Temporary Hearing Loss Affects Brain's Wiring

Scientists have gained new insight into how the brain develops. It turns out, relatively short-term hearing deprivation during childhood can lead to persistent changes in the way the brain understands sound, long after hearing is restored to normal.

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RENEE MONTAGNE, host:

Some young children seem to get ear infections all the time. Their hearing can be muffled for months on end. Now, research suggests that a temporary hearing deficit could twist the way the brain understands sound.

NPR's Deborah Franklin has more.

DEBORAH FRANKLIN: You dont need two ears to hear sound. But Dan Polley, of the Massachusetts Eye and Ear Infirmary, says that having the input from two ears makes it easier to locate where a sound comes from - and more.

Dr. DAN POLLEY (Hearing Scientist, Massachusetts Eye and Ear Infirmary): Our ability to hear speech in a noisy background, to hear the wonderful compliments that your date is paying you when youve taken her out to dinner, or when you have multiple people talking to you at once and you try to hone in on one speech source - all of these phenomena depend critically upon integrating signals from each ear.

FRANKLIN: Polley says this sophisticated combining of sound signals happens not in the ear but mostly higher up, in the brain. Polley and a colleague wondered about kids with lots of ear infections, whether the kind of periodic, months-long hearing loss could affect the wiring of a child's brain. So they 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. In young rats, the ear that had remained open and clear made a sort of real estate grab in the brain. The blocked ear lost influence. And even after both ears were sending signals to the brain again, the imbalance persisted. That could make triangulating the source of a sound harder, creating subtle but important differences in hearing.

Dr. POLLEY: When you dont correctly identify the position of a sound in space, you may not know it. When youre not able to hear in a noisy environment, you may just not go out to dinner as often. You may end up isolating yourself from the environments that really require good hearing.

FRANKLIN: 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 research shows that's exactly the sort of thing that does happen in some children with chronic ear infections.

But here's some comfort for parents: Though it can take a while, the brain is pretty good at developing workarounds. Restore hearing, and the brain will eventually catch up.

Polley's research is published in the current issue of the journal Neuron.

Deborah Franklin, NPR News.

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