Robin P. Michelson

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.

A cochlear prosthesis: further clinical observations; preliminary results of physiological studies.

A series of simple psychoacoustic studies were conducted on three deaf patients with indwelling scala tympani electrodes to determine better what they hear as a consequence of electrical stimulation. Physiological experiments on cats implanted with a similar electrode were conducted to determine how the sensation heard by these patients is generated and encoded in the auditory nervous system. Some preliminary results of these animal experiments are described. Additional improvements in the surgical implantation procedure are detailed.

Multichannel cochlear implant. Preliminary results in man

Cochlear implantations of multichannel electrode arrays have been recently undertaken at the University of California, San Francisco in several totally deaf investigational subjects. One patient was extensively tested with a psychoacoustical battery and is the subject of this communication. Speech testing with CID sentences and open sets of spondee words revealed that this totally deaf subject could achieve approximately 40 to 50% discrimination level with his implant device alone in the absence of visual cues. This represents the first time that speech discrimination has been definitely shown in a cochlear implant subject, and this result has important implications for the future of cochlear implantation.

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 simulate 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.

Removal of glomus jugulare tumor utilizing complete occlusion of the cerebral circulation.

1. The diagnostic features and natural course of glomus jugulare tumors are presented.

Report of a case of cor biloculare with persistent truncus arteriosus

Abstract A case of cor biloculare with persistent truncus arteriosus in a three-day-old infant, with the unusual feature that cyanosis occurred only shortly before death, is presented. The blood flow through the lesser circulation was markedly increased at the time of birth, so that the systemic blood was well aerated or oxygenated, at least to the subcyanotic level; death was caused by diversion of some of the blood from the lesser to the greater circulation, thereby lessening the total output of oxygenated blood.

Surgical approach for insertion of multichannel electrodes into the scala tympani.

In 1971 Michelson reported the results of electrical stimulation of the cochlea via the scala tympani. A single-channel form-fitting electrode was used.‘ Four deaf volunteers received temporary implants through a tympanotomy approach. After a test procedure in the operating room, the electrodes were removed 6 months and 1 year later and two of these patients were implanted with a complete system with wearable devices. In both patients the round window was obliterated by new bone formation. In both patients a bony cast of the scala tympani was removed after freeing it from the margins of the round window. The scala tympani is an ideal site for establishing electrode-tissue interface because it provides access, via the round window, to a linear display of auditory nerve fibers. Design of an optimal scala tympani electrode must allow for discrete excitation of preselected, restricted regions of auditory nerve. Additionally, the electrode must be fabricated of biocompatible materials to allow for the following requirements: (1) that the handling properties make the electrode safely implantable for sufficient distances into the scala tympani (23 to 25 mm) ; and ( 2 ) that the electrode be form-fitting to displace perilymph and, yet, small enough to account for variation in scala tympani size. An electrode meeting all these requirements has been fabricated at the University of California, San Francicso (FIG. 1 ) . The object of this report is to describe its characteristics and surgical implantation. Due to the restricted access provided by the dimensions of the tapering scala tympani, the handling characteristics of the electrode are fundamental to successful implantation. In design of electrodes, stiffness, flexibility, and elasticity must be controlled in both the vertical and horizontal planes of the cochlear spiral. As the surgeon prepares to insert the electrode, the tip must be able to be straightened without damage to the electrode and must retain some of this straightening as the tip is positioned in the basal turn. At this point the electrode exhibits an “elastic memory,” which allows the tip to curve inward as the array is gently inserted beyond the first cochlear spiral. Several observers have noted, both in cadaver temporal bones and in surgical cases, that some other electrode designs tend to veer upward as they are introduced past the first turn of the cochlea. This effect has frequently resulted either in rupture of the basilar membrane or fracture of the osseous spiral lamina at a point opposite the round window. To prevent this natural tendency to veer upward, this electrode must be stiffened in the vertical plane to reduce anterior bending while allowing full flexibility in the horizontal plane of the spiral (FIG. 2).

Protocol for determining the auditory percepts of electrical stimulation of the cochlea.

The percepts of electrical stimulation of the auditory nerve can best be determined in man. An eight channel bipolar electrode array has been developed which allows discreet stimulation of segments of auditory nerve via the scala tympani. In addition, a programmable stimulator has been developed which allows the delivery of different waveforms and stimulus patterns. Hopefully this will begin to allow the generation of the elements of speech.