Search

Introduction: In patients with severe auditory impairment, partial hearing restoration can be achieved by sensory prostheses for the electrical stimulation of the central nervous system. However, these state-of-the-art approaches suffer from limited spectral resolution: electrical field spread depends on the impedance of the surrounding medium, impeding spatially focused electrical stimulation in neural tissue. To overcome these limitations, optogenetic activation could be applied in such prostheses to achieve enhanced resolution through precise and differential stimulation of nearby neuronal ensembles. Previous experiments have provided a first proof for behavioral detectability of optogenetic activation in the rodent auditory system, but little is known about the generation of complex and behaviorally relevant sensory patterns involving differential activation. Methods: In this study, we developed and behaviorally tested an optogenetic implant to excite two spatially separated points along the tonotopy of the murine inferior colliculus (ICc). Results: Using a reward based operant Go/No-Go paradigm, we show that differential optogenetic activation of a sub-cortical sensory pathway is possible and efficient. We demonstrate how animals which were previously trained in a frequency discrimination paradigm (a) rapidly respond to either sound or optogenetic stimulation, (b) generally detect optogenetic stimulation of two different neuronal ensembles, and (c) discriminate between them. Discussion: Our results demonstrate that optogenetic excitatory stimulation at different points of the ICc tonotopy elicits a stable response behavior over time periods of several months. With this study, we provide the first proof of principle for sub-cortical differential stimulation of sensory systems using complex artificial cues in freely moving animals. [ABSTRACT FROM AUTHOR]

IntroductionIn patients with severe auditory impairment, partial hearing restoration can be achieved by sensory prostheses for the electrical stimulation of the central nervous system. However, these state-of-the-art approaches suffer from limited spectral resolution: electrical field spread depends on the impedance of the surrounding medium, impeding spatially focused electrical stimulation in neural tissue. To overcome these limitations, optogenetic activation could be applied in such prostheses to achieve enhanced resolution through precise and differential stimulation of nearby neuronal ensembles. Previous experiments have provided a first proof for behavioral detectability of optogenetic activation in the rodent auditory system, but little is known about the generation of complex and behaviorally relevant sensory patterns involving differential activation.MethodsIn this study, we developed and behaviorally tested an optogenetic implant to excite two spatially separated points along the tonotopy of the murine inferior colliculus (ICc).ResultsUsing a reward based operant Go/No-Go paradigm, we show that differential optogenetic activation of a sub-cortical sensory pathway is possible and efficient. We demonstrate how animals which were previously trained in a frequency discrimination paradigm (a) rapidly respond to either sound or optogenetic stimulation, (b) generally detect optogenetic stimulation of two different neuronal ensembles, and (c) discriminate between them.DiscussionOur results demonstrate that optogenetic excitatory stimulation at different points of the ICc tonotopy elicits a stable response behavior over time periods of several months. With this study, we provide the first proof of principle for sub-cortical differential stimulation of sensory systems using complex artificial cues in freely moving animals.

Introduction: The new Osia® OSI200 implant incorporates a receiver coil and Piezo Power™ Transducer into one monolithic unit. Appropriate planning and surgical approach is needed for suitable positioning of the device. Method: To optimise the surgical field and provide tension-free wound closure our team have adopted a versatile 'Sheffield-S' post-auricular incision which remains hidden within the hairline. Conclusion: This incision provides adequate exposure for device placement and bone polishing/recessing. The soft tissue approach has resulted in improved operative efficacy particularly in those patients with irregular cortical bone or where pre-existing osseointegrated implants need to be removed or avoided. [ABSTRACT FROM AUTHOR]

The cochlear implant (CI) is widely considered to be one of the most innovative and successful neuroprosthetic treatments developed to date. Although outcomes vary, CIs are able to effectively improve hearing in nearly all recipients and can substantially improve speech understanding and quality of life for patients with significant hearing loss. A wealth of research has focused on underlying factors that contribute to success with a CI, and recent evidence suggests that the overall health of the cochlea could potentially play a larger role than previously recognized. This article defines and reviews attributes of cochlear health and describes procedures to evaluate cochlear health in humans and animal models in order to examine the effects of cochlear health on performance with a CI. Lastly, we describe how future biologic approaches can be used to preserve and/or enhance cochlear health in order to maximize performance for individual CI recipients. [ABSTRACT FROM AUTHOR]

The scalar position of the electrode array is assumed to be associated with auditory performance after cochlear implantation. We propose a new method that can be routinely applied in clinical practice to assess the position of an electrode array. Ten basilar membrane templates were generated using micro-computed tomography (micro-CT), based on the dimensions of 100 cochleae. Five surgeons were blinded to determine the position of the electrode array in 30 cadaveric cochleae. The procedure consisted of selecting the appropriate template based on cochlear dimensions, merging the electrode array reconstruction with the template using four landmarks, determining the position of the array according to the template position, and comparing the results obtained to histology data. The time taken to analyze each implanted cochlea was approximately 12 min. We found that, according to histology, surgeons were in almost perfect agreement when determining an electrode translocated to the scala vestibuli with the perimodiolar MidScala array (Fleiss' kappa (κ) = 0.82), and in moderate agreement when using the lateral wall EVO array (κ = 0.42). Our data indicate that an adapted basilar membrane template can be used as a rapid and reproducible method to assess the position of the electrode array after cochlear implantation. [ABSTRACT FROM AUTHOR]

Background: The Cochlear™ Osia® System leaves a retroauricular bump that can cause discomfort and poor aesthetic outcome. Method: To reduce the retroauricular bump, we introduced an implant well in the bone behind the ear for the transducer. We used cutting and diamond drills to create the implant well with an average depth of 4–5 mm. The surgical time including the implant well (40 min) was within the range of reported average surgical time (52 min). Conclusion: Introduction of an implant well resulted in a better aesthetic outcome and improved patients' comfort. The reduced distance between BI300 and ear canal might improve audiological outcome. [ABSTRACT FROM AUTHOR]

Background: Conventional technique for transcutaneous bone-anchored hearing implants surgery requires the elevation of a large retroauricular skin flap to expose the site of implantation, that may cause a large scar and the interruption of retroauricular vasculature. Methods: A less invasive, endoscopic-assisted modification of the surgical technique is described, which provides access to the implant site through two small skin incisions and the creation of a pocket in the retroauricular area. Conclusion: Endoscopic-assisted bone-anchored hearing implant surgery could be a viable option for transcutaneous bone-anchored hearing implant surgery, especially for pediatric patients. [ABSTRACT FROM AUTHOR]

Background Both the Roger remote microphone and on-ear, adaptive beamforming technologies (e.g., Phonak UltraZoom) have been shown to improve speech understanding in noise for cochlear implant (CI) listeners when tested in audio-only (A-only) test environments. Purpose Our aim was to determine if adult and pediatric CI recipients benefited from these technologies in a more common environment--one in which both audio and visual cues were available and when overall performance was high. Study Sample Ten adult CI listeners (Experiment 1) and seven pediatric CI listeners (Experiment 2) were tested. Design Adults were tested in quiet and in two levels of noise (level 1 and level 2) in Aonly and audio-visual (AV) environments. There were four device conditions: (1) an ear canal-level, omnidirectional microphone (T-mic) in quiet, (2) the T-mic in noise, (3) an adaptive directional mic (UltraZoom) in noise, and (4) a wireless, remote mic (Roger Pen) in noise. Pediatric listeners were tested in quiet and in level 1 noise in A-only and AV environments. The test conditions were: (1) a behind-the-ear level omnidirectional mic (processor mic) in quiet, (2) the processor mic in noise, (3) the T-mic in noise, and (4) the Roger Pen in noise. Data Collection and Analyses In each test condition, sentence understanding was assessed (percent correct) and ease of listening ratings were obtained. The sentence understanding data were entered into repeated-measures analyses of variance. Results For both adult and pediatric listeners in the AV test conditions in level 1 noise, performance with the Roger Pen was significantly higher than with the T-mic. For both populations, performance in level 1 noisewith the Roger Pen approached the level of baseline performance in quiet. Ease of listening in noise was rated higher in the Roger Pen conditions than in the T-mic or processor mic conditions in both A-only and AV test conditions. Conclusion The Roger remote mic and on-ear directional mic technologies benefit both speech understanding and ease of listening in a realistic laboratory test environment and are likely do the same in real-world listening environments. [ABSTRACT FROM AUTHOR]

Fibrous tissue and/or new bone are often found surrounding a cochlear implant in the cochlear scalae. This new intrascalar tissue could potentially limit cochlear implant function by increasing impedance and altering signaling pathways between the implant and the auditory nerve. In this study, we investigated the relationship between intrascalar tissue and 5 measures of implant function in guinea pigs. Variation in both spiral ganglion neuron (SGN) survival and intrascalar tissue was produced by implanting hearing ears, ears deafened with neomycin, and neomycin-deafened ears treated with a neurotrophin. We found significant effects of SGN density on 4 functional measures but adding intrascalar tissue level to the analysis did not explain more variation in any measure than was explained by SGN density alone. These results suggest that effects of intrascalar tissue on electrical hearing are relatively unimportant in comparison to degeneration of the auditory nerve, although additional studies in human implant recipients are still needed to assess the effects of this tissue on complex hearing tasks like speech perception. The results also suggest that efforts to minimize the trauma that aggravates both tissue development and SGN loss could be beneficial. [ABSTRACT FROM AUTHOR]

A cochlear implant (CI) is a surgically implanted device for the treatment of severe to profound sensorineural hearing loss in children and adults. It works by transducing acoustic energy into an electrical signal, which is used to stimulate surviving spiral ganglion cells of the auditory nerve. The past 2 decades have witnessed an exponential rise in the number of CI surgeries performed. Continual developments in programming strategies, device design, and minimally traumatic surgical technique have demonstrated the safety and efficacy of CI surgery. As a result, candidacy guidelines have expanded to include both pre and postlingually deaf children as young as 1 year of age, and those with greater degrees of residual hearing. A growing proportion of patients are undergoing CI for off-label or nontraditional indications including single-sided deafness, retrocochlear hearing loss, asymmetrical sensorineural hearing loss (SNHL) in adults and children with at least 1 ear that is better than performance cut-off for age, and children less than 12 months of age. Herein, we review CI design, clinical evaluation, indications, operative technique, and outcomes. We also discuss the expanding indications for CI surgery as it relates to lateral skull base pathology, comparing CI to auditory brainstem implants, and address the concerns with obtaining magnetic resonance imaging (MRI) in CI recipients. [ABSTRACT FROM AUTHOR]

An auditory brainstem implant (ABI) is a surgically implanted central neural auditory prosthesis for the treatment of profound sensorineural hearing loss in children and adults who are not cochlear implant candidates due to a lack of anatomically intact cochlear nerves or implantable cochleae. The device consists of a multielectrode surface array which is placed within the lateral recess of the fourth ventricle along the brainstem and directly stimulates the cochlear nucleus, thereby bypassing the peripheral auditory system. In the United States, candidacy criteria for ABI include deaf patients with neurofibromatosis type 2 (NF2) who are 12 years or older undergoing first- or second-side vestibular schwannoma resection. In recent years, several non-NF2 indications for ABI have been explored, including bilateral cochlear nerve avulsion from trauma, complete ossification of the cochlea due to meningitis, or a severe cochlear malformation not amenable to cochlear implantation. In addition, growing experience with ABI in infants and children has been documented with encouraging outcomes. While cochlear implantation generally remains the first-line option for hearing rehabilitation in NF2 patients with stable tumors or post hearing preservation surgery where hearing is lost but a cochlear nerve remains accessible for stimulation, an ABI is the next alternative in cases where the cochlear nerve is absent and/or if the cochlea cannot be implanted. Herein, we review ABI device design, clinical evaluation, indications, operative technique, and outcomes as it relates to lateral skull base pathology. [ABSTRACT FROM AUTHOR]

The present study explored the application of auditory cortical evoked potentials (ACEP) in the auditory assessment of people using an auditory prosthesis. There were 126 patients with prelingual deafness who were selected from January 2012-June 2017 from the First People's Hospital of Kunshan (Kunshan, China). HEARLab™ system was used to induce a P1-N1-P2 waveform under the condition of 60 dB sound pressure level at /m/, /g/ and /t/ acoustic stimulations. Speech production ability and auditory perception ability of patients were evaluated by speech intelligibility rating (SIR) and categories of auditory performance (CAP). Extraction rate of P1 waves of patients with auditory prosthesis was higher than that of N1 and P2 waves under different acoustic stimulations. A younger initial age and shorter deafness duration before patients used an auditory prosthesis led to more marked P1-N1-P2 waveforms and longer P1 latencies. At /m/ acoustic stimulation, P1 latency and amplitude were negatively associated with the usage time of auditory prosthesis. There were significant differences in the results of SIR and CAP and the initial age of use of auditory prosthesis and deafness duration before patients used the auditory prosthesis. These findings suggest that ACEP can be used to evaluate the auditory assessment of people using an auditory prosthesis. The initial age of use of an auditory prosthesis and deafness duration can affect the P1-N1-P2 waveform and P1 latency of prelingual deafness. [ABSTRACT FROM AUTHOR]

As the only medical device used in the treatment for deafness, the cochlear implant has benefited to more than half a million individuals worldwide. However, the device has limited penetration due to its high cost, especially in low- and middle-income countries. China alone has 27.8 million deaf people, but less than 100,000 of them have received a cochlear implant. The Nurotron Venus device was developed to address the need for an affordable yet safe and effective cochlear implant. The present study describes the design, development, and evaluation of the Nurotron intracochlear electrode array. The standard array is 22 mm in length from the round window marker to the apical tip of the carrier and has 24 electrodes, with a surface area of 0.32 mm² and center-to-center spacing of 0.85 mm. The Nurotron array has been tested to meet the mechanical, chemical, and electrical requirements specified by the ISO Standard 14708-07. Human temporal bone and clinical trial results showed that the Nurotron array is easy to insert (7.8/10 rating with 10 indicating the highest ease of use) and has a low complication rate (12.5%) of severe insertion trauma while achieving high device stability and reliability (6 array failures in 43,000 patient years of experience). As a critical component, the Nurotron array has contributed to the high level of Nurotron implant speech performance, equivalent to that produced by other existing devices. The Nurotron device has benefited 10,000 deaf people and helped reduce the unit cost from US$25,000 in 2011 to US$4,620 in 2017 through the Chinese Government Tender Program. New, slim, and micromachined electrodes are being developed to further improve performance and accessibility. [ABSTRACT FROM AUTHOR]

A case study approach is used to explore the surgical management of a patient with bilateral vestibular schwannomas and the insertion of auditory brainstem implants to restore her hearing. Effective collaborative intra-operative and post-operative care is critical in complex surgery involving a number of specialty services and the intra-operative nursing considerations are described. A brief outline of the differential use of auditory brainstem implants verses cochlea implants is also provided. [ABSTRACT FROM AUTHOR]

Because cochlear implants function by stimulating the auditory nerve, it is assumed that the condition of the nerve plays an important role in the efficacy of the prosthesis. Thus, considerable research has been devoted to methods of preserving the nerve following deafness. Neurotrophins have been identified as a potential contributor to neural health, but most of the research to date has been done in young animals and for short periods (less than 3 to 6 months) after the onset of treatment. The first objective of the current experiment was to examine the effects of a neurotrophin gene therapy delivery method on spiral ganglion neuron (SGN) preservation and function in the long term (5 to 14 months) in mature guinea pigs with cochlear implants. The second objective was to examine several potential non-invasive monitors of auditory nerve health following the neurotrophin gene therapy procedure. Eighteen mature adult male guinea pigs were deafened by cochlear perfusion of neomycin and then one ear was inoculated with an adeno-associated viral vector with an Nft3-gene insert (AAV.Ntf3) and implanted with a cochlear implant electrode array. Five control animals were deafened and inoculated with an empty AAV and implanted. Data from 43 other guinea pig ears from this and previous experiments were used for comparison: 24 animals implanted in a hearing ear, nine animals deafened and implanted with no inoculation, and ten normal-hearing non-implanted ears. After 4 to 21 months of psychophysical and electrophysiological testing, the animals were prepared for histological examination of SGN densities and inner hair cell (IHC) survival. Seventy-eight percent of the ears deafened and inoculated with AAV.Ntf3 showed better SGN survival than the 14 deafened-control ears. The degree of SGN preservation following the gene therapy procedure was variable across animals and across cochlear turns. Slopes of psychophysical multipulse integration (MPI) functions were predictive of SGN density, but only in animals with preserved IHCs. MPI was not affected by the AAV.Ntf3 treatment, but there was a minor improvement in temporal integration (TI). AAV.Ntf3 treatment had significant effects on ECAP and EABR amplitude growth func-tion (AGF) slopes; the reduction in slope in deafened ears was ameliorated by the AAV.Ntf3 treatment. Slopes of the ECAP and EABR AGFs were predictive of SGN density in a broad area near and just apical to the implant. The highest ensemble spontaneous activity (ESA) values were seen in animals with surviving IHCs, but AAV.Ntf3 treatment in deafened ears resulted in slightly higher ESA values compared to deafened untreated ears. Overall, a combination of the psychophysical and electrophysiological measures can be useful for monitoring the health of the implanted cochlea in guinea pigs. These measures should be applicable for assessing cochlear health in human subjects. [ABSTRACT FROM AUTHOR]

The cochlear implant (CI) is widely considered to be one of the most innovative and successful neuroprosthetic treatments developed to date. Although outcomes vary, CIs are able to effectively improve hearing in nearly all recipients and can substantially improve speech understanding and quality of life for patients with significant hearing loss. A wealth of research has focused on underlying factors that contribute to success with a CI, and recent evidence suggests that the overall health of the cochlea could potentially play a larger role than previously recognized. This article defines and reviews attributes of cochlear health and describes procedures to evaluate cochlear health in humans and animal models in order to examine the effects of cochlear health on performance with a CI. Lastly, we describe how future biologic approaches can be used to preserve and/or enhance cochlear health in order to maximize performance for individual CI recipients., (© 2022. The Author(s) under exclusive licence to Association for Research in Otolaryngology.)

Auditory nerve fibers (ANFs) play a crucial role in hearing by encoding and transporting the synaptic input from inner hair cells into afferent spiking information for higher stages of the auditory system. If the inner hair cells are degenerated, cochlear implants may restore hearing by directly stimulating the ANFs. The response of an ANF is affected by several characteristics of the electrical stimulus and of the ANF, and neurophysiological measurements are needed to know how the ANF responds to a particular stimulus. However, recording from individual nerve fibers in humans is not feasible and obtaining compound neural or psychophysical responses is often time-consuming. This motivates the design and use of models to estimate the ANF response to the electrical stimulation. Phenomenological models reproduce the ANF response based on a simplified description of ANF functionality and on a limited parameter space by not directly describing detailed biophysical mechanisms. Here, we give an overview of phenomenological models published to date and demonstrate how different modeling approaches can account for the diverse phenomena affecting the ANF response. To highlight the success achieved in designing such models, we also describe a number of applications of phenomenological models to predict percepts of cochlear implant listeners. [ABSTRACT FROM PUBLISHER]

Bilateral cochlear implants (CIs) provide improvements in sound localization and speech perception in noise over unilateral CIs. However, the benefits arise mainly from the perception of interaural level differences, while bilateral CI listeners' sensitivity to interaural time difference (ITD) is poorer than normal. To help understand this limitation, a set of ITD-sensitive neural models was developed to study binaural responses to electric stimulation. Our working hypothesis was that central auditory processing is normal with bilateral CIs so that the abnormality in the response to electric stimulation at the level of the auditory nerve fibers (ANFs) is the source of the limited ITD sensitivity. A descriptive model of ANF response to both acoustic and electric stimulation was implemented and used to drive a simplified biophysical model of neurons in the medial superior olive (MSO). The model's ITD sensitivity was found to depend strongly on the specific configurations of membrane and synaptic parameters for different stimulation rates. Specifically, stronger excitatory synaptic inputs and faster membrane responses were required for the model neurons to be ITD-sensitive at high stimulation rates, whereas weaker excitatory synaptic input and slower membrane responses were necessary at low stimulation rates, for both electric and acoustic stimulation. This finding raises the possibility of frequency-dependent differences in neural mechanisms of binaural processing; limitations in ITD sensitivity with bilateral CIs may be due to a mismatch between stimulation rate and cell parameters in ITD-sensitive neurons. [ABSTRACT FROM AUTHOR]

Cochlear implant listeners receive auditory stimulation through amplitude-modulated electric pulse trains. Auditory nerve studies in animals demonstrate qualitatively different patterns of firing elicited by low versus high pulse rates, suggesting that stimulus pulse rate might influence the transmission of temporal information through the auditory pathway. We tested in awake guinea pigs the temporal acuity of auditory cortical neurons for gaps in cochlear implant pulse trains. Consistent with results using anesthetized conditions, temporal acuity improved with increasing pulse rates. Unlike the anesthetized condition, however, cortical neurons responded in the awake state to multiple distinct features of the gap-containing pulse trains, with the dominant features varying with stimulus pulse rate. Responses to the onset of the trailing pulse train (Trail-ON) provided the most sensitive gap detection at 1,017 and 4,069 pulse-per-second (pps) rates, particularly for short (25 ms) leading pulse trains. In contrast, under conditions of 254 pps rate and long (200 ms) leading pulse trains, a sizeable fraction of units demonstrated greater temporal acuity in the form of robust responses to the offsets of the leading pulse train (Lead-OFF). Finally, TONIC responses exhibited decrements in firing rate during gaps, but were rarely the most sensitive feature. Unlike results from anesthetized conditions, temporal acuity of the most sensitive units was nearly as sharp for brief as for long leading bursts. The differences in stimulus coding across pulse rates likely originate from pulse rate-dependent variations in adaptation in the auditory nerve. Two marked differences from responses to acoustic stimulation were: first, Trail-ON responses to 4,069 pps trains encoded substantially shorter gaps than have been observed with acoustic stimuli; and second, the Lead-OFF gap coding seen for <15 ms gaps in 254 pps stimuli is not seen in responses to sounds. The current results may help to explain why moderate pulse rates around 1,000 pps are favored by many cochlear implant listeners. [ABSTRACT FROM AUTHOR]

Abstract: As a step to develop auditory prosthesis by cortical stimulation, we tested whether a single train of pulses applied to the primary auditory cortex could elicit classically conditioned behavior in guinea pigs. Animals were trained using a tone as the conditioned stimulus and an electrical shock to the right eyelid as the unconditioned stimulus. After conditioning, a train of 11 pulses applied to the left AI induced the conditioned eye-blink response. Cortical stimulation induced no response after extinction. Our results support the feasibility of auditory prosthesis by electrical stimulation of the cortex. [Copyright &y& Elsevier]

Abstract: Objective: In the last years, technology has made it possible for deaf people, and especially for deaf children, to enter the world of sound and, as a consequence, to facilitate communicative competence in oral language, which used to be an insurmountable obstacle. With this article we are trying to carry out a revision of the descriptions that have traditionally featured deaf children''s voice. Methods: We analyse the voice quality obtained out of a research with 62 children (35 girls and 27 boys) with profound deafness of several degrees, aged on average 7 years 4 months, and users of different types of auditory prostheses (analogue/digital hearing aids or cochlear implants) experimental group – Voice quality was evaluated from the production of a sustained vowel /a:/ for four-five seconds, considering F 0, jitter, shimmer and NHR values. Results: The results are compared with the ones obtained from a control group of hearing children (n=54). The Experimental Group shows altered voice quality parameters. Particularly in F 0 (294.079Hz) and shimmer (0.568dB), there are statistically significant values in comparison with the control group (p<0.001). With regards to jitter (1.474%), differences were smaller. Nevertheless, the results show how the profiles reached by deaf children are nowadays more similar to those reached by the hearing control group. However, the degree of hearing impairment and the type of prosthesis used can determine the parameters of deaf children''s voice quality to a great extent. Thus, the digital hearing aid users are the ones who present better voice quality values: F 0 (265.50z); jitter (1.009%) and shimmer (0.486dB); whereas implant users: F 0 (287.93z); jitter (1.344%) and shimmer (0.526dB), and particularly analogue hearing aid users: F 0 (323.80z); jitter (1.999%) and shimmer (0.687dB), did show significant differences in comparison with the control group of hearing children. Conclusions: In the study of voice quality in children with profound hearing loss, it is very important to have information both about the degree of hearing loss and the kind of prosthesis used. Implant users show more altered voice quality than digital hearing aid users. However, the hearing loss they compensate is much more important than the hearing loss compensated by the hearing aids. Therefore, we consider that both prostheses help children with hearing loss to have a more normalized voice quality than what scientific literature has traditionally stated. Finally, we question the validity of using some acoustic parameters as indicators of voice quality in deaf children having no laryngeal problems. [Copyright &y& Elsevier]

Previous studies have shown that residual acoustic hearing supplements cochlear implant function to improve speech recognition in noise as well as perception of music. The current study had two primary objectives. First, we sought to determine how cochlear implantation and electrical stimulation over a time period of 14 to 21 months influence cochlear structures such as hair cells and spiral ganglion neurons. Second, we sought to investigate whether the structures that provide acoustic hearing also affect the perception of electrical stimulation. We compared psychophysical responses to cochlear implant stimulation in two groups of adult guinea pigs. Group I (11 animals) received a cochlear implant in a previously untreated ear, while group II (ten animals) received a cochlear implant in an ear that had been previously infused with neomycin to destroy hearing. Psychophysical thresholds were measured in response to pulse-train and sinusoidal stimuli. Histological analysis of all group I animals and a subset of group II animals was performed. Nine of the 11 group I animals showed survival of the organ of Corti and spiral ganglion neurons adjacent to the electrode array. All group I animals showed survival of these elements in regions apical to the electrode array. Group II animals that were examined histologically showed complete loss of the organ of Corti in regions adjacent and apical to the electrode array and severe spiral ganglion neuron loss, consistent with previous reports for neomycin-treated ears. Behaviorally, group II animals had significantly lower thresholds than group I animals in response to 100 Hz sinusoidal stimuli. However, group I animals had significantly lower thresholds than group II animals in response to pulse-train stimuli (0.02 ms/phase; 156 to 5,000 pps). Additionally, the two groups showed distinct threshold versus pulse rate functions. We hypothesize that the differences in detection thresholds between groups are caused by the electrical activation of the hair cells in group I animals and/or differences between groups in the condition of the spiral ganglion neurons. [ABSTRACT FROM AUTHOR]

Electrical interaural time delay (ITD) discrimination was measured using 300-ms bursts applied to binaural pitch matched electrodes at basal, mid, and apical locations in each ear. Six bilateral implant users, who had previously shown good ITD sensitivity at a pulse rate of 100 pulses per second (pps), were assessed. Thresholds were measured as a function of pulse rate between 100 and 1,000 Hz, as well as modulation rate over that same range for high-rate pulse trains at 6,000 pps. Results were similar for all three places of stimulation and showed decreasing ITD sensitivity as either pulse rate or modulation rate increased, although the extent of that effect varied across subjects. The results support a model comprising a common ITD mechanism for high- and low-frequency places of stimulation, which, for electrical stimulation, is rate-limited in the same way across electrodes because peripheral temporal responses are largely place invariant. Overall, ITD sensitivity was somewhat better with unmodulated pulse trains than with high-rate pulse trains modulated at matched rates, although comparisons at individual rates showed that difference to be significant only at 300 Hz. Electrodes presenting with the lowest thresholds at 600 Hz were further assessed using bursts with a ramped onset of 10 ms. The slower rise time resulted in decreased performance in four of the listeners, but not in the two best performers, indicating that those two could use ongoing cues at 600 Hz. Performance at each place was also measured using single-pulse stimuli. Comparison of those data with the unmodulated 300-ms burst thresholds showed that on average, the addition of ongoing cues beyond the onset enhanced overall ITD sensitivity at 100 and 300 Hz, but not at 600 Hz. At 1,000 Hz, the added ongoing cues actually decreased performance. That result is attributed to the introduction of ambiguous cues within the physiologically relevant range and increased dichotic firing. [ABSTRACT FROM AUTHOR]

Recent data from feline auditory nerve fibers (ANFs) indicate that electrically stimulated fibers can undergo large degrees of rate adaptation to pulse-train stimuli using pulse rates within the range used by clinical auditory prostheses. However, the application of Hodgkin-Huxley-type models does not produce such adaptation, which occurs over time periods on the order of 100 ms. We describe our development of a computational ANF axon model that incorporates a time-changing external potassium concentration ([K+]cxt) that depends on potassium currents produced by active nodal channel activity. This relatively simple and computationally tractable approach produces poststimulus time histograms that are similar to experimental (cat) data. Furthermore, this mechanism could be easily incorporated into other models to produce much more realistic estimates of the neural coding produced by repeated electric stimulation. [ABSTRACT FROM AUTHOR]

The cochlear implant is the most successful of all neural prostheses developed to date. It is the most effective prosthesis in terms of restoration of function, and the people who have received a cochlear implant outnumber the recipients of other types of neural prostheses by orders of magnitude. The primary purpose of this article is to provide an overview of contemporary cochlear implants from the perspective of two designers of implant systems. That perspective includes the anatomical situation presented by the deaf cochlea and how the different parts of an implant system (including the user's brain) must work together to produce the best results. In particular, we present the design considerations just mentioned and then describe in detail how the current levels of performance have been achieved. We also describe two recent advances in implant design and performance. In concluding sections, we first present strengths and limitations of present systems and then offer some possibilities for further improvements in this technology. In all, remarkable progress has been made in the development of cochlear implants but much room still remains for improvements, especially for patients presently at the low end of the performance spectrum. [ABSTRACT FROM AUTHOR]

Abstract: Auditory prostheses use implanted electrode arrays that permit stimulation at many sites along the tonotopic axis of auditory neurons. Psychophysical studies demonstrate that measures of implant function, such as detection and discrimination thresholds, vary considerably across these sites, that the across-site patterns of these measures differ across subjects, and that the likely mechanisms underlying this variability differ across measures. Psychophysical and speech recognition studies suggest that not all stimulation sites contribute equally to perception with the prosthesis and that some sites might have negative effects on perception. Studies that reduce the number of active stimulation sites indicate that most cochlear implant users can effectively utilize a maximum of only about seven sites in their processors. These findings support a strategy for improving implant performance by selecting only the best stimulation sites for the processor map. Another approach is to revise stimulation parameters for ineffective sites in an effort to improve acuity at those sites. In this paper, we discuss data supporting these approaches and some potential pitfalls. [Copyright &y& Elsevier]

Abstract: Cochlear implants (CIs) stimulate the auditory nerve with trains of symmetric biphasic (BI) pulses. We review studies showing that more efficient stimulation can be achieved by modifying these pulses by (1) increasing the inter-phase gap (IPG) between the two phases of each pulse, thereby delaying the recovery of charge, (2) increasing the duration and decreasing the amplitude of one phase – so-called “pseudomonophasic (PS)” waveforms, and (3) combining the pseudomonophasic stimulus with an IPG in a “delayed pseudomonophasic” waveform (PS_IPG). These efficiency gains, measured using changes in threshold and loudness, occur at a wide range of pulse rates, including those commonly used in current CI systems. In monopolar mode, dynamic ranges are larger for PS and for long-IPG pulse shapes than for BI pulses, but this increase in DR is not accompanied by a higher number of discriminable loudness steps, and hence, in a better coding of loudness. Moreover, waveforms with relatively short and long interphase gaps do not yield different patterns of excitation despite the relatively large differences in threshold. Two important findings are that, contrary to data obtained in animal experiments, anodic currents are more effective than cathodic stimulation for human CI patients and that the thresholds decrease with increases in IPG over a much longer time course (more than 3ms) than for animals. In this review it is discussed how these alternative pulse shapes may be beneficial in terms of reducing power consumption and channel interactions, which issues remain to be addressed, and how models contribute to guiding our research. [Copyright &y& Elsevier]

Abstract: The aims of this paper are to (i) provide a brief history of cochlear implants; (ii) present a status report on the current state of implant engineering and the levels of speech understanding enabled by that engineering; (iii) describe limitations of current signal processing strategies; and (iv) suggest new directions for research. With current technology the “average” implant patient, when listening to predictable conversations in quiet, is able to communicate with relative ease. However, in an environment typical of a workplace the average patient has a great deal of difficulty. Patients who are “above average” in terms of speech understanding, can achieve 100% correct scores on the most difficult tests of speech understanding in quiet but also have significant difficulty when signals are presented in noise. The major factors in these outcomes appear to be (i) a loss of low-frequency, fine structure information possibly due to the envelope extraction algorithms common to cochlear implant signal processing; (ii) a limitation in the number of effective channels of stimulation due to overlap in electric fields from electrodes; and (iii) central processing deficits, especially for patients with poor speech understanding. Two recent developments, bilateral implants and combined electric and acoustic stimulation, have promise to remediate some of the difficulties experienced by patients in noise and to reinstate low-frequency fine structure information. If other possibilities are realized, e.g., electrodes that emit drugs to inhibit cell death following trauma and to induce the growth of neurites toward electrodes, then the future is very bright indeed. [Copyright &y& Elsevier]

A new implantable hearing system, the direct acoustic cochlear stimulator (DACS) is presented. This system is based on the principle of a power-driven stapes prosthesis and intended for the treatment of severe mixed hearing loss due to advanced otosclerosis. It consists of an implantable electromagnetic transducer, which transfers acoustic energy directly to the inner ear, and an audio processor worn externally behind the implanted ear. The device is implanted using a specially developed retromeatal microsurgical approach. After removal of the stapes, a conventional stapes prosthesis is attached to the transducer and placed in the oval window to allow direct acoustical coupling to the perilymph of the inner ear. In order to restore the natural sound transmission of the ossicular chain, a second stapes prosthesis is placed in parallel to the first one into the oval window and attached to the patient’s own incus, as in a conventional stapedectomy. Four patients were implanted with an investigational DACS device. The hearing threshold of the implanted ears before implantation ranged from 78 to 101 dB (air conduction, pure tone average, 0.5–4 kHz) with air-bone gaps of 33–44 dB in the same frequency range. Postoperatively, substantial improvements in sound field thresholds, speech intelligibility as well as in the subjective assessment of everyday situations were found in all patients. Two years after the implantations, monosyllabic word recognition scores in quiet at 75 dB improved by 45–100 percent points when using the DACS. Furthermore, hearing thresholds were already improved by the second stapes prosthesis alone by 14–28 dB (pure tone average 0.5–4 kHz, DACS switched off). No device-related serious medical complications occurred and all patients have continued to use their device on a daily basis for over 2 years. Copyright © 2008 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Abstract: The auditory midbrain implant (AMI), which is designed for stimulation of the inferior colliculus (IC), is now in clinical trials. The AMI consists of a single shank array (20 contacts) and uses a stimulation strategy originally designed for cochlear implants since it is already approved for human use and we do not yet know how to optimally activate the auditory midbrain. The goal of this study was to investigate the effects of different pulse rates and phase durations on loudness and pitch percepts because these parameters are required to implement the AMI stimulation strategy. Although each patient was implanted into a different region (i.e. lateral lemniscus, central nucleus of IC, dorsal cortex of IC), they generally exhibited similar threshold versus phase duration, threshold versus pulse rate, and pitch versus pulse rate curves. In particular, stimulation with 100 μs/phase, 250 pulse per second (pps) pulse trains achieved an optimal balance among safety, energy, and current threshold requirements while avoiding rate pitch effects. However, we observed large differences across patients in loudness adaptation to continuous pulse stimulation over long time scales. One patient (implanted in dorsal cortex of IC) even experienced complete loudness decay and elevation of thresholds with daily stimulation. Comparing these results with those of cochlear implant and auditory brainstem implant patients, it appears that stimulation of higher order neurons exhibits less and even no loudness summation for higher rate stimuli and greater current leakage for longer phase durations than that of cochlear neurons. The fact that all midbrain regions we stimulated, which includes three distinctly different nuclei, exhibited similar loudness summation effects (i.e. none for pulse rates above 250 pps) suggests a possible shift in some coding properties that is affected more by which stage along the auditory pathway rather than the types of neurons are being stimulated. However, loudness adaptation occurs at multiple stages from the cochlea up to the midbrain. [Copyright &y& Elsevier]

Abstract: The multichannel design of contemporary cochlear implants (CIs) is predicated on the assumption that each channel activates a relatively restricted and independent sector of the deaf auditory nerve array, just as a sound within a restricted frequency band activates a restricted region of the normal cochlea The independence of CI channels, however, is limited; and the factors that determine their independence, the relative overlap of the activity patterns that they evoke, are poorly understood. In this study, we evaluate the spread of activity evoked by cochlear implant channels by monitoring activity at 16 sites along the tonotopic axis of the guinea pig inferior colliculus (IC). “Spatial tuning curves” (STCs) measured in this way serve as an estimate of activation spread within the cochlea and the ascending auditory pathways. We contrast natural stimulation using acoustic tones with two kinds of electrical stimulation either (1) a loose fitting banded array consisting of a cylindrical silicone elastomer carrier with a linear series of ring contacts; or (2) a space-filling array consisting of a tapered silicone elastomer carrier that is designed to fit snugly into the guinea pig scala tympani with a linear series of ball contacts positioned along it Spatial tuning curves evoked by individual acoustic tones, and by activation of each contact of each array as a monopole, bipole or tripole were recorded. Several channel configurations and a wide range of electrode separations were tested for each array, and their thresholds and selectivity were estimated. The results indicate that the tapered space-filling arrays evoked more restricted activity patterns at lower thresholds than did the banded arrays. Monopolar stimulation (one intracochlear contact activated with an extracochlear return) using either array evoked broad activation patterns that involved the entire recording array at current levels <6dBSL, but at relatively low thresholds. Bi- and tri-polar configurations of both array types evoked more restricted activity patterns, but their thresholds were higher than those of monopolar configurations. Bipolar and tripolar configurations with closely spaced contacts evoked activity patterns that were comparable to those evoked by pure tones. As the spacing of bipolar electrodes was increased (separations >1mm), the activity patterns became broader and evoked patterns with two distinct threshold minima, one associated with each contact. [Copyright &y& Elsevier]

The cochlear implant is the most successful neural prosthesis to date and may serve as a paradigm for the development or further development of other systems to interface sensors with the nervous system, e.g., visual or vestibular prostheses. This paper traces the history of cochlear implants and describes how the current levels of performance have been achieved. Lessons and insights from this experience are presented in concluding sections. [ABSTRACT FROM PUBLISHER]

The speech reception performance of a recipient of the Clarion CII implant was evaluated with a comprehensive set of tests. The same tests were administered for a group of six subjects with normal hearing. Scores for the implant subject were not different from the scores for the normal-hearing subjects, for seven of the nine tests, including the most difficult test used in standard clinical practice. These results are both surprising and encouraging, in that the implant provides only a very crude mimicking of only some aspects of the normal physiology. [ABSTRACT FROM AUTHOR]

A simplified cochlear implant (CI) system would be appropriate for widespread use in developing countries. Here, we describe a CI that we have designed to realize such a concept. The system implements 8 channels of processing and stimulation using the continuous interleaved sampling (CIS) strategy. A generic digital signal processing (DSP) chip is used for the processing, and the filtering functions are performed with a fast Fourier transform (FFT) of a microphone or other input. Data derived from the processing are transmitted through an inductive link using pulse width modulation (PWM) encoding and amplitude shift keying (ASK) modulation. The same link is used in the reverse direction for backward telemetry of electrode and system information. A custom receiver-stimulator chip has been developed that demodulates incoming data using pulse counting and produces charge balanced biphasic pulses at 1000 pulses/s/electrode. This chip is encased in a titanium package that is hermetically sealed using a simple but effective method. A low cost metal-silicon hybrid mold has been developed for fabricating an intracochlear electrode array with 16 ball-shaped stimulating contacts. [ABSTRACT FROM AUTHOR]

Objectives/Hypothesis: Although cochlear implants have been successfully used by many individuals with profound hearing impairment, limitations still remain with this approach to hearing restoration, including poor stimulation selectivity because of cross-talk between electrodes and poor low-frequency percepts. These limitations may be mitigated by direct intraneural stimulation of the auditory nerve by way of an array of penetrating microelectrodes. Such an approach should provide focal stimulation and selective activation of the nerve fibers,thereby minimizing cross-talk among implanted stimulating electrodes and evoking narrow-band frequency percepts. Study Design: We investigated the activation of primary auditory cortex evoked by such direct intraneural electrical stimulation of the auditory nerve. Methods: We implanted 11 penetrating microelectrodes in the cat auditory nerve, simulated the nerve byway of these electrodes, and recorded the evoked neuronal activity patterns in cat primary auditory cortex. We compared these activation patterns with acoustically evoked cortical activity patterns obtained in a different animal. Results: Our results showed that direct stimulation of the auditory nerve evoked localized activity patterns in primary auditory cortex similar in spatial extent to those evoked by acoustic stimulation and that the extent of cortical activation by both acoustic and electrical stimuli was graded with stimulus intensity. These results suggest that the implanted electrodes can excite independent and small populations of nerve fibers.Conclusion: This study demonstrates the functional feasibility of direct intraneural auditory nerve stimulation with an array of penetrating microelectrodes and that such an approach could form the foundation for an auditory prosthesis with improved frequency coding. [ABSTRACT FROM AUTHOR]

The auditory midbrain implant (AMI) is a new treatment for hearing restoration in patients with neural deafness or surgically inaccessible cochleae who cannot benefit from cochlear implants (CI). This includes neurofibromatosis type II (NF2) patients who, due to development and/or removal of vestibular schwannomas, usually experience complete damage of their auditory nerves. Although the auditory brainstem implant (ABI) provides sound awareness and aids lip-reading capabilities for these NF2 patients, it generally only achieves hearing performance levels comparable with a single-channel CI. In collaboration with Cochlear Ltd. (Lane Cove, Australia), we developed a human prototype AMI, which is designed for electrical stimulation along the well-defined tonotopic gradient of the inferior colliculus central nucleus (ICC). Considering that better speech perception and hearing performance has been correlated with a greater number of discriminable frequency channels of information available, the ability of the AMI to effectively activate discrete frequency regions within the ICC may enable better hearing performance than achieved by the ABI. Therefore, the goal of this study was to investigate if our AMI array could achieve low-threshold, frequency-specific activation within the ICC, and whether the levels for ICC activation via AMI stimulation were within safe limits for human application. We electrically stimulated different frequency regions within the ICC via the AMI array and recorded the corresponding neural activity in the primary auditory cortex (A1) using a multisite silicon probe in ketamine-anesthetized guinea pigs. Based on our results, AMI stimulation achieves lower thresholds and more localized, frequency-specific activation than CI stimulation. Furthermore, AMI stimulation achieves cortical activation with current levels that are within safe limits for central nervous system stimulation. This study confirms that our AMI design is sufficient for ensuring safe and effective activation of the ICC, and warrants further studies to translate the AMI into clinical application. [ABSTRACT FROM AUTHOR]

Abstract: Sensory cortical prostheses have potential to aid people suffering from blindness, deafness and other sensory deficits. However, research to date has shown that sensation thresholds via epicortical stimulation are surprisingly large. These thresholds result in potentially deleterious electrical currents, as well as large activation volumes. Large activation volumes putatively limit the corresponding number of independent stimulation channels in a neural prosthesis. In this study, penetrating stimulation of the auditory cortex was tested for its ability to transmit salient information to behaving rat subjects. Here, we show that subjects that were previously trained to discriminate natural stimuli immediately discriminated different microstimulation cues more accurately and with shorter response latencies than the natural stimuli. Additionally, the cortical microstimulation resulted in a generalization gradient across locations within the cortex. The results demonstrate the efficacy of using closely spaced cortical microstimulation to efficiently transmit highly salient and discriminable information to a behaving subject. [Copyright &y& Elsevier]

Intersubject variability in perception is a prominent characteristic of people with cochlear implants. This study characterized intersubject differences using simple metrics based on psychophysical measures: maximum comfortable loudness levels (C levels) and dynamic ranges (DRs). In a group of 17 subjects, we assessed across-site variation (ASV) and across-site mean (ASM) values of C levels and DRs for bipolar (BP) and monopolar (MP) stimulation, and examined the relation of these metrics to speech recognition across subjects. Significant negative correlations with speech recognition were found for ASVs of C levels for BP stimulation; i.e., subjects with high ASVs of BP C levels had poor speech recognition. Positive correlations with speech recognition were found for ASMs of C levels and ASMs of DRs for both BP and MP stimulation; i.e., subjects with high mean C levels and large mean DRs had better speech recognition. Thus, these psychophysical metrics are effective for diagnosis of individual differences in performance of subjects with cochlear implants. Furthermore, they point to some potentially useful treatment procedures. [ABSTRACT FROM AUTHOR]

Functional implications of across-site variation in detection thresholds in subjects with cochlear implants were evaluated by comparing thresholds to speech recognition performance. Detection thresholds for bipolar (BP) and monopolar (MP) stimulation of all available stimulation sites were assessed in 21 subjects with Nucleus CI24M and CI24R(CS) implants. We found significant negative correlations between speech recognition and within-subject across-site threshold variance for both BP and MP stimulation, but no significant correlation of speech recognition with mean threshold levels. These results suggest that across-site variance of detection thresholds could provide a useful early indication of the prognosis for speech recognition and might serve as an indicator for specific therapeutic approaches in individual subjects. [ABSTRACT FROM AUTHOR]

The design of contemporary multichannel cochlear implants is predicated on the presumption that they activate multiple independent sectors of the auditory nerve array. The independence of these channels, however, is limited by the spread of activation from each intracochlear electrode across the auditory nerve array. In this study, we evaluated factors that influence intracochlear spread of activation using two types of intracochlear electrodes: (1) a clinical-type device consisting of a linear series of ring contacts positioned along a silicon elastomer carrier, and (2) a pair of visually placed (VP) ball electrodes that could be positioned independently relative to particular intracochlear structures, e.g., the spiral ganglion. Activation spread was estimated by recording multineuronal evoked activity along the cochleotopic axis of the central nucleus of the inferior colliculus (ICC). This activity was recorded using silicon-based single-shank, 16-site recording probes, which were fixed within the ICC at a depth defined by responses to acoustic tones. After deafening, electric stimuli consisting of single biphasic electric pulses were presented with each electrode type in various stimulation configurations (monopolar, bipolar, tripolar) and/or various electrode orientations (radial, off-radial, longitudinal). The results indicate that monopolar (MP) stimulation with either electrode type produced widepread excitation across the ICC. Bipolar (BP) stimulation with banded pairs of electrodes oriented longitudinally produced activation that was somewhat less broad than MP stimulation, and tripolar (TP) stimulation produced activation that was more restricted than MP or BP stimulation. Bipolar stimulation with radially oriented pairs of VP ball electrodes produced the most restricted activation. The activity patterns evoked by radial VP balls were comparable to those produced by pure tones in normal-hearing animals. Variations in distance between radially oriented VP balls had little effect on activation spread, although increases in interelectrode spacing tended to reduce thresholds. Bipolar stimulation with longitudinally oriented VP electrodes produced broad activation that tended to broaden as the separation between electrodes increased. [ABSTRACT FROM AUTHOR]

Electrical stimulation of the auditory nerve produces highly synchronized responses. As a consequence, electrical stimulation may result in a narrow dynamic range of hearing and poor temporal representation of an input signal. The electrically evoked compound action potential (ECAP) is an electrophysiologic response used for neural assessment in individuals with auditory prostheses. Because the ECAP arises from the activity of a population of auditory nerve fibers, within- and across-fiber synchrony should be evident in the responses. Due to its clinical relevance and reflection of neural response properties, the ECAP is used in the present study to examine changes in neural synchrony. Empirical and modeled single-fiber data indicate that stimulation with electrical pulses of a sufficiently high rate may induce stochastic neural response behaviors. This study investigated the effects of adding high-rate conditioning pulses (5000 pps) on the ECAP in response to 100 Hz electrical sinusoids. The results showed that high-rate conditioning pulses increased response amplitudes at low sinusoidal levels and decreased the amplitudes at high sinusoidal levels, indicating a decrease in the slope of the ECAP growth functions to sinusoidal stimuli. The results are consistent with a hypothesis that high-rate conditioning pulses increase single-fiber relative spread (RS) in response to sinusoidal stimuli, and the effect is highly dependent on the level of the high-rate conditioning pulses. [Copyright &y& Elsevier]

In cochlear implants, variation across stimulation sites in psychophysical detection thresholds (T levels) and maximum comfortable loudness levels (C levels) can be large when narrow-bipolar (BP) stimulation is used. This across-site variation is typically smaller when monopolar (MP) stimulation is used. At least two models can account for across-site variation and the effects of electrode configuration on the magnitude of the variation. According to one model, across-site variation reflects site-to-site differences in the distances between the stimulating electrodes and the sites of action-potential initiation. Under this model, the lower across-site variation with MP stimulation is due to shallower current versus distance gradients. An alternative model assumes that T and C levels depend on integration of activity across the whole population of neurons and that MP stimulation activates neurons over a larger spatial extent than does BP stimulation. If T and C levels are determined by integration of activity across large overlapping populations of neurons, then their values at adjacent sites should be more similar than if these levels result from integration across smaller, more independent populations. We tested the models by examining the effects on across-site variation of three variables believed to affect the spatial extent of activation: electrode configuration, stimulus level within the dynamic range, and electrode-array design. T levels and C levels were measured in 13 subjects with Nucleus® CI24M (straight array) and 9 subjects with Nucleus® CI24R(CS) (Contour) cochlear implants using bipolar (BP) and monopolar (MP) electrode configurations. Site-to-site variation in T and C levels for BP stimulation was 2.1–3.3 times larger than that for MP stimulation. Contrary to the across-neuron integration hypothesis, no significant differences were found between across-site variation for T levels and that for C levels for the BP configuration. There was considerable overlap in site-to-site variation values for the two types of implants but mean site-to-site variation in C levels for CI24M implants was significantly lower than that for CI24R(CS) implants. Control studies suggested that these results were not an artifact of the scale, and not due to differences in inherent variability of the psychophysical measures, or to the method of quantifying across-site variation. [ABSTRACT FROM AUTHOR]

This study is one of a series that examines stimulus features important for cochlear implant function. Here, we examine effects of stimulus level. In subjects with cochlear implants, a number of psychophysical tests of temporal discrimination (pulse rate discrimination, gap detection, etc.) show marked improvement as a function of stimulus level through most or all of the dynamic range, while electrode-place discrimination can improve or degrade as a function of level. In this study, effects of these combined potential influences were studied by examining the effects of stimulus level on syllable identification. We tested two hypotheses: that syllable identification varies as a function of stimulus level and that level and electrode configuration interact in affecting syllable identification. We examined vowel and consonant identification as a function of stimulus level for bipolar and monopolar electrode configurations. We used experimental processor maps where upper and lower stimulation limits of each electrode pair were equated to eliminate confounding effects of dynamic range, which varies across subjects and electrodes. For each channel, stimulation amplitude was set to a fixed percentage of its dynamic range. Eight adult subjects with Nucleus CI24M implants were tested using the SPEAK processing strategy. With each electrode configuration, stimulus levels were tested from 0% to 90% of the dynamic range in nine steps. The effects on consonant and vowel identification were similar. Phoneme identification was usually better for monopolar than for bipolar stimulation. In the lower half of the dynamic range, syllable identification usually increased as a function of stimulus level. In the upper half of the dynamic range, syllable identification continued to increase as a function of level to 90% of the dynamic range for some subjects, while for others there was no appreciable change or a decrease as a function of level. Decreases in performance at high levels were more common with monopolar than bipolar stimulation. These results suggest that if speech processors are programmed to optimize level for each individual, speech perception performance could be improved. [ABSTRACT FROM AUTHOR]

The configuration of intracochlear electrodes used to electrically stimulate the auditory nerve influences the ensemble fiber response. For example, monopolar stimulation produces lower thresholds and greater spread of excitation than does bipolar stimulation. We used two approaches to investigate how the ensemble of auditory-nerve fibers responds to stimulation delivered by different electrode configurations. As the electrically evoked compound action potential (ECAP) reflects the ensemble response of the nerve, we used its morphology and changes with stimulus level to assess issues related to site-of-excitation and fiber recruitment. In our first approach, feline ECAPs were obtained using a nucleus-style banded electrode array. ECAP latency functions indicated that bipolar stimulation can initiate action potentials at more peripheral sites than does monopolar stimulation. We observed double-peaked ECAPs with bipolar and tripolar stimulation, suggesting excitation of both peripheral and central neural processes. Finally, we observed in some cases a tendency for monopolar stimulation to produce wider ECAP potentials, consistent with the notion that monopolar stimulation excites a broader spatial extent of the fiber population. In our second approach, we applied a simple model to published surveys of single-fiber responses to provide insight into the stochastic properties of the ensemble response. Our results suggest that broader recruitment of fiber activity produced by monopolar stimulation results in a population response with more probabilistic response characteristics and ensemble spike jitter. These observations and our ECAP results are consistent with reports of perceptual advantages attributed to monopolar or other less-focused modes of stimulation. [Copyright &y& Elsevier]