At a crowded reception, an older person asks their conversation partner to repeat themselves again and again: the words drown in the hum of voices. This is what the bridge looks like, the one that carries sound from the ear to meaning. While that bridge holds, noise merely gets in the way, but it doesn't cut the conversation short. With age, the bridge's supports wear out, and the first to give way are the ones holding up not just hearing but the very ability to think.
Researchers from the Human Connectome Project-Aging followed 446 adults aged 45 to 90. Each one took a test on distinguishing words in noise, a short cognitive test, and underwent brain scans with two methods: diffusion MRI revealed the density of nerve fibers, while structural MRI showed the volume of auditory areas. It turned out that in people with lower cognitive test scores, the hearing threshold in noise rose noticeably more steeply with age. By age 80, the difference reached about 5 decibels, meaning these people needed noise nearly twice as quiet to make out the same words.
The arcuate fasciculus is like a cable that transmits the sound signal from the area where a word is heard to the area where it is understood. If the cable's insulation thins, the signal arrives with interference. In noise, the interference grows stronger, and the word falls apart. At the same time, the volume of the primary auditory cortex, the transverse temporal gyrus, also shrank with age and was linked to speech intelligibility. In people with cognitive difficulties, this link was stronger: the cortex shrank, and along with it, the ability to pick speech out of the background faded.
Previously, auditory and cognitive decline in aging were studied separately: some work looked at the ears, others at memory and thinking. The new study brought both directions together in the same people. It showed that early cognitive changes accelerate the wear of specific auditory pathways, and this wear directly predicts how much worse a person hears speech in noise.
The bridge mentioned at the start doesn't wear out evenly. First the supports tied to thinking sag, and only later does the deck itself suffer. That's why an older person at a reception asks for repeats not because their ears hear poorly, but because their brain no longer reconstructs words lost in the general hum as quickly.
The density of white matter in the brainstem auditory pathways turned out to be higher in the right hemisphere, while in the arcuate fasciculus it was higher in the left. Such mirror asymmetry within a single sensory system had not been described before.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.