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.

It has become increasingly evident that human motor circuits are active during speech perception. However, the conditions under which the motor system modulates speech perception are not clear. Two prominent accounts make distinct predictions for how listening to speech engages speech motor representations. The first account suggests that the motor system is most strongly activated when observing familiar actions (Pickering and Garrod, 2013). Conversely, Wilson and Knoblich's account asserts that motor excitability is greatest when observing less familiar, ambiguous actions (Wilson and Knoblich, 2005). We investigated these predictions using transcranial magnetic stimulation (TMS). Stimulation of the lip and hand representations in the left primary motor cortex elicited motor evoked potentials (MEPs) indexing the excitability of the underlying motor representation. MEPs for lip, but not for hand, were larger during perception of distorted speech produced using a tongue depressor, relative to naturally produced speech. Additional somatotopic facilitation yielded significantly larger MEPs during perception of lip-articulated distorted speech sounds relative to distorted tongue-articulated sounds. Critically, there was a positive correlation between MEP size and the perception of distorted speech sounds. These findings were consistent with predictions made by Wilson & Knoblich (Wilson and Knoblich, 2005), and provide direct evidence of increased motor excitability when speech perception is difficult.

Multisensory integration is essential for the quick and accurate perception of our environment, particularly in everyday tasks like speech perception. Research has highlighted the importance of investigating bottom-up and top-down contributions to multisensory integration and how these change as a function of ageing. Specifically, perceptual factors like the temporal binding window and cognitive factors like attention and inhibition appear to be fundamental in the integration of visual and auditory information—integration that may become less efficient as we age. These factors have been linked to brain areas like the superior temporal sulcus, with neural oscillations in the alpha-band frequency also being implicated in multisensory processing. Age-related changes in multisensory integration may have significant consequences for the well-being of our increasingly ageing population, affecting their ability to communicate with others and safely move through their environment; it is crucial that the evidence surrounding this subject continues to be carefully investigated. This review will discuss research into age-related changes in the perceptual and cognitive mechanisms of multisensory integration and the impact that these changes have on speech perception and fall risk. The role of oscillatory alpha activity is of particular interest, as it may be key in the modulation of multisensory integration.

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.