Biomedical subjects
Publications and source records attributed to F A Spelman.
A pseudo-inverse technique has been applied to a lumped-element model of the first turn of an implanted cochlea of a guinea pig. The method calculates the currents necessary to focus or distribute stimuli in desired patterns across the location of the family of auditory nerve cells in the implanted ear. Studies in animals are being undertaken to validate the technique.
A compartmental model of spiral ganglion cells in a potential field was used to predict spike discharges to electrical stimuli. The field was generated by monopolar, bipolar, or quadrupolar point source electrode geometries. The discharges of a population of afferent neurons were modeled to compare the relative spread of excitation. At the same stimulus intensity the spread of excitation was least for the quadrupolar and greatest for the radial bipolar, and this ordering held after equating for maximal spike discharge rates over the three geometries. Excitation spread from quadrupolar stimulation was highly dependent on stimulus intensity.
Psychophysical detection of electrical stimulation of the cochlea was studied as a function of electrical-current configuration. Subjects were postlingually deaf humans with Nucleus 20 + 2, Nucleus 22, and Ineraid cochlear implants and nonhuman primates unilaterally deafened and implanted with a multielectrode array similar to the Nucleus implant. In nonhuman primate and human Ineraid subjects, which had percutaneous connectors, we compared threshold functions for sinusoids and pulse trains for quadrupolar, bipolar, monopolar, and parallel multipolar stimulation. Thresholds decreased across this set of configurations. In some cases, the effects of current configuration were dependent on sinusoidal frequency and pulse duration. Pulse duration-dependent effects were also seen when comparing bipolar, monopolar, and common-ground configurations. Bipolar and monopolar stimulation were compared in Nucleus subjects using pulse trains at 50 microseconds per phase. For bipolar stimulation, thresholds decreased as a function of electrode separation, reaching a level near that for monopolar stimulation at separations of 3.5 to 6.5 mm in most cases. These results may be interpreted in terms of effects of current configuration on the magnitude and shape of electrical-potential fields produced in the cochlea, although more central factors also play a role in determining psychophysical detection thresholds.
Potential distributions measured within the scala tympani of the anesthetized guinea pig support the assertion that focusing is possible when currents are appropriately delivered to the electrodes in the scala tympani. Results obtained with a lumped-element model agree with measurements made in the inner ears of monkeys during monopolar and bipolar stimulation. The predictions are closer for potential distributions apical to the stimulating electrode than they are for basal distributions. In one monkey, in which electrodes were implanted in the middle ear as well as in the inner ear, we obtained measurements of the impedance from inside the scala tympani to points within the middle ear. These impedances are smaller that those initially used in the model, in which the round window membrane was assumed to have a relatively high impedance. A model of the common ground configuration was developed using finite electrode impedances. Finite impedances broaden the potential distributions in this model. Potential distributions from the lumped element model are compared with those obtained with an analytical model, to suggest ways in which focused and unfocused stimuli can affect the excitation of neurons in the implanted ear.
The development of the anteroventral cochlear nucleus (AVCN) in fetal and infant monkeys (Macaca nemestrina) was analyzed for gross morphologic changes together with growth-related modifications in constituent cell size and cell distribution. Rapid and extensive prenatal volumetric changes were followed by slow and limited postnatal volumetric changes. The time course of packing density and cell size changes paralleled the volumetric changes. At each age the packing density along the rostrocaudal axis of the AVCN was constant except in the youngest specimens (mid- to late-fetal), where local variations occurred. Similarly, the size of AVCN cells along the rostrocaudal axis remained approximately constant at any given age. In comparison with the human and mouse, the macaque exhibits relatively less pronounced postnatal change in overall volume and cellular growth features.
Stimulation of the renal sympathetic nerve in young pigs with biphasic pulses of current (3 mA, 800 microseconds per phase, 5, 10 and 50 Hz) produced decreases in arterial and cortical blood flow in the kidney, with the greatest decreases occurring at the highest stimulus frequencies. The decrease in cortical flow lagged that in arterial flow by 1.53-1.99 s; the delay increased with decreasing frequency but was unaffected by captopril, an angiotensin-converting enzyme-blocking agent. This result was consistent with the hypothesis that stimulation of the sympathetic nerve causes constriction first in the afferent arteriole and then in the efferent arteriole. Systemic arterial pressure increased during stimulation of the nerve; the increase was greater in intact nerves than in nerves that had been crushed proximal to the point of the stimulus, indicating that pigs do have renal afferent nerves. Pressure increased after the stimulus ended, but the increase abated or changed to a decrease after administration of captopril. The changes in flow were unaffected by administration of captopril, but were markedly reduced by the blocking agent labetalol (renal arterial flow, 77 +/- 14 per cent; cortical flow, 70 +/- 12 per cent). Thus, the observed changes in flow resulted from direct stimulation of the sympathetic nerves and not from stimulation of the renin-angiotensin system, which affects the pressure response after the stimulus.
We describe an integrated system to record physiological and behavioral variables from nonhuman primates in social groups. The system records data simultaneously from two animals in family groups of five. It synchronizes behavioral and physiological data within 16 ms, either on-line or from recordings. Behavioral data are entered by trained observers on-line or from videotape. Recordings of physiological data are produced on-line as stripchart records, tape recordings on the audio channels of video cassettes, and magnetic disk files. The physiological data include two arterial blood flows, arterial blood pressure and heart rate. The data are transmitted from freely behaving animals to a central site via radio telemetry. The infrared link controls the radio transmitter and physiological signal processing electronics, as well as two sources of drugs for each animal. All of the electronics are contained in a small, light backpack that can be worn by either male or female baboons.
Potential fields induced by nonrecessed, simple recessed, and radially varying recessed electrode designs were measured in vitro. Comparison of experimental results with theoretical analyses substantiated the experimental measurement technique and emphasized the importance of considering both nonuniform charge injection and surface electrochemistry when designing implantable stimulating electrodes. Radially varying recesses produced uniform charge injection at the electrode surface and at the aperture-tissue interface. In general, the radially varying recessed electrodes provided a combination of uniform charge injection and flexibility in design and fabrication that warrants their incorporation into all appropriate planar stimulating electrode designs.
The cable model of a passive, unmyelinated fiber in an applied extracellular field is derived. The solution is valid for an arbitrary, time-varying, applied field, which may be determined analytically or numerically. Simple analytical computations are presented. They explain a variety of known phenomena and predict some previously undescribed properties of extracellular electrical stimulation. The polarization of a fiber in an applied field behaves like the output of a spatial high-pass and temporal low-pass filter of the stimulus. High-frequency stimulation results in a more spatially restricted region of fiber excitation, effectively reducing current spread relative to that produced by low-frequency stimulation. Chronaxie measured extracellularly is a function of electrode position relative to the stimulated fiber, and its value may differ substantially from that obtained intracellularly. Frequency dependence of psychophysical threshold obtained by electrical stimulation of the macaque cochlea closely follows the frequency dependence of single-fiber passive response.
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Localized neural control of renal cortical blood flow was observed in five chloralose-anaesthetized baboons. Renal artery blood flow was measured with an electromagnetic flowmeter, while superficial renal cortical blood flow was measured with a laser Doppler flowmeter. Stimulation of all renal nerve bundles isolated produced significant reductions in total renal blood flow, but reduced the superficial renal cortical blood flow only in localized areas. Systemic arterial blood pressure did not vary significantly, which suggests that the changes in renal cortical blood flow were neurally mediated and not the result of autoregulation. The renal cortical blood flow response was delayed 1-4 s with respect to the total renal blood flow response. The localization of flow control is consistent with the neuroanatomy of the kidney, and the delayed renal cortical blood flow response is consistent with the vascular anatomy of the kidney and with previous measurements of red cell transit times through the renal cortex.
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The destructive potential of the electrically active electrode is believed to be related to charge density at the electrode-tissue surface. Charge density has been calculated with the use of both geometric and electrochemical "real" values for surface area. To further assess the accuracy of surface area measurements determined by means of the previously mentioned methods, an eight-electrode modiolar array cochlear implant was examined. Electrode surface areas were measured first electrochemically, then by means of a light microscope, and finally with the use of the scanning electron microscope. Comparison of these measurements demonstrated a high correlation between the scanning and light microscopic values. However, there was no correlation between the surface areas determined by light and scanning electron microscopy and the electrochemical measurements.
The paper reviews briefly processing of auditory information in the normal ear and uses the normal system as a basis of comparison with the auditory prosthesis. While brief mention is made of tactile and visual prostheses, the paper places emphasis on neural prostheses. Designs of prostheses, both early attempts and recent systems, are discussed in terms of speech processing, their parallel to the normal auditory system, neural stimulation, potential tissue damage, and longevity. The future of auditory prostheses is discussed and critical paths in the design process are suggested.
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Twenty-three electromagnetic flow transducers with lumen diameters of 3.5-6.0 mm were implanted in rhesus monkeys and baboonss for 12 h to 120 days. Each flow transducer was calibrated 1) in vitro on dialysis tubing with saline before implantation, 2) in vivo the last day of the implant period, and 3) again in vitro after the flow transducer was recovered. Three other flow transducers were implanted on femoral arteries of baboon just central to an arteriovenous Silastic shunt, and were calibrated in vivo daily for 23-47 days. In vitro sensitivity was not affected by implant durations of up to 120 days. In vivo sensitivity fluctuated unpredictably for the first 3-4 wk of implant, after which it followed a systematic course that depended on the lumen size. In vivo sensitivity at any time during implant (after the initial period) could be accurately predicted by knowing either the in vitro sensitivity or the terminal in vivo sensitivity.
The conduction velocity of peripheral nerves is an indication of their growth. Ulnar nerve conduction velocity, forearm length, postpartum age and weight were measured in 25 monkeys (Macaca nemestrina) to determine the best way to estimate gestational age. Correlation analysis showed that while conduction velocity is strongly correlated with gestational age (r = 0.68), both forearm length (r = 0.91) and weight (r = 0.94) are more strongly correlated. Of the three simple measurements made, weight provides the best estimate of gestational age.