Anatomy and Physiology of Cochlear Implants

Cochlear Implants

Introduction

Hearing loss impacts a large number of people at various points in their lives¹. Whether a parent or grandparent has a hearing aid, or a loud concert leaves the ears ringing, there is a demand for hearing improvements. According to the National Center for Health Statistics, around 18 percent of American adults reported trouble with hearing in 2012¹. The World Health Organization estimates that 5 percent of the world’s population currently has disabling hearing loss and expects that number to grow to 10 percent by 2050². People with hearing loss have used different types of hearing aids throughout history. From ear trumpets and animal horns used to funnel and amplify sound, to the electronic hearing aids of today³, hearing technology has come a long way. The most recent invention in hearing technology does what was once thought impossible: restore hearing in patients with sensorineural or “nerve” hearing loss⁴. Videos^5,6 have been emerging across the internet of loved ones whose hearing has been restored once a device called a cochlear implant is turned on, but the mechanism to restore their hearing is much more complicated. The implant uses electrical stimulation in different areas of the patient’s cochlea to simulate sound⁴. This paper presents a closer look at how cochlear implants work, engineering considerations in their design, and the need for biomedical engineers in their research and development.

Why Cochlear Implants are Necessary

There are two major categories of hearing loss: conductive and sensorineural⁷. Each is treated by a different device: hearing aids treat conductive hearing loss and cochlear implants treat sensorineural hearing loss. Conductive hearing loss occurs when sound waves are not able to be transmitted through the outer and middle ear due to blockages within the ear⁷. Sensorineural hearing loss occurs when the transmission of nerve impulses in the inner ear is impaired by damage to the hair cells in the cochlea⁷. The damage is usually caused by exposure to loud noises but may also be caused by various diseases, head trauma, genetics, or aging⁷. Hearing aids do not help people with sensorineural hearing loss⁷, so a different device is necessary for treatment.

History of Cochlear Implants

The development of the first cochlear implant was influenced by the discovery that electric current could convey sounds to the brain⁴. In the 1960s, three groups of scientists began to research how that discovery could be used to help patients with sensorineural hearing loss⁴. One group, William House and John Doyle, found that the strength of the stimulus affected the volume of the “sound” in the brain, and the frequency of stimulus affected the pitch⁸. These discoveries led them to create the first single channel cochlear system, meaning only one electrode was used⁸. Around the same time, the Cochlear Company built the first multichannel cochlear system, meaning an array of electrodes was used⁹. In 1982, MED-EL launched its first multichannel cochlear implant¹⁰. Today, the three major cochlear implant manufacturers marketed in the United States are MED-EL, the Cochlear Company, and Advanced Bionics⁴.

Anatomy and Physiology of the Ear

To understand sensorineural hearing loss and how it is rectified, the anatomy of the ear must be explained. Sound travels from the outer ear to the inner ear through a series of steps. An image of the ear’s anatomy can be found in the Appendix as Figure 1. First, sound must funnel through the pinna into the external auditory canal, and vibrate the tympanic membrane (the eardrum)^7,11. These vibrations then pass through three bones in the middle ear: the malleus, the incus, and the stapes^7,11. The stapes covers the oval window, which leads to the inner ear^7,11 where the cochlea is located^7,11. The vibrations of the stapes lead to vibrations in the cochlea and the formation of action potentials that the brain interprets as sound⁷.

The cochlea is a shell-like structure that consists of a tube coiled around itself^7,11 and is composed of three channels: the scala vestibuli, the cochlear duct, and the scala tympani⁷ (see Figure 2). The auditory nerve and the organ of Corti (the organ responsible for hearing) pass through all turns of the cochlea⁷. Vibrations pass through the scala vestibuli to the cochlear duct, causing vibrations in the tectorial membrane to move against hair cells⁷. Hair cell movement causes generator potentials when their stereocilia are displaced⁷. The potentials travel down the auditory nerve to the auditory center of the brain to be interpreted^7,11. Different portions of the cochlea detect different frequencies, with the innermost area of the spiral detecting lower frequencies and the outermost detecting higher frequencies⁷. Depending on the frequency of the sound that travels to the ear, hair cells in the designated regions of the cochlea will be displaced⁷. The displacement sends signals to the brain that indicate a sound at a certain pitch is being heard⁷. If a sound is louder, the stereocilia bend more, which causes more frequent action potentials from that region of the cochlea⁷.

Engineering Aspects of Cochlear Implants: How They Work

Cochlear implants consist of an external microphone, processor, and transmitter, as well as an internal receiver and electrode array⁷ (see Figure 3). The microphone takes in sound from the environment and transmits that sound to the processor^12,13. The processor is a bit more complicated. It takes the acoustic vibrations from the microphone and converts them into electrical stimuli that the auditory nerve can interpret^12,13. The processor sends the electrical stimulation patterns to the transmitter¹⁴, which is housed externally behind the ear^12-14. The transmitter sends the signals to the receiver, which is housed internally on the temporal bone⁷, in a style known as a transcutaneous link¹⁴. A percutaneous connector can also be used¹⁴ and will be discussed in the Design Considerations section. The receiver sends the signals to the electrode array in the cochlea, which directly stimulate different sections of the auditory nerve^{12, 14}. The location of stimulus causes the electrical stimuli to be perceived as different pitches of sound⁷.

Engineering Aspects of Cochlear Implants: Design Considerations

The design considerations to be discussed include: sound coding strategies, transmission link style, electrode design and placement, and the patient. A major engineering design consideration for cochlear implants, especially in terms of improving implants¹⁴, is the sound coding strategy. The strategy chosen can affect speech perception, directional hearing, sound quality, and perception of music and tone¹⁵. There are a number of strategies to choose from, including continuous interleaved sampling (CIS), spectral peak (SPEAK), advanced combination encoder (ACE), and countless others¹⁴. The listed strategies are found in the three FDA-approved cochlear implant companies’ devices¹⁴. CIS is the simplest coding strategy used in cochlear implant design and will be explained in detail because it is the most widely used among cochlear implant manufacturers¹⁴. CIS was developed in response to the Compressed Analog (CA) approach, which filters and sends four different analog waveforms to four electrodes in the cochlea¹⁶. All of the electrodes can stimulate different parts of the cochlea at the same time, which produces problems with distortion caused by nearby electrodes¹⁶.

The CIS approach corrects the distortion by sending non-simultaneous, interleaved pulses to the electrodes, meaning only one electrode is stimulated at a time¹⁶. Figure 4 shows a diagrammatic representation of the CIS approach. To cause the desired electrode stimulation pattern, the signal must first be pre-emphasized¹⁶. Pre-emphasis causes certain frequencies of the signal to be accentuated by increasing their amplitudes¹⁷. The pre-emphasis allows weak consonants to compete with vowels that are commonly more intense in the signal¹⁴. The signal is then sent through a series of band-pass filters that split it into parts¹⁶. Full wave rectification and low-pass filtering are used to modify the waveforms^14,16. Full wave rectification means the signal is converted from alternating current to direct current¹⁸ (see Figure 5). Low pass filtering in CIS means that frequencies of less than 200 or 400 Hz will be outputted¹⁶. These steps result in envelopes: smooth curves outlining the extremes of the signal¹⁹ (see Figure 6). A logarithmic compression function ensures that the output envelopes will be within the range of the patient’s electrically-evoked hearing¹⁶. The envelopes modulate trains of biphasic pulses that are delivered to the electrodes at a non-overlapping, constant rate^14,16. The modulation results in the biphasic pulses matching the amplitudes of the envelopes and delivered to the correct area of the cochlea¹⁶. The faster the biphasic pulse rate, the better the patient will recognize speech and sounds¹⁶.

ACE and SPEAK are both “n-of-m” strategies that split speech signals into m bands, where a subset of n bands with the largest amplitudes is then used for stimulation²⁰. The goal is to only allow important signals to be heard by the patients, and implants using an n-of-m strategy have a significant user preference over CIS strategies²⁰. Cochlear implant manufacturers and researchers have developed a wide number of other coding technologies that improve speech articulation, hearing in either low or high frequencies, or the ability to recognize environmental sounds¹⁴. A description for each of the technologies will not be given.

Another engineering design consideration for cochlear implants is whether the transmission link should be transcutaneous or percutaneous. A transcutaneous link was described in the How They Work section. In summary, the transmitter of the cochlear implant is external, and the receiver is internal; they are held together by magnets¹⁴. The components of the implant create two disadvantages for the transcutaneous link: lack of compatibility with MRI machines and the need for surgery if the implanted electronics fail²¹. In a percutaneous connection, the speech processor and stimulator are located externally, with the stimulator directly connected to both the processor and the electrodes on the cochlea¹⁴. The principal advantage of a percutaneous connector is that there is no need to encode the signals that are sent to the receiver¹⁴. However, it poses an increased risk of infection due to the opening in the skin¹⁴. All commercially available cochlear implants use a transcutaneous link¹⁴.

Electrode design and placement must also be considered when designing a cochlear implant. There are two commercially available electrode array types: straight lateral wall (LW) and pre-curved modiolar hugging (MH)²². Both electrode array types are inserted into the scala tympani of the cochlea²². The straight LW design comes in a variety of lengths, which allows for more custom fits for different cochlea sizes²². They stimulate nerve fiber endings of the auditory nerves at the organ of Corti, and implantation may cause damage to the cochlea²². The pre-curved MH design only spans to the basal turn of the cochlea and rests along the central bony wall of the inner ear^{22, 23}. It stimulates the spiral ganglion cells of the auditory nerve, and implantation causes more frequent damage than straight LW electrodes²². However, it is easier to insert than a straight LW design²².

A very important design consideration is the patient¹⁴. Each patient has a different cochlea size and structure, with differing amounts of damage to cochlear cells¹⁴. For example, a pre-curved MH electrode array may not perfectly fit one person’s cochlea, so they might need a straight LW array²². Patients may have differing amounts of auditory nerve cells in different locations, so the cochlear implant must have electrodes that can stimulate the correct locations^{14, 22}. Overall, the physician fitting the implant must be willing to identify the factors that affect the fit of a cochlear implant to the patient^{14, 22}.

Engineering Aspects of Cochlear Implants: Skills for Success

Biomedical engineers are at the center of design and implementation of cochlear implants, but a large team of diverse individuals is needed for the success of the implant. The team includes physicians, psychologists, educators, entrepreneurs, engineers, and physiologists²⁴. Biomedical engineers are responsible for interpreting the information from all team members in order to improve the cochlear implant²⁴. The original engineers who created the implant needed to understand how the electrical components worked and how to assemble them in order to transmit sound stimulus in a way the inner ear and nervous system could interpret⁴. Therefore, the skills needed for the original design of the implant were knowledge and assembly of electrical components⁴, the anatomy and physiology of the ear, and the anatomy and physiology of the nervous system²⁴. The same skills are needed in the biomedical engineers of today who are continuously improving the technology of the implant²⁴. However, they also need other technical skills such as an understanding of sound coding and signal processing¹⁴. Biomedical engineers must work closely with electrical engineers to develop the various electrical components of the implant⁴, so they must have the ability to understand or interpret that information for others. They must also work closely with physicians to determine what needs to be improved⁴. The continuous interaction between team members with diverse backgrounds requires both oral and written communication skills in the engineer. Overall, a biomedical engineer is necessary for the design and implementation of a cochlear implant because they have both the engineering skills and medical knowledge needed to create a successful implant²⁴.

Conclusion

Cochlear implants restore hearing in patients with sensorineural hearing loss²¹ in a profound way. By bypassing the patient’s normal hearing mechanisms, the implant can simulate sound by directly stimulating different areas of the cochlea in the ear^{21, 22}. The correct stimulation requires research^{25, 26} and development to produce sound processing techniques and electrode placements that will maximize hearing restoration in patients²⁶. Engineers must consider the different sound coding strategies^{14, 16}, connection types¹⁴, electrode placements²², and patients¹⁴ when designing the implant. The listed design considerations require a diverse team to interact with patients and trial different methodologies²⁴. Biomedical engineers are at the center of the development process because they have the engineering skillset and the medical knowledge needed to design and construct these implants. In the future, cochlear implants may be able to completely restore the natural hearing of people with sensorineural hearing loss.

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Appendix

Figure 1: The anatomy of the ear⁷.

Figure 2: A cross section of the cochlea²⁷. Note: the vestibular canal and tympanic canal refer to the scala vestibuli and scala tympani, respectively.

Figure 3: The components of a transcutaneous cochlear implant²⁸.

Figure 4: A schematic of CIS sound coding²¹.

Figure 5: Full wave rectification when the input is a sine wave¹⁸.

Figure 6: Development of a signal envelope²⁹.