Sensory consequences of ageing on the neurobiology of speech perception

By the age of 70 years, 71% of people experience some degree of hearing loss, which can cause severe changes to quality of life. We have all experienced the frustration of struggling to understand speech in background noise. This is particularly true for older adults with hearing loss, who are more likely to disengage from social interactions that take place in noisy environments, increasing social isolation and decreasing mental wellbeing. Indeed, several studies have found a significant relation between hearing loss and reduced brain health. We need new knowledge of how age-related hearing loss affects the brain in order to develop effective interventions.

Hearing loss usually reflects a problem with hair cells in the ear, which degrades how the speech signal is broken down by the ear. This means that a degraded signal is subsequently sent to the brain. Our brains then have to work hard to tune in on the speech and understand the message. This is a bit like using a tuning dial to tune into a radio station. However, we do not know how our brain's tuning ability is affected by hearing loss. We need to know this in order to develop effective interventions for age-related hearing loss.

My first research question is to find out if the higher-level auditory areas of the brain help to tune the way speech is represented in lower-level areas of the auditory system. Knowing whether this first tuning mechanism is broken or functioning will help to clarify what type of computer-based auditory training program will work best to help people with hearing loss.

My second research question is to find out if higher-level motor areas of the brain help to tune the way speech is represented in higher-level auditory areas. Knowing whether this second tuning mechanism is broken or functioning will help to clarify whether intervention strategies should be geared towards supporting remaining auditory resources, or additionally towards supporting compensatory motor resources, such as articulation training to strengthen the link between speech motor pathways and auditory information.

My third research question is to find out whether we can improve a tuning mechanism linked to attention that helps us to keep focused on speech and ignore background sounds in noisy places. Knowing whether we can improve this third tuning mechanism is important because if we can improve it, it could represent a new intervention pathway for age-related hearing loss.

To answer these questions, I will combine three major lines of enquiry. I will use brain stimulation to manipulate how the brain is working. For example, I will use brain stimulation to increase or decrease activity in the higher-level hearing areas of the brain. Brain stimulation works by applying magnetic pulses to the brain, which can temporarily, and safely, affect how the brain cells work together in a particular area of the brain. I will use brain imaging to take a picture of how electrical activity in the brain has been affected by the brain stimulation. I will lastly use behavioural tasks to record people's ability to hear and recognise speech after the brain stimulation. By comparing brain imaging and behaviour before and after the stimulation, I will be able to tell if stimulation of higher-level areas of the brain affects tuning (using brain imaging), and behaviour (using hearing and listening tests) in people with age-related hearing loss.

This project will inform and guide future intervention strategies for age-related hearing loss by determining how use of cortical systems involved in speech perception changes after hearing loss, using a novel combination of brain stimulation, electroencephalography, and behavioural tests. We will investigate three cortical resources and determine their influence on auditory encoding and speech perception following age-related hearing loss. First, we will determine the influence of top-down pathways from auditory cortex on the subcortical representation of speech after age-related hearing loss, in order to establish if top-down auditory pathways are spared or compromised. This is important because it will provide direction for how to maximise efficacy of computer-based auditory training protocols aimed at improving speech perception abilities after age-related hearing loss. Second, we will determine the relation between auditory cortex and premotor cortex after age-related hearing loss, in order to establish if encoding of speech in auditory cortex is complemented by top-down motor networks during speech perception. This is important because it will provide direction for whether intervention strategies should be directed towards supporting remaining auditory resources, or towards supporting additional compensatory motor resources, such as articulation training to strengthen auditory-motor linkages. Third, we will determine if top-down neural alpha oscillations associated with attentive perception can be facilitated by brain stimulation, in order to establish if oscillatory alpha facilitation can assist speech perception in adults with age-related hearing loss. This is important because if alpha oscillations associated with attention can be strengthened, this could represent a new intervention pathway for age-related hearing loss.

Although this project is primarily a basic science project, the work has the potential to have impact in a number of fields. The main non-academic beneficiaries of this research will be clinical practitioners working in audiology and speech and language therapy (SLTs), and industry developers of auditory and neuroscientific technology. In addition, a better understanding of the effect of ageing on speech perception is of wider relevance to the general public, as difficulties hearing in background noise are experienced by many people in their day-to-day lives.

Audiologists: It is well-documented that understanding speech in noisy environments is a major cause of difficulty for listeners suffering with age-related hearing loss. Although standardised audiometric tests of speech perception in noise exist, they are limited by patient compliance, attention, and engagement, and are therefore not suitable for obtaining accurate estimates from all individuals. Similarly, the subjective nature of questionnaires which ask an individual to rate their auditory disability in different situations may not be able to capture responses from all clients, and can potentially lead to incorrect assessments in those able to respond. The findings of this project will provide audiologists with a better understanding of neurobiological auditory and motor system factors influencing hearing following age-related hearing loss, and how they may use findings as potential biomarkers.

SLTs: Although a wide range of instruments are available to SLTs for evaluating speech production and perception abilities in their clients in all age groups, these instruments do not directly test the ability to listen and communicate in an ecologically-valid way. Our method of recording subcortical responses to speech in speech noise has potential as such a tool, which could be of benefit to SLTs in diagnosing and managing speech and language conditions. The findings of the project will also provide SLTs with a better understanding of the role of the motor system in hearing loss, and how this finding might be used to inform articulation training tuition as a compensatory strategy for hearing loss.

Developers of auditory technology: Auditory training programs are available through computers, the internet, and smartphones. The findings of this project will provide guidance as to which types of auditory stimuli could be maximally effective in such training programs, i.e., syllables versus sentences, and whether auditory information should be complemented by articulatory information. Self-management of hearing loss through such technology can provide more patient-centered care and potentially improve retention and satisfaction with hearing aid devices.

Developers of neuroscientific technology: There is great current interest in integrating TMS with EEG. Currently, there is only one EEG system capable of recording subcortical brainstem responses that is TMS-compatible. Gaining a better understanding of how TMS impacts subcortical EEG will be of great interest to technology developers. There is therefore great scope for this project to influence i) future TMS compatibility of subcortical EEG systems, and ii) TMS-compatible cortical EEG systems to be extended to subcortical capabilities (please see Supporting Statement 2).

Wider public: A large proportion of the population have direct or indirect experience of age-related hearing loss, and will have experienced difficulty listening in noisy environments. A better understanding of the effect of ageing on hearing could lead to simple adjustments being made to reduce listening difficulties around the home and in social settings.