Turning down ear sensitivity could help humans retain their hearing.
A strange kind of humming fish has evolved a clever way to avoid deafening itself with its own noise, researchers have found. They say the same mechanism could be at work in other animals, including humans, helping to tone down the senses and avoid overpowering them with self-generated signals.
Andrew Bass, 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. The hum, described by some as similar to the chanting of monks, is so loud that houseboat owners near San Francisco have sometimes complained of their homes vibrating at night.
Bass and his fellow authors have shown that the brains of these fish regulate their hearing so that they are not deafened and can hear predators or incoming females even while humming.
“You never smell yourself, but someone else might. Robert Baker , New York University”
The fish control both sound and hearing through nerve impulses from the same part of the brain. Some impulses signal to muscles around the swim bladder, 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, 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, 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.
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, 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, but someone else might," 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, 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, it might play an important role in how we recognize our own voice. "This could have important implications in learning to speak," Bass says.
WeegM. S., LandB. R. & BassA. H. J. Neurosci., 25. 5967 - 5974 (2005).
New York University
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European Journal of Physics (2019)
Wiley Interdisciplinary Reviews: Cognitive Science (2015)