医用生体工学：補聴器での音の圧縮は難聴の害になることがある

Algorithms for sound processing used in hearing aids can diminish the wearer’s ability to distinguish between similar sounds, reports an animal study published in Nature Biomedical Engineering. The findings indicate that simpler devices may be more effective for people in the short term, and suggest a new approach for improving hearing aids in the future.

Hearing aids are the main treatment for mild-to-moderate hearing loss. However, many people who could benefit from hearing aids do not wear them. This is partly because hearing aids do not adequately restore hearing in real-world settings. Basic hearing aids are designed with linear amplification, which provides a constant level of amplification regardless of the incoming sound level. Many hearing aids are also built with compression algorithms designed to selectively amplify soft sounds.

Nicholas Lesica and colleagues investigated the activity of neurons in the inferior colliculus — one of the brain’s hearing centres — of hearing-impaired gerbils while they were exposed to sounds from human speech. The authors recorded how neurons in this region of the brain responded to sounds that were unprocessed, processed by linear amplification, or processed with amplification and compression. They found that the responses of neurons with amplification and compression were distorted, and that removing compression corrected some, but not all, of the distortions. They also found, however, that amplification alone distorted neural responses even in the absence of hearing loss. This suggests that many of the perceptual problems experienced by hearing aid users are, in fact, normal.

The authors conclude that uptake of hearing aids could be improved rapidly without loss of benefit by making simple devices with linear amplification widely accessible, but that improving hearing aids will ultimately require new sound processing algorithms that compensate for the negative effects of amplification.