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.