EACH human ear processes different sounds, with the right ear responsible for deciphering rapid signals such as speech and the left ear in charge of tones and music, scientists in the United States have found.
Infancy suggests that our two ears are structured differently to decode different sounds, challenging the decades-old belief that such a division was conducted by the brain.
The study, reported in the journal Science, could hold profound implications for the rehabilitation of people with hearing loss in one or both ears, and help doctors to enhance speech and language development in hearingimpaired babies.
Scientists from UCLA in California and the University of Arizona have demonstrated that right-left differences for the auditory processing of sound start at the ear. Yvonne Sininger, professor of head and neck surgery at UCLA, said: “From birth, the ear is structured to distinguish between various types of sounds and to send them to the optimal side in the brain for processing. Yet no one has looked closely at the role played by the ear in processing auditory signals.”
Scientists have long understood that the auditory regions of the two halves of the brain sort out sound differently. The right side dominates in processing of rapidly changing signals, while the left side leads in tones.
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Because of how the brain’s neural network is organised, the left half of the brain controls the right side of the body, and the left ear is more directly connected to the right side of the brain. Prior research had assumed that a mechanism arising from properties unique to each brain hemisphere explained why the two sides of the brain process sound differently. But Dr Sininger’s findings suggest that the difference is inherent in the ear itself. “We always assumed that our left and right ears worked the same way,” she said. “We tended to think it didn’t matter which ear was impaired in a person.
“Now we see that it may have profound implications for the individual’s speech and language development.” Researchers studied tiny amplifiers in the outer hair cells of the inner ear, which expand and contract to amplify the vibrations. These cells then convert the vibrations to neural cells and send them to the brain, which decodes the sound.
In a six-year study, the team evaluated more than 3,000 babies for hearing ability. Researchers were surprised to find that the left ear provides extra amplification for tones like music, while the right ear provides extra amplification for rapid sounds timed like speech.
Barbara Cone-Wesson, professor of speech and hearing sciences at the University of Arizona, said: “Auditory processing starts in the ear before it is ever seen in the brain. Even at birth, the ear is structured to distinguish between different types of sound and to send it to the right place in the brain.”
Earlier research into children with hearing problems supports the findings. A study showed that children with impairment in the right ear encounter more trouble learning than those children with hearing loss in the left ear.
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According to scientists, the research may also help to improve cochlear implants, which offer the profoundly deaf the sensation of hearing.
These implants, which can now be surgically inserted into the ear, are devices which process sounds to the brain. The latest models aim to help preserve what little residual hearing the wearers may still have.