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Humming fish solves noisy clash,[link widoczny dla zalogowanych]
A male plainfin midshipman fish hovers over some eggs,[link widoczny dla zalogowanych]. The graph shows nerve impulses to make sound (yellow) and to inhibit hearing (orange),[link widoczny dla zalogowanych].© Cornell University
A strange kind of humming fish has evolved a clever way to avoid deafening itself with its own noise,[link widoczny dla zalogowanych], researchers have found. They say the same mechanism could be at work in other animals, including humans,[link widoczny dla zalogowanych], helping to tone down the senses and avoid overpowering them with self-generated signals.
Andrew Bass,[link widoczny dla zalogowanych], a neuroscientist with a name amply suited to studying both fish and acoustics, looked at the male plainfin midshipman fish (Porichthys notatus) to study this effect. These 25-centimetre-long fish live off the west coast of the United States from California to Alaska. During summer nights, they hum to attract females and encourage them to lay their eggs. Some impulses signal to muscles around the swim bladder,[link widoczny dla zalogowanych], which is the fish's buoyancy organ, making it generate sound by vibrating. The same area of the brain sends signals to inhibit the sensitivity of the ear's hair cells, which translate sound into electrical signals that the brain can understand.
When the researchers looked at these signals in detail,[link widoczny dla zalogowanych], they found that both happened about 100 times per second. They were also perfectly coordinated so that the bladder vibrated at the exact same time that the ear's sensitivity was reduced. The researchers checked that the ears weren't simply tuning out in response to a loud blast of noise: they paralysed the fish to silence their hums,[link widoczny dla zalogowanych], and found that the two signals were still synchronized.
"We never expected to see this fine temporal control that really matches to every phase of the sound," says Bass. "That was really incredible." The team, based at Cornell University in Ithaca, New York, reports its results in the Journal of Neuroscience1.
Hearing aid Studies in crickets, bats, monkeys and even humans have shown that hearing can become less sensitive during sound production. But it has not been clear how this happens. The team notes that all vertebrates have a nerve connection between the brain and ear that is similar to that found in the plainfin midshipman, so it is probable that they all use the same mechanism to adjust their hearing,[link widoczny dla zalogowanych], they say.
"Without a doubt humans use this principle," agrees Robert Baker, a neuroscientist at New York University. Baker adds that humans might use the same mechanism for other senses, including touch and smell. "You never smell yourself,[link widoczny dla zalogowanych], but someone else might,[link widoczny dla zalogowanych]," Baker says.
Humans have a second mechanism to protect their ears when exposed to loud noise: a reflex tightens muscles in the inner ear to stiffen the eardrum and inner ear bones so they become less efficient at transmitting sound. But this response gets weaker with repeated exposure to noise,[link widoczny dla zalogowanych], and can only protect us from short-lived sounds. It also cannot protect us from noises that reach the ear through the bones in the head. The mechanism identified in the humming fish can.
Bass speculates that if the same process happens in humans,[link widoczny dla zalogowanych], it might play an important role in how we recognize our own voice. "This could have important implications in learning to speak,[link widoczny dla zalogowanych]," Bass says.
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