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New Discovery on Loudness Perception

Submitted By The Department of Health and Human Services National Institute of Deafness and Other Communication Disorders (NIDCD)

New findings about how the brain determines the loudness of the sounds humans are capable of hearing will benefit many of the millions of Americans who have hearing impairments. The findings published in the April 22, 1994 issue of Science, show that two mechanisms, rather than one, help the brain to determine variations in loudness. Research has been conducted by Dr. Fan-Gang Zeng and Dr. Robert V. Shannon of the House Ear Institute, Los Angeles, California.

The cochlea of the inner ear transforms sound waves into electrical signals that are carried by the hearing, or auditory nerve, to the brain. Loudness, which is determined in the brain, is a person's judgment of the intensity of a sound. Scientists have thought for some time that the cochlea not only transforms sound, but also begins processing sound to help the brain detect loudness variations. Scientists did not know, however, that more than one mechanism was used to process loudness.

"Our research suggests that very low pitched sounds, those with less than 300 Hertz are not coded in the cochlea", said Dr. Zeng. He added, "They appear to be transformed in the cochlea and then travel along the auditory nerve to the brain stem where they are then processed. Pitches higher than 300 hertz, however still appear to be coded in the cochlea. The research team tested loudness variations of several tones in eight adults with cochlear implants and three adults with brain stem implants. While patients with brain stem implants processed the loudness of all pitches the same, patients with cochlear implants processed the loudness of low pitches differently."

"We are not sure how and why this happens, but these results strongly suggest that two mechanisms code for loudness at two different levels of the auditory pathway," said Dr. Shannon. Dr. Zeng cautioned that more work is needed. The investigators in this study only tested simple tones. How the brain processes the loudness of complex sounds, such as speech, that consist of many tones, needs to be examined. In addition, loudness is only one feature of sound. Similar work needs to continue on how the brain processes other aspects of sound such as pitch. "This work will not only improve the capabilities of hearing devices, it may lead the way for further research that will improve our understanding of sound perception," said James B. Snow, Jr. M.D.

 





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