Neural Reorganization as a Consequence of Age-Related Sensory Decline

Lay Summary

Hearing loss may create brain changes that affect cognition

This study will investigate how common age-related hearing loss changes brain structure and function, and what consequences these neurobiological effects have on understanding and remembering speech.
Behind only arthritis and hypertension, hearing loss is the third most prevalent chronic condition among older adults. Approximately 80% of people over the age of 70 develop some degree of hearing loss, and this is not merely a sensory issue. Those with reduced hearing ability require more cognitive processing to understand speech, limiting what would normally be available for other cognitive functions. Since hearing loss can make conversation frustrating, some people reduce their social activity, feel socially excluded, and may develop depression. Epidemiological evidence increasingly suggests links between hearing loss and cognitive decline or dementia.

This suggested link may occur because speech degradation and semantic ambiguity from impoverished acoustic signals place increased demands on executive functioning, impairing memory and cognitive performance. The poorer speech comprehension in those with hearing impairments can be explained in part by changes in brain structure and function that occur as a result of hearing loss.

To better understand the relationship between hearing ability and brain function, 80 older adults with mild hearing loss will be imaged by structural MRI scans, with a subset undergoing high-density diffuse optical tomography (HD-DOT) as they undergo auditory behavioral tests. HD-DOT, like functional MRI, measures blood flow to active brain regions. Unlike MRI, though, HD-DOT is completely silent. Investigators will conduct three experiments, examining individual differences in four key areas: hearing ability, brain structure, functional brain connectivity, and speech-related brain activity.

In Experiment 1, participants will hear short stories that have been acoustically degraded but are still intelligible. Following the stories participants’ memory for the content will be tested to assess the degree to which acoustic challenge might interfere with memory.

In Experiment 2, participants will listen to spoken sentences that may contain ambiguous words (for example the word “bark”, which could be the sound of a dog or the covering of a tree). Again, memory for sentences and key words will be tested to see whether acoustic challenge interferes with language processing and memory.

Experiment 3 will directly assess brain activity during speech comprehension using HD-DOT. The results will test the hypotheses that, like young adults, older adults show an increased reliance on the frontal cortex when speech is degraded, and that when compared individually, better performers will show increased frontal cortex activity.

Significance: Understanding how hearing loss impacts brain structure and function has the potential to lead to innovations that are designed to further improve everyday communication for millions of people and to preserve overall cognitive function well into older age.

Abstract

Neural Reorganization as a Consequence of Age-Related Sensory Decline

Normal aging is frequently associated with some degree of hearing loss, affecting more than 80% of adults over the age of 70 years. Of special interest is the increasing realization that hearing loss increases listeners’ reliance on executive processes in order to extract meaning from an impoverished acoustic signal. This includes greater dependence on short-term memory, decision-making skills, and linguistic context. As a result, poorer hearing requires increased perceptual effort as listeners struggle to comprehend spoken language. However, the degree to which hearing loss impacts perceptual effort and concomitant brain activity, and whether hearing loss has longer-term effects on brain structure and organization, are still open questions. In this project we investigate how hearing loss impacts brain structure and function, and the consequences of these neurobiological effects for speech processing. We will test 80 adults over the age of 65 years with hearing abilities that range from normal to a moderate hearing impairment; none will wear a hearing aid. All participants will be audiometrically tested, and participate in MRI scanning and two behavioral tasks. MRI scanning will include structural scans (T1, T2, and DTI) and resting state functional connectivity. The first behavioral task will involve listening to short stories (~30 seconds) presented as either unprocessed speech or speech that was degraded through a noise vocoding process that leaves speech intelligible, but increases perceptual effort. Subjects will be asked to provide either immediate or delayed recall of the main ideas of the narrative (i.e., a gist-based scoring procedure). The second behavioral task will involve listening to sentences that contain ambiguous words, known to increase reliance on frontotemporal semantic systems during comprehension. For example, the word “bark” could refer to the covering of a tree or the sound a dog makes, and listeners must engage additional cognitive resources to (unconsciously) determine which meaning is appropriate. Subjects will listen to these sentences and sentences that do not contain ambiguous words, both as unprocessed speech or degraded speech. Subsequently subjects will be tested on both explicit and implicit memory for the sentences. Finally, a subset of participants will perform task-based functional imaging using high-density diffuse optical tomography (HD-DOT), which permits the measurement of localized brain response in the absence of background noise. We hypothesize that acoustic challenge (speech degradation) and linguistic challenge (semantic ambiguity) will both place increased demands on executive processes, impairing memory performance for degraded speech compared to clear speech. We predict this effect will be exaggerated in listeners with worse hearing. We will use both behavioral and audiometric measures to analyze the imaging data; we expect that individual differences in brain structure and functional connectivity will mediate the effect of hearing loss on behavioral performance. These results will demonstrate the widespread cognitive effects of hearing loss and pave the way for interventions that can facilitate communication.