Something must be going on in the brains of older adults that causes them to struggle to follow speech amid background noise, even when their hearing would be considered normal, researchers from University of Maryland have determined.
Researchers Samira Anderson, Jonathan Z. Simon and Alessandro Presacco found that adults aged 61-73 with normal hearing scored significantly worse on speech understanding in noisy environments than adults aged 18-30 with normal hearing.
The researchers studied two areas of the brain. They looked at the more 'ancestral' midbrain area which does basic processing of all sounds.
They also looked at the cortex which is particularly large in humans and part of which specialises in speech processing.
In the young group, the midbrain generated a signal that matched its task in each case - looking like speech in the quiet environment, and speech clearly discernable against a noisy background in the noise environment.
But in the older group, the quality of the response to the speech signal was degraded even when in the quiet environment, and the response was even worse in the noisy environment.
"For older listeners, even when there isn't any noise, the brain is already having trouble processing the speech," said Simon.
Neural signals recorded from cortex showed that younger adults could process speech well in a relatively short amount of time.
But the auditory cortex of older test subjects took longer to represent the same amount of information.
"Part of the comprehension problems experienced by older adults in both quiet and noise conditions could be linked to age-related imbalance between excitatory and inhibitory neural processes in the brain," Presacco added.
This imbalance could impair the brain's ability to correctly process auditory stimuli and could be the main cause of the abnormally high cortical response observed in the study.
"Older people need more time to figure out what a speaker is saying. They are dedicating more of their resources and exerting more effort than younger adults when they are listening to speech," Simon noted in a paper published by the Journal of Neurophysiology.
This eroding of brain function appears to be typical for older adults and a natural part of the ageing process.
The researchers are now looking into whether brain training techniques may be able to help older adults improve their speech comprehension.
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