A normal result in a hearing test does not necessarily mean that the aging auditory system functions as it does for a younger person. Measurements from 105 people suggest that the nerve signal from the inner ear weakens with age, while the brain amplifies its responses to sound. Surprisingly, the two processes appear largely to develop in parallel rather than being directly linked.
You may be able to hear every tone in a standard hearing test and still lose track of the words when several people are speaking at once. The test says normal, but the world still sounds more muddled.
A new study suggests that the explanation may lie in several parts of the auditory system. With age, the signals from the ear become weaker, while the brain responds more strongly and has difficulty adapting to repeated sounds.
“Hidden hearing loss refers to the situation where you experience listening problems even though the hearing test indicates that your hearing is normal. One possible explanation is that some of the connections between the ear’s sensory cells and the auditory nerve have been weakened, but our results suggest that ageing in the brain may also play a role,” explains the study’s senior author, Jens Hjortkjær, Associate Professor in the Department of Health Technology at the Technical University of Denmark.
The researchers measured responses from the ear, the brainstem and the cerebral cortex of 105 people aged 18 to 77 years whose hearing was normal according to standard clinical criteria.
“The counterintuitive finding is that the signal from the ear becomes weaker with age, while the brain responds more strongly. If the brain were simply amplifying a reduced signal, we would expect the two changes to go hand in hand from person to person. They did so only to a limited extent,” says Jens Hjortkjær.
The study therefore raises the question of whether changes in several parts of the auditory system may contribute to problems that a standard hearing test does not detect.
A standard hearing test measures only part of hearing
When certain sensory cells in the inner ear are damaged, detecting faint sounds becomes more difficult. But the connections that pass information to the auditory nerve may weaken before hearing thresholds change noticeably.
“A standard hearing test measures the softest level of a tone you can hear. It tells us much less about how well the signal is transmitted from the inner ear to the brain,” notes Jens Hjortkjær.
The outer hair cells act as a biological amplifier, making faint sounds easier to detect. When they are damaged, a tone must be louder before it can be heard. The inner hair cells, by contrast, convert sound into nerve signals, which are passed through synapses to the auditory nerve.
“Because each inner hair cell transmits signals through many nerve fibres, some of these connections can be lost without affecting the ability to hear a faint tone. The audiogram may therefore look normal, even when fewer nerve fibres are carrying the signal,” adds Jens Hjortkjær.
Animal experiments revealed what a standard hearing test may miss
Animal experiments have shown that loud noise can damage the synapses between the inner hair cells and the auditory nerve without destroying the outer hair cells or making faint sounds harder to hear.
“This research showed that neural loss in the ear can occur without the standard form of hearing loss. It raised the question of how widespread this neural degeneration is in humans and what consequences it may have,” says Jens Hjortkjær.
Researchers cannot count the synapses or nerve fibres of living people directly. They therefore have to measure the electrical responses and reflexes that occur when the auditory nerve transmits sound signals.
What drives these changes remains unclear. Long-term exposure to noise is one possible explanation, but normal ageing and other biological processes may also play a role.
“We cannot determine from this study how much these changes in sound-evoked responses are due to a lifetime of noise exposure and how much is due to other ageing processes,” says Jens Hjortkjær.
The challenge is not always hearing the tone – but picking out the voice
An audiogram primarily tests the faintest tone a person can hear. Understanding speech in noise, by contrast, requires the auditory system to separate many simultaneous sounds and preserve the subtle differences in timing that make them recognisable.
“You may be able to hear a single tone in a quiet room and still struggle when many sounds occur at the same time. That is where we suspect that the loss of neural connections may matter,” emphasises Jens Hjortkjær.
In addition, the brain itself changes the way it processes sound as we age. The ageing brain often responds more strongly and finds it harder to suppress its response to repeated or irrelevant sounds.
This may be a way of compensating for a weaker signal from the ear, but it may also reflect more general age-related changes in the brain.
“The two processes can produce the same observed result, even though they do not have the same cause,” says Jens Hjortkjær.
The researchers therefore wanted to investigate whether changes in the ear drive changes in the brain – or whether the two develop in parallel.
The researchers followed the same sound from the ear to the cerebral cortex
Using electrodes in the ear canal and on the scalp, the researchers tracked how the auditory system processed loud clicks, pure tones and repeated sound sequences.
“We wanted to measure both the signal from the ear and the brain’s later response to the same sounds. That allowed us to investigate whether age-related changes at the two levels are correlated,” says Jens Hjortkjær.
The researchers recorded three responses that depend in different ways on activity in the auditory nerve: the early electrical response to a click; how precisely the brainstem followed a tone oscillating 326 times per second; and how strongly a small muscle in the middle ear contracted in response to sound.
“No single test can document the loss of nerve fibres in humans. That is why we combined several measures that could potentially point in the same direction,” adds Jens Hjortkjær.
Simultaneously, the researchers investigated how the cerebral cortex adapted to a sequence of six tones played at a rate of two tones per second.
If the stronger activity in the cerebral cortex was compensating for a weaker signal from the ear, the people with the weakest auditory nerve signals should also have the most amplified and persistent responses in the brain. The researchers therefore compared the measurements for each participant.
The ear sent weaker signals – but the brain responded more strongly
The three peripheral measures changed with age: the first electrical response after a click became weaker, the brainstem followed the tone’s rapid oscillations less precisely, and the muscle reflex in the middle ear weakened.
“We cannot count the nerve fibres directly. But when three different measures, all of which depend on activity in the auditory nerve, change similarly with age, it supports the interpretation that fewer or weaker nerve connections are carrying the signal,” says Jens Hjortkjær.
By contrast, measurements of the outer hair cells in the low-frequency part of the cochlea examined in standard hearing tests did not change significantly with age in these normal-hearing participants. This suggests that the weaker nerve signals were not simply caused by the classic loss of sensitivity to faint sounds.
Further up the auditory system, the brainstem’s later response to a click remained strong, even though the earlier response from the auditory nerve became weaker with advancing age. Its ability to follow the fast oscillations of the tone, however, became less precise.
“The brainstem appears able to maintain the strength of the response but not the precise timing. This matters because small differences in timing help us distinguish between voices and other sound sources,” says Jens Hjortkjær.
In the cerebral cortex, the observed pattern was different. The older participants responded strongly to the onset of individual sounds and adapted more slowly when the tones were repeated.
“The young brain quickly adapts to the regularity and begins to follow the rhythm of the sequence. With advancing age, the response to each individual tone becomes more distinct,” notes Jens Hjortkjær.
The changes in the ear and the brain did not go hand in hand
The different measures of decline early in the auditory system were interrelated. The same was true of the measures of altered activity in the cerebral cortex. But changes in the ear were not necessarily associated with changes in the brain – and vice versa.
“If the brain’s increased activity directly compensated for a weaker signal from the ear, the two patterns should have gone hand in hand. They did so only to a limited degree. This points to parallel ageing processes in the ear and the brain,” says Jens Hjortkjær.
This does not mean that reduced signals from the ear have no effect on the brain. The relationship may be more complex and depend on such factors as experience, the sound environment and the brain’s ability to adapt. But changes in cortical responses to sound cannot, on their own, be taken as evidence of hidden damage in the inner ear.
“We found the same weak relationship with different sounds and different measurements. This strengthens the suspicion that ageing in the brain itself plays a significant role,” says Jens Hjortkjær.
A standard hearing test may be accurate – and still not be enough
The study does not yet show whether these changes explain why some older people struggle to understand speech against background noise. The participants listened passively to simple clicks and tones and were not tested in complex sound environments.
“We do not yet understand the consequences well enough. But understanding speech in noise requires the ear and brain to handle many simultaneous signals and pick out the relevant ones. We therefore want to investigate whether these different changes can explain problems that a standard audiogram does not detect,” notes Jens Hjortkjær.
The researchers are now studying people who experience hearing difficulties but are found to have clinically normal hearing. The participants will use hearing aids while the researchers measure neurophysiological markers from the ear and brain.
“By comparing the measurements with the participants’ experience of using the hearing aids, we hope to understand who benefits from the technology and why,” says Jens Hjortkjær.
The researchers will also compare the brain’s response to sound with its response to flashes of light.
“If we see the same stronger response to visual stimulation, that would point towards more general ageing effects in the brain. If it occurs only in response to sound, it may be more closely linked to the auditory system,” concludes Jens Hjortkjær.
The study does not provide a new clinical test. But it shows why a normal audiogram does not necessarily tell the whole story: ageing can both weaken the signal from the ear and change how the brain processes and adapts to sound – without the two processes necessarily going hand in hand.
