Temporary hearing loss may actually be the ear's way of protecting itself from lasting damage, suggests a study published Monday in the Proceedings of the National Academy of Sciences.
Well, if you're a mouse, at any rate.
An international team of researchers reports that when mice are exposed to loud noise, their inner ears release a chemical that causes a temporary reduction in hearing sensitivity (determined by monitoring electrical signals in their brains). As in humans, this change in sensitivity can last for more than 24 hours, says Gary Housley, chairman of physiology at Australia's University of New South Wales and lead author of the study.
But mice bred to lack the receptor for this chemical showed no change in hearing sensitivity in response to loud noise. "It was really remarkable that hours of sound at that quite high level had no effect on the hearing at all," says Housley.
At even higher levels of noise, though, the mice that lacked the receptor were much more vulnerable than regular mice to permanent hearing loss.
According to Housley, this suggests, somewhat counterintuitively, that temporary loss of sensitivity to loud noise can be beneficial, as it protects the inner ear from more serious, long-lasting damage.
In the past, "change in hearing sensitivity was thought to reflect a transient, damaging response to noise," says Housley. "In fact, that's not the case. ... This really is adaptation."
And even though this study was done with mice, Housley says that it's relevant to human health as well. He points to a recent study that found members of two Chinese families with a mutation in the gene encoding the same chemical receptor exhibited progressive hearing loss over time.
Scientists have been aware of the protective effect of short-term hearing loss for quite some time, says Jochen Schacht, director of the Kresge Hearing Research Institute at the University of Michigan in Ann Arbor. "If you give a low-intensity noise, you can protect against a high-intensity noise," he says. "This is a story that ... has been around for quite a while."
Schacht, who wasn't a member of the research team, says that the real significance of this study is that it confirms a biochemical mechanism for how the ear protects itself from permanent damage. Understanding this mechanism could be useful in the future for developing drugs to guard against loud noise.
So if temporary hearing loss actually does protect our ears, does that mean that it's safe to crank up the volume?
No, says Housley. "When you're exposed to loud sound, [you] have no way of really appreciating what the absolute sound levels are," he says. "The actual sound level coming into your ear hasn't changed and can still be damaging."
Housley says that people often compensate for reduced sensitivity to noise by turning up the volume too high or tolerating loud noise for much longer than they should.
Our ears' adaptive mechanisms "provide us with an illusion that we're in a safe, moderate sound environment when, in fact, we may well not be," he says.