The Neuroplasticity of the Brain After a Cochlear Implant

Within a muted city, wherein the traffic roars are unheard and the bustling crowds are silent, a burst of sound suddenly comes alive - all thanks to a tiny device nestled behind the ear: the cochlear implant (CI). CIs have revolutionized auditory communication for individuals with severe to profound hearing loss. Designed to bypass certain damaged or impaired structures in the ear, these devices showcase the brain’s capacity for adaptation through the interplay between technology and neuroplasticity. However, to understand how CIs function, the structures and processes involved in audition must  be recognized first.

Structurally, the ear can be divided into three parts: the outer ear, the middle ear, and the inner ear [1]. The ear canal, which is located in the outer ear, is the narrow tunnel through which sound waves enter [1]. The pinna, which is the visible flap of the outer ear, plays a key role in helping our ears capture sound, by directing sound waves into the ear canal in a process that is similar to the effect of a funnel [1]. As sound waves travel along the ear canal, they then reach the tympanic membrane, commonly known as the eardrum [1]. Sound waves that hit the tympanic membrane cause it to vibrate, and these vibrations are then transferred to the ossicles, which are the three tiny bones of the middle ear [1].  The mechanical vibrations of the ossicles amplify the sound vibrations generated from the tympanic membrane, which then cause vibrations in the cochlea, a structure to which the ossicles are connected [1]. The cochlea is a snail-shell-shaped, fluid-filled sensory organ that contains numerous receptors, called hair cells, lining a membrane known as the basilar membrane [2]. The vibrations from the ossicles, upon reaching the cochlea, cause the fluid within the cochlea to move [2]. The motion of this cochlear fluid, which corresponds to the sounds that are heard from outside, stimulates the hair cells within the cochlea, which are arranged based on the pitch of sound, from high to low pitch [2]. Once these hair cell receptors are activated, they send electrical signals through the auditory nerve, which transmits auditory information to the auditory cortex in our brain. The cortex is where information about sound is processed, integrated with our other senses, and becomes a part of our cognitive and other psychological processes in an extremely complex manner [2].

With such an intricate system, a failure in any one component of hearing may have devastating effects on the sensation and perception of sound. Such hearing loss can occur in two forms: conductive and sensorineural [3]. Conductive hearing loss is caused by an impediment in the outer ear that prevents the conduction of sound, which could be as minor as a lodged piece of dried earwax, or as severe as a ruptured eardrum or the accumulation of fluid in the middle ear during otitis media, which is a middle ear infection [3]. In these cases, hearing can simply be restored by clearing any existing obstructions [3]. On the other hand, sensorineural hearing loss deals with damage to the cochlea’s hair cells or the auditory nerve, which may result from genetics, trauma, or aging [3]. CIs are primarily used for individuals who have sensorineural hearing loss. However, a CI cannot be used for these patients if the auditory nerve is severely damaged, as the brain is unable to receive auditory information through its neural pathways even with a CI implanted in such instances [3]. Hence, the main goal of CIs is to bypass any problems in the transduction of sound into neural signals that may occur at the level of the cochlear hair cells.

 To achieve this purpose, CIs use a sound processor that is positioned behind the ear on the surface of the head to capture sound from the outside environment.Then, it transmits the sound to a surgically implanted receiver that is  underneath the skin behind the ear [4]. This receiver then sends signals to electrodes that are attached to the cochlea, which thereupon directly activate and stimulate the auditory nerve, so that it sends electrical auditory signals to the brain corresponding to the sounds that are “heard” by the sound processor, hence bypassing the transduction of sound by hair cells during audition [4]. Though this mechanism may seem straightforward at a glance, the brain must first learn how to decipher the messages sent by a newly implanted CI, a process that takes several months or even years [4]. At the foundation of this rehabilitative learning mechanism is plasticity, which serves a key role in allowing individuals to adapt to CIs.

More specifically, plasticity is the brain’s ability to rewire its neural pathways [5]. For example, it is crucial to help us learn new information and recover from traumatic injuries [5]. In the auditory cortex, there are two different types of plasticity: short-term and long-term plasticity [6]. Short-term plasticity, for instance,can occur in the form of stimulus-specific adaptation (SSA), a mechanism through which neurons that respond to a long-lasting or repetitive stimulus become less responsive to it for a short amount of time, usually until the stimulus is taken away or removed [6]. This plays a role in the brain’s ability to ignore irrelevant background noise [6]. Long-term neuroplasticity, however, is involved in processes that are longer lasting and result in changes that prevail for extended periods of time, such as when the brains of individuals who had been recently implanted with CIs need to make necessary adjustments in the auditory cortex and other auditory processing pathways to successfully hear sound as normal   [6]. 

Additionally, long-term neuroplasticity can also be either adaptive or cross-modal, both of which are particularly important when examining plasticity in the context of CIs [7]. Adaptive plasticity relates to the brain’s ability to alter its circuitry to compensate for and adjust to processing the newer “styles” of electrical information, like those sent from CIs when they are first implanted [7]. Alternatively, cross-modal plasticity is the brain's ability to compensate for the deterioration or loss of a sensory process such as hearing, vision, smell, or touch [7]. For individuals with CIs, the ear(s) affected by sensorineural hearing loss have been deprived of auditory sensation [7]. As a result, the brain compensates for the loss of hearing by reorganizing its neural pathways in a way that heightens the sensitivity of other sensory processes [7]. As a result of long periods of auditory deprivation, extensive reorganization occurs, inhibiting the brain from adapting easily to a CI [7]. To avoid such unfavorable outcomes with CIs, the timing of the implant must be considered, especially with regard to the sensitivity periods for audition  [7]. 

For patients with sensorineural hearing loss, sensitivity periods are indicated as the optimal times for CIs to be implanted[8]. For instance, CIs are most effective when they are implanted into prelingual children before the age of two [8]. However, in mature auditory cortices where there are few to no sensitivity periods, the brain’s ability to respond to CIs becomes difficult due to one’s previous experiences in life without the CI [7]. To achieve better outcomes for mature auditory cortices, post-implantation treatment plans most often require  speech and language therapists to more efficiently achieve functioning levels of auditory perception [9]. Furthermore, this therapy is usually individualized to optimize rehabilitation for individuals [9].

The transformative impact of the CI through adaptive adjustments and cross-modal adjustments showcases the brain’s neuroplasticity. The brain’s ability to adapt and integrate new pathways for electrical signals allows individuals with sensorineural hearing loss to gain auditory comprehension. Understanding the factors involved to optimize rehabilitation is crucial for regaining the sense of sound. Diving deep into the nuances of the brain’s auditory cortex with its ever changing neural networks allows for a redefinition of sound for thousands.

References

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8. Moreno-Torres, I., Madrid-Cánovas, S., & Blanco-Montañez, G. (2016). Sensitive periods and language in cochlear implant users. Journal of child language, 43(3), 479–504. https://doi.org/10.1017/S0305000915000823

9. De Raeve, L., Cumpăt, M.-C., van Loo, A., Costa, I. M., Matos, M. A., Dias, J. C., Mârțu, C., Cavaleriu, B., Gherguț, A., Maftei, A., Tudorean, O.-C., Butnaru, C., Șerban, R., Meriacre, T., & Rădulescu, L. (2023). Quality Standard for Rehabilitation of Young Deaf Children Receiving Cochlear Implants. Medicina, 59(7), 1354. https://doi.org/10.3390/medicina59071354