Two bodyguards stand in a building's entryway. One hears a loud footstep and grabs the door so it will not slam. The other turns a knob to dim the lights. From the outside you cannot tell who did what: the door is shut and the lights are dimmer. That is how the protection of our hearing is set up, and for a long time scientists could not figure out how these two bodyguards operate.
The first bodyguard is a tiny muscle inside the ear. When a sound is loud, it tenses and pulls the eardrum taut so that it vibrates less. The second is a nerve pathway that runs from the brain to the cochlea, the organ of hearing, and there it reduces the amplification of sound. Both respond quickly and almost simultaneously. Scientists compared three groups of people: young people with good hearing, older people with good hearing, and older people with poor hearing. It turned out that in older people the muscle contracts more weakly than in young people. But once the scientists accounted for the state of the cochlea, the difference disappeared. So the muscle does not weaken from age, but from tiny damage in the cochlea that a standard hearing test cannot see.
Now that the two bodyguards can be told apart, doctors will be able to spot hearing problems earlier. If the muscle contracts more weakly, that may point to hidden damage in the cochlea, even when a standard test comes back normal. And clean measurements of the nerve pathway will help explain why some people hear speech well in a noisy place and others do not. The bodyguards at the entrance to the ear are not just standing guard, they are telling us about the state of the whole building.
The middle-ear reflex fires not only in response to external loud sounds but also to your own vocalizations: when a person speaks or sings, the muscle tenses to protect the cochlea from excessive stimulation.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.