Brain, Not Just Ears, Loses Hearing

Even with a hearing aid turned to full volume, some elderly people can't make sense of speech. Now researchers suggest that the brain, not just the ears, is to blame. The aging brain loses the capacity to distinguish high-speed speech sounds, a problem that might be alleviated by polite conversation.

The gradual hearing loss that comes with old age, called presbycusis, is one of the three most common chronic medical problems in the elderly. It's caused by the gradual death of sensory cells in the inner ear. But this normal decline doesn't seem to explain the specific problem that some people have processing speech sounds, which depends on detecting the difference between milliseconds-long gaps between the murmur of vowels and high-pitched hisses of consonants.

To pin down the role of the brain, neuroscientist Joseph Walton and colleagues from the University of Rochester School of Medicine and Dentistry in New York examined nerve cells in the mouse. They focused on the auditory midbrain, which helps process sounds. Older animals had lost half of the nerve cells devoted to detecting the shortest gaps, they reported on 31 May at a meeting of the Acoustical Society of America in Atlanta.

In another study, neuroscientist Robert Frisina of the Rochester Institute of Technology in New York used positron emission tomography (PET) imaging to uncover the amount of activity in the human auditory midbrain, and they found that that region wasn't as active as in younger adults. They also found evidence that fewer nerve fibers infiltrated those brain regions, and that there were different levels of calcium entering and leaving the nerve cells themselves.

The researchers hope that therapies to correct these problems can be developed. In the meantime, it may help not to speak all at once, says audiologist Michael Chial of the University of Wisconsin, Madison. The elderly have additional trouble deciphering speech when background noise obscures the key sounds of words. "In polite conversation, people take turns," Chial says. "It's a social adaptation that works well to maximize communication."