Restoring hearing with light

The World Health Organization estimates that 466 million people—6% of the world’s population—suffer from debilitating hearing loss. For many of them, cochlear implants can partially restore lost functionality. Consisting of a string of electrodes, a cochlear implant is inserted into the snail-shaped cochlea of the inner ear, where it stimulates the spiral ganglion, the cluster of neurons that, in a healthy ear, encode auditory input in a train of nerve pulses. (See the article by Mario Svirsky, Physics Today, August 2017, page 52 .) A typical implant has 12–24 electrodes, each corresponding to a different frequency band. But the electrodes stimulate relatively large regions, making the effective number smaller, and the reduced spectral encoding can hamper speech perception in noisy environments.

One alternative being explored is optogenetics. In cells transfected with genes for light-sensitive proteins (opsins), light can trigger events with high spatial and temporal precision. Scientists at the University Medical Center Göttingen in Germany have shown that optogenetic stimulation of spiral ganglia in deaf adult gerbils can generate neuronal responses whose intensity dependence and timing closely mimic those of normal-hearing gerbils and partially restore auditory function. And when normal-hearing gerbils trained to respond to audible cues were subsequently deafened, they quickly transferred that learned behavior to optical cues.

In this x-ray tomograph of a gerbil’s inner ear, the colored region denotes the location of the ganglion. The team used viruses to transfer the opsin gene to the ganglion neurons; an optical fiber (not to scale) inserted into the ear delivered the triggering light pulses. The next step will be to examine the spectral resolution that can be achieved using multiple LEDs or other multichannel light triggers. (C. Wrobel et al., Sci. Transl. Med. 10, eaao0540, 2018, doi:10.1126/scitranslmed.aao0540 ; image courtesy of University Medical Center Göttingen.

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