The Effects of Age-Related Hearing Loss on the Brain and Cognitive Function

Hearing loss has been identified as potentially the biggest modifiable risk factor for dementia and cognitive decline, but the causal link between these two conditions that affect older adults is not clear.Age-related hearing loss presents as a constellation of dysfunctions that affect both the auditory periphery, the auditory cortex, and global cortical organisation.There is evidence for compensatory neural resource allocation, suggestive of cognitive compensation, which may have a significant impact on cognitive functioning.Several hypotheses have been proposed to explain the potential relationship between auditory and cognitive impairment: Some hypotheses suggest that the relationship is underpinned by general neurodegeneration in ageing; others suggest that auditory impairment and sensory deprivation are causally linked to cognitive impairment.Limitations in the methods used for quantifying both age-related hearing loss and cognitive decline may lead to either over- or under-estimation of the association between age-related hearing loss and cognitive decline. Age-related hearing loss (ARHL) is a common problem for older adults, leading to communication difficulties, isolation, and cognitive decline. Recently, hearing loss has been identified as potentially the most modifiable risk factor for dementia. Listening in challenging situations, or when the auditory system is damaged, strains cortical resources, and this may change how the brain responds to cognitively demanding situations more generally. We review the effects of ARHL on brain areas involved in speech perception, from the auditory cortex, through attentional networks, to the motor system. We explore current perspectives on the possible causal relationship between hearing loss, neural reorganisation, and cognitive impairment. Through this synthesis we aim to inspire innovative research and novel interventions for alleviating hearing loss and cognitive decline. Age-related hearing loss (ARHL) is a common problem for older adults, leading to communication difficulties, isolation, and cognitive decline. Recently, hearing loss has been identified as potentially the most modifiable risk factor for dementia. Listening in challenging situations, or when the auditory system is damaged, strains cortical resources, and this may change how the brain responds to cognitively demanding situations more generally. We review the effects of ARHL on brain areas involved in speech perception, from the auditory cortex, through attentional networks, to the motor system. We explore current perspectives on the possible causal relationship between hearing loss, neural reorganisation, and cognitive impairment. Through this synthesis we aim to inspire innovative research and novel interventions for alleviating hearing loss and cognitive decline. ARHL, or presbycusis, is characterised by gradually developing high-frequency hearing loss, often accompanied by poor speech discrimination, and may begin to surface in the fourth decade of life [1.Davis A. et al.Aging and hearing health: the life-course approach.Gerontologist. 2016; 56: S256-S267Crossref PubMed Scopus (195) Google Scholar]. The prevalence of ARHL increases with age, affecting >40% of people over 50 years old, rising to ~71% of people over 70 years [2.Action on Hearing Loss Hearing Matters. Action on Hearing Loss, 2015Google Scholar]. For most people this is a relatively unremarkable part of the ageing process (Box 1), but some individuals with ARHL experience effort and difficulties in understanding speech, hindering communication and socialisation [3.Cardin V. Effects of aging and adult-onset hearing loss on cortical auditory regions.Front. Neurosci. 2016; 10: 199Crossref PubMed Scopus (71) Google Scholar]. Increased listening effort may lead older adults to avoid social interaction, exacerbating loneliness and depression, and reducing well-being [4.Rutherford B.R. et al.Sensation and psychiatry: linking age-related hearing loss to late-life depression and cognitive decline.Am. J. Psychiatry. 2018; 175: 215-224Crossref PubMed Scopus (168) Google Scholar]. Recent research further shows that hearing loss is associated with cognitive decline and dementia [5.Uchida Y. et al.Age-related hearing loss and cognitive decline - the potential mechanisms linking the two.Auris Nasus Larynx. 2019; 46: 1-9Abstract Full Text Full Text PDF PubMed Scopus (118) Google Scholar,6.Livingston G. et al.Dementia prevention, intervention, and care.Lancet. 2017; 390: 2673-2734Abstract Full Text Full Text PDF PubMed Scopus (3078) Google Scholar]. However, although there is reasonable evidence for hearing loss as a marker for risk of cognitive decline, it is not yet clear whether there is a causal effect of hearing loss on cognitive decline. Collating the most recent evidence on how ARHL affects the brain provides valuable information on the possible underlying mechanisms and causal relationships between hearing loss, neural changes, and dementia.Box 1Defining ARHL in Terms of Hearing ThresholdsHearing thresholds are usually measured using pure-tone audiometry, which estimates the lowest detectable levels of pure tones at a range of frequencies. The pure-tone average (PTA) is the average of hearing-threshold levels at frequencies of 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz in the individual’s better ear. The World Health Organisation (WHO) defines the onset of mild hearing impairment as a PTA of >20 dB HL [85.World Health Organization Deafness and Hearing Loss. WHO, 2020Google Scholar]. Further hearing impairment categories are defined at subsequent 15 dB steps; a hearing threshold of >35 dB HL would quantify moderate hearing loss, >50 dB HL for moderately severe loss, >65 dB HL for severe loss, and >80 dB HL for profound hearing loss [86.Humes L.E. The World Health Organization’s hearing-impairment grading system: an evaluation for unaided communication in age-related hearing loss.Int. J. Audiol. 2019; 58: 12-20Crossref PubMed Scopus (39) Google Scholar]. A person with normal hearing can hear tones in the frequency range 500–4000 Hz presented at 20 dB HL or softer. ARHL presents following cumulative effects of ageing on the sensory system [87.Bowl M.R. Dawson S.J. Age-related hearing loss.Cold Spring Harb. Perspect. Med. 2019; 9a033217Crossref PubMed Scopus (98) Google Scholar] (Figure I).Pure-tone audiometry remains the primary, gold-standard method for quantifying ARHL in practice and research. It is employed to understand changes in cochlear function and structure. However, to understand hearing ability more generally, it is also necessary to evaluate an individual's ability to function and participate in daily life activities [76.Meyer C. et al.What is the international classification of functioning, disability and health and why is it relevant to audiology?.Semin. Hear. 2016; 37: 163-186Crossref PubMed Scopus (19) Google Scholar]. Pure-tone thresholds do not account well for speech comprehension, which is a major complaint in ARHL [75.Pichora-Fuller M.K. et al.Hearing, cognition, and healthy aging: social and public health implications of the links between age-related declines in hearing and cognition.Semin. Hear. 2015; 36: 122-139Crossref PubMed Scopus (58) Google Scholar]. There are numerous potential causes of damage to the peripheral and central auditory system, which can be categorised into various subtypes of ARHL. The damages can manifest not only in high-frequency threshold elevations but also in the perception of supra-threshold sounds [75.Pichora-Fuller M.K. et al.Hearing, cognition, and healthy aging: social and public health implications of the links between age-related declines in hearing and cognition.Semin. Hear. 2015; 36: 122-139Crossref PubMed Scopus (58) Google Scholar]. Hearing thresholds are usually measured using pure-tone audiometry, which estimates the lowest detectable levels of pure tones at a range of frequencies. The pure-tone average (PTA) is the average of hearing-threshold levels at frequencies of 500 Hz, 1000 Hz, 2000 Hz, and 4000 Hz in the individual’s better ear. The World Health Organisation (WHO) defines the onset of mild hearing impairment as a PTA of >20 dB HL [85.World Health Organization Deafness and Hearing Loss. WHO, 2020Google Scholar]. Further hearing impairment categories are defined at subsequent 15 dB steps; a hearing threshold of >35 dB HL would quantify moderate hearing loss, >50 dB HL for moderately severe loss, >65 dB HL for severe loss, and >80 dB HL for profound hearing loss [86.Humes L.E. The World Health Organization’s hearing-impairment grading system: an evaluation for unaided communication in age-related hearing loss.Int. J. Audiol. 2019; 58: 12-20Crossref PubMed Scopus (39) Google Scholar]. A person with normal hearing can hear tones in the frequency range 500–4000 Hz presented at 20 dB HL or softer. ARHL presents following cumulative effects of ageing on the sensory system [87.Bowl M.R. Dawson S.J. Age-related hearing loss.Cold Spring Harb. Perspect. Med. 2019; 9a033217Crossref PubMed Scopus (98) Google Scholar] (Figure I). Pure-tone audiometry remains the primary, gold-standard method for quantifying ARHL in practice and research. It is employed to understand changes in cochlear function and structure. However, to understand hearing ability more generally, it is also necessary to evaluate an individual's ability to function and participate in daily life activities [76.Meyer C. et al.What is the international classification of functioning, disability and health and why is it relevant to audiology?.Semin. Hear. 2016; 37: 163-186Crossref PubMed Scopus (19) Google Scholar]. Pure-tone thresholds do not account well for speech comprehension, which is a major complaint in ARHL [75.Pichora-Fuller M.K. et al.Hearing, cognition, and healthy aging: social and public health implications of the links between age-related declines in hearing and cognition.Semin. Hear. 2015; 36: 122-139Crossref PubMed Scopus (58) Google Scholar]. There are numerous potential causes of damage to the peripheral and central auditory system, which can be categorised into various subtypes of ARHL. The damages can manifest not only in high-frequency threshold elevations but also in the perception of supra-threshold sounds [75.Pichora-Fuller M.K. et al.Hearing, cognition, and healthy aging: social and public health implications of the links between age-related declines in hearing and cognition.Semin. Hear. 2015; 36: 122-139Crossref PubMed Scopus (58) Google Scholar]. This review discusses the physiology of ARHL, from the peripheral auditory system to the auditory cortex, and to global neural changes that accompany ARHL. We focus on the impact of these cortical changes on cognitive functioning during ageing, and explore the evidence for a possible causal relationship between ARHL-related changes in neural functioning and cognitive decline. ARHL is attributed to sensory, metabolic, or neural changes in the peripheral auditory system which affect hearing ability. Sensory ARHL is characterised by degeneration of outer and inner hair cells within the cochlea, of which the inner cells are responsible for the transduction of auditory signals. Atrophy originates in the basal end of the cochlea, and over time progresses to the apex. Basal atrophy manifests in the high-frequency hearing loss typical of sensory ARHL [7.Liberman M.C. Kujawa S.G. Cochlear synaptopathy in acquired sensorineural hearing loss: manifestations and mechanisms.Hear. Res. 2017; 349: 138-147Crossref PubMed Scopus (371) Google Scholar]. It has been suggested that degeneration of basal sensory receptor cells is often a consequence of accumulated environmental noise exposure rather than of ageing [8.Pichora-Fuller M.K. MacDonald E. Sensory aging: hearing.in: Reference Module in Neuroscience and Biobehavioral Psychology. Elsevier, 2017Crossref Scopus (1) Google Scholar]. Sensory ARHL is quantifiable using pure-tone audiometry. The audiogram showing sensory ARHL will display normal hearing thresholds in the lower frequencies, and a steep increase in thresholds at higher frequencies [9.Dubno J.R. et al.Classifying human audiometric phenotypes of age-related hearing loss from animal models.J. Assoc. Res. Otolaryngol. 2013; 14: 687-701Crossref PubMed Scopus (108) Google Scholar]. However, older adults with similar pure-tone thresholds can differ in their ability to understand degraded speech, even after the effects of age are controlled for [10.Vermiglio A.J. et al.The relationship between high-frequency pure-tone hearing loss, hearing in noise test (HINT) thresholds, and the articulation index.J. Am. Acad. Audiol. 2012; 23: 779-788Crossref PubMed Scopus (63) Google Scholar]. The effect of ARHL on the wider auditory periphery, auditory cortices, and nonauditory neural systems has a greater effect on communication owing to increased difficulty with speech perception. Metabolic (or strial) ARHL is characterised by atrophy of the stria vascularis, on the outer wall of the cochlear duct, which is responsible for metabolic processes in the cochlea. Degeneration of this structure decreases the endocochlear potential (EP), impairing the EP-dependent cochlear amplifier. The entire cochlea is affected, but the amplifier in particular is necessary for the perception of high-frequency sounds [11.Keithley E.M. Pathology and mechanisms of cochlear aging.J. Neurosci. Res. 2019; (Published online May 7, 2019. https://doi.org/10.1002/jnr.24439)Crossref PubMed Scopus (72) Google Scholar]. The audiogram in metabolic ARHL displays a constant hearing loss at lower frequencies, with a gradual increase in threshold at higher frequencies owing to the loss of EP [9.Dubno J.R. et al.Classifying human audiometric phenotypes of age-related hearing loss from animal models.J. Assoc. Res. Otolaryngol. 2013; 14: 687-701Crossref PubMed Scopus (108) Google Scholar,12.Frisina R.D. et al.Age-related hearing loss: prevention of threshold declines, cell loss and apoptosis in spiral ganglion neurons.Aging. 2016; 8: 2081-2099Crossref PubMed Scopus (53) Google Scholar]. The flat loss at lower frequencies and gradual sloping loss at higher frequencies in metabolic ARHL, compared with the normal lower-frequency thresholds and drastic sloping loss at higher frequencies in sensory ARHL, is key in differentiating between these two subtypes of hearing loss [9.Dubno J.R. et al.Classifying human audiometric phenotypes of age-related hearing loss from animal models.J. Assoc. Res. Otolaryngol. 2013; 14: 687-701Crossref PubMed Scopus (108) Google Scholar]. Neural ARHL is characterised by atrophy of the spiral ganglion cells, the first afferent neurons in the neural pathway from the ear to the brain. The audiogram is not affected until a critical number of cells have degenerated (80–90%) [13.Wu P.Z. et al.Primary neural degeneration in the human cochlea: evidence for hidden hearing loss in the aging ear.Neuroscience. 2019; 407: 8-20Crossref PubMed Scopus (178) Google Scholar]. This type of hearing loss may precede sensory hair loss and is accompanied by a dramatic decrease in speech discrimination ability [14.Liberman M.C. Noise-induced and age-related hearing loss: new perspectives and potential therapies.F1000Research. 2017; 6: 927Crossref PubMed Scopus (140) Google Scholar]. This neural degeneration may provide insight into why older adults with similar hearing acuity (measured by pure-tone audiometry) differ in their speech-in-noise perception [15.Lopez-Poveda E.A. Why do I hear but not understand? Stochastic under sampling as a model of degraded neural encoding of speech.Front. Neurosci. 2014; 8: 348Crossref PubMed Scopus (61) Google Scholar] (Figure 1). Auditory perception involves not only peripheral ‘hearing’ and the transduction of sounds but also decoding and comprehension of the auditory message, which occurs in higher brainstem and cortical regions. Studies suggest that ageing may impact on supra-threshold auditory processes (which cannot be identified by a clinical audiogram), including temporal coding, which involves the synchronisation of neural firing to the temporal fine structure or temporal envelope of sound [16.Plack C.J. et al.Perceptual consequences of 'hidden' hearing loss.Trends Hear. 2014; 18: 1-11Google Scholar]. Animal models suggest that this temporal coding may be affected by age-related cochlear synaptopathy – the loss of connections between the sensory hair cells and the auditory nerve [17.Parthasarathy A. Kujawa S.G. Synaptopathy in the aging cochlea: characterizing early-neural deficits in auditory temporal envelope processing.J. Neurosci. 2018; 38: 7108-7119Crossref PubMed Scopus (87) Google Scholar]. Brainstem temporal processing may also decline owing to age-related demyelination [18.Peters A. The effects of normal aging on myelinated nerve fibers in monkey central nervous system.Front. Neuroanat. 2009; 3: 11Crossref PubMed Scopus (136) Google Scholar] and a reduction in neural inhibition [19.Caspary D.M. et al.Age-related changes in the inhibitory response properties of dorsal cochlear nucleus output neurons: role of inhibitory inputs.J. Neurosci. 2005; 25: 10952-10959Crossref PubMed Scopus (131) Google Scholar]. Brainstem neural function can be measured using the auditory brainstem response (ABR), a measure of synchronous activation of successive nuclei within the auditory pathway in response to a brief click or tone. Amplitudes of ABR waves are reduced in older listeners [20.Grose J.H. et al.Age-related change in the auditory brainstem response and suprathreshold processing of temporal and spectral modulation.Trends Hear. 2019; 23: 1-11Google Scholar]. The frequency-following response (FFR) is a sustained brainstem potential that reflects neural synchronisation to the frequency components of a sound wave. The FFR can be used to measure the temporal precision of subcortical neural coding of musical pitch and speech [21.Schoof T. Rosen S. The role of age-related declines in subcortical auditory processing in speech perception in noise.J. Assoc. Res. Otolaryngol. 2016; 17: 441-460Crossref PubMed Scopus (24) Google Scholar]. Research has demonstrated stronger FFR responses in younger compared with older listeners in response to speech stimuli [22.Presacco A. et al.Speech-in-noise representation in the aging midbrain and cortex: effects of hearing loss.PLoS One. 2019; 14e0213899Crossref PubMed Scopus (46) Google Scholar,23.Roque L. et al.Effects of age, cognition, and neural encoding on the perception of temporal speech cues.Front. Neurosci. 2019; 13: 749Crossref PubMed Scopus (31) Google Scholar], particularly speech in noise [24.Presacco A. et al.Evidence of degraded representation of speech in noise, in the aging midbrain and cortex.J. Neurophysiol. 2016; 116: 2346-2355Crossref PubMed Scopus (106) Google Scholar]. It is possible that age-related supra-threshold temporal processing deficits in the brainstem and midbrain account in part for the speech-in-noise perception difficulties facing older listeners, which are not well predicted by pure-tone audiometry [10.Vermiglio A.J. et al.The relationship between high-frequency pure-tone hearing loss, hearing in noise test (HINT) thresholds, and the articulation index.J. Am. Acad. Audiol. 2012; 23: 779-788Crossref PubMed Scopus (63) Google Scholar]. When the auditory periphery is damaged, the cochlea is less effective in converting sound into neural activity. A reduction in the precision of subcortical neural coding can also impact on the representation of sounds. The resultant auditory signal is therefore diminished, and this may significantly affect how the brain processes this information. One might hypothesise that this altered neural processing may in turn affect nonauditory cognitive processes as a result of atrophy, or cortical reorganisation, changing the way in which resources in the brain are allocated during perception and comprehension of speech. The auditory cortex encompasses several brain regions in the temporal lobes which are organised in a functional hierarchy for the processing of sound. The primary auditory cortex, at the bottom of this functional hierarchy located on Heschl's gyrus, receives direct information from the cochlea via the ascending auditory pathway. The wider auditory cortex, extending from Heschl's gyrus to the superior temporal gyrus, receives projections from the primary auditory cortex and is involved (among other functions) in sound localisation, as well as in integration with other sensory networks. Evidence indicates that older adults with hearing loss show a constellation of changes in primary auditory cortex. For example, dysfunctional neurotransmission as a result of decreased GABA levels has been observed in older adults with hearing loss compared with individuals with normal hearing [25.Gao F. et al.Decreased auditory GABA+ concentrations in presbycusis demonstrated by edited magnetic resonance spectroscopy.Neuroimage. 2015; 106: 311-316Crossref PubMed Scopus (51) Google Scholar]. However, there is evidence for a general age-related decline in GABA concentration in the auditory cortex that is independent of hearing loss [26.Lalwani P. et al.Neural distinctiveness declines with age in auditory cortex and is associated with auditory GABA levels.Neuroimage. 2019; 201: 116033Crossref PubMed Scopus (33) Google Scholar]. As well as potential defective neurotransmission, there is evidence that diminished grey matter volume in the primary auditory cortex is associated with poorer hearing [27.Alfandari D. et al.Brain volume differences associated with hearing impairment in adults.Trends Hear. 2018; 22: 1-8Google Scholar]. However, global decreases in grey matter volume, as well as cortical thinning and increased cerebrospinal fluid, are neural characteristics of general ageing [28.Fjell A.M. et al.One-year brain atrophy evident in healthy aging.J. Neurosci. 2009; 29: 15223-15231Crossref PubMed Scopus (441) Google Scholar,29.Lockhart S.N. DeCarli C. Structural imaging measures of brain aging.Neuropsychol. Rev. 2014; 24: 271-289Crossref PubMed Scopus (150) Google Scholar]. An important question is whether deprivation of auditory input caused by ARHL exacerbates the brain atrophy typical of ageing, and whether this has consequences for cortical organisation. Studies provide evidence for a link between changes in brain morphology and ARHL (assessed using audiometric thresholds), including cortical thinning [30.Giroud N. et al.The impact of hearing aids and age-related hearing loss on auditory plasticity across three months – an electrical neuroimaging study.Hear. Res. 2017; 353: 162-175Crossref PubMed Scopus (29) Google Scholar] and reduced grey matter volume in the auditory cortices [31.Rigters S.C. et al.Hearing impairment is associated with smaller brain volume in aging.Front. Aging Neurosci. 2017; 9: 2Crossref PubMed Scopus (40) Google Scholar,32.Ren F. et al.Gray matter atrophy is associated with cognitive impairment in patients with presbycusis: a comprehensive morphometric study.Front. Neurosci. 2018; 12: 744Crossref PubMed Scopus (29) Google Scholar]. There are two proposed explanations for the changes in brain morphology in older adults who display age-related hearing threshold elevations. The first is that there is a direct causal relationship between auditory impairment and declines in brain volume owing to auditory deprivation (sometimes referred to as the auditory deprivation hypothesis) [32.Ren F. et al.Gray matter atrophy is associated with cognitive impairment in patients with presbycusis: a comprehensive morphometric study.Front. Neurosci. 2018; 12: 744Crossref PubMed Scopus (29) Google Scholar]. The second is that ageing leads to concurrent declines in the auditory periphery and the CNS [33.Eckert M.A. et al.Age-related hearing loss associations with changes in brain morphology.Trends Hear. 2019; 23: 1-14Google Scholar,34.Peelle J.E. Wingfield A. The neural consequences of age-related hearing loss.Trends Neurosci. 2016; 39: 486-497Abstract Full Text Full Text PDF PubMed Scopus (140) Google Scholar]. One longitudinal study provides evidence supporting the idea of a causal relationship between ARHL [quantified as a pure-tone average (PTA, see Glossary) of >25 dB hearing level (HL) in older adult participants] and neural atrophy in support of the auditory deprivation hypothesis. Differences in brain volume between older adults with normal versus clinically significant pure-tone hearing loss were not present in a baseline magnetic resonance imaging (MRI) scan. However, 6.4 years later, those with pure-tone hearing loss showed an accelerated decline in brain volume, especially in the right temporal lobe [35.Lin F.R. et al.Association of hearing impairment with brain volume changes in older adults.Neuroimage. 2014; 90: 84-92Crossref PubMed Scopus (270) Google Scholar]. Nevertheless, others have contested the auditory deprivation hypothesis. Indeed, a more recent longitudinal study found no evidence that clinically significant pure-tone hearing loss affected brain morphology [33.Eckert M.A. et al.Age-related hearing loss associations with changes in brain morphology.Trends Hear. 2019; 23: 1-14Google Scholar]. These inconsistent findings could be explained by the different longitudinal time-windows employed – 6.4 years in the former study compared with a shorter window ranging from ~1.3 to 5 years in the latter. It is possible that there is a causal relationship between clinically significant pure-tone hearing loss and reduced grey matter in the auditory cortex, but only presents after a longer time-period (>5 years). In addition to structural changes in the cortex, older adults with clinically significant pure-tone hearing loss also display functional differences in auditory processing compared with younger adults with normal pure-tone thresholds. For example, functional MRI (fMRI) studies to determine age-related changes in the auditory cortex showed that the older adults with pure-tone threshold elevations exhibited increased activation in response to pink noise (i.e., 1/f noise) in the temporal lobes, particularly in the right hemisphere, compared with younger adults with normal audiometric thresholds who showed reduced activation and left lateralisation [36.Profant O. et al.Functional changes in the human auditory cortex in ageing.PLoS One. 2015; 10e0116692Crossref PubMed Scopus (37) Google Scholar]. The authors suggested that this activation may be due to reduced inhibition associated with ageing, or potentially reflects a compensatory mechanism for elevated audiometric thresholds [36.Profant O. et al.Functional changes in the human auditory cortex in ageing.PLoS One. 2015; 10e0116692Crossref PubMed Scopus (37) Google Scholar]. However, there were no significant differences in activation between older adults with mild (audiometric thresholds >20 dB HL at frequencies ≥4000 Hz) versus moderate (audiometric thresholds >20 dB HL at frequencies ≥1000 Hz) pure-tone hearing loss. The lack of an effect of hearing-loss severity on neural activity may cast doubt on the existence of a causal relationship between pure-tone hearing loss and neural changes. Other researchers who used more complex auditory stimuli, consisting of monosyllabic words, also found similar effects of age on auditory cortex activity, but age-related pure-tone hearing loss (PTA 26–40 dB HL) did not significantly affect activation [37.Chen X. et al.Language processing of auditory cortex revealed by functional magnetic resonance imaging in presbycusis patients.Acta Otolaryngol. 2016; 136: 113-119Crossref PubMed Scopus (5) Google Scholar]. These data can be interpreted to support the theory that general ageing, or indeed other subtypes of hearing loss not identified by the audiogram, rather than clinically significant pure-tone hearing loss, leads to functional changes in the auditory cortex. The perception, and particularly comprehension, of auditory information is reliant on integration among brain networks to interpret auditory stimuli. Studies have found important differences in functional connectivity among brain areas involved in auditory processing in older adults with ARHL, which may hinder speech perception [38.Wolak T. et al.Altered functional connectivity in patients with sloping sensorineural hearing loss.Front. Hum. Neurosci. 2019; 13: 284Crossref PubMed Scopus (15) Google Scholar]. Specifically, findings show reduced connectivity between visual and auditory sensory cortices in ARHL [39.Puschmann S. Thiel C.M. Changed crossmodal functional connectivity in older adults with hearing loss.Cortex. 2017; 86: 109-122Crossref PubMed Scopus (33) Google Scholar], as well as in the attention and default mode networks [40.Husain F.T. Schmidt S.A. Using resting state functional connectivity to unravel networks of tinnitus.Hear. Res. 2014; 307: 153-162Crossref PubMed Scopus (140) Google Scholar]. These data suggest that, in individuals with hearing loss, there are changes in the organisation of the cortical networks that support speech perception. In the following section of this review cortical reorganisation observed in ARHL is explored further. This section focuses on three brain networks that are known to support auditory perception – the attentional, visual, and motor networks. Evidence indicates that ARHL not only affects auditory brain areas but also nonauditory regions. This is because nonauditory regions are potentially upregulated to support speech perception after hearing loss. It is possible that this suggested reorganisation of resources causes complications for cognitive and neural functioning. The cingulo-opercular network is suggested to be of importance for speech processing in both normal-hearing and hearing-impaired individuals [41.Vaden K.I. et al.Cortical activity predicts which old