Perceptual consequences of disrupted auditory nerve activity were systematically studied in 21 subjects who had been clinically diagnosed with auditory neuropathy (AN), a recently defined disorder characterized by normal outer hair cell function but disrupted auditory nerve function. Neurological and electrophysical evidence suggests that disrupted auditory nerve activity is due to desynchronized or reduced neural activity or both. Psychophysical measures showed that the disrupted neural activity has minimal effects on intensity-related perception, such as loudness discrimination, pitch discrimination at high frequencies, and sound localization using interaural level differences. In contrast, the disrupted neural activity significantly impairs timing related perception, such as pitch discrimination at low frequencies, temporal integration, gap detection, temporal modulation detection, backward and forward masking, signal detection in noise, binaural beats, and sound localization using interaural time differences. These perceptual consequences are the opposite of what is typically observed in cochlear-impaired subjects who have impaired intensity perception but relatively normal temporal processing after taking their impaired intensity perception into account. These differences in perceptual consequences between auditory neuropathy and cochlear damage suggest the use of different neural codes in auditory perception: a suboptimal spike count code for intensity processing, a synchronized spike code for temporal processing, and a duplex code for frequency processing. We also proposed two underlying physiological models based on desynchronized and reduced discharge in the auditory nerve to successfully account for the observed neurological and behavioral data. These methods and measures cannot differentiate between these two AN models, but future studies using electric stimulation of the auditory nerve via a cochlear implant might. These results not only show the unique contribution of neural synchrony to sensory perception but also provide guidance for translational research in terms of better diagnosis and management of human communication disorders.

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Perceptual consequences of disrupted auditory nerve activity

A totally implantable cochlear prosthesis is presented. Said prosthesis is designed to function for hours or days without any external components. An externally-activated subcutaneous pressure switch enables the user to adjust the perceived volume by pressing against the skin with a finger. A similar second pressure switch provides a safety 'off/on' feature. A pliable bridge connecting two hermetically sealed housing sections, and corrugated conductor lines, provide for future dimensional changes due to head growth. A titanium encapsulated microphone is positioned underneath the skin in the posterior wall of the external auditory canal. A modiolus-hugging electrode array is inserted into one of the cochlea scala. Periodic charging of the implanted battery (contained within one of the housing sections) is required, such charging accomplished via an external mechanically held head-mounted device containing an external coil, said external coil inductively coupling electrical power to an implanted receiving coil. The external and implanted coils can also be used as a communication link to program the implanted electronics.

A totally implantable cochlear prosthesis

Object. Neurofibromatosis Type 2 (NF2) has typically resulted in deafness after surgical removal of bilateral vestibular schwannomas (VSs). Cochlear implants are generally ineffective for this kind of deafness because of the loss of continuity in the auditory nerve after tumor removal. The first auditory brainstem implant (ABI) in such a patient was performed in 1979 at the House Ear Institute, and this individual continues to benefit from electrical stimulation of the cochlear nucleus complex. In 1992, an advanced multichannel ABI was developed and a series of patients with NF2 received this implant to study the safety and efficacy of the device. Methods. At the time of first- or second-side VS removal, patients received an eight-electrode array applied to the surface of the cochlear nucleus within the confines of the lateral recess of the fourth ventricle. The device was activated approximately 6 weeks after implantation, and patients were tested every 3 months for the 1st year after the initial stimula...

Multichannel auditory brainstem implant: update on performance in 61 patients

Cochlear implants have provided hearing to more than 120,000 deaf people. Recent surgical developments include direct electrical stimulation of the brain, bilateral implants and implantation in children less than 1 year old. However, research is beginning to refocus on the role of the brain in providing benefits to implant users. The auditory system is able to use the highly impoverished input provided by implants to interpret speech, but this only works well in those who have developed language before their deafness or in those who receive their implant at a very young age. We discuss recent evidence suggesting that developing the ability of the brain to learn how to use an implant may be as important as further improvements of the implant technology.

Beyond cochlear implants: awakening the deafened brain.

An apparatus and method for enabling far-field radio-frequency communications with an implantable medical device in which an antenna is embedded within a dielectric compartment of the device. A helical antenna may be employed to save space while still permitting far-field telemetry over a desired range of frequencies.

Antenna for an implantable medical device

Most patients with neurofibromatosis type 2 (NF2) are totally deaf after removal of their bilateral acoustic neuromas. Twenty-five patients with neurofibromatosis type 2 have been implanted with a brainstem electrode during surgery to remove an acoustic neuroma. The electrode is positioned in the lateral recess of the fourth ventricle, adjacent to the cochlear nuclei. The present electrode consists of three platinum plates mounted on a Dacron mesh backing, a design that has been demonstrated to be biocompatible and positionally stable in an animal model. Correct electrode placement depends on accurate identification of anatomic landmarks from the translabyrinthine surgical approach and also on intrasurgical electrophysiologic monitoring. Some tumors and their removal can result in significant distortion of the brainstem and surrounding structures. Even in the absence of identifiable anatomic landmarks, electrode location can be adjusted during surgical placement to find the location that maximizes the auditory evoked response and minimizes activation of other monitored cranial nerves. Stimulation of the electrodes produces auditory sensations in most patients, with results similar to those of single-channel cochlear implants. A coordinated multidisciplinary team is essential for successful application of an auditory brainstem implant.

Auditory Brainstem Implant: I. Issues in Surgical Implantation:

A low-cost, four-channel cochlear stimulation system utilizes a completely passive, implantable receiver/electrode array that is inductively coupled to an external wearable processor. The receiver/electrode array is formed in a silicone rubber carrier adapted to be implanted in a deaf patient. At one end of the receiver/electrode array, positioned subcutaneously near the surface of skin above the ear, four receiving coils are arranged in an appropriate pattern. Such receiving coils are held within an hermetically-sealed titanium case. At the other end of the receiver/electrode array, which may be pre-formed in a spiral to match the basal turn of the cochlea, and which is inserted in the cochlea, four ball electrodes are spaced apart along an inner radius of the spiral. Each electrode is electrically connected to a respective receiving coil. Each receiving coil is also electrically connected to a reference electrode typically located near the receiver-coil end of the array. The wearable processor senses audible sounds, converts the sensed sounds to corresponding electrical signals, and splits the electrical signals into four frequency bands or channels. A speech processing strategy applies the processed signals of each channel to each of four external coils, as a series of biphasic current pulses. The four external coils are aligned, using a suitable headpiece, with corresponding coils of the receiver/electrode array, thereby inductively coupling the biphasic current to a respective electrode of the implanted electrode array.

Low-cost, four-channel cochlear implant

Auditory Brainstem Implant

Threshold current levels for electrical stimulation of a single human brainstem via an auditory prosthesis are compared with postmortem measures of the distance between the electrode and stimulated structures. The results compare well with the summary of threshold-distance measures from animal experiments compiled by J. Ranck (see Brain Res., vol. 98, p. 417-40, 1975). The correspondence between the human and animal data gives confidence that the extent of current spread (distance to stimulable neural units) can be well estimated from the current level at threshold for 200 /spl mu/s/phase biphasic pulses. This is of particular interest in electrical stimulation of the human central nervous system, where localization of stimulation is of paramount importance.

Franco Portillo

Papers

Auditory Brainstem Implant: I. Issues in Surgical Implantation:

Low-cost, four-channel cochlear implant

Auditory Brainstem Implant

Threshold-distance measures from electrical stimulation of human brainstem

Auditory brain stem implant: electrical stimulation of the human cochlear nucleus.