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...

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