This blog post explores how cochlear implants help those who have lost their hearing regain sound, examining their principles and limitations.
What should you do if a tire loses air? The answer is simple: refill it with air. If the tire has a puncture or other issue making refilling impossible, you can purchase and replace it with a new tire. While replacing a tire can be a bit of a hassle and cost money, this method can be the ultimate solution. But what if your ears can’t hear well? In this case, the situation can be a bit more complicated. Of course, wearing a hearing aid can help you hear better. Hearing aids provide a simple yet effective solution for many people experiencing hearing loss. But what if you still can’t hear well even with a hearing aid? The answer is similarly simple: replace your ears!
Hearing sound involves vibrations originating from a sound source being transmitted as vibrations through a medium. These vibrations then cause the outer and middle ear to vibrate. This leads to vibrations in the inner ear’s fluid and tiny hair cells. This stimulates the auditory nerve, sending electrical signals to the auditory center. If any part of this process, starting from the outer ear (the sound’s entry point into the body) and extending to the brain (where sound is interpreted), malfunctions, problems with hearing occur. The causes of hearing loss are highly diverse. When problems occur in the outer or middle ear, preventing the physical transmission of sound waves, this is called ‘conductive hearing loss’. In such cases, wearing a hearing aid to amplify sound can help. However, amplifying sound does not solve all types of hearing loss. Hearing aids only increase the volume of sound; they do not address the sensory elements necessary for recognizing sound.
On the other hand, when problems arise in the hair cells inside the cochlea, preventing the physical signal from being transmitted to the auditory nerve, this is called ‘sensorineural hearing loss’. In this case, hearing aids alone are often insufficient. Hearing aids receive sound input, convert it into a digital signal, amplify it, and then transmit it back as sound waves. This method is limited to simply amplifying sound. Therefore, it is inevitably less effective for patients with sensorineural hearing loss who have difficulty processing the sound stimulus itself. For them, it is not about increasing the volume of sound, but about regaining the ability to hear the sound itself.
A ‘cochlear implant’ is a device surgically inserted into the cochlea, where the damaged hair cells are located. Much like replacing a flat tire with a normal one, it takes over the role of the hair cells: converting sound waves into electrical signals. A cochlear implant consists of two main parts: the external sound processor worn on the body and the implanted component placed inside the body. First, when sound arrives, the external microphone receives the analog signal (sound waves). This signal is then converted into a digital signal by the sound processor. This signal is transmitted via a coil to the implant inserted inside the cochlea. The implant converts this digital signal into electrical stimulation signals, outputting electrical stimulation through electrodes. This outputted electrical stimulation directly stimulates the auditory nerve, enabling patients with hearing loss to hear sounds at a normal level. Because it directly stimulates the auditory nerve, the cochlear implant can provide a solution for all cases of hearing loss, except when the auditory nerve or the auditory center itself is damaged.
However, the cochlear implant is not a perfect solution. First, unlike hearing aids that can be used simply by wearing them, a cochlear implant requires a separate surgical procedure to insert the implant into the cochlea. Surgery is not suitable for all patients, and even after surgery, a certain recovery and rehabilitation process is necessary. In particular, training to learn new auditory experiences is essential after cochlear implant surgery. Additionally, the cable connecting the implant to the external sound processor must also be placed inside the body. Therefore, the possibility of side effects from the surgery cannot be ruled out. Furthermore, while cochlear implant surgery is intended for patients with minimal residual hearing, the process of inserting the implant removes any remaining hair cells, resulting in permanent loss of that residual hearing. Consequently, a cochlear implant is not suitable for all individuals with hearing loss.
Finally, the sounds delivered to the person through the cochlear implant do not precisely match the sounds heard by a person. This is because the cochlear implant ultimately converts sound waves into electrical stimulation, and this process does not perfectly match how the human auditory system ultimately converts sound waves into electrical stimulation. Because of this, patients who undergo cochlear implant surgery must relearn how to process auditory stimuli delivered through the device, necessitating a rehabilitation training process. Auditory training can take time, and the quality of perceived sound may vary depending on the extent of training.
Therefore, the ultimate goal of cochlear implants is to make the sounds heard through the implant indistinguishable from those heard through a normal ear. Progressive research continues across multiple fronts, including the signal transmission process between hair cells and the auditory nerve, the form of the final stimulus received by the auditory nerve, and the technology for generating artificial stimuli. While technological advancements in recent years have significantly improved cochlear implant performance, there remains considerable room for further enhancement. This is because the human auditory system is highly complex, and perfectly mimicking its intricate processes is challenging. However, thanks to these efforts, creating a perfect spare ear is no longer just a dream.
When faced with situations where desired sound information cannot be obtained—such as malfunctioning speakers or earphones, or videos with out-of-sync audio—one is reminded anew of the importance of sound. As a technology that gifts a new ear to hearing-impaired patients living in an unintentionally muted world, the cochlear implant will serve as a signal flare shattering the silence that once seemed eternal.
