University at Buffalo (UB)
Release Date: June 15, 1995 This content is archived.
BUFFALO, N.Y. -- University at Buffalo researchers studying the inner ears of chinchillas have discovered that some ears can spontaneously broadcast intense sounds that are transmitted into the brain and mask external sounds of similar frequencies. These sounds, called spontaneous otoacoustic emissions, are loud enough to be heard by others standing nearby.
The results of the research, reported in the current issue of Nature, indicate that this masking of external sounds -- a phenomenon the researchers call a "line busy" signal -- leads to a significant hearing loss that is completely different from that caused by damage to sensory cells.
Nicholas Powers, Ph.D., UB assistant professor of communicative disorders and sciences, and Richard J. Salvi, UB professor of communicative disorders and sciences, said many human ears emit low-intensity sounds that are so weak that they are not noticed by most listeners and do not affect hearing. However, these sounds are very strong in a small percentage of individuals and are associated with noticeable hearing loss.
The researchers, who are members of the Center for Hearing and Deafness at UB, have found these high-intensity sounds in 15 chinchillas and conducted extensive measurements on two animals. The sounds being emitted from these two animals were so loud that they could be heard by the researchers when the animals were taken into a quiet room.
While it has been known for years that the ears of some humans and animals can produce sound (objective tinnitus), it was thought that these inner-ear vibrations did not reach the brain. The UB research team is the first to determine that these sounds actually are transmitted into the brain by way of the auditory nerve.
The researchers measured the sensitivity of the chinchillas' auditory nerves and found a significant hearing loss. In these ears, the threshold for hearing -- the lowest sound intensity that can activate the neurons -- was elevated by about 40 decibels, but only at frequencies near those of the spontaneous emissions.
The team also inserted a microelectrode into the auditory nerve to pick up activity from individual auditory neurons that connect to different inner hair cells. They found that when they recorded activity from a neuron that was "tuned into the spontaneous emission," there was a high rate of activity in the absence of any external sounds. "We think the neural activity is produced by the spontaneous vibrations in the outer hair cells, which, in turn, activate the inner hair cells and auditory nerve," Salvi said.
"We also found that if we tried to activate these neurons using external sound, they didn't respond well. They can't respond to external sound because they already are responding to sounds generated within the ear," Powers noted, calling the phenomenon "line busy."
"The same phenomenon is likely to occur in humans with intense spontaneous emission," Salvi added.
This "line busy" signal -- created by this "internal biological noise" -- "significantly degrades a neuron's ability to respond to external sound and results in a hearing loss completely different from that caused by damage to sensory cells," the researchers reported.
When Vlasta Spongr, another member of the research team, examined the chinchillas' ears anatomically, she found no pathology. The hearing loss, Salvi said, is due to this competition between the internal sounds -- the spontaneous emissions -- and external sounds. External sounds of the same frequency as the spontaneous emissions cannot activate a neuron that is already "busy;" the listener cannot hear those external sounds.
Other members of the research team were Jian Wang, a research scientist, and Chun-Xiao Qiu, a doctoral student, both in the UB Department of Communicative Disorders and Sciences.