The damage occurs to the dendrites, the delicate branches on the end of neurons in the inner ear. Ordinarily, these dendrites receive auditory signals from the ear's adjacent "hair cells" and carry them on to the brain.
But with sustained exposure to loud noise, the dendrites pull away from the hair cells and can no longer receive that signal - like unplugging the earbuds from an iPod.
Once the dendrites retract, they do not grow back, said Liberman, also a Harvard Medical School professor. His team measured the number of connections between the hair cells and the adjacent dendrites, and found that after two hours of exposure to 100 decibels, half of these connections were gone. Thomas Willcox, director of Thomas Jefferson University's Balance and Hearing Center, said the findings are cause for concern. "It could reshape the way we think about noise-induced hearing loss," said Willcox.
People may think they can recover from a noise-induced hearing loss. Indeed, the hair cells, which convert sound into an electrochemical signal for transmission, can recover from temporary harm caused by loud noise.
But even if the hair cells recover, any lost connections with the adjacent neurons do not, Liberman said. A person with this type of damage can still perceive soft noise as well as before. But with fewer connections to the cochlear neurons, that person will do so with less resolution - almost like with a blurry photograph - and will be less able to interpret it, he said.
This loss of resolution explains why someone who seems to have otherwise normal hearing can have a hard time understanding speech in restaurants and other loud places.
There may yet be hope. Using lab animals, Liberman's team is testing the use of neurotrophic proteins to spur the regrowth of dendrites.
Meanwhile, wear earplugs.