An experimental technique using light has restored some sense of hearing to deaf gerbils. And while it is just a proof-of-concept study at this stage, it does offer hope that one day a similar method could be used to treat hearing loss in humans.
Worldwide, roughly 360 million people suffer from a form of disabling hearing impairment, the most common type being sensorineural hearing loss (SNHL). This is caused by damage to the cochlea, the fluid-filled inner part of the ear that somewhat resembles a snail's shell.
Today, patients with SNHL can be treated with cochlear implants. These involve electrical currents that directly stimulate the auditory nerve, jumping over any parts of the inner ear that have been damaged. But the issue with cochlear implants is their sound resolution is poor and it can be difficult for patients to separate background noise from conversation in the foreground.
To get around this problem, a team from Germany has looked to a field of research called optogenetics, which involves using light to control cells in living tissue. After preliminary studies in mice and rats, the researchers tested the technique on adult Mongolian gerbils, whose auditory pathways are more similar to those of humans than other rodents. The results of the study have been published in the journal Science Translational Medicine.
First, with their hearing still intact, the gerbils were trained to complete the shuttle box test. This involves placing the critters inside a box that is separated in two by an obstacle. The gerbil then has to jump over the obstacle whenever they hear a particular sound – if they fail to do so, they will be hit with a mild electric shock.
Then came the treatment. Using a virus, the researchers inserted a light-detecting gene into the gerbils' cochleas. Next, they implanted optical fibers that could be turned on and off to send light signals to the inner ear. When they completed the shuttle box test post-surgery, the gerbils responded to the light signals as if they were sounds, which would imply they could "hear" the light signals. This was confirmed when some gerbils were deafened. The deaf gerbils no longer responded to the sound but they continued to jump across the obstacle when the light signals were activated.
In reality, the technique is a long way from being applied to humans and scientists haven't yet been able to find a way to recreate different sounds. Hypothetically, however, it has the potential to stimulate cells more precisely than the treatments currently available.
“Since light can be better confined than electrical current, the number of independent frequency channels is expected to be greater in the optical CI," Tobias Moser, study author and professor of auditory neuroscience at the University Medical Center Goettingen, told Futurism.
"This, we think, will provide more frequency resolution of sound coding enabling the user to better separate similar sounds, understand speech in the noise, and appreciate melodies.”