Researchers from Massachusetts Eye and Ear/Harvard Medical School say it is possible for the brain of the human adult to compensate for inner ear damage by bringing back auditory abilities, according to a latest study published in the journal Neuron.
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The problem however is that while nerve fibers can be restored to link the ear back to the brain for hearing abilities, speech intelligibility is not recovered; and this is why some people with restored hearing still complain of not understanding speech even though they still hear normally.
Daniel B. Polley, director of the Amelia Peabody Neural Plasticity Laboratory at the Massachusetts Eye and Ear and an Associate Professor of Otolaryngology at Harvard Medical School, explained that the plasticity of the adult brain at higher stages of processing sound information serves as an amplifier to amplify sound signals.
"It seems that even just 3 percent of the normal complement of inputs is enough for the brain to operate on; however, the compensation is incomplete,” Dr. Polley. “There is a cost, and the cost is that the neurons that recover cannot decode complex sounds, such as speech, which are central to our ability to communicate."
Thousands of nerve fibers make up the auditory nerve in order to carry sound signals from the ear to the brain and back, but scientists say these auditory nerve fibers die easily from excessive noise, medications, and aging making hearing loss a natural thing to some people.
Physicians before this time never understood why people with hearing difficulties would complain of not understanding speech despite the fact that they hear normally within accepted thresholds; but physicians now know that loss of nerve fibers over time may be part of the problem. Meanwhile, the plasticity of the brain or the ability of the brain to adapt hearing to new environments contribute to the same problem.
"Someone with a substantial depletion of auditory nerve fibers would be sitting across from you and could hear the sound of your voice but would not be able to extract any intelligible information from it, particularly if other people were talking nearby," said Dr. Polley. "The loss of nerve fibers reduces the bandwidth of information that can be transmitted from the inner ear to the brain, which leads to a struggle to process sound information, even if hearing thresholds are normal."
Using chemicals to clean the nerve fibers that process sound in the inner ears of lab mice, researchers found that the mice could hear normally and recorded top processing in the brain as can be seen with enhanced cortex activity, showing that the “amplifier” is located within the cortex.
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The researchers were able to see that the brain’s natural plasticity has a limit to recovering sounds. "Like feedback from a microphone, having too much gain in the system can push neural circuits toward becoming pathologically hyperactive and hypersensitive," said Dr. Polley.