Tinnitus location found in the brain.

The chronic sensation of loud ringing in the ears, or tinnitus, plagues roughly 12 million people in the United States, including about 10 percent of the elderly. An additional 40 million people have a milder form of the disease. Loud noise or other injury to the cochlea, a portion of the inner ear, can trigger the disorder, which often accompanies hearing loss. The progression of tinnitus is poorly understood, and there is no cure.

Now, scientists have located the area of the brain that seems to register this ringing sound--a first step, they hope, toward treating the condition.

The finding emerged from a study of unusual tinnitus patients, who hear changes in the ringing when they move their jaws. Researchers used positron emission tomography (PET) to examine the brains of four such people and compared the images produced when the people clenched and relaxed their jaws.

PET scans showed that jaw clenching changed blood flow, an indicator of neural activity, to certain regions of the brain--mainly an area of the temporal lobe associated with hearing. In three of the patients, who had tinnitus only in the right ear, blood flow changes showed up on the left side of the brain. Blood flow also increased somewhat in the hippocampus, a part of the brain that controls emotions. Other, more predictable activation occurred in parts of the brain responsible for motor activity, in this case the jaw clenching, the researchers report in the Jan. 22 Neurology.

Six people without tinnitus showed no blood flow change in auditory regions, whether they clenched their jaws or not.

The researchers then exposed all of the participants to a tone in the right ear. This activated responses in auditory portions of the brain on both the right and left sides, says study coauthor Alan H. Lockwood, a neurologist at the State University of New York and at the Veterans Administration Western New York Health Care System, both in Buffalo. The findings suggest that sound, even in one ear, is processed on both sides of the brain, whereas the tinnitus arose on one side only.

The result indicates that "this form of tinnitus is riot cochlear," because a disturbance arising in the ear would register on both sides of the brain, says Kenneth A. Gruber, a scientist at the National Institute on Deafness and Other Communications Disorders in Bethesda, Md. In these people, tinnitus apparently originates within the brain, Lockwood agrees.

Just as some amputees feel pain in a limb they no longer have, people with tinnitus experience sounds they don't actually hear. "We think that tinnitus may be a sort of sensory analog to phantom limb pain," Lockwood says.

Although the brain appears to be largely "hard-wired" for sensory input, it "is capable of undergoing some fairly substantial reorganization, especially in altered sensory perception," says Lockwood, whose current work focuses on this brain remodeling, or plasticity.

The researchers also consider activation of the hippocampus unusual. In the control group, PET scans showed no hippocampal activation. The findings support past research hinting that the frustration of tinnitus can induce emotional problems Lockwood says.

Counseling helps some tinnitus patients, whereas others find relief from devices that mask the ringing with more soothing sounds. Few drugs ease tinnitus. Lidocaine, an anesthetic, works only when given intravenously and then only for a short time. Knowing the location of tinnil us processing in the brain may enable researchers to test the effects of new medications, Gruber says.