Faculty Bibliography

OBJECTIVE: To validate a method of measuring angular depth of insertion (aDOI) as well as positional depth of each electrode contact in a cochlear implant by using intraoperative postinsertion skull radiographs. STUDY DESIGN: Retrospective review. SETTING: Tertiary referral center. PATIENTS: Intraoperative postinsertion radiographs obtained from 18 cochlear implant recipients were chosen for analysis. One high-resolution computer tomography scan of the head with the electrode in place was also analyzed. One cadaveric temporal bone with an inserted electrode provided additional data for analysis. INTERVENTION: aDOI and position of each electrode contact were measured from the radiographs using readily available software. High-resolution computer tomography imaging of the cochlea and electrode were reconstructed in three dimensions and used to simulate head rotation during intraoperative radiographs. The cadaveric temporal bone was imaged by x-ray at various acquisition angles. MAIN OUTCOME MEASURES: We evaluated the error introduced in measuring aDOI by assessing intra- and inter-rater variability. We also evaluated the error introduced by x-ray acquisition at nonstandardized angles by analyzing the three-dimensional construct and the cadaveric temporal bone. RESULTS: The concordance correlation coefficients for intrarater (0.991) and inter-rater (0.996) variability in aDOI measurement were excellent. The error introduced by nonstandardized x-ray acquisition angles was only -12.5 degrees to +15.8 degrees even at the limits of clinically relevant head rotation. CONCLUSIONS: The intraoperative postinsertion radiograph is sufficient for estimating positional depth of electrode contacts and the aDOI. This measure is robust in the face of nonstandardized x-ray acquisition angles, and shows good intra- and inter-rater variability.

OBJECTIVES: Commercially available cochlear implant systems attempt to deliver frequency information going down to a few hundred Hertz, but the electrode arrays are not designed to reach the most apical regions of the cochlea, which correspond to these low frequencies. This may cause a mismatch between the frequencies presented by a cochlear implant electrode array and the frequencies represented at the corresponding location in a normal-hearing cochlea. In the following study, the mismatch between the frequency presented at a given cochlear angle and the frequency expected by an acoustic hearing ear at the corresponding angle is examined for the cochlear implant systems that are most commonly used in the United States. DESIGN: The angular insertion of each of the electrodes on four different electrode arrays (MED-EL Standard, MED-EL Flex28, Advanced Bionics HiFocus 1J, and Cochlear Contour Advance) was estimated from X-ray. For the angular location of each electrode on each electrode array, the predicted spiral ganglion frequency was estimated. The predicted spiral ganglion frequency was compared with the center frequency provided by the corresponding electrode using the manufacturer's default frequency-to-electrode allocation. RESULTS: Differences across devices were observed for the place of stimulation for frequencies below 650 Hz. Longer electrode arrays (i.e., the MED-EL Standard and Flex28) demonstrated smaller deviations from the spiral ganglion map than the other electrode arrays. For insertion angles up to approximately 270 degrees , the frequencies presented at a given location were typically approximately an octave below what would be expected by a spiral ganglion frequency map, while the deviations were larger for angles deeper than 270 degrees . For frequencies above 650 Hz, the frequency to angle relationship was consistent across all four electrode models. CONCLUSIONS: A mismatch was observed between the predicted frequency and the default frequency provided by every electrode on all electrode arrays. The mismatch can be reduced by changing the default frequency allocations, inserting electrodes deeper into the cochlea, or allowing cochlear implant users to adapt to the mismatch. Further studies are required to fully assess the clinical significance of the frequency mismatch.

What is the best way to help humans adapt to a distorted sensory input? Interest in this question is more than academic. The answer may help facilitate auditory learning by people who became deaf after learning language and later received a cochlear implant (a neural prosthesis that restores hearing through direct electrical stimulation of the auditory nerve). There is evidence that some cochlear implants (which provide information that is spectrally degraded to begin with) stimulate neurons with higher characteristic frequency than the acoustic frequency of the original stimulus. In other words, the stimulus is shifted in frequency with respect to what the listener expects to hear. This frequency misalignment may have a negative influence on speech perception by CI users. However, a perfect frequency-place alignment may result in the loss of important low frequency speech information. A trade-off may involve a gradual approach: start with correct frequency-place alignment to allow listeners to adapt to the spectrally degraded signal first, and then gradually increase the frequency shift to allow them to adapt to it over time. We used an acoustic model of a cochlear implant to measure adaptation to a frequency-shifted signal, using either the gradual approach or the "standard" approach (sudden imposition of the frequency shift). Listeners in both groups showed substantial auditory learning, as measured by increases in speech perception scores over the course of fifteen one-hour training sessions. However, the learning process was faster for listeners who were exposed to the gradual approach. These results suggest that gradual rather than sudden exposure may facilitate perceptual learning in the face of a spectrally degraded, frequency-shifted input. This article is part of a Special Issue entitled .

Abstract Conclusion: The human frequency-to-place map may be modified by experience, even in adult listeners. However, such plasticity has limitations. Knowledge of the extent and the limitations of human auditory plasticity can help optimize parameter settings in users of auditory prostheses. Objectives: To what extent can adults adapt to sharply different frequency-to-place maps across ears? This question was investigated in two bilateral cochlear implant users who had a full electrode insertion in one ear, a much shallower insertion in the other ear, and standard frequency-to-electrode maps in both ears. Methods: Three methods were used to assess adaptation to the frequency-to-electrode maps in each ear: (1) pitch matching of electrodes in opposite ears, (2) listener-driven selection of the most intelligible frequency-to-electrode map, and (3) speech perception tests. Based on these measurements, one subject was fitted with an alternative frequency-to-electrode map, which sought to compensate for her incomplete adaptation to the standard frequency-to-electrode map. Results: Both listeners showed remarkable ability to adapt, but such adaptation remained incomplete for the ear with the shallower electrode insertion, even after extended experience. The alternative frequency-to-electrode map that was tested resulted in substantial increases in speech perception for one subject in the short insertion ear.

OBJECTIVES: Auditory brainstem implants (ABI) use the same processing strategy as was developed for cochlear implants (CI). However, the cochlear nucleus (CN), the stimulation site of ABIs, is anatomically and physiologically more complex than the auditory nerve and consists of neurons with differing roles in auditory processing. The aim of this study was to evaluate the hypotheses that ABI users are less able than CI users to access speech spectro-temporal information delivered by the existing strategies and that the sites stimulated by different locations of CI and ABI electrode arrays differ in encoding of temporal patterns in the stimulation. DESIGN: Six CI users and four ABI users of Nucleus implants with ACE processing strategy participated in this study. Closed-set perception of aCa syllables (16 consonants) and bVd words (11 vowels) was evaluated via experimental processing strategies that activated one, two, or four of the electrodes of the array in a CIS manner as well as subjects' clinical strategies. Three single-channel strategies presented the overall temporal envelope variations of the signal on a single-implant electrode located at the high-, medium-, and low-frequency regions of the array. Implantees' ability to discriminate within electrode temporal patterns of stimulation for phoneme perception and their ability to make use of spectral information presented by increased number of active electrodes were assessed in the single- and multiple-channel strategies, respectively. Overall percentages and information transmission of phonetic features were obtained for each experimental program. RESULTS: Phoneme perception performance of three ABI users was within the range of CI users in most of the experimental strategies and improved as the number of active electrodes increased. One ABI user performed close to chance with all the single and multiple electrode strategies. There was no significant difference between apical, basal, and middle CI electrodes in transmitting speech temporal information, except a trend that the voicing feature was the least transmitted by the basal electrode. A similar electrode-location pattern could be observed in most ABI subjects. CONCLUSIONS: Although the number of tested ABI subjects was small, their wide range of phoneme perception performance was consistent with previous reports of overall speech perception in ABI patients. The better-performing ABI user participants had access to speech temporal and spectral information that was comparable to that of average CI user. The poor-performing ABI user did not have access to within-channel speech temporal information and did not benefit from an increased number of spectral channels. The within-subject variability between different ABI electrodes was less than the variability across users in transmission of speech temporal information. The difference in the performance of ABI users could be related to the location of their electrode array on the CN, anatomy, and physiology of their CN or the damage to their auditory brainstem due to tumor or surgery.

A non-parametric bootstrapping statistical method is introduced and investigated for estimating confidence intervals resulting from information transfer (IT) analysis of confusion matrices. Confidence intervals can be used to statistically compare ITs from two or more confusion matrices obtained in an experiment. Information transfer is a nonlinear analysis and does not satisfy many of the assumptions of a parametric method. The bootstrapping method accurately estimated IT confidence intervals as long as the confusion matrices contained a sufficiently large number of presentations per stimulus category, which is also a condition for reduced bias in IT analysis.

A growing number of minimally invasive surgical and diagnostic procedures require the insertion of an optical, mechanical, or electronic device in narrow spaces inside a human body. In such procedures, precise motion control is essential to avoid damage to the patient's tissues and/or the device itself. A typical example is the insertion of a cochlear implant which should ideally be done with minimum physical contact between the moving device and the cochlear canal walls or the basilar membrane. Because optical monitoring is not possible, alternative techniques for sub millimeter-scale distance control can be very useful for such procedures. The first requirement for distance control is distance sensing. We developed a novel approach to distance sensing based on the principles of scanning electrochemical microscopy (SECM). The SECM signal, i.e., the diffusion current to a microelectrode, is very sensitive to the distance between the probe surface and any electrically insulating object present in its proximity. With several amperometric microprobes fabricated on the surface of an insertable device, one can monitor the distances between different parts of the moving implant and the surrounding tissues. Unlike typical SECM experiments, in which a disk-shaped tip approaches a relatively smooth sample, complex geometries of the mobile device and its surroundings make distance sensing challenging. Additional issues include the possibility of electrode surface contamination in biological fluids and the requirement for a biologically compatible redox mediator.

Interleaved masking in cochlear implants is analogous to acoustic simultaneous masking and is relevant to speech processing strategies that interleave pulses on concurrently activated electrodes. In this study, spatial decay of masking as the distance between masker and probe increases was compared between forward and interleaved masking in the same group of cochlear implant users. Spatial masking patterns and the measures of place specificity were similar between forward and interleaved masking. Unlike acoustic hearing where broader tuning curves are obtained in simultaneous masking, the type of masking experiment did not influence the measure of place specificity in cochlear implants.