Many people benefit from technological development. Being able to use a phone or personal computer makes your life a lot easier. Although it sometimes seems some people are not able to function without their device, as if it has become a part of their body, other people actually rely on devices to take over bodily or sensory functions. One such example is a cochlear implant (CI). This is a medical device that is used to provide the brain with input; more specifically it can restore some access to sound to people with severe to profound hearing loss. In this article, I will explain how a CI works and some of the benefits and challenges that come with this technology.
From pressure changes to electricity
In order to understand how a CI works, it is important to know a bit more about the auditory system and what sound actually is. Physically, sound is pressure changes in the air (or another medium) caused by an object's movements or vibrations. Usually, we can perceive this sound since the ear can transfer these pressure changes into electrical signals, and subsequently meaningful sounds and speech. The auditory signal takes the following path for this: first, the pinna picks up the sound, which then travels through the ear canal unit it arrives at the ear drum. The eardrum is set into vibration by the air pressure changes from the sound. This, you could say, causes a chain reaction: the vibrating of the eardrum causes the three smallest bones in your body, the ossicles (hammer, anvil and stirrup) to vibrate in succession, which amplify the vibration and transmit it to the oval window. This oval window is part of the cochlea, a snail-like structure filled with liquid, that consequently vibrates too. Within the cochlear lie various structures: the basilar membrane, organ of Corti and tectorial membrane, which are all set into motion. Most importantly, the hair cells start to move, which transduce the environmental stimulus waves into electrical signals. In these hair cells, the movement (bending of the hair cells) triggers a sequence of chemical reactions that lead to action potentials. Interestingly enough, these hair cells do not vibrate in a random fashion. Both their pattern of vibration and intensity convey properties of the sound signal. The pattern or place of the vibration within the cochlea conveys the pitch of the sound; this is called tonotopy. These vibrations trigger electrical signals, action potentials, which are then sent through the auditory nerve to the brainstem and from there are transmitted to higher-order structures of the brain, such as the auditory cortex.
Technology to the rescue
However, some parts of this auditory pathway might not function properly. For every step up to the brainstem there is a device that can be used to restore hearing: from a hearing aid that amplifies incoming sound to bone-conduction devices, a cochlear implant and an auditory brainstem implant. Of these devices the CI is, after the conventional hearing aid, the most widely used. CIs can help people with severe to profound hearing loss to receive a sensation of sound by directly stimulating the auditory nerve fibers in the inner ear. In terms of technology, it consists of an external microphone that picks up the sound, a speech processor that processes the sound, a transmitter that sends the signal to the implanted receiver (coil), and an electrode array that is placed inside the cochlea. That sounds amazing, right? A device that lets almost and completely deaf people hear again. And not only that, but indirectly it also leads to less depression, social isolation, unemployment, and more independence.
Do not forget the brain
Although a CI might lead to great results, it is not “plug and play”. The sound that is produced by a CI is less nuanced than natural hearing because it is processed by a little computer. The sound can be described as sounding similar to a robot voice. It, therefore, takes some time to get used to before speech can be understood from this distorted signal. This shows that a CI really is a BCI (Brian Computer Interface), it is not only important to develop a good device, but it interacts with the brain which in turn has to adjust to the device and vice versa. Luckily there are professionals, called audiologists, that fit (tune) the device to the CI user's needs and guide them during rehabilitation. There are many different settings, from the way that sound (and background noise) is processed, to the way that the auditory nerve is stimulated. But researchers are also trying to understand how the brain actually deals with the signal and how effortful listening with a CI is. Because even though someone might understand perfectly with a CI, it might be that they still need to put a lot of effort into achieving this.
If you are interested to learn more about this topic
For Dutch-speaking people, in the podcast “Met Hertz & Ziel – de rol van cochleaire implantaten” three Flemish audiologists discuss cochlear implants. They not only describe, for example, how a CI works, but they also discuss how CIs changed the lives of some of the users, why they cannot predict how well someone will perform with a CI, how audiologists fit the device when someone is not able to communicate how well they perceive sounds, or what drives the decision to get or not get a CI from both a clinical and user perspective. https://open.spotify.com/episode/135M0GXTfEAIdY06gxFRed
If you like to learn more about hearing loss in general: https://www.who.int/news-room/fact-sheets/detail/deafness-and-hearing-loss
Or about what it might cost to not address hearing loss (which ranges from 6-30 billion for several European countries): https://adulthearing.com/wp-content/uploads/2019/12/Spend_to_Save_The_Ear_Foundation_2016_1-1.pdf
Author & Illustrations : Loes Beckers
Goldstein, E. B. (1999). Sensation and perception (5th ed.). Brooks/Cole Pub.