Modern cochlear implant

Whispers of a solution

In normal hearing, auditory structures capture sound and translate it into information that the brain can interpret. The ear canal funnels vibrations onto the eardrum, and movements there perturb tiny bones, which propagate the fluctuations to the inner ear. Waves roll through the fluid-filled cochlea, a snail-shaped tube, and bend the thousands of delicate hair cells that lie within this organ. The displacements cause the hair cells to produce an electrical signal that stimulates the eighth cranial nerve — sometimes called the auditory nerve — and the message continues to the brain.

Within that chain of events, numerous things can go wrong. In most cases, severe hearing loss arises from damage to or absence of the sensory hair cells. Cochlear implants bypass the need for these cells by using electrical stimuli to directly excite the auditory nerve.

In the late 1950s, French doctors made a rudimentary effort to implement this idea in an individual whose hearing gear was largely gone. The system worked poorly and failed quickly, but news of the attempt reached and inspired the late otologist William House in California. He dreamed of designing a robust, easily usable prosthesis that would help deaf people hear for their lifetimes.

House developed an apparatus that delivered current through a single wire, or electrode, to a single spot on the cochlea. In 1961, two individuals received this implant. It enabled them to perceive environmental sounds, and its descendant devices helped people read lips, but speech was extremely distorted. Nevertheless, this triumph kicked off a bustle of activity among a few scientists who aimed to improve the device.

Most experts, however, did not join these pioneers, but instead cast skepticism on the enterprise. Restoration of meaningful hearing was impossible with electrical stimulation, they decried. The required neural response was far too complicated to replicate by simple and coarse signals.

Channels to hearing

Fortunately, Ingeborg Hochmair and Graeme Clark did not shy away from daunting challenges. Independently, they set out to craft an apparatus that would not only enhance awareness of the surroundings and facilitate lip reading, but would also enable deaf people to comprehend speech without visual cues. To do so, the investigators exploited a part of the auditory network that the single-electrode approach ignored.

In a person with intact cochlear hair cells, these structures beat every time sound waves hit. The perturbations cause neurons to fire in synch with the pulses, and single-electrode tactics rely on this ‘time coding’ system (so named because its operation depends on how often vibrations arrive). By the end of the 1960s, studies by the late Blair Simmons (Stanford University) suggested that single electrodes could reliably convey tones only up to a certain frequency. Above that point, further increases did not register as higher pitches to most people.

Simmons proposed that the time code alone would not allow faithful auditory perception; a second strategy that our brains use to decipher sound was needed in addition. The hair cells along the cochlea do not react uniformly to a given incoming pitch; their locations matter. A baby’s cry awakens hair cells — and thus, the nerves they excite — at the cochlea’s opening, whereas a rumbling truck disturbs those in the spiral’s center. As a result, the unfurled cochlea bears similarities to a piano, with pitch proceeding in an orderly fashion from highest to lowest. Together, the ‘notes’ in different places communicate all tones to the brain.

Clark and Hochmair — with electrical engineer (and future husband) Erwin Hochmair — harnessed this ‘place coding’ phenomenon by routing particular speech sounds to different parts of the cochlea. In this way, they selectively targeted nerves that respond especially well to the frequencies received. The scientists deployed multiple electrodes, each of which resided at a different site on the cochlea’s inner surface.

In addition to solving the electrical challenges, they and others in the field also tackled numerous safety and mechanical issues. For example, the devices — and the techniques for implanting them — needed to minimize infection risk, tissue damage, and hazards associated with running current through a person’s body. The investigators had to find nontoxic materials that were inert to biological activities and formulate ways to thread electrode arrays deep into the cochlea and position them there.

Each of their designs includes components that, together, transform acoustical information into electrical signals that excite the auditory nerve (see Figure).

Patients were first implanted with the Hochmair and Clark inventions in December 1977 and August 1978, respectively. These multichannel prototypes dramatically upgraded speech perception. Many of today’s systems mirror the basic blueprint of the original devices.

In 1985, the US Food and Drug Administration gave its inaugural approval to a multichannel cochlear implant— for treating adults who could hear before they went deaf. Three years later, an NIH consensus statement concluded that multichannel stimulation would probably produce superior speech recognition than single-channel stimulation.

Breaking sound barriers

Although the early-generation multichannel devices had propelled implants to a new performance level, many recipients could not grasp spoken words or sentences without contextual or visual hints. The next major development catapulted the technology over this hurdle.

In 1991, Blake Wilson reported a new speech-processing strategy that provides time- and place-coding information in a particularly clear way. It rapidly presents a wide range of frequency pulses that are slightly offset in time from one another, or ‘interleaved’. Because no two electrodes receive a signal simultaneously, the scheme minimizes distortions and omissions. Through this and other important features, Wilson’s ‘continuous interleaved sampling’ (CIS) system has allowed the majority of cochlear implant recipients — for the first time — to understand words and sentences with no visual cues. CIS supplies the basis for the sound-processing strategies that are now widespread and fueled an exponential growth in implant use that began in the early 1990s. Its rapid introduction, utilization, and dissemination stemmed in large part from a policy that donates to the public domain all intellectual property produced by Wilson and his colleagues from their NIH-funded cochlear-implant research.

Today, most people with cochlear implants can talk on their cell phones and follow conversations in relatively quiet environments. Experts complain that patients are doing so well, they’re hitting ceilings on standard hearing tests, which lack the sensitivity to demonstrate the subtle deficits in speech comprehension that a typical recipient experiences. Additional advances are moving the field toward the ability to fine-tune hearing and thus help people fully appreciate music and understand tone languages, which are spoken by a large fraction of the planet’s population.

Amplifying improvements

As of 2010, approximately 219,000 people across the globe had received cochlear implants, and more than 80% of the prostheses had been dispensed since 2000. The numbers are increasing rapidly; by the middle of 2013, more than 320,000 individuals were using cochlear implants, and almost 40,000 had one in each ear.

As the device’s effectiveness has grown, so too has the number of potential candidates. Now, adults who have severe age-related hearing loss, for instance, are taking advantage of the invention.

Cochlear implants have delivered exceptionally dramatic effects to children. If a person can’t hear during the first few years of life, the brain can’t fully equip itself to understand and acquire speech. Early intervention is therefore crucial. Of every 1000 babies born, more than one is deaf.

Today, about 60% of implants worldwide go to children; those who live in an industrialized nation routinely receive them between age 1 and 2 years. With this intervention, many youngsters can more easily mainstream into regular schools.

Some individuals now receive implants in both ears, which is especially helpful not only to enrich hearing in general, but also for localizing speech in noisy settings. People are also combining cochlear implants with hearing aids to get the most out of both instruments; for example, electrical stimulation in the areas of the cochlea that respond to high frequencies can synergize well with hearing aids that enhance low-frequency perception.

Brilliance and relentless commitment have fueled the reverberating success of Clark, Hochmair, and Wilson. Less than a generation ago, deaf individuals had no hope of hearing again. These scientists have cracked the barriers that formerly isolated huge numbers of people from the realm of sound and have made many lives hum in new ways.

by Evelyn Strauss

Key publications of Graeme M. Clark

Clark, G.M., Pyman, B.C., and Bailey, Q.R. (1979). The surgery for multiple-electrode cochlear implantations. J. Laryngol. Otol. 93, 215-223.

Clark, G.M., Blamey, P.J., Brown, A.M., Gusby, P.A., Dowell, R.C., Franz, B.K.-H., Pyman, B.C., Shepherd, R.K., Tong, Y.C., Webb, R.L., Hirshorn, M.S., Kuzma, J., Mecklenburg, D.J., Money, D.K., Patrick, J.F., and Seligman, P.M. (1987). The University of Melbourne — Nucleus multi-electrode cochlear implant. Adv. Otorhinolaryngol. 38, 1-181.

Busby, P.A., Whitford, L.A., Blamey, P.J., Richardson, L.M., and Clark, G.M. (1994). Pitch perception for different modes of stimulation using the cochlear multiple-electrode prosthesis. J. Acoust. Soc. Am. 95, 2653-2669.

Key publications of Ingeborg Hochmair

Hochmair-Desoyer, I.J., Hochmair, E.S., Rischer, R.E., and Burian, K. (1980). Cochlear prostheses in use: Recent speech comprehension results. Arch. Otorhinolaryngol. 229, 81-98.

Hochmair-Desoyer, I.J., Hochmair, E.S., Burian, K., and Fischer, R.E. (1981). Four years of experience with cochlear prostheses. Med. Prog. Technol. 8, 107-119.

Hochmair-Desoyer, I.J., Hochmair, E.S., Burian, K., and Stiglbrunner, H.K. (1983). Percepts from the Vienna cochlear prosthesis. Ann. N. Y. Acad. Sci. 405, 295-306.

Key publications of Blake S. Wilson

Wilson, B.S., Finley, C.C., Lawson, D.T., Wolford, R.D., Eddington, D.K., and Rabinowitz, W.M. (1991). Better speech recognition with cochlear implants. Nature. 352, 236-238.

Wilson, B.S., Finley, C.C., Lawson, D.T., Wolford, R.D., and Zerbi, M. (1993). Design and evaluation of a continuous interleaved sampling (CIS) processing strategy for multichannel cochlear implants. J. Rehabil. Res. Dev. 30, 110-116.

Wilson, B.S. and Dorman, M.F. (2008). Cochlear implants: A remarkable past and a brilliant future. Hear. Res. 242, 3-21.