A cochlear implant allows users to hear (again). For them, this means more than simply hearing sounds and speech. Achieving closest to natural hearing remains the ultimate goal.

Hearing a child‘s cheerful laughter, the voices of friends and all those favorite songs – all of this is associated with emotions. To experience them as authentically as possible, cochlear implant (CI) users rely on a device that mimics hearing as naturally as possible. Close to natural hearing comes with additional benefits: An almost natural hearing impression helps the brain to better process and understand sounds, including complex ones. This facilitates understanding conversations in larger groups or in noisy surroundings.^{[1] [2]}

How does close to natural hearing work with a CI?

In natural hearing, sound waves travel through the outer and the middle ear before reaching the inner ear. The inner ear contains the cochlea with thousands of fine hair cells that react specifically at certain frequencies. If the hair cells cannot process sound (sensorineural hearing loss), a CI system can remedy the problem. It consists of an externally worn audio processor and an internal implant that is surgically placed underneath the skin behind the ear. The implant comprises a housing with a coil and a magnet as well as an electrode which is inserted into the cochlea. The electrode plays a crucial role in delivering a close to natural hearing experience to users. The more precisely it is placed, the better it can activate the nerve structures matching the corresponding pitch.

Electrode length makes a difference

To successfully mimic sounds as naturally and precisely as possible, the cochlear implant electrode needs to be long enough to stimulate the entire cochlea up to the apex (720°). This allows access to the whole spectrum of sound. In the cochlea, every frequency is coded at a certain place: high frequencies are allocated at <240°, medium frequencies <480°, low frequencies <720°. Cochlear implants with shorter electrode arrays can also stimulate low pitches, but only in those cochlear regions they actually reach.^{[1] [3] [4]} This requires upward alterations of pitches. Consequently, low frequencies sound tinny and thin and the richness of sound is lost.^{[5] [6] [7] [8] [9]} Detailed information on the link between natural hearing and electrode length can be found here.

In short videos, international experts give an insight into the current state of research on natural hearing with cochlear implants. Click here  to watch them.

Right place, right time

But a longer electrode not automatically delivers more natural hearing. Why not? Well, the second turn of the cochlea comes with a feature that cochlear implants need to consider – a special sound coding feature. In addition to place matching, sound is coded using a temporal aspect. Place coding (tonotopicity) defines which hair cells are stimulated by a certain sound. Temporal rate coding defines how fast or slow hair cells are activated and deactivated. If the implant neglects temporal coding, the brain will confuse pitches. It would interpret them as a fixed rate of upwards shifted pitches.^{[4] [10] [11]} FineHearing, MED-EL‘s unique rate-specific coding strategy, ensures that stimulation rates for electrode contacts in the second cochlear turn are matched to the corresponding frequencies without delay. This is the only way to process sounds picked up by the audio processor at each frequency as naturally as possible, giving users an ideal basis for close to natural hearing.

Find more detailed information on temporal coding on our professional blog.

Literature

[1] – Canfarotta, M. W., Dillon, M. T., Buss, E., Pillsbury, H. C., Brown, K. D., & O’Connell, B. P. (2020). Frequency-to-place mismatch: Characterizing variability and the influence on speech perception outcomes in cochlear implant recipients. Ear & Hearing, 41(5), 1349–1361.

[2] – Canfarotta, M. W., Dillon, M. T., Buchman, C. A., Buss, E., O’Connell, B. P., Rooth, M. A., King, E. R., Pillsbury, H. C., Adunka, O. F., & Brown, K. D. (2020). Long‐term influence of electrode array length on speech recognition in cochlear implant users. The Laryngoscope, 131(4), 892–897.

[3] – Li, H., Schart-Moren, N., Rohani, S., A., Ladak, H., M., Rask-Andersen, A., & Agrawal, S. (2020). Synchrotron Radiation-Based Reconstruction of the Human Spiral Ganglion: Implications for Cochlear Implantation. Ear Hear. 41(1).

[4] – Landsberger, D.M., Vermeire, K., Claes, A., Van Rompaey, V., & Van de Heyning, P. (2016). Qualities of single electrode stimulation as a function of rate and place of stimulation with a cochlear implant. Ear Hear., 37(3), 149–159.

[5] – Dorman, M.F., Cook Natale, S., Baxter, L., Zeitler, D.M., Carlson, M.L., Lorens, A., Skarzynski, H., Peters, J.P.M., Torres, J.H., & Noble, J.H. (2020). Approximations to the voice of a cochlear implant: explorations with single-sided deaf listeners. Trends Hear. 24:2331216520920079.

[6] – Dorman, M. F., Natale, S. C., Zeitler, D. M., Baxter, L., & Noble, J. H. (2019). Looking for Mickey Mouse™ But Finding a Munchkin: The Perceptual Effects of Frequency Upshifts for Single-Sided Deaf, Cochlear Implant Patients. Journal of sprache, sprachverständnis, and hörbeeinträchtigte research: JSLHR, 62(9), 3493–3499.

[7] – Roy, A.T., Penninger, R.T., Pearl, M.S., Wuerfel, W., Jiradejvong, P., Carver, C., Buechner, A., & Limb, C.J. (2016). Deeper cochlear implant electrode insertion angle improves detection of musical sound quality deterioration related to bass frequency removal. Otol Neurotol., 37(2), 146–151.

[8] – McDermott, H., Sucher, C., & Simpson, A. (2009). Electro-acoustic stimulation. Acoustic and electric pitch comparisons. Audiol Neurootol., 14(1), 2–7.

[9] – Harris, R.L., Gibson, W.P. Johnson, M., Brew, J., Bray, M., & Psarros, C. (2011). Intra-individual assessment of speech and music perception in cochlear implant users with contralateral Cochlear and MED-EL systems. Acta Otolaryngol., 131(12), 1270–1278.

[10] – Schatzer, R., Vermeire, K., Visser, D., Krenmayr, A., Kals, M., Voormolen, M., Van de Heyning, P., & Zierhofer, C. (2014). Electric-acoustic pitch comparisons in single-sided-deaf cochlear implant users: frequency-place functions and rate pitch. Hören Res., 309, 26–35.

[11] – Rader, T., Döge, J., Adel, Y., Weissgerber, T., & Baumann, U. (2016). Place dependent stimulation rates improve pitch perception in cochlear implantees with single-sided deafness. Hören Res., 339, 94–103.