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Biomedical subjects

F A Spelman

Publications and source records attributed to F A Spelman.

At least 37 records · Page 2Linked to original sources
PubMed1988

Analytical theory for extracellular electrical stimulation of nerve with focal electrodes. I. Passive unmyelinated axon.

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.

Animals↗
PubMed1986

Localized neural control of blood flow in the renal cortex of the anaesthetized baboon.

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.

Animals↗
PubMed1983

Comparison of geometric and "real" surface areas of cochlear electrodes.

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.

Cochlear Implants↗
PubMed1982

The cochlear prosthesis: a review of the design and evaluation of electrode implants for the profoundly deaf.

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.

Cochlear Implants↗
PubMed1979

Effect of implant duration on in vivo sensitivity of electromagnetic flow transducers.

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.

Animals↗
PubMed1979

Conduction velocity of the ulnar nerve, length of the forearm and body weight as correlates of gestational age in the infant pigtail macaque.

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.

Action Potentials↗
PubMedDate not supplied

Three-dimensional reconstruction of the cochlea from two-dimensional images of optical sections.

This paper describes a methodology for three-dimensional (3D) computer-aided reconstruction of the guinea pig cochlea using orthogonal-plane fluorescence optical sectioning. Specimens are sectioned optically, allowing them to remain intact during observation. Equations to correct the data for specimen translation and rotation are developed and 3D reconstructions of the scala tympani, round window membrane, and cochlear aqueduct are presented. The error associated with the reconstruction is estimated to be < 19 microns.

Animals↗
PubMedDate not supplied

Psychophysical evaluation of cochlear prostheses in a monkey model.

Functional aspects of cochlear prostheses implanted in the scala tympani were tested in monkeys trained to perform a simple reaction-time task. Thresholds for detection of electrical stimulation and dynamic ranges were tested for a wide range of frequencies of sinusoidal stimulation and for biphasic rectangular pulses of various durations and repetition rates. The results are comparable with available data from implanted human patients and extend these findings, exploring various aspects of electrical stimulation in greater detail.

Acoustic Stimulation↗