Cochlear Implants: My Perspective

By William F. House, D.D.S., M.D. (Edited by David House)

Footnotes

[1]Dijourno A, Eyries C: "Prosthese auditive par excitation electrique a distance du nerf sensoriel a P'aide d'un bobinage inclus a demeure", 1957 Presse Med. 35, 14-17.

[2]For a more detailed accounting of the history of cochlear implants the reader is referred to an earlier publication: House W, Berliner K: Cochlear implants: From idea to clinical practice: Chapter 2 in Cooper H, ed. Cochlear Implants: A Practical Guide; London Whurr Publishers, 1991.

[3]Simmons FB: "History of cochlear implants in the United States: A personal perspective", In Schindler RA, and Merzenich, MM (eds) Cochlear Implants 1985 pp 1-7, New York: Raven Press.

[4] As the reader is likely aware, the cochlea is the fluid-filled, snail-shell shaped organ in the inner ear where the vibrations of sound are converted or transduced into nerve impulses. If the cochlea were unrolled, it would look like a very narrow funnel about an inch (35 mm) long. Within that funnel, the basilar membrane separates the cochlea into two tubes, connected at the narrow end. Sound vibrations enter the cochlea because the bones of hearing communicate sound from the eardrum along their length (much like vibrations might be transmitted by a steel rod) to what is called the oval window. The sound vibrations then travel down the basilar membrane toward the narrow end of the cochlea at its apex.

The basilar membrane is studded with hair cells. These hair cells are so called because they have short hair-like processes which project out of the basilar membrane; and below, they are each associated with the end or dendrite of one or more nerve fibers. The current thought is that the vibration of the basilar membrane causes the hair cells to fire the nerve fibers, so that the mechanical sorting of sound is thought to be an important part of frequency discrimination.

[5] "Refractory" is a binary condition analogous to what happens with a machine gun, which either fires at a given rate or does not fire at all. In this context, we would expect that the cochlear neurons would either be quiet, or would fire universally, repetitiously, and continuously.

[6] House WF, Urban J: "Long term results of electrode implantation and electronic stimulation of the cochlea in man", Ann. Otol. Rhinol. Laryngol. 82,(1973) 504-514.

[7]Merzenich MM, Schindler RA and Sooy F, (eds): "Proceedings of the First International Conference on Electrical Stimulation of the Acoustic Nerve as a Treatment for Profound Sensorineural Deafness in Man", 1974 San Francisco, University of California. Both of the quotes which follow are from page vii.

[8]Just to reiterate: The term "multi-channel" derived from the idea that the frequency spectrum would have to be divided into parts, and that each part — as if it were a separate stream or channel of information — would have to be presented locally to its "correct" portion of the cochlea. This the tonotopic approach. By contrast, "single channel" implants were seen as unable to provide any information separation, and therefore they could not provide the patient with frequency discrimination: they were not "tonotopically correct."

[9]Ref 7, pg. 209

[10]Gault RH: "Touch as a substitute for hearing in the interpretation and control of speech", Archives of Otolaryn 3 (1926) 121-135.

[11]Geers, AE, Moog, JS: "The sensory aids study at central institute for the deaf", The Volta Review, vol 96, No 5, Nov. 1994, CI-4 208 and CI-6.

[12]As the reader may be aware, the intracochlear fluid is essentially the same as spinal fluid, and they are connected.

[13]Bilger RC, Black FO, Hopkinson NT, Myers EN, Stenson NR, Vega A, Wolf RV: "Evaluation of subjects presently fitted with implanted auditory prostheses", Ann Oto Rhin & Laryn, Supp 38-Vol 86. May-June, 1977. No. 3, Part 2.

[14]As an example of the kind of barriers to progress referred to in the opening paragraphs, consider that at that time, in 1976, there was a strong feeling that implants did nothing worthwhile for the patients and that the surgery on these deaf patients was immoral, offering only a placebo and thereby taking advantage of the patient's suffering.

[15]Jack Urban and I thought, given our years of experiments and testing, that we had paid a lot of attention to the "stimulus coding and to the external electronic package"; that was the whole point of the hard-wired devices, and the many experiments we had done with different signals. Still, that's life.

[16]Around that time, I suggested privately to some of the writers of such articles that such implants be referred to as "elegantly simple", but that phrase somehow never caught on.

[17]Snyder, RL, Rebscher SJ, Cao K, Leake, PA, Kelly K: "Chronic intracochlear electrical stimulation in the neonatally deafened cat, I: Expansion of central representation", Hearing Res, 50 (1990) 7-34.

[18]Leake PA, Hradek GT, Rebscher SJ, Snyder, RL: Chronic intracochlear electrical stimulation induces selective survival of spiral ganglion neurons in neonatally deafened cats: Hearing Res 54 (1991) 251-271.

[19]Snyder, RL, Rebscher SJ, Leake, PA, Kelly K, Cao K: "Chronic intracochlear electrical stimulation in the neonatally deafened cat, II: Temporal properties of neurons in the inferior colliculus", Hearing Res, 56 (1991) 246-264.

[20]Quite the reverse, in fact: it appears that stimulation may be beneficial, as the Leake et al reference cited above evidences.

[21]Clark GM, Tong YC, Black R, Forster IC, Patrick JF and Dewhurst DJ: "A multiple-electrode cochlear implant", 1977 J. Laryngol. Otol. 91, 935-945.

[22]Nobel laureate, 1961

[23]Schuknecht, HF: "Pathology of The Ear", 2nd ed; Lea & Febiger, 1993: Pages 66-67 give a lucid description of the type I and type II neurons.

[24]Linthicum FH, Fayad J, Otto SR, Galey FR, House WF: "Cochlear Implant Histopathology"; Am J Otol 12 No 4 1991 245-311.

[25]For an excellent review of the dendrite and spiral ganglion cell populations of many types of congenital hearing loss see Chapter 4 in Schuknecht, HF: "Pathology of The Ear", 2nd ed; Lea & Febiger 1993. Also see Figure 3.17, page 90, showing normal dendrite and neuronal population in a case of total congenital deafness due to Scheibe faulty embryogenesis. Although dendrite survival in total sensory-neural deafness is rare, it apparently is occasionally possible.

[26]Reports and Panel discussions: Cochlear Implants in Children Conference in New York Feb 4-5, 1994.

[27]As an aside, the cochlear microphonic, an AC response to acoustic stimulation appears to be generated at the boundary of the hair cells and the scala media (Schuknecht Ref 21 pg 85). The voltage of the cochlear microphonic increases linearly with stimulus intensity up to 105 dB. The cochlear microphonic is absent if the hair cells are missing. No definite function has been attributed to the cochlear microphonic. Is it possible that this AC potential has a lot to do with spiral ganglion cell function? Are cochlear implants replacing this AC potential?

[28] 2000 Hz is far above the theoretical firing rate of about 400 Hz of the VIII nerve. That this was possible with a single-electrode system was never commented on. I reason that it is further evidence that we do not know how implants work.

It is curious that when multi-electrode systems were introduced later in an effort to improve frequency discrimination, the frequency difference limens were never studied. It was simply assumed that they were better and this assumption was never validated scientifically. Given that there is reason to believe these assumptions to be invalid, psychoacoustic studies should certainly be reinstituted.

[29]Unpublished data submitted to the FDA regarding the AllHear implant system.

[30]Welling DB, Hinojosa R, Gantz, BJ, Lee JT: "Insertion Trauma of Multi-channel Cochlear Implants", Laryngoscope 103: Sept 1993 995-1001.

[31]Kennedy TWO: "Multi-channel Intracochlear Electrodes: Mechanism of Insertion Trauma", Laryngoscope 97: 1987 42-49.

[32]Shepherd RK, Clark GM, Pyman BC, et al.: "Banded Intracochlear Electrode Array: Evaluation of Insertion Trauma in Human Temporal Bones", Ann Otol Rhino Laryngol 94: 1985 55-59.

[33]Schuknecht, HF: "Pathology of The Ear", 2nd ed; Lea & Febiger 1993: page 319 under A., Degeneration of the Cochlear Neurons.

[34]In the Scheibe case illustrated in Figure 3.17 page 90 in Schuknect's book, inserting a long electrode would not result in loss of residual hearing since none exists, but could cause extensive loss of dendrites and 40% of their associated spiral ganglion cells.

[35]Found, respectively, on pages 256, 278, and 288: Linthicum ref 24.

[36]Bogies MS, Baker JE, Balkany TJ: "Loss of residual hearing after cochlear implantation", Laryngoscope: 99: 1989 1002-1005.

[37]Personal Communication: Arlie Adams, Tucker Maxon School, Portland OR; Pat Chute, Children's Hearing Institute, NY; Donna Mills, House Ear Clinic, CA.

[38]Cochlear Corporation: Severely Hearing Impaired Study.

[39]Dye et al: "Measurable Residual Hearing Following Cochlear Implantation", Reported in Berliner et al: Cochlear Implants In Children. In Meyers et al: Advances in Otolaryngology - Head and Neck Surgery. Vol 4; St. Louis Mosby Year Book: 1990 61-79.

[40]For those who would quibble, the phrase might be more accurately rendered as "implants provide a stimulus which is interpreted as sound," but my point is that a functionally significant difference has never been proven.

[41]Boothroyd A: "Assessment of speech perception capacity in profoundly deaf children", Am J Otol Supp (1991), 12:67-72.

[42]Carhart, R: "Tests for selection of hearing aids," Laryngoscope (1946), 56:780-794.

[43]Jerger J, Malmquist C, Speaks C: "Comparison of some speech intelligibility tests in the evaluation of hearing aid performance", J Speech and Hearing Research (1966) 9:258.

[44]As the field matures and we have clearer information about which sound processing strategies are better, and as the acceptance of implants improves to the point that even very young deaf children are routinely implanted, the results of speech tests for a group of patients within one program will be seen as an important indicator of the strength of that program: not merely an indicator of the value of one specific implant, if only one is used, but rather an indicator of the strength of the whole program, its training, its people, and so on.

[45]Northern JL: "Probe Microphone measurements", Hearing Aid Selection and Assessment, ed. Muller HG, Hawkins DB, Northern JL (Singular Publishing Group, Inc., 1992), San Diego p. 9.

[46]Preves D: "Levels of realism in hearing aid measurement techniques", Hearing Journal (1984), 37:19.

[47]Those who wish to take advantage of the contaminating errors such a measure provides will tend to more eagerly implant those with good language skills. Likewise, programs which develop better after-implant training for their patients will be seen, within such a process, as "having better implants", a conclusion which may be patently false.

[48]Rankovic, C: "An application of the articulation index to hearing aid fitting", Journal of Speech and Hearing Research (1991), 34:391-403.

[49]Pavlovic, C: "Speech recognition and five articulation indexes", Hearing Instruments (1991), 42:20-24.

[50]By "indication of the expectation" I mean that even though we can calculate an articulation index rather exactly, we cannot by the same token develop an exact specification for the future success of that patient, as we shall see: too much else bears on the matter. What we will have, and all we will have, is a convenient indicator of how well the patient can hear the critical sounds of speech, such as the consonant sounds. I would also note that the articulation index was developed by English-speakers, and reflects the significant sound characteristics of that language. Some other languages — particularly non-Romance languages — have very different qualities, and thus may perhaps need development of an articulation index calculated on a different basis.

[51]Mueller H, Killion M: "An easy method for calculating the articulation index," Hearing Journal (1990), 43:14-17.

[52]For the details of the mathematical approach, the reader is referred to the articles previously referenced. It also bears mention that AllHear is developing a computer program which will do these calculations automatically. The details of distribution are not settled as of this writing, however. If you are interested, you may contact us via e-mail at the address given on the cover page.

[53]As the reader may be aware, phonemes are to spoken words what letters are to written words and atoms to molecules: they are the smallest units into which the object can ordinarily be divided. For further information, see Ling D, Ling AH: "Aural Habilitation", A. G. Bell Association for the Deaf, 1978.

[54]Moog JS, Geers AE, "Early speech perception (ESP) test for profoundly hearing-impaired children", St. Louis, MO: Central Institute for the Deaf, 1990.

[55]Ibid, pg 6.

[56]Tye-Murray N, Tyler RS, Woodworth GG, Gantz BJ, "Performance over time with a Nucleus or Ineraid cochlear implant", Ear and Hearing (1992), 13 (#3):200-208.

[57]Boothroyd A: "Assessment of speech perception capacity in profoundly deaf children", Am J Otol Supp (1991), 12:67.

[58]Watson, CS: "Auditory perceptual learning and the cochlear implant", Am J Otol (1991), 12:77.

[59]Ref. 56.

[60]A "closed set", in this context, would be a list of words which are provided to the patient in a printed list. One of these words would be spoken or presented so that the patient could not see the speaker, and the patient would choose one of the words from the list. By contrast, "open set" would be where no such list is provided. Any word might be spoken or presented to be repeated by the patient. Thus, "open set" speech recognition is similar to what we do whenever speaking on the telephone: that is, discrimination of speech in a situation where few or no non-auditory clues are being provided about the words being spoken.

[61]Cohen NL, Waltzman SB, Fisher SG: "A Prospective Randomized Study of Cochlear Implants", New England J Med (1993), 328 (#4):236.

[62]Skinner, MW, Holden LK, Holden TA, et al: "Performance of Post-linguistically deaf adults with the Wearable Speech Processor (WSP III) and the Mini Speech Processor (MSP) of the Nucleus Multi-Electrode cochlear implant", Ear and Hearing (1991), 12:3-22.

[63]This broadly similar experience leads us to believe that (for younger patients), parental involvement (which is difficult to quantify or precisely define) is very likely a significant factor in addition to the four above mentioned.

[64]Interestingly, the ability to be or become a good lip reader seems also to depend on the first three factors, with the addition that those who have become deafened post-lingually will have a greater chance at excelling in this art.

[65] In practice, most oral programs work with severely deafened children and after a few months or a year, if there is no progress in the development of the child's speech and lip-reading skills, a total communication program would be recommended.