Cochlear implants can bring the experience of sound to people with hearing loss, but results may vary – here’s why

Cochlear implants are among the most successful neural prostheses on the market. These artificial ears have enabled nearly one million people worldwide with severe to profound hearing loss to regain access to the sounds around them or experience the sense of hearing for the first time. times.

However, the effectiveness of cochlear implants varies considerably from user to user due to a range of factors, such as the duration of hearing loss and age at the time of implantation. Children who receive implants at a younger age may be able to learn hearing skills similar to their peers with natural hearing.

I am a researcher studying pitch perception with cochlear implants. Understanding the mechanics of this technology and its limitations can help lead to potential new developments and improvements in the future.

How does a cochlear implant work?

In fully functional hearing, sound waves enter the ear canal and are converted into neural impulses as they travel through hair-like sensory cells in the cochlea or inner ear. These neural signals then travel through the auditory nerve behind the cochlea to the central auditory areas of the brain, resulting in perception of sound.

People with severe to profound hearing loss often have damaged or missing sensory cells and are unable to convert sound waves into electrical signals. Cochlear implants bypass these hair cells by directly stimulating the auditory nerve with electrical impulses.

Cochlear implants consist of an outer part rolled up behind the ear and an inner part implanted under the skin.

The external unit, which includes a microphone, a signal processor and a transmitter, picks up and processes sound waves from the environment. It divides sounds into different frequency bands, which are like different channels on a radio, with each band representing a specific range of frequencies in an overall sound spectrum. It also extracts information about the amplitude, or volume, of each frequency band.

It then transmits this information to the receiver of the internal unit implanted in the cochlea. The internal unit’s electrodes directly stimulate the auditory nerve with electrical impulses based on amplitude information. Electrodes at the base of the cochlea transmit electrical signals containing high-frequency auditory information while electrodes at the top transmit electrical signals containing low-frequency information to the brain, mimicking frequency scanning in a fully functioning ear.

Where cochlear implants fall short

While people with cochlear implants are able to reasonably detect sounds and perceive speech in quiet environments, they often have great difficulty understanding speech in noisy environments, appreciating music, and localizing sounds. , i.e. to determine from which direction a sound comes.

Cochlear implants are fundamentally limited by their poor ability to discriminate between sound frequencies and to transmit rapid changes in sound amplitude over time. For example, current cochlear implant systems use only 12 to 22 electrodes to stimulate surviving auditory nerve fibers, whereas natural hearing has 30,000 auditory nerve fibers to encode detailed information about incoming sounds. In addition, stimulation by electrodes inside the cochlea excites a large group of auditory nerve fibers without much precision.

These factors lead to poor frequency resolution. Imagine it as a painting with a thick brush that can only show an overall shape without the fine details, or only blurry details.

Why cochlear implants work better for some

It remains difficult to accurately predict the performance of cochlear implants for each user.

There are a variety of factors that can affect the number of healthy auditory nerve fibers available to transmit acoustic information to the brain. Cochlear implant users with better survival of their auditory nerve fibers may have improved frequency and timing representations of sounds represented by electrical stimulation, which may lead to improved speech and pitch perception.

Neural health is not the only factor that contributes to variability in the effectiveness of cochlear implants. A 2012 study of 2,251 cochlear implant users found that speech recognition varied widely and only 22% of the difference could be explained by clinical factors such as length of experience with the implant and the cause of the hearing loss. Moreover, it is difficult to directly assess the effects of neural survival on the performance of cochlear implants. This suggests that other factors also play a role in determining the success of speech recognition with cochlear implants.

For example, research has shown that cognitive skills such as working memory can influence how well a person can understand speech after implantation. Cochlear implants increase cognitive load, or the amount of mental effort needed to perform a task, because the sound quality users hear is often lower than that of natural hearing. Aging can also negatively affect cognitive processing abilities, including attention deficits and slower processing speed of listening tasks.

Additionally, most of the implant’s electrode arrays do not reach the top of the cochlea where low-frequency information is transmitted in natural hearing. This leads to mismatches between the frequencies carried by the implant and those of natural hearing, which results in reduced sound quality.

Improving cochlear implants

Scientists are studying a number of potential ways to improve the effectiveness of cochlear implants.

Hearing sound through electrical stimulation is a new experience for those accustomed to hearing without an implant. Auditory training exercises can help familiarize users with this new form of hearing and can even improve overall speech and music perception. However, even with training, conventional cochlear implants may not fully replicate the rich experience of natural hearing.

Researchers are investigating the potential use of light beams instead of electrical pulses to achieve better frequency resolution. This is done by genetically modifying the fibers of the auditory nerve to make them sensitive to light. Since light beams are able to more selectively stimulate auditory neurons compared to electrical impulses, this tactic can result in more accurate frequency information. The research team behind this approach aims to start clinical trials in 2026.

Another approach is to insert electrodes directly into the auditory nerve fibers instead of the cochlea. By increasing the number of available electrodes, this strategy can improve sound frequency and timing information from the implant, and improve speech understanding in noisy environments and music perception.

Finally, another development uses magnetic stimulation to transmit acoustic information via small implantable microcoils. This approach allows for finer stimulation patterns than generalized electrical activation of traditional electrodes, potentially leading to more accurate sound representation.

Research into new technologies can provide solutions to further improve the hearing experience of people with hearing loss.

Niyazi Arslan, Ph.D. Speech and Hearing Science Candidate, Arizona State University

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