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.The Ageing Ear: Tired of Listening? 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]. The prevalence of ARHL increases with age, affecting N40% of people over 50 years old, rising to~71% of people over 70 years [2]. 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]. Increased listening effort may lead older adults to avoid social interaction, exacerbating loneliness and depression, and reducing well-being [4]. Recent research further shows that hearing loss is associated with cognitive decline and dementia [5,6]. 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. 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.The Peripheral and Subcortical Auditory Systems in Age-Related Hearing Loss 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]. It has b...

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Excitability of articulatory motor cortex is facilitated when listening to speech in challenging conditions. Beyond this, however, we have little knowledge of what listener-specific and speech-specific factors engage articulatory facilitation during speech perception. For example, it is unknown whether speech motor activity is independent or dependent on the form of distortion in the speech signal. It is also unknown if speech motor facilitation is moderated by hearing ability. We investigated these questions in two experiments. We applied transcranial magnetic stimulation (TMS) to the lip area of primary motor cortex (M1) in young, normally hearing participants to test if lip M1 is sensitive to the quality (Experiment 1) or quantity (Experiment 2) of distortion in the speech signal, and if lip M1 facilitation relates to the hearing ability of the listener. Experiment 1 found that lip motor evoked potentials (MEPs) were larger during perception of motor-distorted speech that had been produced using a tongue depressor, and during perception of speech presented in background noise, relative to natural speech in quiet. Experiment 2 did not find evidence of motor system facilitation when speech was presented in noise at signal-to-noise ratios where speech intelligibility was at 50% or 75%, which were significantly less severe noise levels than used in Experiment 1. However, there was a significant interaction between noise condition and hearing ability, which indicated that when speech stimuli were correctly classified at 50%, speech motor facilitation was observed in individuals with better hearing, whereas individuals with relatively worse but still normal hearing showed more activation during perception of clear speech. These findings indicate that the motor system may be sensitive to the quantity, but not quality, of degradation in the speech signal. Data support the notion that motor cortex complements auditory cortex during speech perception, and point to a role for the motor cortex in compensating for differences in hearing ability.

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