You may have heard of cochlear implants. It is an electronic device also referred to as neuroprosthesis, which acts as a bionic replacement for the human ear. These implants have improved the hearing of thousands of people around the world.
However, cochlear implants are not the only sport in town. The auditory brain stem implant is another device that promises to bring them a sense of sound without it, albeit on a different route.
Although the cochlear implant itself is a very complex device, the basic idea behind it is simple. The normal mechanics of the ears, which receive vibrations from the air and turn them into nerve signals, are completely bypassed. Instead, a small electronic device captures sound with a microphone. The word is then processed, with priority on the highest perception of audible speech. This processed term is used to drive an array of electrodes implanted in the cochlea. These electrodes stimulate the cochlear auditory nerve, enabling the wearer to perceive sound.
Auditory brain stem implants (ABI) are similar to cochlear implants in many ways. The basic theory is exactly the same: the audio is captured electronically, and then used to stimulate the nerves to provide hearing to the brain. What makes ABI different is that it completely crosses the cochlea inside the ear. Instead, ABI stimulates electrodes located in the cochlear nucleus of the brainstem.
Thus ABI has the advantage that it can provide a hearing aid to patients who, for whatever reason, cannot have a cochlear implant inserted into the auditory nerve of the inner ear. Patients with a condition called neurofibromatosis type 2 (NF2) were the primary group to use ABI. NF2 is a condition that affects the nervous system and treatment associated with it often causes damage to the auditory nerve. Thus, for patients with this condition, an ABI is appropriate where a conventional cochlear implant would be ineffective. In cases where the cochlear auditory nerve may be damaged or destroyed, an ABI may apply.
However, the drawback of ABI is that it requires much more complex implants than a cochlear implant. Surgery involves opening the skull to access the brain stem, which is much more invasive than the simple procedure required to implant a cochlear device in the inner ear.
Outcomes for patients like cochlear implants are not as successful when it comes to comprehension. Combining the use of ABI with leapreading techniques, many patients go on to learn to understand speech, but very few people can understand speech by relying solely on ABI.
It is basically down to place the electrodes. The cochlea itself has a fairly simple map of an area that corresponds to high and low tones, which can be stimulated directly by implants. However, when electrodes are inserted into the brainstem, these regions are difficult to accurately map and stimulate, and thus an ABI will struggle to deliver as much tonal information to the brain as a cochlear implant.
Low performance, more invasive implantation methods, and unclear application of ABI mean that cochlear implants are much more commonly used in practice. More than 700,000 cochlear implants have been implanted worldwide. However, only a few thousand ABI devices have been installed the most.
Although ABI results may not be as good as cochlear implant standards, these bionic devices still have value. For patients who cannot use cochlear implants at all, ABI still provides a basic hearing aid that can be effective, especially when it comes to environmental noise. Overall, this is an interesting application of the same technology as cochlear implants, but dialed in a uniquely specific use case.