Ears Work in Stereo Before Brain Does
They respond differently to certain stimuli, baby study says
THURSDAY, Sept. 9, 2004 (HealthDayNews) -- Depending on the type of sound you hear -- such as speech or music -- one ear may be more adeptly processing that information and sending it to a part of the brain specialized to hear that type of sound.
That's the conclusion of a study in the Sept. 10 issue of Science, which found that when infants were exposed to different sounds, the hair cells in each ear responded more vigorously to certain ones.
Study author Yvonne Sininger, a professor in residence in the division of head and neck surgery at the University of California, Los Angeles, said researchers had previously known that different areas of the brain were specialized to respond to certain sounds. "But, it was never considered that there was some facilitation starting at the level of the ear," she said.
Dr. Anil Lalwani, chairman of otolaryngology at New York University Medical Center, said, "It's interesting that such nervous system specialization may have origins in peripheral hearing organs.
"We've always assumed the supremacy of the brain, but this raises the possibility that peripheral organs may drive some of the differences seen in the central nervous system," said Lalwani.
Sounds are processed in different parts of the brain. The left hemisphere generally dominates in the processing of speech sounds, while auditory regions in the right hemisphere are often responsible for processing tones and music.
Sininger and her colleague, Barbara Cone-Wesson of the University of Arizona in Tucson, wondered if these differences might have developed in response to information received from the ears.
So they studied the results of otoacoustic emissions (OAE) tests from more than 1,500 infants in Los Angeles. The original information had been gathered to evaluate the hearing tests used to screen hearing in newborns.
Infants who had hearing loss or an ear disease were excluded from the study.
The OAE test measures the activity of hair cells in the outer ear. These hair cells amplify sound that enters the cochlea, a small, snail-shell shaped bone in the ear that transmits sound information to the acoustic nerve in the brain.
The babies were subjected to a series of sounds, either clicks or tones separately, through the use of a probe in each ear. Clicks were meant to simulate speech, according to the study.
Overall, the babies' right ears responded more to the speech-type clicks and less to tones, while their left ears responded more strongly to tones than to the clicks.
This specialization in the ears suggests that the information sent to the brain by the ears may lead to the development of auditory specialization in the brain.
"Initial processing of sound in the auditory system at the level of the cochlea and brain stem may serve to facilitate later development of hemispheric specialization for sound processing," the authors concluded in the study.
Sininger explained that one of a newborn's most important tasks is to learn and sort out her auditory environment, and to process speech sounds and figure out what they mean.
So, she said, "It makes sense that a baby would be born with a mechanism to help facilitate that."
Sininger said she looked for anatomical differences between the ears to explain the difference, but could find none.
Neither Sininger nor Lalwani could think of any immediate practical implications from this study. But Sininger said down the road perhaps some consideration would be given to ear specialization when dealing with rehabilitation for hearing loss, or when deciding on which side a cochlear implant should be placed. Also, the study raises the possibility that other peripheral organs, such as the eyes, may play a role in the development of brain specialization.
"We always assumed that our left and right ears worked exactly the same way. As a result, 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," Sininger said in a statement.
To learn more about how babies hear and learn language, visit the American Speech-Language-Hearing Association.