The Brain and HearingThe brain plays a vital role in hearing but how can we unlock its mysteries and maximise its potential?

Professor Catherine McMahon, at Macquarie University’s Hearing CRC, investigated.

The world’s population is rapidly ageing due to longer life expectancies and declining fertility rates, particularly in the developed world. By 2050, for the first time in history, there will be more people aged 60 or over than those 15 and under. The United Nations World Population Ageing Report predicts the number of people 60-plus will double from 11.7 per cent in 2013 to 21.1 per cent by 2050 meaning there will be over two billion older adults. This provides a major challenge for developing financially sustainable policies and systems to maximise the health and social wellbeing of older people.

There is increasingly good evidence that suggests as we age, sensory and cognitive abilities decline. This can lead to reduced independence, increased reliance on and use of health care and community services, and a higher risk of death. As recent studies show, the brain and ears are intertwined in this process. For example, researchers from America’s John Hopkins Medical Centre discovered that people with mild hearing loss have a two-fold increased risk of acquiring dementia compared to those with normal hearing. This statistic spirals to five times more likely for people with severe hearing loss. Furthermore, an Australian study in older adults showed that hearing loss is associated with an increased risk of mortality. This was supported by the findings of John Hopkins researchers, who discovered the greater a person’s hearing impairment the greater their risk of death (as reported in the medical journal JAMA).

Catherine McMahon

Associate Professor Catherine McMahon

Such declines are typically associated with reductions in brain volume and changes to brain and brainstem neural wiring patterns. For example, advanced cases of Alzheimer’s disease, the most commonly known type of dementia, show brain shrinkage and neural cell death while adult-onset hearing loss is associated with brain volume and white matter changes.

But it is not all doom and gloom – contrary to outdated ideas that the brain was fixed and unable to be rewired from a very young age – it is now known that it is an incredible, ever-changing organ that complements our hearing, among many other senses, throughout our lives. In addition, humans are resourceful and are able to find ways to adapt to reductions in sensory and cognitive information, despite significant brain changes. For example, the brain changes that occur in Alzheimer’s disease might occur approximately 20 years before symptoms of memory loss start to show.

In the case of communication, while cognitive abilities decline from 20 years of age, knowledge of language and worldly experience increase with age, so older adults are able to use this knowledge and context to compensate for a reduced cognitive and auditory function to make sense of what is being said.
The ability of the human brain to adapt to change is known as neuroplasticity. Neuroplasticity can be beneficial, where brain function can be enhanced with increased use. However, sometimes it can also be detrimental. When there is a disconnect between the ear and brain, things can go wrong. Following are three auditory problems that highlight the interactions that occur between the ear and the brain: cross-modal plasticity; tinnitus; and auditory processing disorder.

CROSS-MODAL PLASTICITY
In people with severe-profound hearing loss, the auditory part of the brain can be partly taken over by the visual system through a process of competitive advantage known as cross-modal plasticity. Certainly, studies have identified that congenitally deaf adults show superior visual attention to hearing people. However, the question that researchers are seeking to address is whether cross-modal plasticity can limit the ability of individuals to perceive speech when audition is restored, such as through a cochlear implant. There is some research to suggest that this is the case. Researchers in Hanover, Germany, showed that visual patterns presented to post-lingually deafened cochlear implant users activate both visual and auditory centres in the brain. Importantly, they showed that people with a more active auditory cortex from visual stimuli had poorer speech perception abilities.

TINNITUS
Another adverse outcome of deafness is tinnitus – the ringing in the ear. Scientists now believe that tinnitus is caused by increased neural activity in the brain, often after the ear is damaged after exposure to loud noise. Tinnitus is likely to be a by-product of the brain’s effort to compensate for the reduced sound activity coming from the ear by turning up the volume as neural information is transmitted to the brain. Several animal and human studies of tinnitus have demonstrated that a reduction of neural input from the ear, can lead to enhanced activity at higher auditory centres – in the brainstem or the brain. Around 25 per cent of people who have tinnitus are said to be bothered by it. These people are likely to also show greater than average activity in the emotional (limbic) and stress (autonomic) systems of the brain.

AUDITORY PROCESSING DISORDER
Auditory processing disorder was originally considered a disorder of people with normal hearing. It remains a broad umbrella term that is diagnosed by behavioural tests and the causes of this are highly debated, but it results in considerable difficulties understanding speech in noisy environments. It is likely due to multiple disruptions to the auditory pathway, such as poor integration of signals from each ear (which is important for sound localisation), or being unable to detect a difference in sounds that are played close in time or close in pitch. It is now becoming clear that auditory processing disorders can occur at the same time as hearing loss, making it more difficult to understand speech in noisy environments.

THE IMPORTANCE OF RESEARCH
The healthy adult brain has over 100 billion neurones and 100 trillion neural connections (synapses). Therefore it will take a concerted effort from scientists, funders, and patients to understand how it normally works, how disease or injury can affect this, and how therapies can restore function. America’s National Institute of Health (NIH) has committed billions of dollars to develop techniques and technologies to explore how the brain works. Known as the BRAIN (Brain Research through Advancing Innovative Neurotechnologies) Initiative (see www.braininitiative.nih.gov), NIH has already allocated US$127 million since 2014 (around A$179m) to the project. High-resolution imaging techniques, such as functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG), enable scientists to assess the brain’s structural and functional integrity. These technologies provide insight for scientists to understand how rehabilitation techniques after damage or disease can restore the brain’s map.

In our own research, conducted through the HEARing Co-operative Research Centre program, we investigated how tinnitus affects the auditory brain of adults, and how treatment restores its function. In a 30-week treatment program of 12 adults with significant tinnitus, everyone with the condition showed a positive improvement. We compared the brain’s response to a range of pure tones. At 500Hz (the sound level where all participants, including 10 control subjects, had normal hearing), we compared its location and amplitude. Our results showed two main points.

Firstly, people with tinnitus had larger waveforms than non-tinnitus participants which did not change significantly throughout treatment. This suggests the brain’s auditory volume control in people with tinnitus is turned up higher than in people without it and that treatment was not effective in turning this down.

Secondly, the tonotopic map was disrupted in people with tinnitus, where the cerebral response to the 500Hz tone was located at the front area of the brain. However, this did shift towards a more normal location throughout the program. Brain plasticity in adults during rehabilitation from injury has been observed in other areas of healthcare. For example, studies in stroke rehabilitation show associations between reorganisation of the motor and sensory cortices and functional recovery of limb and hand movements. Understanding brain plasticity might enable new tools for intervention to be designed or fine-tuned for people in need.

The association between the sensory input received through the ear and the processing of this input by the brain has long been overlooked. For many years, the ability to perceive and understand speech has been considered to be a function of the ear and auditory pathways. However, recent research highlights the importance of the brain in this process, particularly in situations in which the auditory signal is degraded, either from background noise or from hearing loss. That is, when the signal is poor, the brain must work harder to understand what is said. This is now more commonly referred to as cognitive load, or listening effort. Anecdotal reports suggest that people with hearing loss are often fatigued by the end of the day, which probably results from maintaining high levels of concentration or effort to participate in conversations. For some, this could lead to social isolation. It might also explain why two people can obtain the same speech scores on a clinical test but only one believes that it reflects how they hear in everyday life. Therefore understanding the brain’s capacity and how this interacts with incoming auditory information is critical for developing more targeted therapies and signal processing strategies for hearing devices.

THE BRAIN AND HEARING AIDS
For over 10 years, we have known that the brain’s capacity could be useful in programming a hearing device. In 72 people with similar levels of hearing fitted with hearing aids, a study from Denmark (published in the International Journal of Audiology) demonstrated that an individual’s cognitive ability or capacity (measured by the reading span test) provided a good predictor of speech perception performance. The research team later demonstrated that adults with higher cognitive capacity are able to understand speech in noise better using fast wide-dynamic range compression algorithms than those with lower cognitive capacity.

Fast compression, also known as syllabic compression, was designed to provide the user with greater access to the speech signal within varying acoustic environments and is therefore typically used in programming hearing aids. Based on this research, scientists at Eriksholm Research Centre are now looking at how brain signals which measure aspects of cognition can be used to adaptively program a hearing aid. Importantly, there is now emerging
evidence from large population-based studies in the UK, US and France which shows that simply wearing hearing aids might be able to reduce the accelerated mental decline that occurs in adults with hearing loss.

In a study published by the American Geriatrics Association of 1,276 people with varying hearing difficulties and 2,394 with none, the ones with hearing loss, but not those wearing hearing aids, had greater cognitive decline over a 25-year period than control subjects. European studies have shown that people who need and wear auditory devices are also less depressed, less tired in the evening and find sleeping easier. Add to this the fact that many people take 10 years or more to decide whether to get hearing aids – hoping their condition will improve. But, in this time, the brain suffers by losing sounds it used to recognise so seeing an audiologist early is wise.

TRAIN YOUR BRAIN While it is important to understand how cognitive capacity affects speech perception and programming of devices, it is of considerable interest to know whether we can increase cognitive capacity or at least avoid the reductions that occur with age. Todd Sampson, who starred in the ABC science documentary
series, Redesign My Brain, reports that there are multiple factors that can enhance the brain’s capacity.

Certainly, there is now a substantial body of evidence to show that the most important factors include good nutrition, exercise and having sufficient sleep. However, there is considerable interest in the role of brain training to enhance or maintain brain function and brain health. There are numerous online computer based brain-training programs that are available.

At this stage, most studies have demonstrated improvement on the tasks that were trained, but a limited improvement on untrained tasks. Despite this, it remains a multi-million dollar industry. It is important to note that well designed, targeted training programs, used with the support of an audiologist, could show benefit for some people, such as those with hearing loss, on trained and untrained tasks. In any case, keeping the brain active in this way is great exercise for your grey matter.

This article was originally published in Hearing HQ Magazine Dec 15.