Biomedical subjects
Publications and source records attributed to F A Spelman.
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.
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.
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.
Prolonged endotracheal intubation in critically ill patients has become an accepted routine. One notable consequence of this form of management is injury to the posterior aspect of the glottis, apparently an injury caused by the presence of an endotracheal tube. Utilizing two types of sensing devices, the pressure exerted by endotracheal tubes was measured in the canine larynx via a laryngofissure approach. Pressure in excess of 200 mm Hg was consistently noted in the region of the arytenoid cartilage. A variety of tubes were tested with similar results.
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.
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Electrical stimulation via implanted electrode 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.
Tissue impedance was measured in the cochleas of monkeys and guinea pigs implanted with electrode arrays. The impedances were measured within the scala tympani and between the scala tympani and the internal auditory meatus of the modiolus. The measurements revealed several impedance properties. 1) The impedances inside and outside of the scala tympani are resistive for frequencies between 8 Hz and 12.5 kHz. 2). The impedances measured between the scala tympani and the internal auditory meatus were larger in magnitude than the impedances measured in the scala tympani. 3) Impedance was not affected significantly by changing stimulus current flow from 0.45 to 45 microA rms. 4) Impedance magnitude increased rapidly by different factors inside and outside the scala tympani after sacrifice of the subject animals. Measurements of the tissue impedance made in conjunction with measurements of threshold of response to electrical stimulation revealed that the threshold of response to electrical stimuli was lowest when the stimulating currents were highest outside the scala tympani. We conclude that the excitable elements of the auditory nerve are being stimulated within the modiolus rather than within the scala tympani.
A cochlear prosthesis designed to electrically stimulate eighth nerve fibers in deaf subjects was studied in acute guinea pig preparations. The prosthesis, which consisted of two pairs of platinum-iridium electrodes, was inserted into the scala tympani through the round window. The electrical impedances of different current paths and their effectiveness in evoking neural responses in the central auditory system are compared. Assuming lumped neural elements, the results indicate that the sites of neural excitation lie outside the scala tympani. Frequency-response characteristics of neural excitation are in agreement with resistive spread of current through tissue to the site of stimulation. Neural membrane characteristics similar to those previously studied in nonmammalian nerves seem to account for the response initiation.