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Health Center Research Shows
Hearing Loss Can Affect the Brain
December 13, 1999

The cochlea - part of the inner ear - has long been the focus for much of the research on hearing loss, research that has led to various prosthetic devices and treatments. Although this work is very important, it largely presumes there are no changes in the brain after a hearing loss.

"The adult nervous system is fairly well formed and, in the past, most biologists assumed that it doesn't undergo much change," says Steven Potashner, a professor of anatomy at the Health Center.

By focusing on changes that occur in the adult brain, Potashner's work takes a different direction. Potashner and co-principal investigator Sanoj Suneja, a research associate in the Department of Anatomy, recently received a five-year, $1.4 million research grant from the National Institute on Deafness and Other Communicative Disorders.

"In the early '90s we were studying the anatomy and biochemistry of the parts of the brain that receive the information we hear," says Potashner. "Our work suggested that a severe hearing loss caused changes in the connections between the nerve cells that convey and interpret acoustic information in the adult brain."

In 1991, he and Kent Morest, also a professor of anatomy, received a federal grant to take a look at changes in the brain after the cochlea was damaged by loud noise.

The grants have made it possible to look at hearing loss caused by:

  • destruction of the cochlea, which may be a consequence of an infection or caused by the surgical removal of a tumor;

  • damage to the middle ear bones, which provide a sound conduction path: this kind of hearing loss may occur in those with chronic, untreated ear infections;

  • damage to the cochlea from very loud noises: this kind of hearing loss often occurs in construction workers, soldiers, and pilots.

"Each of these events caused a distinct pattern of change in the connections between nerve cells that process acoustic information in the brain," says Potashner.

The hearing loss caused by damage or destruction of the cochlea left certain nerve cells in the cochlear nucleus of the brain partially denuded of the contacts that usually convey acoustic information from the cochlea.

"We predicted the picture would stay that way," says Potashner. "And it did, for a few days after cochlear destruction and for about six months after noise damage. Then we found evidence that the empty contact sites became re-occupied, that is, we found evidence that nerve cells grew new contacts.

"Our studies also suggested that the newly grown contacts provide an inappropriate excitatory drive to many of the brain cells that convey acoustic information," he says.

This enhanced excitability in the system may be experienced as tinnitus, or ringing in the ears, which often follows hearing loss. Tinnitus can be caused by cochlear malfunctions, but about half the cases are not related to the cochlea. Instead, these cases may be driven by the mechanisms described by Potashner.

"Most research on hearing loss has focused on the cochlea," says Potashner. "Very little has focused on what happens in the brain because of hearing loss, but it may very well be an important component.

"There may well be treatments in the future that address these problems in the brain," he adds. "It's too early to say what form they will take, but without this kind of information, new developments may not occur at all."

Kristina Goodnough