Intracochlear Electrical Stimulation Research Articles

Intracochlear Electrical Stimulation of Normal and DEAF CATS Investigated Using Brainstem Response Audiometry

Brainstem response audiometry for intracochlear electrical stimulation of normal-hearing and deafened cats was investigated. In normal cochleas the brainstem response amplitude grew slowly near threshold as a current-amplitude dependent process, identified as electrophonic in origin. This terminated in a rapidly growing charge-dependent process at approximately 20 dB above threshold, identified as direct electrical stimulation of the auditory nerve. Small levels of white noise (25-35 dB SPL) were sufficient to mask most of the electrophonic response, leaving the direct stimulation process essentially unmodified. In cochleas damaged with d.c. currents and loud sounds, only a rapidly growing charge-dependent process was observed which grew similarly to that in normal-hearing cats but occurred at lower currents. This indicates that possibly the electrical properties of the cochlea were altered in the deafening process, suggesting the inadequacy of normal animals as deaf models for electrical stimulation. Using the technique of derived brainstem responses, it was shown that direct electrical stimulus components were localized to the vicinity of the stimulus electrode with electrophonic components distributed more widely. However, at high currents there was some evidence of the stimulus spreading into the internal auditory meatus.

Deaf animal models for studies of a multichannel cochlear prosthesis.

Pathological alterations of the cochlea were studied in three different deaf animal (cat) populations. The ototoxic drug neomycin sulfate, was administered in one experimental series by direct infusion into the cochlear perilymph; a second group was given a series of intramuscular injections of the drug; and in a third experiment a mechanical lesion was made in the basilar membrane of the basal turn and the animals subsequently deafened by systemic neomycin. Hearing losses were tracked by monitoring thresholds of auditory brainstem responses to click stimulation. These deaf cat preparations fairly efficiently model pathologies recorded in man and are highly predictable over an acceptable time frame. Such preparations are of practical value for experiments involving intracochlear electrical stimulation (e.g., with model cochlear prosthesis electrodes).

Electrical stimulation of the eighth nerve in cat: Temporal properties of unit responses in the large spherical cell region of the AVCN

Single‐unit responses to intracochlear electrical stimulation were studied. Responses were highly phase‐locked to sinusoidal stimulation at frequencies less than 4 kHz. At any given stimulus frequency, synchronization indexes were greater than those that have been reported for eighth nerve responses to acoustic stimuli. At the onset of a low‐frequency (i.e., below 800 Hz) sinusoidal stimulus, firing rate gradually increased over many tens of milliseconds. Rise times were as long as 50–100 ms for low‐intensity, low‐frequency sinusoidal stimulus bursts. At greater intensities and/or higher stimulus frequencies, the response rise times were considerably shorter. Preliminary results indicate that responses to trains of short‐duration biphasic pulses do not exhibit such slow rise times. These results may relate to Eddington's [Ann. Otol. Rhinol. Laryngol. 87 (Suppl. 53), 1–37 (1978)] finding that behavioral threshold increases sharply as the burst duration of a sinusoidal stimulus is decreased. However, threshold is only a weak function of the duration of a train of short‐duration biphasic pulses. For sinusoids below 300 Hz, multiple discharges per cycle were common at levels considerably above threshold. [Work supported by NIH.]

Cochlear pathology following chronic intracochlear electrical stimulation.

Cochlear pathology following chronic intracochlear electrical stimulation.

Implanted Cochlear Prosthesis Analogue In Primates

Electronic cochlear prostheses are currently being implanted in the profoundly deaf. Little, however, is known about the biologic effects of electrical stimulation of the auditory system. To study some effects of intracochlear electrical stimulation in monkeys, a totally implantable device that produces a constant current similar to that produced by devices now used in human implantation was designed and miniaturized. This paper reports on the characteristics of the device and a pilot project designed to test it in vivo. The feasibility of primate implantation and behavioral observation, as well as histopathologic study, was also determined. The device was implanted in each of two Macacca nemestrina monkeys, which were then observed. One monkey was killed and the implanted cochlea studied with scanning electron microscopy. Evidence of osteogenesis of the scala tympani was found.

Plasticity of binaural interaction. I. Effect of early auditory deprivation

Plasticity of binaural interaction. I. Effect of early auditory deprivation

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

A series of psychoacoustic experiments was conducted in subjects implanted with a permanent intracochlear bipolar electrode. These experiments were designed to reveal the nature of the sensation evoked by direct sinusoidal electrical stimulation of the acoustic nerve. A series of single unit experiments in the inferior colliculus of cats was then conducted, using intracochlear stimulus electrodes identical to those implanted in human subjects in all respects except size, and using identical stimuli. These physiological experiments were designed to reveal how sounds evoked by intracochlear electrical stimulation in humans are generated and encoded in the auditory nervous system. Among the results were the following: 1) The sensation arises from direct electrical stimulation of the acoustic nerve. It is not “electrophonic” hearing arising from electro-mechanical excitation of hair cells. 2) While sounds are heard with electrical stimulation at frequencies from below 25 to above 10,000 Hz, the useful range of discriminative hearing is limited to frequencies below 400–600 Hz. 3) There is no “place” coding of electrical stimuli of different frequency. Tonal sensations generated by electrical stimulation must be encoded by the time order of discharge of auditory neurons. 4) The periods of sinusoidal electrical stimuli are encoded in discharges of inferior colliculus neurons at frequencies up to 400–600 Hz. 5) Both psychoacoustic and physiological evidence indicates that the low tone sensations evoked by electrical stimulation are akin to the sensations of “periodicity pitch” generated in the normal cochlea. 6) Most cochlear hair cells are lost with intracochlear implantation with this electrode. Most ganglion cells survive implantation. Implications of these experiments for further development of an acoustic prosthesis are discussed.

Intracochlear Electrical Stimulation: Results of Psychoacoustic Studies in Man and Physiological Studies in Cats

A series of simple psychoacoustic experiments has been conducted on subjects previously implanted with an intracochlear bipolar electrode. With use of simple ac electrical stimulation, subjects heard sounds described as “tones” over a frequency range from about 100 to at least several thousand Hz. The apparent pitch rose rapidly up to 400–600 Hz; at higher frequencies, it changed slowly if at all over a broad range of frequency. The dF was about 1–3 Hz at 100 Hz, 5–10 Hz at 500 Hz, and 50–100 Hz at 1 kHz. Physiological experiments were conducted to determine how sensation heard by these subjects is generated and encoded in the auditory nervous system. Study of single units of the inferior colliculus in cats using standard microelectrode techniques has revealed: (1) Tuning curves of colliculus neurons excited by electrical stimuli were flat, and all were roughly parallel. The normal “place” representation of stimulus frequency is obscured. (2) The period of the stimulus is represented in the time order of discharge of colliculus neurons at frequencies up to 500–600 Hz. (3) With the stimulus electrode employed, the threshold of the acoustic nerve at the cochlear base was 30–40 dB lower than at the cochlear apex. Psychoacoustic and physiological evidence strongly suggests that hearing in these subjects is akin to the described sensation of “periodicity pitch.”

EFFECTS OF VISUAL DEPRIVATION ON MORPHOLOGY AND PHYSIOLOGY OF CELLS IN THE CATS LATERAL GENICULATE BODY.

EFFECTS OF VISUAL DEPRIVATION ON MORPHOLOGY AND PHYSIOLOGY OF CELLS IN THE CATS LATERAL GENICULATE BODY.