Fundamental considerations in designing auditory implants

Audiological and Surgical Outcomes of Pediatric Cochlear Implantation in Mondini's Dysplasia: Our Experience

OBJECTIVES

Aim of present study is to compare audiological and surgical outcomes in prelingual deaf children with Mondini's dysplasia (MD) and those with normal inner ear anatomy.

MATERIALS AND METHODS

Retrospective data was collected from Jan 2008 to Dec 2016. Children with bony IEM other than MD, syndromic association, multiple disabilities, those lost to follow up, and perilingual or postlingual deafness were excluded from study. Audiological outcomes for auditory perception (CAP score) and speech intelligibility (SIR score) was noted for a follow up period of 1 year.

RESULTS

Mean age at implantation was 2.8 years (Range of 2 to 6 years). 2 patients had intraoperative CSF ooze which was controlled intraoperatively by conservative measures. Post operative facial nerve function was normal in all patients. None of the patient in either group had any complications at one year of follow up period. There was statistically significant improvement in CAP - SIR score in Group A at 6 - 12 months compared to pretreatment. There was no statistically significant difference between the 2 groups in terms of CAP - SIR score at 6 - 12 months.

CONCLUSION

The study stresses the fact that cochlear implantation can be safely performed in children with MD although there is a risk of intraoperative CSF leak which can be controlled intraoperatively. Cochlear implantation in children with MD has good surgical, auditory and speech outcomes at par with children with normal bony inner ear anatomy.

The development of auditory skills in young children with Mondini dysplasia after cochlear implantation

The aim of this study is to survey and compare the development of auditory skills in young children with Mondini dysplasia and profoundly-deaf young children with radiologically normal inner ears over a period of 3 years after cochlear implantation. A total of 545 young children (age 7 to 36 months) with prelingual, severe to profound hearing loss participated in this study. All children received cochlear implantation. Based on whether or not there was a Mondini dysplasia as diagnosed with CT scanning, the subjects were divided into 2 groups: (A) 514 young children with radiologically normal inner ears and (B) 31 young children with Mondini dysplasia. The Infant-Toddler Meaningful Auditory Integration Scale (IT-MAIS) was used to assess the children's auditory skills that include vocalization changes, spontaneous alerting to sounds in everyday living environments, and the ability to derive meaning from sounds. The assessment was performed prior to surgery and at 1, 3, 6, 9, 12, 24, and 36 months after implant device switch-on. The mean scores for overall auditory skills were not significantly different between groups A and B at pre-surgery, 1, 12, 24, and 36 months post-surgery, but were significantly different at 3, 6, and 9 months post-surgery. The mean scores for all auditory skills in children with Mondini dysplasia showed significant improvement over time. The mean scores for the three subcategories of auditory skills in children with Mondini dysplasia also showed significant differences at pre-surgery, 1, 3, 6, and 9 months, however, there were no significant differences at 12, 24, and 36 months. Overall, the auditory skills of young children with Mondini dysplasia developed rapidly after cochlear implantation, in a similar manner to that of young children with radiologically normal inner ears. Cochlear implantation is an effective intervention for young children with Mondini dysplasia.

Phase locking of auditory-nerve fibers to the envelopes of high-frequency sounds: implications for sound localization

Although listeners are sensitive to interaural time differences (ITDs) in the envelope of high-frequency sounds, both ITD discrimination performance and the extent of lateralization are poorer for high-frequency sinusoidally amplitude-modulated (SAM) tones than for low-frequency pure tones. Psychophysical studies have shown that ITD discrimination at high frequencies can be improved by using novel transposed-tone stimuli, formed by modulating a high-frequency carrier by a half-wave-rectified sinusoid. Transposed tones are designed to produce the same temporal discharge patterns in high-characteristic frequency (CF) neurons as occur in low-CF neurons for pure-tone stimuli. To directly test this hypothesis, we compared responses of auditory-nerve fibers in anesthetized cats to pure tones, SAM tones, and transposed tones. Phase locking was characterized using both the synchronization index and autocorrelograms. With both measures, phase locking was better for transposed tones than for SAM tones, consistent with the rationale for using transposed tones. However, phase locking to transposed tones and that to pure tones were comparable only when all three conditions were met: stimulus levels near thresholds, low modulation frequencies (<250 Hz), and low spontaneous discharge rates. In particular, phase locking to both SAM tones and transposed tones substantially degraded with increasing stimulus level, while remaining more stable for pure tones. These results suggest caution in assuming a close similarity between temporal patterns of peripheral activity produced by transposed tones and pure tones in both psychophysical studies and neurophysiological studies of central neurons.

Cochlear implantation in children with anomalous cochleovestibular anatomy

OBJECTIVES/HYPOTHESIS

To evaluate outcomes after cochlear implantation in children with anomalous cochleovestibular anatomy, a review of radiological classification, surgical implantation, and outcome of 103 children with such anomalies was performed. The hypothesis was that children with anomalous cochleovestibular anatomy would have poorer outcomes and therefore be poorer candidates as a result of their diminished ability to interpolate and use auditory information delivered through a cochlear implant.

STUDY DESIGN

A series of studies was carried out to review the cochleovestibular anomalies among 298 children implanted over the decade ending in January 2002. Children were grouped based on cochleovestibular anatomy as follows: normal (n = 195), common cavity deformity (n = 8), hypoplastic cochlea (n = 16), incomplete partition (n = 42), and vestibular aqueduct enlargement (n = 37). Concomitant anomalies of the posterior labyrinth (n = 26) and internal auditory canal/cochlear canal (n = 11) were also identified. Findings at surgery, postoperative speech perception outcomes, and speech processor programmability were examined as a function of cochleovestibular anatomy.

METHODS

A database containing demographics (age at implant, duration of implant use), audiological characteristics, pure-tone average, surgical findings (cerebrospinal fluid leak/perilymph leak, abnormal facial nerve anatomy), speech perception data (from two closed-set and three open-set tests), and data relating to speech processor programmability were used for analysis. Electrically evoked auditory brainstem response was measured in 94 of the children (2 cases of common cavity deformity, 7 of hypoplastic cochlea; 10 of incomplete partition; and 12 of vestibular aqueduct enlargement). Response morphological findings were assessed by visual inspection of the waveforms. Data were analyzed using analyses of variance with post hoc testing using the Bonferroni multiple-comparisons test. To further assess differences in outcomes between different categories of cochleovestibular anomalies, linear regression analyses were performed. The significance level was set at P < .05.

RESULTS

The use of high-resolution imaging techniques resulted in the detection of a cochleovestibular anomaly in 35% of implanted ears. Implantation was more challenging in 24% of the children as a result of abnormal middle ear anatomy (17.5%) or cerebrospinal fluid leak/perilymph leak (6.7%). There was no significant difference in speech perception scores in children with anomalous cochleae compared with children with normal cochleovestibular anatomy. Children with narrowing of the internal auditory canal/cochlear canal performed more poorly than all other groups. Children with common cavity deformity and hypoplastic cochlea had reduced dynamic range and increased incidence of facial simulation and were judged to be more difficult to program despite the fact that no fewer electrodes were inserted. Children with common cavity deformity and hypoplastic cochlea tended to require wider pulse widths more often than children in other groups, and these requirements were associated with abnormal morphological findings on evoked auditory brainstem response testing.

CONCLUSION

The authors have been continuing to assess the candidacy of each child applying for cochlear implantation individually, and the results of present study have suggested that the presence of anomalous cochleovestibular anatomy, with the exception of narrowing of the internal auditory canal/cochlear canal, should not play a significant role in candidacy assessment. Children with narrow internal auditory canal/cochlear canal should be carefully and individually considered. In children with anomalous cochleovestibular anatomy, the potentially increased difficulty in the establishment of optimal stimulation levels and the higher potential for surgical difficulty must be weighed in candidacy decisions but do not universally preclude successful implantation and a good outcome.

Sex- and age-related elevation of cochlear nerve envelope response (CNER) and auditory brainstem response (ABR) thresholds in C57BL/6 mice

The C57BL/6 mouse has long been considered, in scores of published studies, as a model of early adult-onset, progressive sensorineural hearing loss (presbycusis). The auditory brainstem response (ABR) has most often been used in these studies as a measure of functional loss. Whereas the ABR measures the response to a rapid acoustic onset, the cochlear nerve envelope response (CNER) measures the ability of cochlear nerve axons to respond to the low frequency modulations of the entire acoustic waveform, acoustic changes that are utilized in vocalizations and music. The present study compared the ability of these two measures to assess presbycusis in male and female C57BL/6 mice, at ages ranging from 50 to 400 days. Thresholds to the CNER were almost invariably more sensitive than the ABR, in response to stimulus frequencies ranging from 8 to 56 kHz. By 100 days of age, mice showed elevation of thresholds in response to high frequency stimuli, and this loss was greater in females than in males. These trends persisted for both measures over the next 300 days, involving successively lower frequencies.

Cochlear implantation in common cavity malformations using a transmastoid labyrinthotomy approach

The transmastoid facial recess approach has become the standard technique for cochlear implantation. Although this approach has been used for implantation in patients with common cavity deformities, it is not without increased risk to the facial nerve. Using a direct approach to the common cavity that circumvents the facial recess, we have successfully implanted four patients with common cavity deformities. An aberrant facial nerve in one patient would have precluded placement of the electrode array using standard cochlear implant techniques. As demonstrated in these four patients, the direct approach to the common cavity is an effective approach for placement of the electrode array, minimizes risk to the facial nerve, and may decrease the likelihood of postoperative cerebrospinal fluid leaks. Intraoperative video footage demonstrates the feasibility and facility of this approach in patients with common cavity deformities.

Human offset auditory brainstem response: effects of stimulus acoustic ringing and rise-fall time

Offset auditory brainstem response (ABR) traditionally has been thought to be an artifactual response elicited by stimulus acoustic ringing. Additionally, offset ABR's sensitivity to stimulus rise-fall time has been associated with concurrent changes in acoustic ringing. The present study tested the validity of offset ABR by recording the response in 40 young, normal-hearing adults using tone burst stimuli with varying degrees of acoustic ringing and various rise-fall times. Stimuli were computer-generated 10-ms tone bursts of 500 and 2000 Hz. In Experiment 1, offset ABR was recorded using stimuli with no acoustic ringing, normal ringing, and excessive ringing. Rise-fall time was held constant at 0.5 ms. In Experiment 2, rise-fall time was manipulated in a stimulus with no ringing. In Experiment 3, only rise time was manipulated in a no-ringing stimulus, while fall time was held constant at 0.5 ms. Reliable offset ABRs were recorded for all degrees of acoustic ringing, including the "no-ringing' condition. Offset ABR was sensitive to rise and fall times, and was elicited best with a 500-Hz stimulus. The results indicate that offset ABR is a real response and not an artifact produced by acoustic ringing.

[The phenomenon of hearing: an interdisciplinary discussion. II]

This part of the paper deals with the neurophysiological background of speech analysis and hearing of music. Single vowel- and consonant-detector cells could be clearly separated at the colliculus and geniculate level (Kallert, Keidel). Musical stimulation is decoded at three levels: hair cells, geniculate, and auditory cortex. Human cortical evoked potentials represent rhythm, consonance, and (as DC-shift) the emotional content of music. Marked harmonics, even of single hair-cell vibrations and in single units of medial geniculate in combination with clock-cell networks, are the physiological basis of "harmony" in music. Dissonant stimuli are detectable at the cortical level in man (Finkenzeller, Keidel).

[The phenomenon of hearing: an interdisciplinary discourse. I]

For the last century hearing has been considered a purely passive process. G. S. Ohm's and H. v. Helmholtz's resonance theory of hearing was widely accepted until Wien's fundamental objection led to G. v. Békésy's and O. F. Ranke's travelling wave theory, the hydrodynamics of the inner ear that distributes the frequencies along the basilar membrane, which acts as the "first filter". Seebeck, Licklider, Schouten, and deBoer emphasized also the time domain and developed the concept of periodicity hearing and the "residue phenomenon". However, it was not until the concept of the "cochlear amplifier" and "active hearing" with energy generation during the transduction process within the ear itself was introduced that the "second filter" could be understood. Kemp's echo was the key to this revolutionary step in the theory of hearing.

Phase-locking of auditory-nerve discharges to sinusoidal electric stimulation of the cochlea

The activity of auditory-nerve fibers was recorded in anesthetized cats in response to sinusoidal electric stimuli applied through a bipolar electrode pair inserted about 5 mm into the cochlea through the round window. The synchronization index was calculated from period histograms for frequencies ranging from 0.2 to over 10 kHz. The stimulus artifact was largely eliminated through the use of differential micropipettes and an adaptive digital filter. Measured synchronization indices were many times larger than the indices that could be attributed to the residual stimulus artifact. Synchronization indices at each stimulus frequency varied considerably from fiber to fiber, even in the same animal. The dependence of synchrony on stimulus frequency was also variable, decreasing monotonically in some fibers and nonmonotonically in others. The average electric synchronization index for all fibers did not fall as steeply with frequency as does the average synchrony for acoustic stimuli. The finding of significant phase locking to electric stimuli well above 1 kHz suggests that the poor frequency discrimination of cochlear-implant recipients for single-channel stimulation above this frequency may be due to the inability of the central processor to make effective use of the available phase-locking information for monaural stimulation.

Latency and amplitude compound action potential tuning curves for tonal stimuli with nontraditional envelopes

To evaluate the influence of the acoustic context on the latency and amplitude tuning curves (TCs) of cochlear nerve compound action potentials (CAPs), tonal stimuli were generated with a variety of amplitude modulation envelopes. CAPs were produced by intensity increases (onsets) and decreases (offsets) from the low ambient sound level and by intensity changes from a preexisting tonal level. Onset CAPs from ambient levels generated V-shaped TCs. However, when simultaneous masking was used with onset CAPs which were produced by a 5- to 12-dB increase from preexisting levels of approximately 65 dB SPL, TCs were W-shaped and similar in appearance to those produced by simultaneous masking of offset CAPs. The forward masking of this same CAP resulted in a very sharp V-shaped TC. These data suggest that the preadaptation to 10 ms of a moderate level of a tonal stimulus can increase the tuning of ensembles of cochlear neurons to subsequent transient amplitude changes.

Discrimination of synthetic vowels by using tactile vocoder and a comparison to that of an eight-channel cochlear implant

A vowel discrimination test using a tactual vocoder was administered and the results were compared to that of an eight-channel cochlear implant. Both the tactile vocoder and the cochlear implant divided the speech signals into 16 frequency components using band-pass filters and lateral inhibition circuits. In the tactile vocoder, these 16 components were converted into a vibration with 200 Hz frequency and applied to a 3 x 16 element vibrator array using bimorph piezoelectric elements. The vibratory patterns were sensed on the fingertip. In the cochlear implant, the 16 components were reduced to eight current stimulation signals, consisting of biphasic pulses with 200 Hz frequency, which were applied to an eight-channel electrode array implanted in the scala tympani. The electrode array passed through the round window into the scala tympani to a depth of 23 mm. These psychophysical experiments investigate the ability of human subjects to discriminate synthetic vowels as a function of the number of channels employed. The results suggested that an eight-channel and a 16-channel tactile vocoder provided essentially the same discrimination scores. However, the ability to discriminate synthetic vowels decreased rapidly when less than eight channels were employed. The ability of an eight-channel tactile vocoder is expected to be better than that of the eight-channel cochlear implant because it is supposed that vowel discrimination is degraded by a phenomenon known as "current spreading" in the case of cochlear stimulation. However, the comparison between the two devices was not done on the cochlear implant subject.

Temporal response patterns of auditory nerve fibers to electrical stimulation in deafened squirrel monkeys

Electroneural response patterns of single auditory-nerve neurons were studied in aminoglycoside-deafened squirrel monkeys. The electrical stimuli were delivered through bipolar electrodes implanted in the scala tympani. The effects of pulse width, shape, frequency, and intensity on neural adaptation, phase locking, and spectral content were evaluated. Our results did not demonstrate the characteristic adaptation seen in auditory-nerve neurons in response to acoustic stimulation. Phase locking to a broad stimulus pulse (3200 microseconds/phase) was found to a very restricted phase angle of the electrical stimulus which was broader for square wave than for sine wave stimulation. The latency of the phase locked response varied inversely with stimulus intensity with greater variation for square wave stimulation than for sine wave stimulation. Auditory neurons were capable of a very high degree of phase locking to a 200-microseconds/phase pulse presented at 156 pulses per second (PPS) and to the first pulse of a 2500-Hz pulse burst. Phase locking was much poorer for the subsequent 200-microseconds/phase pulses comprising the 2500-Hz pulse burst where the neuron's response was determined by its relative recovery status. These findings can be explained by an interaction between the neuron's relative refractory status and its integration of charge over the stimulatory half cycle of the electrical stimulus. These two factors also appear to determine the interspike interval of the neural response. This interval decreased monotonically with increasing stimulus intensity. The neural spike rate (150-500 Hz) producing this interval increased with intensity and may be a source of periodicity information which the central auditory nervous system could interpret as pitch. This may account for the proportional relationship between pitch and stimulus intensity seen in some cochlear implant patients. Our study demonstrates that auditory-nerve neurons comply with basic neurophysiological principles in their responses to electrical stimulation. These principles should be incorporated into the cochlear prosthesis stimulator if more normal neural response patterns are desired in the cochlear prosthesis patient.

Single-channel versus multichannel electrical stimulation. Voicing-frequency and formant-transition difference limens

The 'frequency' processing ability of 11 cochlear-implant patients wearing the Symbion device was evaluated with the stimuli presented in a multichannel mode and in a single-channel mode. A fixed-formant synthetic vowel /a/ was used to measure voicing-frequency difference limen. A voiced, synthetic single formant was used to measure the formant-transition difference limen. On the voicing-frequency test, patients obtained a median difference limen of 8 Hz when stimuli were presented in the multichannel mode and 14.7 Hz in the single-channel mode. On the formant-transition test, patients obtained a median difference limen of 589 Hz in the multichannel mode and 611 Hz in the single-channel mode. Three patients were unable to obtain a difference limen within the range of available stimuli. The performance with multichannel stimulation was significantly better (lower difference limen) than performance in the single channel stimulation. Temporal information in more channels and place information may contribute to the superior performance with multichannel stimulation.

Cerebral magnetic responses to noise bursts and pauses of different durations

We compared magnetic-evoked responses of human auditory cortex to short (5, 10, 20, 40, 80 and 160 ms) noise bursts and to pauses of identical durations in continuous noise. Onsets of both stimuli evoked responses with the most prominent deflection (N100m) peaking at about 100 ms. Both field maps could be explained by current dipoles, which agree with activity at the supratemporal cortex at slightly different locations. At the shortest 5-ms duration the noise bursts evoked a clear N100m whereas pauses elicited very low-amplitude responses or no response at all. For both stimuli, N100m increased in amplitude when the stimulus duration was increased from 5 up to 20-40 ms. The latencies were 10-20 ms longer for pauses than noise bursts with the longest latencies at the shortest stimulus durations. The differences in amplitudes and latencies as a function of stimulus duration and the slightly different source areas indicate that the generators of the on- and off-responses are not identical.

Physiological properties of the electrically stimulated auditory nerve. I. Compound action potential recordings

The electrically evoked compound action potential (CAP) of the auditory nerve exhibits two peaks, termed N0, at 350 microseconds latency, and N1, at 550 microseconds latency. At low stimulus intensities the CAP consists solely of the long latency N1 peak. As the stimulus strength is increased the higher threshold N0 appears. At high stimulus intensities N1 disappears and only the N0 component of the CAP remains. It is postulated that N1 represents action potentials propagated from the dendritic processes of the auditory neurons and that N0 represents action potentials initiated on the axons of these cells. The N1 peak exhibits anomalous refractory behavior which can be identified in the electrically evoked auditory brainstem response (EABR). That behavior may be useful diagnostically in assessing the extent of dendrite degeneration in cochlear implant candidates and users.

Multichannel electrical stimulation of the auditory nerve in man. II. Channel interaction

A multichannel cochlear implant can be an effective prosthesis only if its channels are independent of each other. Presumably independence is achieved by stimulating different populations of surviving neurons. Two types of interaction might occur between channels: electrical current field summation peripheral to stimulation of the nerves and neural-perceptual interaction following stimulation. Two psychophysical techniques to assess channel independence are discussed. In one technique a masker is presented on one channel in order to adapt the nerves responding to that channel. The forward masked threshold of a signal is then measured on all other channels and elevation of threshold is assumed to indicate overlapping neural populations. In the second procedure channel interaction is evaluated by measuring the loudness summation of stimuli presented simultaneously to two channels. The magnitude, distribution, and phasic components of the loudness summation are measures of interaction between channels. Data from two subjects suggests that monopolar stimulation produces broader interaction patterns than bipolar stimulation as a function of electrode separation. Considerable differences in the extent of channel interaction were observed between the two subjects, possibly because of the difference in the absolute current levels needed for equivalent sensation levels.

Multichannel electrical stimulation of the auditory nerve in man. I. Basic psychophysics

Basic psychophysical measurements were obtained from three patients implanted with multichannel cochlear implants. This paper presents measurements from stimulation of a single channel at a time (either monopolar or bipolar). The shape of the threshold vs. frequency curve can be partially related to the membrane biophysics of the remaining spiral ganglion and/or dendrites. Nerve survival in the region of the electrode may produce some increase in the dynamic range on that electrode. Loudness was related to the stimulus amplitude by a power law with exponents between 1.6 and 3.4, depending on frequency. Intensity discrimination was better than for normal auditory stimulation, but not enough to offset the small dynamic range for electrical stimulation. Measures of temporal integration were comparable to normals, indicating a central mechanism that is still intact in implant patients. No frequency analysis of the electrical signal was observed. Each electrode produced a unique pitch sensation, but they were not simply related to the tonotopic position of the stimulated electrode. Pitch increased over more than 4 octaves (for one patient) as the frequency was increased from 100 to 300 Hz, but above 300 Hz no pitch change was observed. Possibly the major limitation of single channel cochlear implants is the 1-2 ms integration time (probably due to the capacitative properties of the nerve membrane which acts as a low-pass filter at 100 Hz). Another limitation of electrical stimulation is that there is no spectral analysis of the electrical waveform so that temporal waveform alone determines the effective stimulus.

Spatial cross-correlation. A proposed mechanism for acoustic pitch perception

We propose in this paper a new class of model processes for the extraction of spectral information from the neural representation of acoustic signals in mammals. We are concerned particularly with mechanisms for detecting the phase-locked activity of auditory neurons in response to frequencies and intensities of sound associated with speech perception. Recent psychophysical tests on deaf human subjects implanted with intracochlear stimulating electrodes as an auditory prosthesis have produced results which are in conflict with the predictions of the classical place-pitch and periodicity-pitch theories. In our model, the detection of synchronicity between two phase-locked signals derived from sources spaced a finite distance apart on the basilar membrane can be used to extract spectral information from the spatiotemporal pattern of basilar membrane motion. Computer simulations of this process suggest an optimal spacing of about 0.3-0.4 of the wavelength of the frequency to be detected. This interval is consistent with a number of psychophysical, neurophysiological, and anatomical observations, including the results of high resolution frequency-mapping of the anteroventral cochlear nucleus which are presented here. One particular version of this model, invoking the binaurally sensitive cells of the medial superior olive as the critical detecting elements, has properties which are useful in accounting for certain complex binaural psychophysical observations.

Coding considerations in design of cochlear prostheses

Some basic considerations in design of multichannel auditory nerve array stimulators and sound processors for an electrical stimulation cochlear prosthesis are briefly reviewed. Considerations specific to the design of strict auditory nerve simulation-based prostheses are discussed, as are considerations for production of channel vocoder-based and more purely information-based prostheses.