Temporary Hearing Loss May Rewire Kids' Brains

Some kids seem to have near-constant ear infections. Even after the pain is gone, a parent's got to wonder: Are there lasting effects from all that muffling of sound in the formative years?

A child's developing brain needs sound from both ears. (iStockphoto.com)

Research in rats just published in the journal Neuron suggests there might be effects in the brain that, while not permanent, can last for years. Apparently, hearing loss in one ear during critical periods of brain development can rewire the auditory cortex, changing the way it processes sound.
Neurobiologist Dan Polley, who recently moved to Harvard and the Massachusetts Eye and Ear Infirmary in Boston, conducted the research with a colleague, Maria Popescu, while at Vanderbilt University.
Polley says that while we don't need two ears to hear sound, figuring out where that twitter of birds or the shout from a friend is coming from requires the sort of depth perception that input from two ears provides. Plus, there are other benefits from a nuanced fusion of the two signals in the brain.
"Our ability to hear speech in a noisy background; to hear the wonderful compliments that your date is paying when you've taken her out to dinner; or when you have multiple people talking to you at once, and you try to home in on one speech source -- all these phenomena depend critically upon integrating signals from each ear," he says.
Polley wondered if the kind of periodic, months-long hearing loss experienced by some children with chronic infections and resulting blockage of the middle ear might actually affect the wiring of the brain. So he and his colleague tried a little test in rats of different ages: In each animal, they blocked the sound in one ear for a couple of months, and then unblocked that ear.
The result: In young rats, the ear that had remained open and clear made a sort of real estate grab in the auditory cortex, developing a much richer network of neural connections. The blocked ear lost influence. And even after both ears were once again sending clear signals to the brain, the imbalance in the brain persisted.
It's the sort of thing, Polley says, that could make triangulating the source of a sound harder, he says, and create subtle, but important deficits in hearing.
"When you don't correctly identify the position of a sound a in space, you may not know it," he says. When you're not able to hear in a noisy background, you may just not go out to dinner as often. You may end up isolating yourself from the environments that really require good hearing."
A child with that sort of problem might withdraw in a noisy classroom, Polley says, or--depending on when the imbalance occurs--might miss milestones in language or learning.Other studies have shown that's just the sort of thing that's been reported among some children with chronic middle ear infections.
Here's some comfort for parents: Though it can take a while, the brain is pretty good at developing workarounds, Polley says. Restore hearing, and the brain will eventually catch up.