Neural encoding of sound sensation evoked by electrical stimulation of the acoustic nerve

Five historical innovations that have shaped modern otolaryngological surgery

Throughout history, many innovations have contributed to the development of modern otolaryngological surgery, improving patient outcomes and expanding the range of treatment options available to patients. This article explores five key historical innovations that have shaped modern otolaryngological surgery: Operative Microscope, Hopkins Rigid Endoscope, Laryngeal Nerve monitoring, Cochlear implants and Laser surgery. The selection of innovations for inclusion in this article was meticulously determined through expert consensus and an extensive literature review. We will review the development, impact and significance of each innovation, highlighting their contributions to the field of otolaryngological surgery and their ongoing relevance in contemporary and perioperative practice.

A Five-Decade Text Mining Analysis of Cochlear Implant Research: Where We Started and Where We Are Heading

Background and Objectives: Since its invention in the 1970s, the cochlear implant (CI) has been substantially developed. We aimed to assess the trends in the published literature to characterize CI. Materials and Methods: We queried PubMed for all CI-related entries published during 1970-2022. The following data were extracted: year of publication, publishing journal, title, keywords, and abstract text. Search terms belonged to the patient's age group, etiology for hearing loss, indications for CI, and surgical methodological advancement. Annual trends of publications were plotted. The slopes of publication trends were calculated by fitting regression lines to the yearly number of publications. Results: Overall, 19,428 CIs articles were identified. Pediatric-related CI was the most dominant sub-population among the age groups, with the highest rate and slope during the years (slope 5.2 ± 0.3, p < 0.001), while elderly-related CIs had significantly fewer publications. Entries concerning hearing preservation showed the sharpest rise among the methods, from no entries in 1980 to 46 entries in 2021 (slope 1.7 ± 0.2, p < 0.001). Entries concerning robotic surgery emerged in 2000, with a sharp increase in recent years (slope 0.5 ± 0.1, p < 0.001). Drug-eluting electrodes and CI under local-anesthesia have been reported only in the past five years, with a gradual rise. Conclusions: Publications regarding CI among pediatrics outnumbered all other indications, supporting the rising, pivotal role of CI in the rehabilitation of children with sensorineural hearing loss. Hearing-preservation publications have recently rapidly risen, identified as the primary trend of the current era, followed by a sharp rise of robotic surgery that is evolving and could define the next revolution.

Locus coeruleus activity improves cochlear implant performance

Cochlear implants (CIs) are neuroprosthetic devices that can provide hearing to deaf people1. Despite the benefits offered by CIs, the time taken for hearing to be restored and perceptual accuracy after long-term CI use remain highly variable2,3. CI use is believed to require neuroplasticity in the central auditory system, and differential engagement of neuroplastic mechanisms might contribute to the variability in outcomes4-7. Despite extensive studies on how CIs activate the auditory system4,8-12, the understanding of CI-related neuroplasticity remains limited. One potent factor enabling plasticity is the neuromodulator noradrenaline from the brainstem locus coeruleus (LC). Here we examine behavioural responses and neural activity in LC and auditory cortex of deafened rats fitted with multi-channel CIs. The rats were trained on a reward-based auditory task, and showed considerable individual differences of learning rates and maximum performance. LC photometry predicted when CI subjects began responding to sounds and longer-term perceptual accuracy. Optogenetic LC stimulation produced faster learning and higher long-term accuracy. Auditory cortical responses to CI stimulation reflected behavioural performance, with enhanced responses to rewarded stimuli and decreased distinction between unrewarded stimuli. Adequate engagement of central neuromodulatory systems is thus a potential clinically relevant target for optimizing neuroprosthetic device use.

Auditory cortical plasticity in cochlear implant users

Cochlear implants are one of the most successful neuroprosthetic devices that have been developed to date. Profoundly deaf patients can achieve speech perception after complete loss of sensory input. Despite the improvements many patients experience, there is still a large degree of outcome variability. It has been proposed that central plasticity may be a major factor in the different levels of benefit that patients experience. However, the neural mechanisms of how plasticity impacts cochlear implant learning and the degree of plasticity's influence remain unknown. Here, we review the human and animal research on three of the main ways that central plasticity affects cochlear implant outcomes.

Perceptual Differences Between Low-Frequency Analog and Pulsatile Stimulation as Shown by Single- and Multidimensional Scaling

Cochlear-implant users who have experienced both analog and pulsatile sound coding strategies often have strong preferences for the sound quality of one over the other. This suggests that analog and pulsatile stimulation may provide different information or sound quality to an implant listener. It has been well documented that many implant listeners both prefer and perform better with multichannel analog than multichannel pulsatile strategies, although the reasons for these differences remain unknown. Here, we examine the perceptual differences between analog and pulsatile stimulation on a single electrode. A multidimensional scaling task, analyzed across two dimensions, suggested that pulsatile stimulation was perceived to be considerably different from analog stimulation. Two associated tasks using single-dimensional scaling showed that analog stimulation was perceived to be less Clean on average than pulsatile stimulation and that the perceptual differences were not related to pitch. In a follow-up experiment, it was determined that the perceptual differences between analog and pulsatile stimulation were not dependent on the interpulse gap present in pulsatile stimulation. Although the results suggest that there is a large perceptual difference between analog and pulsatile stimulation, further work is needed to determine the nature of these differences.

Responses of Cat Auditory Nerve Fibers to Biphasic Electrical Current Pulses

Discharge patterns of single auditory nerve fibers were recorded from normal-hearing cats implanted with a 12-band intracochlear electrode array. Stimuli were biphasic current pulses of specifiable width, amplitude, and rate. Acoustic tuning curves were obtained to determine the cochlear positions of the fibers. Response latencies to electrical stimuli formed two groups. Short latency (0.3 to 0.7 ms) responses were attributed to direct activation of spiral ganglion neurons. At high stimulus intensities, these often exhibited abrupt shifts toward even shorter latencies. Long latency (> 1.5 ms) responses were probably caused by electrophonic activation of functional hair cells. Response thresholds to electrical stimuli depended on a fiber's proximity to the stimulating electrodes, and they did not depend on a fiber's acoustic response threshold or spontaneous discharge rate. High intensity (> 1.5 mA) stimuli could excite fibers over a wide range of characteristic frequencies, even for the narrowest (0.45 mm) electrode separations. Response threshold was an exponentially decreasing function of pulse width for widths up to 300 μs/phase. Fiber discharges were highly phase-locked at all suprathreshold intensities, and saturation discharge rates usually equaled stimulus pulse rates for rates up to at least 800 pulses/s. Dynamic ranges were small (1 to 6 dB), increased with pulse rate, and were uncorrelated with electrical response threshold. Within the dynamic range, shapes of poststimulus time and interspike interval histograms resembled those obtained in response to acoustic stimuli. Depolarization block caused fiber activity to cease in 2 to 5 seconds for sustained stimuli presented at high (> 600 pulses/s) pulse rates and intensities.

Biophysical Measurements in The Implanted Cochlea

Electrical stimulation via implanted electrodes has been used to produce perceptions of sound in human subjects. This study describes preliminary work needed to understand the implanted ear and the distribution of current within it so that a stimulus system can be designed that is optimal for longevity, information transfer, and safety.

Biophysical Measurements in the Implanted Cochlea

Electrical stimulation via implanted electrodes has been used to produce perceptions of sound in human subjects. This study describes preliminary work needed to understand the implanted ear and the distribution of current within it so that a stimulus system can be designed that is optimal for longevity, information transfer, and safety.

Early UCSF contributions to the development of multiple-channel cochlear implants

The early contributions of the UCSF cochlear implant (CI) research team to the development of multiple-channel cochlear implants from about 1971 through the mid-1980s are briefly summarized. Scientists at UCSF conducted fundamental studies related to device safety, the control of patterned electrical stimulation, and the designs of intracochlear electrode arrays, coders, and implanted multiple-channel electrode drivers. They conducted many original studies documenting parameters of hearing with cochlear implants relevant to next-generation CI designs. On these bases, the UCSF team constructed early models of multichannel devices that were progenitors of the Advanced Bionics' Clarion CI. This article is part of a Special Issue entitled <Lasker Award>.

System identification of the nonlinear dynamics in the thalamocortical circuit in response to patterned thalamic microstimulation in vivo

OBJECTIVE

Nonlinear system identification approaches were used to develop a dynamical model of the network level response to patterns of microstimulation in vivo.

APPROACH

The thalamocortical circuit of the rodent vibrissa pathway was the model system, with voltage sensitive dye imaging capturing the cortical response to patterns of stimulation delivered from a single electrode in the ventral posteromedial thalamus. The results of simple paired stimulus experiments formed the basis for the development of a phenomenological model explicitly containing nonlinear elements observed experimentally. The phenomenological model was fit using datasets obtained with impulse train inputs, Poisson-distributed in time and uniformly varying in amplitude.

MAIN RESULTS

The phenomenological model explained 58% of the variance in the cortical response to out of sample patterns of thalamic microstimulation. Furthermore, while fit on trial-averaged data, the phenomenological model reproduced single trial response properties when simulated with noise added into the system during stimulus presentation. The simulations indicate that the single trial response properties were dependent on the relative sensitivity of the static nonlinearities in the two stages of the model, and ultimately suggest that electrical stimulation activates local circuitry through linear recruitment, but that this activity propagates in a highly nonlinear fashion to downstream targets.

SIGNIFICANCE

The development of nonlinear dynamical models of neural circuitry will guide information delivery for sensory prosthesis applications, and more generally reveal properties of population coding within neural circuits.

The cochlear implant: historical aspects and future prospects

The cochlear implant (CI) is the first effective treatment for deafness and severe losses in hearing. As such, the CI is now widely regarded as one of the great advances in modern medicine. This article reviews the key events and discoveries that led up to the current CI systems, and we review and present some among the many possibilities for further improvements in device design and performance. The past achievements include: (1) development of reliable devices that can be used over the lifetime of a patient; (2) development of arrays of implanted electrodes that can stimulate more than one site in the cochlea; and (3) progressive and large improvements in sound processing strategies for CIs. In addition, cooperation between research organizations and companies greatly accelerated the widespread availability and use of safe and effective devices. Possibilities for the future include: (1) use of otoprotective drugs; (2) further improvements in electrode designs and placements; (3) further improvements in sound processing strategies; (4) use of stem cells to replace lost sensory hair cells and neural structures in the cochlea; (5) gene therapy; (6) further reductions in the trauma caused by insertions of electrodes and other manipulations during implant surgeries; and (7) optical rather electrical stimulation of the auditory nerve. Each of these possibilities is the subject of active research. Although great progress has been made to date in the development of the CI, including the first substantial restoration of a human sense, much more progress seems likely and certainly would not be a surprise.

The multiple-channel cochlear implant: the interface between sound and the central nervous system for hearing, speech, and language in deaf people-a personal perspective

The multiple-channel cochlear implant is the first sensori-neural prosthesis to effectively and safely bring electronic technology into a direct physiological relation with the central nervous system and human consciousness, and to give speech perception to severely-profoundly deaf people and spoken language to children. Research showed that the place and temporal coding of sound frequencies could be partly replicated by multiple-channel stimulation of the auditory nerve. This required safety studies on how to prevent the effects to the cochlea of trauma, electrical stimuli, biomaterials and middle ear infection. The mechanical properties of an array and mode of stimulation for the place coding of speech frequencies were determined. A fully implantable receiver-stimulator was developed, as well as the procedures for the clinical assessment of deaf people, and the surgical placement of the device. The perception of electrically coded sounds was determined, and a speech processing strategy discovered that enabled late-deafened adults to comprehend running speech. The brain processing systems for patterns of electrical stimuli reproducing speech were elucidated. The research was developed industrially, and improvements in speech processing made through presenting additional speech frequencies by place coding. Finally, the importance of the multiple-channel cochlear implant for early deafened children was established.

Cochlear electrode arrays: past, present and future

Cochlear implants are very successful devices: more than 60000 people use them throughout the world. Key to the success of these prostheses is the development of electrode arrays that place contacts close to the target neurons, survive for decades in the tissues of the inner ear, and that provide reliable and repeatable excitation to the cells of the auditory nerve. This article describes the early electrode arrays and their development into the arrays that are used presently in clinical cochlear prostheses. While integrated circuit techniques were proposed and tested in the laboratory two decades ago, the present clinical devices still are hand built and made of wire-based technologies. Current approaches that seek to automate the construction of cochlear electrode arrays are described and discussed.

The sciatic nerve of the toad Xenopus laevis as a physiological model of the human cochlear nerve

The response of single fibres of the human cochlear nerve to electrical stimulation by a cochlear implant has previously been inferred from the response of the cochlear nerve in other mammals. These experiments are hindered by stimulus artefact and the range of stimulus currents used is therefore much less than the perceptual dynamic range (from threshold to discomfort) of human subjects. We have investigated use of the sciatic nerve of the toad Xenopus laevis as a convenient physiological model of the human cochlear nerve. Use of this completely dissected nerve reduces the problems of stimulus artefact whilst maintaining the advantages of a physiological preparation. The validity of the model was assessed by measuring the refractory periods, excitation time-constant, and relative spread of single fibres using microelectrode recording. We have also investigated the response of nerve fibres to sinusoidal stimulation. Based on these measurements, we propose that the sciatic nerve may be a suitable model of the human cochlear nerve if the timescales of stimuli are decreased by a factor of about five to compensate for the slower dynamics of the sciatic nerve and if noise is added to the stimuli to compensate for the lower internal noise of sciatic nerve fibres.

Cochleotopic fos immunoreactivity in cochlea and cochlear nuclei evoked by bipolar cochlear electrical stimulation

Fos-like immunoreactivity evoked by basal, second or apical turn bipolar intracochlear electrical stimulation was evaluated in the spiral ganglion and cochlear nuclei. At stimulation levels of six times the electrically evoked auditory brain stem response thresholds, immunoreactive neurons were observed at appropriate discrete cochleotopic regions relative to stimulation site. The number of neurons increased with stimulus level and closely correlated to wave I amplitude. At 10 times thresholds, some spread in fos-like immunoreactivity to adjacent cochlear turns was found. However, fos-like immunoreactivity at this high level of stimulation still clearly showed a differential distribution in density of expression. These results indicated that the restricted topographic distribution of activity evoked by high levels of electrical stimulation is initiated at first order primary neurons of the system. For the profoundly deaf with cochlear implants, this indicates that place (channel) information can be maintained in the spiral ganglion and central nervous system even at very high levels of electrical stimulation. Together with our previous studies, these results indicate that fos provides a marker which can be used for evaluation of extent and pattern of cellular activation throughout the central auditory pathways, including activation of auditory nerve cells.

The nerve-electrode interface of the cochlear implant: current spread

One of the fundamental facets of the cochlear implant that must be understood to predict accurately the effect of an electrical stimulus on the auditory nerve is the nerve-electrode interface. One aspect of this interface is the degree to which current delivered by an electrode spreads to neurons distant from it. This paper reports a direct mapping of this current spread using recordings from single units from the cat auditory nerve. Large variations were seen in the degree to which the different units are selective in responding to electrodes at different positions within the scala tympani. Three types of units could be identified based on the selectiveness of their response to the different electrodes in a linear array. The first type of unit exhibited a gradual increase in threshold as the stimulating site was moved from more apical to more basal locations within the scala tympani. The second type of unit exhibited a sharp local minimum, with rapid increases in threshold in excess of 6 dB/mm in the vicinity of the minimum. At electrode sites distant from the local minima the rate of change of the threshold approached that of the first type of units. The final type of unit also demonstrated a gradual change in threshold with changing electrode position, however, two local minima, one apical and one basal, could be identified. These three types are hypothesized to correspond to units which originate apical to the electrode array, along the electrode array and basal to the electrode array.

Single unit activity in the inferior colliculus of the rat elicited by electrical stimulation of the cochlea

The activity of single neurons (n = 182) of the central nucleus of the inferior colliculus (CIC) of the rat was recorded in response to unilateral electrical stimulation of the left cochlea and/or acoustical stimulation of the right ear. The probability of response to both modes of stimulation was comparable (90 per cent for contralateral and 60 per cent for ipsilateral presentation). Response patterns consisted predominantly of onset excitations. Response latencies to electrical stimuli ranged from 3 to 21 ms, with an average value of 9.7 ms (SD = 3.5 ms) in the ipsilateral CIC and 6.6 ms (SD = 3.4 ms) in the contralateral CIC. With respect to binaural inputs, the majority of units were excited by stimulation of either ear (EE; about 60 per cent) while about one third were influenced by one ear only (EO). Units excited by one ear and inhibited by the other (EI) were rare. The main difference between the present implanted rats and normal animals was the virtual absence here of inhibitory effects for both types of stimuli when they were delivered to the ipsilateral ear (very few EI units).

Electrocochleographic analysis of the suppression of tinnitus by electrical promontory stimulation

To investigate the origin, and evaluate the mechanism by which tinnitus is suppressed we performed electrical promontory stimulation (EPS) in 56 patients with tinnitus, and measured the compound action potential (CAP) using electrocochleography before and after EPS. In the group of patients in whom tinnitus was suppressed, the CAP amplitudes increased significantly, whereas the latencies showed no remarkable change. In the group of patients in whom tinnitus was not suppressed, both the CAP amplitudes and latencies exhibited no significant change. These data indicate that the effect on the cochlear nerve plays an important role in the suppression of tinnitus by EPS. The CAP reflects the number of the auditory nerve fibers which discharge synchronously. It is speculated that an increase of the CAP amplitudes is caused by synchronizing discharges of the auditory nerve fibers, and that the mechanism by which EPS suppresses tinnitus may be related to synchronizing these discharges.

Reduction in excitability of the auditory nerve following electrical stimulation at high stimulus rates

While recent studies have suggested that electrical stimulation of the auditory nerve at high stimulus rates (e.g., 1000 pulses/s) may lead to an improved detection of the fine temporal components in speech among cochlear implant patients, neurophysiological studies have indicated that such stimulation could place metabolic stress on the auditory nerve, which may lead to neural degeneration. To examine this issue we recorded the electrically evoked auditory brainstem response (EABR) of guinea pigs following acute bipolar intracochlear electrical stimulation using charge-balanced biphasic current pulses at stimulus rates varying from 100 to 1000 pulses/s and stimulus intensities ranging from 0.16 to 1.0 microC/phase. Charge density was held constant (approximately 75 microC cm-2 geom/phase) in those experiments. To monitor the recovery in excitability of the auditory nerve following this acute stimulation. EABR thresholds, wave I and III amplitudes and their latencies were determined for periods of up to 12 h following the acute stimulation. Higher stimulus rates and, to a lesser extent, higher intensities led to greater decrements in the post-stimulus EABR amplitude and prolonged the recovery period. While continuous stimulation at 100 pulses/s induced no decrement in the EABR, stimulation at 200 and 400 pulses/s produced an increasingly significant post-stimulus reduction of the EABR amplitude, which showed only partial recovery during the monitoring period. No EABR response could be evoked immediately following stimulation at 1000 pulses/s, using a probe intensity 16-19 dB below the stimulus intensity. However, partial EABR recovery was observed for wave III following stimulation at the lowest stimulus intensity (0.16 microC/phase). These stimulus-induced reductions in the EABR amplitude were also reflected in increased thresholds and latencies. Providing stimulus rate and intensity were held constant, stimulation at different charge densities (37.7, 75.5 and 150.7 microC cm-2 geom/phase) had no influence on the post-stimulus EABR recovery. Significantly, the introduction of a 50% duty cycle into the stimulus pulse train resulted in a more rapid and complete post-stimulus recovery of the EABR compared to continuous stimulation. These data suggest that stimulus rate is a major contributor to the observed reduction in excitability of the electrically stimulated auditory nerve. This reduction may be a result of an activity-induced depletion of neural energy resources required to maintain homeostasis. The present findings have implications for the design of safe speech-processing strategies for use in multichannel cochlear implants.

Quantitative comparison of electrically and acoustically evoked auditory perception: implications for the location of perceptual mechanisms

Electrical stimulation of the human auditory system produces different patterns of spatial and temporal neural activity than those that occur in the normal, acoustically stimulated system. Quantitative comparison of psychophysical performance measured with acoustic and electrical stimulation may allow us to infer the physiological locus of perceptual mechanisms. In this paper we compare psychophysical data on temporal resolution from normal-hearing listeners, cochlear implant listeners, and patients electrically stimulated on the cochlear nucleus. Measures of gap detection, forward masking, and modulation detection will be compared. These comparisons demonstrate that temporal processing is relatively similar across these three groups once the obvious differences in dynamic range are taken into consideration. In addition, preliminary results with speech processors indicate that implant patients can utilize all temporal information in speech. Thus, implant patients have relatively normal temporal resolution and can integrate temporal cues normally for the recognition of complex acoustic patterns such as speech. These results imply that the central auditory systems of implant patients are able to fully utilize the non-natural patterns of temporal neural information produced by electrical stimulation. The differences in the microstructure of the neural pattern (phase locking, stochastic independence of fibers, spatial distribution of activity, etc.) between electrical and acoustic stimulation are apparently not necessary for temporal processing. Thus, the physiological locus of temporal processing mechanisms must be more central in the auditory system than the cochlea and cochlear nucleus.

Discrimination of complex electrical stimulation through a multichannel intracochlear implant

A model has been developed to describe the electric fields generated in the inner ear when electrical stimuli are presented through a multichannel implant in the scala tympani of the cochlea. The model relies on the hypothesis that stimuli which excite the largest number of neural elements provide the greatest probability of successful discrimination by the implanted subject. It suggests that the effective stimulus is determined by the linear combination of electrical fields produced by the individual channels, and that excitation takes place in a spatially restricted area of the auditory nerve in the vicinity of the stimulating electrodes. The model was tested by biophysical measurements of the potential developed in the stimulated cochlea, and by a psychophysical study of the ability of a monkey to discriminate complex electrical signals using dual channel stimulation. The experimental findings are in agreement with the computer simulations.

Effects of level on nonspectral frequency difference limens for electrical and acoustic stimuli

The purpose of this experiment was to study the effects of stimulus level on discrimination of frequency as represented in the temporal waveforms of acoustic and electrical signals. The subjects were four nonhuman primates in which one ear had been deafened and implanted with an electrode array and the other ear was untreated. Frequency difference limens for 100 Hz electrical sinusoidal stimulation via a cochlear implant in the deafened ear were compared to those for 100 Hz sinusoidally amplitude-modulated white noise (SAM noise) acoustic stimuli to the normal-hearing contralateral ear. To correct for loudness cues, levels of the test stimuli were varied relative to the reference-stimulus level. The test-stimulus levels at which the percent responses were minimum were determined. These levels were used to measure the frequency difference limens. Frequency difference limens for the electrical stimuli decreased as a function of reference-stimulus level through most of the dynamic range, while those for the acoustic stimuli reached a minimum at 20 dB to 40 dB above threshold. For the electrical stimuli the slopes and relative positions of the frequency difference limen vs. level functions varied from subject to subject, and with changes in electrode configuration within a subject. These differences were related to threshold level and dynamic range. At higher levels of stimulation, frequency difference limens for acoustic and electrical stimuli fell in the same range. The slopes and relative positions of the frequency difference limen vs. level functions for electrical stimuli did not parallel those of level difference limen vs. level functions collected simultaneously from the same ears. The data suggest that nonspectral frequency discrimination may depend on the number of nerve fibers stimulated. With prostheses in cochleas with less than a full complement of auditory nerve fibers, the data suggest that stimulation level is an important variable influencing discriminability.

Cylindrical cochlear electrode array for use in humans

This report describes the fabrication of a flexible multichannel electrode array suitable for use in humans. The conductors, pads, and stimulating tips are made of platinum on a polyimide substrate. Photolithographic techniques are employed in the fabrication of the electrode on a planar surface in the form of a film. The film is rolled subsequently into a cylinder of diameter 0.50 mm and the cylinder is filled with medical grade silicone rubber. The stimulation pads then form rings around the cylinder. In vitro and in vivo tests are ongoing with good results so far.

Comparisons of psychophysical and neurophysiological studies of cochlear implants

This paper compares psychophysical and neural studies of electrical stimulation of the auditory nerve with the goal of evaluating the relevance of single-unit animal models for the development of cochlear prostheses for profoundly deaf humans. Comparative psychophysical studies with implanted deaf subjects indicate that animal models, at least nonhuman primates, provide a close match to humans, though this is not always true for acoustic stimulation of normal-hearing subjects. However, the human-animal comparisons, especially those involving electrical stimuli, need further study using more carefully matched conditions. Comparisons of psychophysical and neurophysiological thresholds for electrical stimulation in animals reveal consistently higher thresholds in the neural studies. A number of factors which may account for these differences are discussed. A partial resolution of the problem could result from conducting neurophysiological and behavioral studies in the same animal. Finally, comparison of psychophysical and neurophysiological studies of temporal encoding suggest that there may be more information encoded in the auditory nerve than is used by the system, at least for nonspectral frequency discrimination. This points to a need for further analysis of the processing of this information at higher levels in the auditory pathway.

Immediate effects of cochlear implantation on voice quality

Seminal quantitative group acoustic data are presented on voice quality changes following electrical stimulation (ES) of the auditory nerve via a single-channel cochlear implant (CI). It was found that the fundamental frequency (Fo) of our patients was significantly lower after the first day of ES, while intensity and speaking duration were not significantly different from pre-CI values. These results suggest that the CI provides enough frequency information less than 300 Hz to permit immediate and independent alterations in voice Fo towards normal-hearing speakers values. Our findings also indicate that intensity and speaking duration require additional time before differences found become significant. Longitudinal data are still needed to determine if Fo continues to lower and if intensity and/or speaking duration change significantly to approximate values observed in normal-hearing individuals.

Experimental Basis and Design of a New Cochlear Prosthesis System

In a multichannel cochlear prosthesis, electrical interactions between electrodes impose severe limitations on dynamic range and selectivity. We present a theoretical model to cope with these limitations. Building a successful cochlear implant requires full custom-integrated circuits. We present the design of such a device, implemented in complementary metal oxide semiconductor technology. The area of the chip is 9 mm2 and it can stimulate 15 cochlear electrodes with current impulses.

Threshold and loudness functions for pulsatile stimulation of cochlear implants

Thresholds and loudness estimates were measured for biphasic pulsatile electrical stimulation of the auditory nerve. Measures were collected as a function of the parameters: pulse duration, and pulse rate. The results indicate that the sensations of threshold and loudness are determined by a complex function of the stimulating current waveform. For stimuli with the same charge, maximum loudness is seen at the shortest pulse durations, and a secondary maximum is seen at pulse durations of 2-3 ms/phase. It is possible that the secondary peak in the loudness function and the slow growth of loudness just above threshold for long pulses are indications of dendrite survival near the electrode. If this interpretation is valid, these measures could lead to perceptual tests of peripheral nerve viability. In addition, a speech processor device could use these measures to equalize the loudness of stimuli with different pulse durations and pulse rates.

Responses of cochlear nucleus units to electrical stimulation through a cochlear prosthesis: channel interaction

The responses of 39 single units in the ventral cochlear nucleus of acute anesthetized guinea-pigs were studied with continuous electrical stimuli presented through a dual-channel implant in the scala tympani. Implants had four electrodes placed along the axis of the cochlea with 1 mm separations. With a specific pair (either apical or basal) of stimulating electrodes, about half of the units responded when current was flowing apically, while the rest responded to current in the opposite direction. No obvious relation existed between the effective polarity of the basal and apical pairs of electrodes. Two response types were observed while stimulating through both pairs simultaneously. Seventy-nine percent of the units responded to the sum of the current waveforms presented through the two pairs. Twenty-one percent responded to the difference between the waveforms. Both types of responses were observed for suprathreshold as well as for some intensities of stimulation that alone were subthreshold. The type of response was not dependent on the absolute threshold or threshold difference between the two pairs. For equal peak-intensities of stimuli, two-channel stimulation evoked larger responses than single-channel stimulation, provided the two channels were in their effective polarities. Responses to dual-channel stimulation were consistently larger than the summed responses to the two individual single-channel stimuli. The observed responses to the dual-channel stimulation indicate that the adequate stimulus was determined by the linear combination of fields produced by the individual channels in the vicinity of the stimulating electrodes before the auditory nerve is stimulated, and that excitation takes place in a spatially restricted area of the auditory nerve.

Electrically-evoked responses in animals with progressive spiral ganglion degeneration

The deafness (dn/dn) mouse has an hereditary cochlear dysfunction throughout its development, and spiral ganglion cell density decreases progressively over the three age groups we examined. We have used this mutant to examine inferior colliculus evoked responses to modiolar electrical stimulation as a function of spiral ganglion degeneration. No differences were found between mutants and control mice or between ages in either threshold for detection of the response or latency of the response. However, peak-to-peak amplitudes of the response were larger in the mutants than in the controls in the young and intermediate age groups. There was a poor correlation between spiral ganglion degeneration and size of the evoked response: for example, mutants in the old age group had similar amplitudes of response as controls while spiral ganglion cell density was reduced to 21% of the value in young mice, and mutants in the intermediate age group with 50% spiral ganglion degeneration showed response amplitudes more than double that in controls. These data may be relevant to the significant numbers of people with hereditary deafness among the hearing-impaired human population.

Inferior colliculus single unit responses to peripheral electrical stimulation in normal and congenitally deaf mice

We have investigated the single unit response in the inferior colliculus of the mouse to electrical pulse stimulation at the auditory periphery. The experimental animals were deafness mutant mice (dn/dn), which have hereditary congenital peripheral pathology. The control animals were normal littermates (+/dn). Monopolar electrical pulses were applied either to the modiolus or the round window and the evoked single unit activity was recorded in the contralateral inferior colliculus (IC). Two types of single unit response were observed: a single-spike response, and a multiple-spike response. There was a predominance of the former in the control animals and of the latter in the mutants. For both modes of stimulation the group mean threshold of response was lower in mutants compared to controls, whilst the maximum number of evoked spikes per 50 presentations was higher in mutants than control animals. The mutant response pattern was similar to that seen in animals with experimentally induced auditory deprivation. The suitability of the deafness mutant as a model for the investigation of coding strategies for cochlear implants is discussed.

Representation of voice pitch in discharge patterns of auditory-nerve fibers

Responses of populations of auditory-nerve fibers were measured for synthesized consonant-vowel stimuli. This paper explores the encoding of fundamental frequency (pitch) in these responses. Post-stimulus time (PST) histograms were computed from 25 ms segments of the spike trains. Discrete Fourier transforms with a 40 Hz resolution were computed from the histograms. Two representations of pitch are considered. The first representation is based on the pitch-related temporal properties of the speech signal. Histograms for individual units can show envelope modulations directly related to the pitch period. These modulations reflect the responses of these fibers to a number of stimulus harmonics near fiber CF. Responses of fibers near formant frequencies are dominated by a single large harmonic component, and thus show small or no pitch-related enveloped modulations. Envelope modulations are reduced in the presence of background noise. The second representation uses both temporal properties of auditory-nerve responses and cochlear place to encode the pitch-related harmonic structure of speech. As a measure of the response of the population of fibers to each harmonic of 40 Hz the magnitude of the component of the Fourier transform at that frequency was averaged across all fibers whose characteristic frequencies were within one-fourth octave of that harmonic. We call this measure the average localized synchronized rate (ALSR). The ALSR provides a good representation of stimulus spectrum, even in the presence of background noise. From the harmonic structure of the ALSR, we are able to extract the stimulus pitch frequency. The relationship of these two representations to pitch perception in both acoustic and electrical stimulation (via cochlear implants) is discussed.

Unit responses at cochlear nucleus to electrical stimulation through a cochlear prosthesis

Afferent auditory fibers of the guinea pig cochlea were electrically stimulated with current introduced through electrodes in the scala tympani. Thresholds were determined for unit responses recorded in the ventral cochlear nuclei to a sinusoid of 98 Hz from response-rate growth functions versus stimulus intensity. Suprathreshold response rates for most units grew rapidly from threshold to saturation at 2-15 dB above threshold. Peristimulus time histograms were collected for responses to single sinusoids and combinations of two and five sinusoids ranging from 86 to 134 Hz. Spike occurrences were highly synchronous with individual cycles of the pure sinusoids, but responses to the more complex waveforms occurred primarily to the more intense peaks. The amplitude envelope was thus a major contributor to responses to multiple sinusoids. Destruction of cochlear structures with neomycin increased unit thresholds and produced some changes in waveform encoding.

Tuning characteristics of cochlear nucleus units in response to electrical stimulation of the cochlea

The responses of single units in the ventral cochlear nucleus of acute anesthetized guinea pigs were studied with continuous sinusoidal electrical stimuli presented through a multi-electrode implant in the scala tympani. Implants had two or four electrodes along the axis of the scala with 1 mm separations. Best frequencies were consistently in the 100 Hz range (50-250 Hz) with thresholds of about 0.063 mA peak-to-peak. Tuning curves were usually symmetrical with slopes of 3-4 dB/octave, both below and above the best frequency. The relative sharpness of the tuning curves, as measured by Q10dB, averaged 0.2. Dynamic ranges as determined by the intensity-rate functions for the various frequencies were 2-15 dB. No significant difference was found between tuning characteristics of units in response to stimulation via the apical or basal pair of implant electrodes. The findings suggest some limitations on the applicability of independent stimulating channels in multi-electrode implants.

Comparison of percepts found with cochlear implant devices

Much of what has been observed in psychoacoustic testing of implanted listeners is similar to that seen in normal or hearing-impaired listeners with only parametric modification. What is lacking is evidence for critical band phenomena and for frequency selectivity. The unique interaction of waveform and sensation level with pitch, loudness, and threshold appears only in implanted listeners. Direct waveform processing, without spectral decomposition, appears to be the means by which information is brought to the nervous system through electrical stimulation. The operations by which this is coded to create percepts of pitch and loudness remain to be elucidated.

Chronic electrical stimulation of auditory nerve in cat: Physiological and histological results

Cats were implanted with two-channel scala tympani bipolar electrode arrays consisting of four PtIr wires in a molded silicone rubber carrier. The electrically evoked auditory brainstem response (ABR) was recorded to monitor the physiological response to biphasic pulsatile stimulation in these chronic preparations. Baseline data were collected over a 1-6 month period. Animals were then subjected to a long period of continuous high level stimulation delivered through a system designed to insure delivery of charge-balanced biphasic waveforms. Subsequent changes in physiological response were interpreted as indicating electrically induced damage to the cochlea. Localized loss of hair cells and growth of connective tissue resulted from the implantation of scala tympani inserts. Electrically evoked ABR responses were not altered by the long-term presence of the electrode, nor by the presence of intervening connective tissue. Physiological manifestations of stimulus-induced change appeared only after hundreds of hours of continuous stimulation. Apparent functional damage was not suspended or reversed with cessation of stimulation, but rather continued for several hundred hours after the stimulation was terminated. Deterioration of physiological response was accompanied by two deleterious histological changes: (a) bone growth within the scala tympani; and (b) loss of nerve fibers and spiral ganglion cells. Both of these changes were restricted to an area corresponding to the implant intracochlear location and were most marked in the region adjacent to the chronically stimulated electrode pair. In cases where stimuli were not charge balanced or surgical trauma was incurred, bone growth was most extensive and nerve damage most pervasive. The data from cases stimulated at lower levels of charge density, i.e. 20-40 muC/cm2, suggest that these may be more feasible levels for safe chronic electrical stimulation in scala tympani.

Preservation of central auditory function in the deafness mouse

Deafness mice are profoundly deaf from birth as a result of genetically determined cochlear dysfunction. Evoked potentials in response to direct electrical stimulation of the cochlear nerve can readily be recorded in the inferior colliculus of deafness mice, and such responses are larger in amplitude than those in control mice. These observations indicate that at least some central connections become functional in the deafness central auditory pathway in the absence of peripheral stimulation, and are relevant to the general problem of restoring function by direct nerve stimulation in the profoundly deaf.

Voice pitch as an aid to lipreading

The totally deafened adult, unable to make use of a hearing aid, has no alternative to lipreading for everyday communication. Lipreading, however, is no substitute for hearing speech. Many lipreaders have great difficulty in ideal conditions and even the best lipreaders find the task demanding and tiring. Prosthetic attempts to substitute for lost hearing have centred on three distinct types of intervention, visual, tactile, and electrocochlear. As none of these is likely to yield a good understanding of a speech independent of lipreading in the near future, we have attempted to isolate relatively simple patterns of stimulation that, although not intelligible in themselves, well aid lipreading. From this point of view, the fundamental frequency or 'pitch' of the voice is the most important pattern element because if provides both segmental and suprasegmental information and is practically invisible. It thus complements the visual information already available on the face. As we show here, with the voice pitch presented acoustically, normal listeners can lipread a speaker reading continuous text at up to two and a half times the rate possible on the basis of lipreading alone. The pitch signal by itself, of course, is completely unintelligible. Although our work is primarily concerned with methods of electrical stimulation of the cochlea, it has implications for other sensory substitution techniques, the design of special purpose hearing aids and current theories of speech perception.

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.

Design of the cochlear prosthesis: effects of the flow of current in the implanted ear

When structures within the temporal bone are stimulated electrically it is desirable to maximize the dynamic range of the stimulus. The maximum dynamic range of electrical stimulus seems to be found when the threshold of stimulation is minimum. The minimum threshold of stimulus is likely to be reached when the electrical current that flow through regions containing excitable cells is maximized. By implanting electrodes throughout the temporal bone, it is possible to apply electrical currents to the ear and to measure the distributions of current flowing within the ear. The results of these measurements demonstrate that when current flow is directed outside the scala tympani, lower thresholds can be obtained. Frequency dependence of the paths of current flow cannot be used to explain the frequency dependence of the frequency-threshold functions measured in animals.

Estimates of essential neural elements for stimulation through a cochlear prosthesis

Electrical stimulation of afferent auditory pathways through electrodes placed within and outside of the cochlea were used to study stimulation and design parameters relevant to a cochlear prosthesis. In the acute guinea pig preparation, the tract response evoked in brachium of the inferior colliculus by electrical stimulation to an ear provided estimates of the effectiveness of various electrode placements. Stimulation between an electrode in the cochlea and a site along the eighth nerve was characterized by the lowest thresholds. Stimulation between intracochlear electrodes was somewhat less effective, and stimulation between external electrodes at the nerve, cochlear nucleus, or distant point was least effective. Thresholds, expressed as current, rose at approximately 6 dB per octave for stimulus frequencies from 1 kHz to 16 kHz. Thresholds below 10 microA rms were seen for optimal placements. These observations suggest that the neural elements being stimulated are the cell bodies of the spiral ganglion cells.

Operating ranges for cochlear implants

The range of simple electrical stimuli that may be used for prosthetic stimulation via scala tympani implants was explored using psychophysical procedures in macaque monkeys. Biophysical considerations placed further limitations on the operating range. The operating range was dependent onstimulus waveform, electrode configuration, and the condition of the implanted cochlea and eighth nerve.

Psychophysical evaluation of cochlear prostheses in a monkey model

Functional aspects of cochlear prostheses implanted in the scala tympani were tested in monkeys trained to perform a simple reaction-time task. Thresholds for detection of electrical stimulation and dynamic ranges were tested for a wide range of frequencies of sinusoidal stimulation and for biphasic rectangular pulses of various durations and repetition rates. The results are comparable with available data from implanted human patients and extend these findings, exploring various aspects of electrical stimulation in greater detail.