Feasibility of multichannel scala tympani stimulation

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.

Simultaneous cochlear implantation as a therapeutic option in vestibular schwannoma surgery: case report

Hearing loss is the most common symptom of vestibular schwannomas (VSs). The management of these lesions includes observation, radiosurgery, and microsurgical resection. Hearing preservation and rehabilitation are the major challenges after the tumor treatment. A 43-year-old man with previous left-sided profound hearing loss and tinnitus presented with a 2-mm left-sided intracanalicular VS. The decision was made to perform a simultaneous cochlear implantation (CI) and microsurgical resection of the tumor. The patient did well postoperatively, with significant improvement of tinnitus, sound localization, and speech recognition in noise. Previous reports of simultaneous CI and VS resection in patients with neurofibromatosis type 2 and sporadic VS in the only hearing ear have been described. The role of CI in patients with VS and normal contralateral hearing has been recently described, showing positive outcomes due to the binaural benefits. Tinnitus also can be treated by the implantation of the cochlear device. The simultaneous microsurgical removal of VS and implantation of a cochlear device is a feasible approach in patients with unilateral hearing loss and severe tinnitus.

Auditory Prostheses Research with Multiple Channel Intracochlear Stimulation in Man

Although single-channel electrode arrays implanted in the scala tympani of deaf patients are useful as an aid to lip reading and for distinguishing some environmental sounds, they do not transmit intelligible speech. However, multichannel electrode arrays, which take advantage of the cochlea's tonotopic organization, may be capable of generating the complex patterns of neural activity necessary for speech discrimination.

In this study, multichannel electrodes were implanted in the cochleas of four volunteers, with access to the connecting wires made through the skin via a percutaneous connector. The major portion of the data presented is from two of these subjects: one has been bilaterally deaf since birth and the other has been unilaterally deaf for 15 years. Preliminary results of experiments with two more recently implanted subjects are described as well as experiments with a fifth volunteer who was implanted with five electrodes by House in 1969.

Data on pitch and loudness discrimination as well as the effects of stimulation parameters on threshold, impedance, and electrode interaction are presented. Place pitch and periodicity pitch were observed in all five volunteers. The results of pitch-matching experiments with the unilaterally deaf volunteer were consistent with tonotopic maps of the cochlea, and experiments indicated that a pitch continuum may be achieved by combining place and periodicity pitch modulation.

Preliminary experiments in tune recognition with one subject demonstrate his ability to recognize simple melodies based on periodicity pitch cues.

These results, coupled with the finding that subjective sensations remain stable over the long-term, support the feasibility of providing artificial hearing with a multichannel cochlear stimulation system.

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>.

A primer on brain-machine interfaces, concepts, and technology: a key element in the future of functional neurorestoration

Conventionally, the practice of neurosurgery has been characterized by the removal of pathology, congenital or acquired. The emerging complement to the removal of pathology is surgery for the specific purpose of restoration of function. Advents in neuroscience, technology, and the understanding of neural circuitry are creating opportunities to intervene in disease processes in a reparative manner, thereby advancing toward the long-sought-after concept of neurorestoration. Approaching the issue of neurorestoration from a biomedical engineering perspective is the rapidly growing arena of implantable devices. Implantable devices are becoming more common in medicine and are making significant advancements to improve a patient's functional outcome. Devices such as deep brain stimulators, vagus nerve stimulators, and spinal cord stimulators are now becoming more commonplace in neurosurgery as we utilize our understanding of the nervous system to interpret neural activity and restore function. One of the most exciting prospects in neurosurgery is the technologically driven field of brain-machine interface, also known as brain-computer interface, or neuroprosthetics. The successful development of this technology will have far-reaching implications for patients suffering from a great number of diseases, including but not limited to spinal cord injury, paralysis, stroke, or loss of limb. This article provides an overview of the issues related to neurorestoration using implantable devices with a specific focus on brain-machine interface technology.

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.

Virtual active touch using randomly patterned intracortical microstimulation

Intracortical microstimulation (ICMS) has promise as a means for delivering somatosensory feedback in neuroprosthetic systems. Various tactile sensations could be encoded by temporal, spatial, or spatiotemporal patterns of ICMS. However, the applicability of temporal patterns of ICMS to artificial tactile sensation during active exploration is unknown, as is the minimum discriminable difference between temporally modulated ICMS patterns. We trained rhesus monkeys in an active exploration task in which they discriminated periodic pulse-trains of ICMS (200 Hz bursts at a 10 Hz secondary frequency) from pulse trains with the same average pulse rate, but distorted periodicity (200 Hz bursts at a variable instantaneous secondary frequency). The statistics of the aperiodic pulse trains were drawn from a gamma distribution with mean inter-burst intervals equal to those of the periodic pulse trains. The monkeys distinguished periodic pulse trains from aperiodic pulse trains with coefficients of variation 0.25 or greater. Reconstruction of movement kinematics, extracted from the activity of neuronal populations recorded in the sensorimotor cortex concurrent with the delivery of ICMS feedback, improved when the recording intervals affected by ICMS artifacts were removed from analysis. These results add to the growing evidence that temporally patterned ICMS can be used to simulate a tactile sense for neuroprosthetic devices.

A brain-machine interface instructed by direct intracortical microstimulation

Brain–machine interfaces (BMIs) establish direct communication between the brain and artificial actuators. As such, they hold considerable promise for restoring mobility and communication in patients suffering from severe body paralysis. To achieve this end, future BMIs must also provide a means for delivering sensory signals from the actuators back to the brain. Prosthetic sensation is needed so that neuroprostheses can be better perceived and controlled. Here we show that a direct intracortical input can be added to a BMI to instruct rhesus monkeys in choosing the direction of reaching movements generated by the BMI. Somatosensory instructions were provided to two monkeys operating the BMI using either: (a) vibrotactile stimulation of the monkey's hands or (b) multi-channel intracortical microstimulation (ICMS) delivered to the primary somatosensory cortex (S1) in one monkey and posterior parietal cortex (PP) in the other. Stimulus delivery was contingent on the position of the computer cursor: the monkey placed it in the center of the screen to receive machine–brain recursive input. After 2 weeks of training, the same level of proficiency in utilizing somatosensory information was achieved with ICMS of S1 as with the stimulus delivered to the hand skin. ICMS of PP was not effective. These results indicate that direct, bi-directional communication between the brain and neuroprosthetic devices can be achieved through the combination of chronic multi-electrode recording and microstimulation of S1. We propose that in the future, bidirectional BMIs incorporating ICMS may become an effective paradigm for sensorizing neuroprosthetic devices.

Strategies to improve electrode positioning and safety in cochlear implants

An injection-molded internal supporting rib has been produced to control the flexibility of silicone rubber encapsulated electrodes designed to electrically stimulate the auditory nerve in human subjects with severe to profound hearing loss. The rib molding dies, and molds for silicone rubber encapsulation of the electrode, were designed and machined using AutoCad and MasterCam software packages in a PC environment. After molding, the prototype plastic ribs were iteratively modified based on observations of the performance of the rib/silicone composite insert in a clear plastic model of the human scala tympani cavity. The rib-based electrodes were reliably inserted farther into these models, required less insertion force and were positioned closer to the target auditory neural elements than currently available cochlear implant electrodes. With further design improvements the injection-molded rib may also function to accurately support metal stimulating contacts and wire leads during assembly to significantly increase the manufacturing efficiency of these devices. This method to reliably control the mechanical properties of miniature implantable devices with multiple electrical leads may be valuable in other areas of biomedical device design.

The inferior collicular potential in acoustic and electrical stimulation of the cochlea: an experimental study of guinea pig

We made experiments of the inferior collicular potentials in acoustic and electrical stimulation for the purpose of studying fundamental issues for cochlear implantation. Guinea pigs with normal Preyer's reflex were used for this study. The results were as follows: (1) in acoustic stimulation relatively wide and large waveforms were gained but in electrical stimulation sharp and narrow ones were gained, (2) in acoustic stimulation the input-output curve of latency and amplitude was biphasic but in electrical stimulation it was monophasic. For this reason, in acoustic stimulation by click, when the intensity is low (under 80 dB SPL) stimulus site would be comparatively low frequency fibers. When the intensity is high (over 80 dB SPL) the stimulus site would shift to high frequency fibers. Therefore, many more neighboring fibers start responding. This results in biphasic input-output curves of latecy and amplitude. By electrical stimulation, however, it would be possible to stimulate only a restricted area among the bipolar electrode. Therefore, as the intensity increases, the response amplitude increases and becomes saturated at a constant level. This results in monophasic input-output curves of latency and amplitude.

Inner ear implants for experimental electrical stimulation of auditory nerve arrays

Electrode arrays chronically implanted in the inner ear are gaining increased use for experimental studies of the auditory nervous system, as well as for studies related to development of improved auditory prostheses. Commercially available electrode arrays are designed for human use and thus may be unsuitable for experimental studies, particularly in small animals. This paper describes a simple, inexpensive method for making custom electrode arrays in a variety of configurations, suitable for animals ranging from small rodents to non-human primates.

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.

[Clinical observations in electric stimulation of the ear (author's transl)]

For 20 years direct electrical stimulation has been used in cases of severe bilateral hearing loss or complete deafness to mediate acoustic percepts. The relevant literature is reviewed. While these attempts were initially thought to be unphysiological and unsuited for transferring speech, acoustic sensations were successfully conveyed even with the very simple unichannel electrodes through percutaneous signal transfer. Patients fitted with such simple systems were able to hear and distinguish environmental noise and speech, their lip reading as well as their speech improved. Speech discrimination was, however, impossible with such simple implants both on theoretical grounds and in practical terms, because frequency analysis is exclusively based on periodicity (up to 400 Hz). Designing bipolar multichannel electrodes which, when introduced into the scala tympani or the modiolus, produce discrete stimulation of several circumscribed groups of nerve fibers, was the logical consequence of earlier attempts along these lines, Implantation of these systems can be done along the transmeatal, meato-mastoidal or mastoidal approach. The electrodes can be implanted in bundles through the round window or into the modiolus; they can, however, also be introduced individually through several drill holes in the promontory for placement in the scala tympani and vestibuli. This produces a far more differentiated stimulation simulating a tonotopic pattern of stimuli. In addition to periodicity, the place principle can thus be utilized for frequency coding. While their dynamic range is rather poor (15 to 30 dB at best), multichannel systems, in theory, offer substantially more favorable conditions for speech intelligibility. Since current knowledge of speech coding is, however, inadequate, the degree of intelligibility obtainable is still insufficient for everyday life. Inspite of this flaw, such implants as are available today substantially benefit the patients, who are able to establish acoustic communications with their environment by distinguishing environmental noise from speech, to discriminate between male and female voices, to recognize musical rhythms and even to understand a few words. Indications for the implantation of prostheses, the requisite conditions and postoperative training programs are discussed. While cochlear implants are still experimental, they appear to be reasonably justified in selected cases, since they have been well tolerated by all patients treated sofar without causing any complications and since many of the data obtainable can only be collected in humans. It is, however, essential that experimental implantation be exclusively dealt with by specialized teams, which should evaluate such data as are available and translate them into practice as soon as possible. A routine impantation of hearing prostheses is currently unwarranted.

[Physiological basis for a cochlear prosthesis (author's transl)]

For the attempt to develop a cochlear prosthesis, which allows some understanding of speech, it seems--at least for the first attempts--to be appropriate to mimic natural conditions as far as possible. The auditory nerve contains about 30,000 afferent fibres. Qualitatively, their behavior is similar but quantitative measures show considerably differences (2.3). Nothing certain can be said at present however about the spiral fibres originating from the outer hair cells. The quantitative differences between single afferents concern tuning, frequency selectivity, thresholds, intensity functions and--of particular interest for electrical stimulation--differences in timing of the activity pattern, brought about by differences in travelling time along the cochlear duct (2.3). The time differences seen in the activity pattern of different fibres are in the order of several ms (2.3.6;2.3.7). Actionpotentials elicited by natural acoustic stimuli show probabilistic behavior, that is they are not strictly determined. It is obvious that with artificial electrical stimulation not every surviving single fibre can be selectively stimulated. An electrode will always stimulate a group of fibres simultaneously. With any conceivable electrical stimulation all fibres in the suprathreshold region of the electrode will be synchronously activated (3.2); a fundamental difference to the natural situation. To estimate the number of channels, necessary to stimulate the auditory nerve with sufficient accuracy to allow speech perception we consider some psychoacoustic data. These have shown that the auditory system possesses the ability to differentiate a great number of different pitches, but on the other hand it is capable of integrating different frequency areas to a so called critical bandwidth. Sound energy falling into one critical bandwidth is integrated to a uniform auditory sensation. If one is to integrate various fibres of the auditory nerve to one channel of stimulation it seems to be adequate to use the critical bandwidth as a measure (3.1). According to this criterion 15 channels would have to be introduced into the speech region of the cochlea. This would allow 1.2 mm of cochlear length for each channel. Perfect electrical separation of the channels is required. Considering the severe distortions in neuronal activity pattern, introduced by electrical stimulation in comparison to the natural conditions it is not clear even whether the number given would be sufficient. On the other hand, current spreading would appear to prohibit any higher electrode density. As far as coding of sound parameters within one channel is concerned it is proposed that full use should be made of frequency analysis according to the place principle. In respect to coding of periodicity and loudness it is proposed to approach natural conditions as far as possible (3.3). Here delay times between the individual channels and a probabilistic character of the stimuli should be introduced to avoid dominance of periodicity pitch...

Cochlear implants

All presently devised single channel devices generate a primitive sensation of hearing by the mechanism of 'periodicity pitch'. No 'place pitch' encoding is possible. Although some enhancement of communicative skills with lip reading results, unaided speech discrimination is not possible. Definite psychological advantages for the totally deaf have been observed with these simple devices. Multiple segments of auditory nerve must be stimulated in a manner which will simulate the complex patterns of neural activity necessary for speech discrimination. Electrode optimization and the pathophysiological consequences of electrical stimulation of the auditory nerve can best be determined in animals. The perceptual consequences of electrical stimulation of the auditory nerve, however, can best be determined in man. How much we will have to innovate the methods of aural rehabilitation will depend upon how well we can generate perceptual speech patterns by electrical excitation of the auditory nerve.

The cochlear histopathology of chronic intracochlear implantation

A fundamental question regarding the feasibility of artificial electrical stimulation of auditory nerve in cases of sensory deafness is the fate of that nerve after electrode implantation. The results of chronic scala tympani electrode implantation in normal and neomycin-deafened cats indicate that the vast majority of primary auditory neurons will survive the initial surgical implantation and the long-term interface with the molded silastic electrode for periods of at least 30 months (the longest normal-implant survivors). While there is histologic evidence that some neurons are lost in the basal coil, especially in neomycin-deafened animals, the majority of spiral ganglion cells and their radial fibres survived neomycin administration and surgical implantation even in this region. The implantation of electrodes in neomycin-deafened cats did not result in heavy neuronal degeneration. Indeed, there was little apparent difference in nerve survival between the implanted and unimplanted ears of the neomycin animals for periods up to five months (the longest neomycin implant survivor). Traumatic electrode insertion with injury to the boney covering of the modiolus or disruption of the basilar membrane resulted in extensive nerve loss in the traumatized region. When carefully inserted by an experienced otologist, the molded-silastic scala tympani electrode permitted discrete differential stimulation of restricted segments of auditory nerve and produced little or no neural degeneration.

Present status and future development of the cochlear prosthesis

Presently devised single channel devices generate relatively primitive sensation of hearing. They provide some enhancement of communication skills for the totally deaf. Definite psychological advantages for the totally deaf have been observed. Pitch discrimination is by the mechanism of "periodicity pitch." No "place" pitch encoding is possible. The recognition of complex sounds is not possible. Multiple segments of auditory nerve must be stimulated in a manner which will stimulate the complex patterns of neural activity necessary for speech discrimination. Electrodes can be optimized and the pathophysiological consequences of electrical stimulation can be determined in experimental animals. The perceptual consequences of electrical stimulation, however, can best be determined in man himself. How much we will have to rely on known and future methods of aural rehabilitation will depend upon how well perceptual speech patterns can be generated by electrical stimulation of the auditory nerve.