Researchers have known since the 1950s that humans process speech and tone sounds in different sides of the brain.
Generally speaking, the left side of the brain processes speech and performs sophisticated language functions. It excels at dealing with rapid, repetitive sounds. And, generally speaking, the right side of the brain is the primary processor of tonal sounds. It excels at hearing pitch, or sound frequency, and interpreting music.
There's an elegant, crossed-pathway of auditory neurons, or nerve cells, that link ears and sound-processing centers in the brain. The right ear auditory nerve pathway leads to the left hemisphere's auditory cortex. Therefore, the right ear reacts faster and more accurately to speech-type stimuli than the left ear does. Conversely, the left ear auditory nerve pathway connects to the right hemisphere's auditory cortex, so it is the preferred ear for hearing music.
Babies aren't born with these neural pathways connecting the ear to the cortex, however. These pathways become apparent only after infants are at least four month old.
So two scientists who screened thousands of newborns as part of a project to optimize infant hearing tests were startled when they realized their data show that infants do have stimulus-related ear differences from birth.
"We don't think that these differences are anatomical," said Barbara Cone-Wesson, an associate professor in the University of Arizona's speech and hearing sciences department. "There are really no physical differences between right and left ears, although no one's really looked at anatomical differences between right and left brain stems. We think a neural pathway much lower in the brain could be causing the stimulus-related ear differences in newborns."
Yvonne S. Sininger of the UCLA School of Medicine and Cone-Wesson report the discovery in the Sept. 10 issue of Science.
Both were involved in a major, multi-center research project that tested hearing in more than 7,000 infants across the country in the 1990s. The project goal was to find the best tool for screening newborns for hearing impairment. Cone-Wesson, who directed the project in Los Angeles County, tested more than 3,500 infants at her hospital.
And, after it all, Cone-Wesson said, "We had a bucketload of data" on newborn babies' inner ear and brain wave responses to different kinds of stimuli.
She and Sininger analyzed the data on infants' responses to two different kinds of 'otoacoustic emissions.' Otoacoustic emissions are sounds that are generated by the inner ear itself in the process of hearing. They can occur in an ear which is completely cut off from the rest of the brain, so long as the inner-ear hair cell system is intact.