Temporary hearing impairment leads to ‘lazy ear’

Brain rewiring after ear infections may cause long-term hearing problems, rat study shows

Some folks who don’t seem to listen may just have a lazy ear. A new study in rats shows that short-term hearing impairments at any stage of life can lead to rewiring in the part of the brain that processes sounds, making the ear seem as if it is loafing on its duty to make sense from noise.

Ear infections and fluid buildup in the middle ear — a condition known as otitis media with effusion — can dampen incoming sound waves. These problems are extremely common in children and represent the top reason children go to the doctor. Such temporary hearing impairment can lead to lingering hearing deficits even after the infection or fluid clears up. The long-term difficulties result from a problem with how the brain adjusts to hearing changes rather than a malfunction in the ear’s ability to detect sounds, researchers report in the March 11 Neuron.

An analogous problem in which the brain has trouble processing visual signals from a perfectly functional eye is often called “lazy eye.” A lazy eye can often be retrained through practice in children up to about 8 years old.

Likewise, the new study shows that the brain’s auditory cortex remains flexible enough that it can partially rewire itself even into adulthood. This gives hope that at least some ear “laziness” problems can be corrected in adults, say study coauthors Daniel Polley, a neuroscientist at the Massachusetts Eye and Ear Infirmary in Boston, and Maria Popescu of Vanderbilt University in Nashville.

Polley and Popescu’s experiments with rats show that the brain has a number of critical windows for rewiring itself. The researchers surgically tied off the ear canal in one ear of infant, juvenile and adult rats to mimic the sound-deadening effects of fluid in the ear. After 60 days, the team measured activity of the rats’ auditory cortex cells in response to sounds of various frequencies. Blocking sound to one ear produced different changes in the rats’ brains, depending on the age when hearing was impaired, the team found.

In 2-week-old rats with blocked ears, more cells in the auditory cortex responded to low-frequency sounds and fewer cells responded to high-frequency sounds compared with rats with no ear blockage, suggesting a diminished range. The infant rats also had a strengthened response to sound signals from the open ear and a weakened response to signals from the closed ear — meaning that one side of the brain loses out in the competition to process sounds. Such losses in people could lead to subtle speech defects or other learning problems, says Takao Hensch, a neuroscientist at Children’s Hospital Boston and Harvard University.

Juvenile rats whose ear canals were tied off at age 4 weeks didn’t have more low-frequency–sensitive cells in their auditory cortex, indicating that the critical window for determining the low- and high-frequency range had already closed. But like the infant rats, the juvenile rats still showed a shift in which ear responded most to sound signals.

If the ear canal wasn’t tied off until the rats were adults, the brain cells had a weakened response to the blocked ear but didn’t strengthen the open ear’s input. That result shows that as animals age, they lose the ability to boost signals from the open ear.

The new study “opens up quite a rich system to study brain plasticity,” Hensch says. Researchers still don’t yet know how long each of the critical rewiring periods last in rats or, assuming the system is similar in humans, in people. Also unclear is exactly what effect the brain rewiring would have on hearing in people.

While there’s been little to no work done on how common lazy ear is in humans, the researchers think the new study could have important implications in medicine, especially for choosing how aggressively to treat childhood ear infections.

Since adults still retain some ability to rewire sound-processing centers, the researchers hope that just as a lazy eye can be retrained, lazy ears might also learn new work habits.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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