cognitive Training in Vital Elderly study (ACTIVE) study⁴⁷ involved 2832 healthy seniors. The goal of the study was to determine the impact of cognitive training regarding everyday function and cognitive ability over a 10 year period. Group 1 was given brain training exercises, including 10 training sessions with a focus on memory, reasoning, or speed of processing, and each participant received 4 sessions of booster training 11 and 35 months after initial training. Group 2 received no training. Upon conclusion of the study, Group 1 had better cognitive function and demonstrated better memory and information processing ability than Group 2. Of note, these advantages were maintained at 5- and 10-year follow-up intervals.

Snowden’s longitudinal study⁴⁸ of 678 nuns, ages 75 to 107 years, and Perls⁴⁹ report on centenarians who avoid dementia indicate the likelihood that “cognitive reserve” plays an important role in healthy aging. That is, brain engagement and stimulation, academic pursuits, and participation in hobbies and socialization may help preserve cognitive ability, despite a genetic tendency for Alzheimer’s Disease.

Jia and colleagues⁵⁰ recently reported 2,102 people aged 65 years and older. Each was assessed from 2011 to 2016. Cognitive reserve was measured and quantified through a combined score based on educational level, occupation complexity, as well as reported engagement in social and other cognitive activities. Modifiable-risk-factor assessments were also measured and quantified based on depression, diabetes, smoking, physical activity, healthy diet, and drinking. The researchers determined cognitive reserve and modifiable-risk-factors impact the incidence of dementia and they stated “cognitive reserve significantly moderates the association between modifiable-risk-factor profiles and dementia.

Discussion

Despite cognitive impairments which may be, or may appear to be, attributed to hearing loss and listening disorders, and despite emerging reports of cognitive benefits associated with hearing aid amplification, we are only at the beginning of our understanding of these correlations. We cannot yet prove or declare causal relationships between hearing loss, listening disorders, cognition, dementia, and amplification.

Nonetheless, is seems likely that better hearing and an improved SNR allows one to engage in mentally stimulating and challenging activities which will generally have a positive impact on our brain, cognitive reserve, QOL, and overall health. Hearing aids are an effective intervention for age-related hearing loss with negligible risk^51,52 while the risk of untreated hearing loss is often substantial.^53,54

It seems reasonable to conclude that for those who acknowledge hearing loss and listening disorders, and act to improve their hearing and listening ability, they may be positively impacting their cognitive trajectory and may reduce their risk of cognitive decline and impairment.