How Metal can regain its Sound: the Science of Cochlear Implants

Posted by megkirch on April 29, 2021 in Neuroprosthetics, Neuroscience in pop culture | Leave a comment

This article discusses various scenes from the film Sound of Metal. Although what is discussed likely will not come as any great surprise to anyone generally familiar with the film, be aware that mild “spoilers” are ahead! If you haven’t yet seen the film but choose to read on, this article contains some clips from the film that are most relevant. 

The Oscar-nominated film Sound of Metal takes a jarring turn just 10 minutes in. One moment there are the typical sounds of conversation and ambient noise you would expect from a film’s audio track; the next, a high-pitched ringing, muffling those previous sounds into a distant, barely perceptible background murmur. This represents the first-person experience of protagonist Ruben (played by Riz Ahmed, a role for which he was nominated for a Best Actor Academy Award), a heavy metal drummer in his first moments of rapid hearing deterioration.

In just a few days time, Ruben’s hearing decomposes into near-complete deafness, rendering him incapable of understanding speech or hearing more than the rhythmic vibrations of his drums. Thus Sound of Metal unfolds into a film about loss, identity, addiction, and grappling with deafness as a culture to be embraced or as a devastating condition to be “cured”. While the film has received some criticism for setting up this false choice, it has also been extensively praised for its sensitive portrayal of the deaf community and for its astonishing, Oscar-winning sound design. In absence of a traditional cinematic score, the movie’s soundtrack consists of alternating tracks of third-person soundscape and Ruben’s acoustic experience – both as he plunges into deafness and as he resurfaces into hearing with the aid of cochlear implants.

This is the point at which neuroscience gains a real presence in the film (and not just because the director Darius Marder and his co-writer and brother Abraham Marder are nephews of neuroscience icon Eve Marder). Ruben’s cochlear implants become a significant plot point – not only for their reusherance of Ruben’s hearing, but also for their limitations. Through both storytelling and innovative sound design, Sound of Metal – whether intentionally or not – highlights the incredible innovation of cochlear implants as well as the immense challenges inherent in trying to override the brain’s built-in systems for perception. 

The neuroscience of cochlear implants

If you aren’t already familiar with the ins-and-outs of the auditory system and the technicalities of cochlear implants (like Ruben in his first meeting with an audiologist), let’s review. Sound waves (i.e. changes in air pressure) are transduced into electrical signals – the currency of the central nervous system – by specialized types of cells called hair cells that reside deep in the inner ear. These hair cells are positioned in such a way that sound waves, when they reach the inner ear, cause these cells to bend. This movement tugs on physical links between the “hairy” parts of the hair cells (called stereocilia), literally pulling open ion channels in the hair cell membrane. This leads to ion flow that in turn triggers the release of neurotransmitters from the hair cells to the auditory nerve fibers, thus initiating the cascade of electrical signals from the ear to the brain that ultimately give rise to auditory perception. 

Importantly, these hair cells reside within an inner ear structure called the cochlea. The cochlea is normally curled up and resembles a conch shell, but were you to uncoil it, you would encounter a cylindrical, fluid-filled structure that is wider on one end and narrower on the other. Hair cells reside in the middle down the cochlea’s full length (typically 35mm!!), and the pattern of where in the cochlea hair cells are activated and when is determined by the frequency(ies) of the sound. Specifically, the narrower end (“base”) is activated by high frequencies and the wider end (“apex”) by low frequencies [1]. 

This discovery, referred to as the place theory of hearing, paved the way for the development of cochlear implants. When the inner ear machinery fails to transduce sound into electrical signals (usually due to hair cell damage), an electrode array that is part of the implant essentially bypasses the hair cells and directly stimulates the auditory nerve fibers down the length of the cochlea. Crucially, this stimulation isn’t uniform but is instead determined by the frequencies contained in the incoming sounds. This is accomplished by the implants’ multiple components: a microphone that receives incoming sound signals, a sound processor that identifies the frequencies in those signals, a transmitter, and an implanted receiver that then stimulates the electrodes in a frequency-dependent manner. Thus, the electrical stimulation provided by the implants simulates the normal effects of sound vibrations on the cochlea. 

The pitfalls and promises of neuroprosthetics and plasticity

Cochlear implants accomplish the remarkable feat of supplanting the auditory signals that go from the ear to the brain when the inner ear machinery fails to produce those signals on its own. However, those initial signals on their own are insufficient for “hearing”; the brain needs to be able to make sense of those signals to achieve auditory perception, and this presents some considerable challenges.

We get our first glimpse of this in Sound of Metal when we witness the activation of Ruben’s cochlear implants. His audiologist connects the sound processor and starts tweaking its settings to adjust volume, pitch, etc., and the film’s sound design allows us to experience how those different settings affect Ruben’s ability to hear. While some settings are better than others, none sound especially “normal”. Even though Ruben can begin to hear and understand his audiologist’s speech, the whole soundscape is intensely tinny, as if run through some electronic filters (which is, essentially, what is actually happening). But why does it sound so…off? At first Ruben appears amazed and elated at the sudden reappearance of his ability to hear, but that excitement dissipates into some amount of disappointment; it’s better than nothing, but far from what he remembers. Importantly, cochlear implant users’ experiences with implant activation is highly heterogenous (for instance, not all are able to immediately understand speech, as was the case for Ruben). But why is cochlear implant-enabled hearing imperfect and so variable?

First, cochlear implants themselves have technical limitations. Most implants may not be able to cover the full length of the cochlea, which can lead to higher-frequency sounds stimulating auditory nerve fibers that normally receive input from lower-frequency areas of the cochlea [2]. Furthermore, the sound resolution that’s possible with the implants’ limited number of stimulating electrodes (usually 22) will inevitably be coarser than what’s possible with thousands of functioning hair cells. This limited resolution will especially impact one’s ability to sense some of the subtler aspects of sound, such as pitch and timbre – aspects that are crucial components of music. In a wrenching scene in Sound of Metal, not long after his implant activation, Ruben is at a party listening to his girlfriend sing while her father accompanies on piano. At first, the viewer experiences their performance from the perspective of the average party-goer, and we appreciate the beauty and emotionality of their duet. But then the perspective shifts to Ruben’s, and we hear an aggressively tinny cacophony that is painfully wanting for beauty. For someone like Ruben whose life once revolved around music, this comes as a gut-punch. 

How the brain receives and uses auditory signals – whether coming through intact inner ear machinery or from a cochlear implant – is what ultimately determines auditory perception. Thus, perhaps Ruben’s deprived auditory experience is due to the auditory areas of his brain being activated differently by his cochlear implants than when his hearing had been intact. Indeed, studies in animal models with only one cochlear implant have allowed for directly examining neuronal activity in the auditory cortex – an important part of the brain for auditory perception that receives auditory input coming from the ear – in response to auditory input through an intact ear compared to through an implant. These studies have revealed pronounced differences in auditory cortical activity, with considerably less activation produced by cochlear implant stimulation [3]. 

Another important factor, and a key source of variability among cochlear implant users’ experiences, is that deafness itself comes in many forms. Some may have been born deaf (congenitally deaf) or lost their hearing before learning to speak, whereas others acquire it later through injury, illness, or some other source of degeneration. Considering that the auditory cortex undergoes a critical period of development and refinement through early-life auditory experience, an early insult to the system or congenital deafness can lead to significant changes in its organization and function. One example of this is cross-modal plasticity; essentially, valuable cortical real estate normally reserved for auditory processing can, in the absence of auditory input, get repurposed to process other types of inputs, such as visual inputs (e.g., sign language; [4]). While some degree of cross-modal plasticity may also occur if hearing is lost later in life, its extent will be less than if the onset of deafness is before or during the auditory cortex’s critical period. 

Particularly important for the utility of cochlear implants is another form of brain plasticity, referred to as adaptive plasticity. After all, cochlear implants provide the brain with new auditory signals, and the brain is then responsible for constructing our conscious perception out of those signals. The auditory cortex (and other auditory centers of the brain) will have been largely deprived of structured input for the extent of deafness, and the precise signals provided by the implant will differ from the signals which would have been delivered organically by an intact auditory system. Thus, the brain must learn and adapt in order to make sense of these new signals. 

Because adaptive brain plasticity presents considerable opportunities for improving cochlear implant outcomes, much current research is focused on how best to promote constructive plasticity.  On one hand, some evidence suggests that cross-modal plasticity (as discussed above) can actually be detrimental [2]. On the other, studies of outcomes with human cochlear implant users indicate that extended training with their new implants can significantly improve their hearing. For instance, the ability to decipher speech often emerges through training even in individuals who were not initially able to do so upon their implant activation. This perceptual change is often accompanied by reductions in other aspects of hearing (such as of background noise), suggesting an improvement in “signal-to-noise” in the brain [2]. 

Sound of Metal leaves open many questions about Ruben’s future with his new implants, and the film concludes before we have the chance to follow him in his journey with auditory adaptation. While some have interpreted the film to be anti-implants, this writer’s perspective was instead that cochlear implants are portrayed with realistic nuance. After all, the immense complexity of the auditory system makes it so that for many, the device may not be an immediate “miracle cure”. But at the same time, that complexity presents hopeful opportunities for adaptation and improvement, and also makes cochlear implants’ ability to rescue hearing to the extent that it does all the more remarkable. 

References:

[1] Goldstein, B.E. Sensation and Perception, 9th edition. Belmont, Calif: Wadsworth Pub. Co.

[2] Glennon, E., Svirsky, M.A., and Froemke, R.C. (2020). Auditory cortical plasticity in cochlear implant users. Curr Opin Neurobiol 60, 108–114.

[3] Johnson, L.A., Santina, C.C.D., and Wang, X. (2016). Selective Neuronal Activation by Cochlear Implant Stimulation in Auditory Cortex of Awake Primate. J Neurosci 36, 12468–12484.

[4] Nishimura, H., Hashikawa, K., Doi, K., Iwaki, T., Watanabe, Y., Kusuoka, H., Nishimura, T., and Kubo, T. (1999). Sign language ‘heard’ in the auditory cortex. Nature 397, 116–116.