SciTech

How Things Work: Cochlear Implants

Superhuman hearing is usually something left to comic books, movies, and even fantasies of a more-advanced, future human race.

But this seemingly impossible future prospect is not as far away as you may think. Cochlear implants are one of the newest advancements in the bionic augmentation of human hearing. These devices are still in the development phase, but even the earliest versions of cochlear implants have successfully restored hearing to people who were once deaf.

Conventional hearing aids are simply crude sound amplifiers mounted outside the ear. Cochlear implants function in a somewhat more sophisticated way. Cochlear implants are essentially microphones connected directly to the auditory nerve endings within the ear, transmitting electrical impulses to the nerve endings whenever sound is detected. Hence, these devices interface directly with the brain, causing the sensation of hearing.

In essence, a cochlear implant replicates the function of the cochlea, which is the primary organ for detecting sound within the ear. The cochlea is composed of a long spiral of tissue covered with nerve endings. Sound that enters the ear travels along this tissue, creating vibrations that are picked up by the nerve cells connected to it. Low-frequency sounds travel much farther along the tissue than high-frequency sounds, and correspondingly, different nerve sections along the cochlea deliver impulses that are interpreted by the brain as different frequencies.

Inserting the implant itself involves disconnecting the biological cochlea to expose the nerves for connections, hence destroying any form of hearing the person might have originally possessed. This is the main reason why tests have been strictly limited to those who already have severe hearing losses and no other means of hearing. As less than one percent of deaf people have actually suffered damages to the auditory nerve, they are currently the only viable candidates for such techniques.

Apart from that, another danger that is associated with cochlear implants is cerebrospinal fluid leakage, which is a leakage of the fluid that surrounds the brain and spinal chord. Meningitis and facial paralysis are also possible dangers, although these are largely due to the surgical procedure rather than the implant itself.

Once implanted, the cochlear implant is directly hooked up to these nerve cells in place of the cochlea. Tiny electrical impulses are generated by the implant when any sound is detected, stimulating the appropriate set of nerves, which then transmit information to the brain.

Current models of cochlear implants, however, are far from achieving the hearing quality of real cochlea. The electrodes used by the implant to stimulate the nerve endings cannot accurately mimic the signals created by the original auditory nerve, causing most sounds to be initially heard as little more than incomprehensible noise.

Fortunately, scientists are discovering that the brain is much more adaptable to these implants than previously thought. As the brain learns to accept the new input, it acquires the ability to differentiate between various sounds and can even begin recognizing speech. Combined with lip-reading techniques, cochlear implants have successfully returned above-average speech recognition in many adults.

Of course, the question remains: Would cochlear implants truly be able to grant us super-hearing someday in the future? In principle, yes. Just like the volume on a microphone can be tuned up or down, the sensitivity of the cochlear implant could theoretically be adjusted for such applications. Similarly, it would not be hard to modulate frequencies received so that we might one day be able to hear sounds outside of human auditory range, such as ultrasonic squeaks or subsonic tremors.

This implant thus opens up a whole new world of possibilities for the human ear. People with the current form of cochlear implants have reported hearing a buzzing noise produced by electromag- netic fields, such as metal detectors or MRI scanners.

If designed specifically for this task, it is conceivable that one day cochlear implants may reveal to us entirely novel forms of communication. This might begin with implanted radios or cell phones and concievably culminate in bionic implants that transmit brainwaves in psychic communication.