Understanding exactly how birds hear sounds differently than humans is a key area of research for scientists studying deafness in humans. This is because birds have the ability to regrow lost hair cells in the inner ear in order to regain hearing loss, while humans do not.
In research published online today in Nature Communications, another key difference between how humans and birds hear points to an evolutionary change that allows humans to hear high frequency sounds for survival advantage (the sounds of different predators rustling in leaves, for example) — sound variations that birds can’t detect.
"We already know that after deafening, birds can regenerate their hair cells whereas mammals cannot," John Oghalai, MD, professor of otolaryngology, told me. "Now, our data argues that an evolutionary trade-off for humans that allowed higher frequency hearing resulted in a loss of some of the hair cell functionality."
Human ears are extremely fragile instruments. Sound waves enter the ear, causing the eardrums to vibrate. These vibrations get sent to the cochlea in the inner ear, where fluid carries them to rows of hair cells, which in turn stimulate auditory nerve fibers. These impulses are then sent to the brain via the auditory nerve, where they get interpreted as sounds.
One of the goals of the Oghalai lab is to better understand the changes in the cochlear function in humans that lead to progressive hearing loss, and ultimately, to develop techniques to treat the problem before it leads to deafness.
This new research showing another variation in how the bird cochlea works compared to the human cochlea is key to the lab’s research.
"There have been limited ways to understand how the cochlea works in birds," Oghalai said. In the past, researchers have removed the inner ear structure of birds and examined them in a petri dish. But in this study, researchers developed a new optical technique that uses a laser beam to penetrate the bone and look inside the inner ear of living chickens.
This technique allowed researchers to take a picture of the tissues inside the inner ear which revealed the evolutionary variations that allowed humans to hear high frequency sounds resulting in a loss of some of the hair cell functionality.
"Our data indicate that while nearly all sensory hair cells in birds are being stimulated by all sounds, only those that are tuned to the correct frequency will pass a signal on to the auditory nerve," Oghalai said. "In contrast, in the mammalian inner ear, sounds selectively stimulate only specific hair cells and then all of those targeted hair cells send signals on to the brain."
Thus, the ability to distinguish between different frequency sounds is based upon tuning mechanisms inside bird hair cells, but it is based upon tuning mechanisms that lie outside of the hair cells in mammals, he said.
"The goal is by understanding these variations in how different species of hair cells work, this will help us understand how hair cell regeneration occurs," Oghalai told me.