Mary W. Lowder

Preservation of Hearing in Cochlear Implant Surgery: Advantages of Combined Electrical and Acoustical Speech Processing†

Objectives/Hypothesis: This study documents the importance of preserving residual low‐frequency acoustic hearing as those with more residual hearing are selected for cochlear implantation. Surgical strategies used for hearing preservation with a short hybrid cochlear implant are outlined. The benefits of preserved residual low‐frequency hearing, improved word understanding in noise, and music appreciation are described.

A longitudinal study of electrode impedance, the electrically evoked compound action potential, and behavioral measures in nucleus 24 cochlear implant users.

Objective The primary goal of this study was to examine changes that may occur in electrode impedance, electrically evoked compound action potential (EAP) threshold and slope of the EAP growth function, and behavioral measures of threshold (MAP T-level) and maximum comfort (C-level) over time in both adult and child cochlear implant users. Secondary goals were to determine whether changes in these measures are consistent between children and adults, and to determine whether behavioral measures (MAP T- and C-levels) and electrophysiologic measures (EAP thresholds) exhibit the same trends over time. Design Thirty-five children and 33 adults implanted with the Nucleus CI24M between November 1996 and August 1999 participated in this study. Subjects were included in this study if 1) they had used their implant for at least 1 yr after device connection, and 2) they had participated in the necessary data collection at a minimum number of the time intervals assessed in this study. EAP threshold, slope of the EAP growth function, and common ground electrode impedance measures were collected intraoperatively, at initial stimulation, and at several subsequent visits up to 2 yr post initial stimulation. MAP T- and C-levels were measured at initial stimulation and at the same time intervals as described above. Results Changes in electrode impedance, EAP thresholds, and slope of the EAP growth function from measures made intraoperatively, at initial stimulation, and at 1 to 2 mo post initial stimulation were similar in both children and adults. Beyond the 1- to 2-mo visit, children exhibited significant increases in electrode impedance, EAP thresholds, slope, and MAP T-levels, whereas these same measures in adults remained relatively stable. EAP thresholds in children stabilized by the 3- to 8-mo visit, and electrode impedance stabilized by the 6- to 8-mo visit, while slope of the EAP growth function, MAP T-levels, and MAP C-levels were stable by 1 yr post initial stimulation. C-levels in adults increased up to 1 yr post initial stimulation; however, the amount of increase was much smaller than that seen in children. In both children and adults, longitudinal trends in EAP thresholds mirrored T-level more closely than C-level. Conclusions The results of this study suggest that peripheral changes occur in many children that do not generally occur in adults within the first year of cochlear implant use. One implication of these results is that if EAP thresholds are to be used to assist in programming the speech processor for children, it is best to make those measures at the same time interval as device programming rather than using measures made intraoperatively or at the initial programming session to set MAP levels at later visits.

Binaural cochlear implants placed during the same operation

Objective To evaluate the binaural listening advantages for speech in quiet and in noise and to localize sound when independently programmed binaural cochlear implants are used, and to determine whether ears with different hearing ability and duration of profound deafness perform differently with cochlear implants as well as to what extent preimplant psychophysical and physiologic assessment could be predictive of performance. Study Design Prospective study in which patients were prospectively selected to undergo bilateral implantation during a single surgical procedure at a tertiary referral center. All testing was performed with patients using their right, left, or both cochlear implants. Preimplant and intraoperative measures used electrical stimulation at the round window and stimulation through the cochlear implant. Results Bilateral implantation during the same operation did not cause any postoperative problems such as severe vertigo or ataxia. At 1 year, results of speech testing in quiet demonstrated a binaural advantage for 2 of 10 subjects. Speech-in-noise testing demonstrated that two implants were beneficial for two individuals. All subjects benefited from a head shadow effect when an ear with a better signal-to-noise ratio was available. The ability to localize sound was improved with binaural implants in all subjects. Preimplant psychophysical or physiologic measures were not predictive of eventual speech perception performance. Conclusion Binaural cochlear implants can assist in the localization of sounds and have the potential in some individuals to improve speech understanding in quiet and in noise.

Performance over time of adult patients using the Ineraid or nucleus cochlear implant.

This study examined the average and individual performance over time of 49 adult cochlear implant subjects. Subjects were randomly assigned to receive either the Ineraid cochlear implant, with analog processing, or the Nucleus cochlear implant, with feature-extraction processing. All subjects had postlingual profound bilateral sensorineural hearing loss and received no significant benefit from hearing aids before implantation. Group data were examined in two ways. First, only subjects who had complete data over the test period were examined. Second, an analysis of all available data was carried out by mixed linear-model analysis. In this analysis, to account for missed follow-ups at the planned intervals, data consisting of the observations closest in time to the planned test times were modeled by natural splines with knots at the planned follow-up times. Contrasts between all pairs of planned follow-up times for each device were tested, as were contrasts between devices at each planned follow-up time. Results indicated little difference between the performance of the Ineraid and Nucleus subjects in their level of performance or their rate of learning. Postimplantation performance was typically superior to preimplantation performance within 9 months, and continued to improve up to 18-30 months depending on the speech perception measure. In some subjects, improvements in speech perception measures were observed up to four or five years postimplantation. There was also evidence that three subjects had a decrement in overall speech perception performance, although their postimplantation scores were always higher than their preimplantation scores. In at least one subjects this was likely a result of age-related cognition decrements.

Three-month results with bilateral cochlear implants.

Objectives To evaluate possible binaural listening advantages for speech in quiet, speech in noise, and for localization in a group of postlingually deafened adults with two cochlear implants functioning independently after 3 mo experience. Design Nine postlingually deafened subjects who had received a Cochlear Corporation CI24M implant in each ear were evaluated on a number of tasks. The subjects all had audiometric or biographical (e.g., duration of deafness) differences between the ears. Word and sentence materials were presented to the subjects in quiet and in noise with the signal always in the front and the noise from the front or either side. Results are reported for each ear and for both ears with the noise on either side. This allowed evaluation of head shadow and squelch effects. Additionally, localization ability was assessed for broadband noise presented either to the right or left of center at 45° azimuth. Localization was assessed for each ear and for both ears. Results Results of speech testing in quiet showed a significant advantage for the binaural condition over the better ear in four subjects. In noise, with both signal and noise in front of the subject, a significant advantage of two ears over the better ear was found for four subjects. For noise to one side of the head, when the ear opposite the noise source was added to the ear ipsilateral to the noise, a significant advantage was demonstrated for seven of seven tested subjects. When the ear ipsilateral to the noise was added to the ear contralateral to the noise, a significant advantage was shown for only one of seven (noise on right) and three of seven (noise on left) tested subjects. The localization task showed that all seven tested subjects could discriminate 45° left from 45° right above chance with bilateral stimulation. Three subjects could perform the discrimination above chance with only one ear. However, performance with both ears was significantly better than performance with one ear for two of these latter subjects. Conclusions We conclude that bilateral cochlear implants can provide real advantages, particularly when it is possible to utilize the ear that is away from a noise source, thus taking advantage of the head shadow effect. In addition, localization ability was generally better with two implants than with one.

Residual speech perception and cochlear implant performance in postlingually deafened adults.

Objective This study aimed at testing the post-hoc validity of the previously reported predictive index for postoperative cochlear implant performance, based on preoperative duration of deafness, and speech reception. Study Design Adult patients with postlingual severe to profound hearing loss, who were implanted with Cochlear Corporation CI-22 and CI-24 devices were included in this study. We studied the relationship between their postoperative word rec-ognition scores and preoperative factors, namely, duration of deafness, and sentence recognition. We used the same predictive index reported in the previous study to predict their postoperative scores and test the model’s agreement with the actual performance. Results We found that postoperative performance as measured by CNC word scores had an inverse relationship with the duration of deafness, and a direct correlation with the preoperative performance on CID sentence recognition tests. A nonlinear term [Duration / (1+CID)] was shown to improve the correlation coefficient of our predictive index. Conclusion Some predictability of cochlear implant outcome is possible depending on the preoperative duration of deafness and speech recognition abilities. Preoperative residual speech recognition acts as a “trophic factor” that protects the spiral ganglion and/ or the central auditory pathways from degeneration. In other words, it improves the expected postoperative word scores.

Longitudinal assessment of physiological and psychophysical measures in cochlear implant users.

Objective The purpose of this study was to evaluate the effects of long-term electrical stimulation on human cochlear implant users. Design Repeated measures of electrically evoked auditory brain stem response (EABR) threshold, slope of the EABR growth function, and behavioral measures of threshold and dynamic range were made for a group of 22 Ineraid cochlear implant users and 19 Nucleus cochlear implant users over a 3- to 5-yr period. Results Data from both Ineraid and Nucleus cochlear implant users suggest that EABR threshold, slope of the EABR growth function, and behavioral measures of threshold and dynamic range remain reasonably stable for periods up to 6 yr postimplant. Conclusions The results of this study show little evidence that prolonged electrical stimulation through daily use of a cochlear implant has deleterious effects on the auditory system.

Effects of extreme tonotopic mismatches between bilateral cochlear implants on electric pitch perception: A case study

Objectives: Recent studies suggest that pitch perceived through cochlear implants (CIs) changes with experience to minimize spectral mismatches between electric and acoustic hearing. This study aimed to test whether perceived spectral mismatches are similarly minimized between two electric inputs. Design: Pitch perception was studied in a subject with a 10-mm CI in one ear and a 24-mm CI in the other ear. Both processors were programmed to allocate information from the same frequency range of 188–7938 Hz, despite the large differences in putative insertion depth and stimulated cochlear locations between the CIs. Results: After 2 and 3 years of experience, pitch-matched electrode pairs between CIs were aligned closer to the processor-provided frequencies than to cochlear position. Conclusions: Pitch perception may have adapted to reduce perceived spectral discrepancies between bilateral CI inputs, despite 2–3 octave differences in tonotopic mapping.

Performance of Adult Ineraid and Nucleus Cochlear Implant Patients after 3.5 Years of Use

Forty-two postlingually deafened adult patients, 21 with a formant extraction version of the Nucleus cochlear implant and 21 with the Ineraid cochlear implant (analog processing), were evaluated on a series of speech perception tests after using their implants for about 3.5 years. A wide range of performance was observed across patients for both devices. All but 4 patients showed an enhancement in their lipreading ability with the implant. Word recognition averaged about 14-19% correct, and word recognition in sentences averaged about 43-49% correct for the two implant groups. Average performance was superior with the Ineraid implant on consonant recognition in noise. An information transmission analysis suggested that vowel perception was influenced by first- and third-formant frequency for the Nucleus, and first-formant and fundamental frequency for the Ineraid patients. It appeared that the Ineraid device was more effective, on average, at conveying information about consonant nasality and frication. For consonant perception, nasality and frication contributed most to the total information transmitted for both implant types. Both devices had difficulty conveying information about vowel second-formant frequency and consonant place information. These scores at 3.5 years are substantially elevated from preoperative performance and, overall, the patients clearly benefit from their implant.

Comparison of the F0F2 and F0F1F2 processing strategies for the Cochlear Corporation cochlear implant.

In April 1985, an updated processing strategy became available for the Cochlear Corporation Nucleus cochlear implant. Whereas the original strategy codes only fundamental frequency, amplitude, and information in the second formant region, the newer strategy also codes frequencies in the first formant region. This investigation evaluated the speech recognition skills of five subjects who were experienced with both designs. On average, the addition of first formant information improved word identification in an audition-only condition and improved spondee recognition in noise. Scores for the NU 6 Monosyllabic Word Test and the Sentence Test Without Context improved from 8% (2-12%) to 28% words correct (10-42%), and from 31% (10-45%) to 64% words correct (39-84%), respectively. Scores for the Four-Choice Spondee Test in noise improved from 37% (25-50%) to 75% (45-90%). The percent correct scores for the Iowa 14-Item Consonant Confusion Test in an audition-only and vision-plus-audition condition did not vary with the change in processing strategy. However, an information transfer analysis performed on the responses to the consonant test in a vision-only and a vision-plus-audition condition suggested that the newer strategy enhances the transmission of the voicing, duration, and envelope features.