Allen Counter (right) tests the hearing of an Inuit hunter in northern Greenland.

Scientists have obtained rare detailed views of a live animal's inner ear. The images show a humanlike cochlea, which converts sound waves into electrical impulses, and the nerve that carries these impulses to the brain.

"We can see details as small as two-thousandths of an inch. That has never been done before with noninvasive imaging," says Allen Counter, associate professor of neurology at Harvard University. "This technology will improve our understanding of how we hear, and help us to better diagnose ear diseases and loss of hearing in humans. About 28 million people in the United States suffer hearing loss or deafness."

The experiments used a special magnetic resonance imaging technology to look into the heads of guinea pigs. The technique employs a combination of magnetic and radio fields to obtain black- and-white images of soft tissue within bone. Guinea pigs were chosen because laws prohibit exposing humans to the high-intensity magnetic fields needed to capture clear images of such small structures.

This image of the inner ear of a live guinea pig shows a cross-section of the spiral cochlea. Running through the cochlea is a nerve (8n) that carries electrical impulses to the brain.

Because they possess the same hearing apparatus as humans, guinea pigs serve as models for human hearing and hearing loss. Counter and his colleagues worked at the Karolinska Institute in Sweden. The images they captured appear on the cover of -- and in a report published in -- the February issue of the international journal NeuroReport.

"If eyes are a window on the brain, ears are its doorways," Counter says.

At the end of the canal leading into the ear lies a tough membrane known as the eardrum. As sound, or vibrations of air, play the eardrum, three small bones inside of it transfer the vibrations to the cochlea. This snail-shaped tube in the bones of the temples contains a coiled membrane lined with microscopic hairs that stimulate nerve cells. Hair-to-nerve connections transform the vibrations into electric signals that the brain perceives as words, warnings, noise, music, or other sounds.

When hair cells in the cochlea are destroyed, hearing is lost. "Deafness is incurable," Counter comments. "Therefore, we want to understand as much as possible about the cochlea and how it's affected by disease and traumas such as loud noises."

In this cross-section of a guinea pig skull, the eye-like objects are cochleae, where sounds are converted to nerve impulses.

Artificial Hearing

Progress has been made on developing artificial cochleae, but Counter describes these devices as "still experimental." A microphone mounted behind the ear sends sounds to tiny electrodes that are implanted in the inner ear and take the place of hair cells.

For those who became deaf in childhood, before learning language, artificial cochleae don't usually provide the ability to understand conversation, but they offer the possibility of enjoying music, or hearing sounds of oncoming danger.

"Success with postlingually deaf people has been better, but mixed," Counter notes. "I have met only one woman who does exceptionally well with an artificial cochlea."

Counter tells the story of a group of Inuits (formerly called Eskimos) that he studied in northern Greenland. The males begin to lose their hearing as young as age 10. By 20 to 30, they are as hearing impaired as Americans who frequently play or listen to loud rock music. Those more than 60 years old have lost most or all of their hearing.

The Inuit women, however, don't experience the same problem.

Males make a living by hunting, and Counter traced their hearing problem to guns. In the last century, they killed seals, walruses, and polar bears with harpoons made mostly of bone and spear tips fashioned out of metal from meteorites.

Around 1909, explorer Robert Peary, during expeditions to the North Pole, began to supply the Inuits with rifles and shotguns. The men have been shooting their prey since.

"Sudden loud noises exert a damaging impact on both ears," Counter explains. "Tests of their hearing show temporary loss each time they fire a gun. Hearing returns, but not all the way to the same level as before. They lose hair cells, and hearing, one gunshot at a time."

Counter fitted the men with hearing aids, but that didn't provide the hoped-for solution. These devices amplify all hearing frequencies, high and low. But gunshots destroy mostly higher frequencies, those sounds associated with consonants. When low frequencies, mainly the vowel sounds, are amplified with the high, it creates a noise level that the men can't stand.

"They can hear you speaking," Counter says, "but can't understand what you're saying."