Selective activation and graded recruitment of functional muscle groups through spinal cord stimulation.
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Animals↗
Biomedical subjects
K W Horch
Publications and source records attributed to K W Horch.
Proposed specifications for a lumbar spinal cord electrode array for control of lower extremities in paraplegia.
The goal of the study was to provide specifications for a stimulating electrode array to be implanted in the lumbosacral spinal cord as part of a functional neuromuscular stimulation (FNS) system for control of lower extremity muscles in paralyzed individuals. Dual channel stimulation of the quadriceps activation pool in the feline ventral lumbo-sacral spinal cord was performed to measure electrode interactions and to explore the effect of various stimulation paradigms on muscle fatigue. There was no measurable overlap in the populations of motor neurons activated from two different electrodes for spacings > or = 1 mm with currents below 100 microA. However, a statistically significant increase in the population of activated fibers due to current summation was observed when stimuli > or = 70 microA were simultaneously presented through pairs of electrodes within 3 mm of each other. Fatigue effects were studied with three paradigms: 1) stimuli were delivered through a single electrode, 2) stimuli were delivered through two electrodes with the stimulus to the second electrode presented during the refractory period of fibers stimulated by the first electrode, and 3) stimuli were interleaved between the two electrodes such that the stimulus to one electrode was presented midway between stimuli to the other electrode, and the rate of stimulation through a single electrode was half that used in the first two paradigms. Dual channel refractory and single channel stimulation did not differ from each other in the rate at which the muscle fatigued, in both cases the force decayed to 30% of its initial level within 2 min of the initiation of the stimulation regime, whereas the force with interleaved stimulation was still above the initial force at this time due to strong potentiation. Based on these results and on and activation pool dimensions obtained in an earlier study, preliminary specifications are presented for an electrode array to be implanted in the human spinal cord for functional neuromuscular stimulation.
Animals↗
Metallized polymer fibers as leadwires and intrafascicular microelectrodes.
We have developed a process for producing fine, very flexible microwires suitable for use as small signal leadwires or nerve electrodes. The process incorporates metallization of high-performance monofilament polymer fibers to yield electrically conductive fibers with greatly improved flexibility over solid metal wires of similar strength. The metallization layers are produced by serial vacuum deposition of a 0.3 micron thick coating of three metals, titanium-tungsten (Ti/W), gold (Au), and platinum (Pt), onto monofilament, poly-p-phenyl-terephthalate aramid fibers (Kevlar). The metallized fibers are then insulated with an approx. 1 micron thick layer of silicone elastomer. The result is a microlead with high electrical conductivity (linear resistance = 30 omega/cm), desirable interfacial properties, excellent mechanical stability and extremely high flexibility. These physical characteristics are appropriate for application as signal leadwires or recording/stimulating electrodes where small size and high flexibility are paramount. In this paper we report on the electrical and mechanical properties of these metallized fibers and demonstrate their use as intrafascicular electrodes for recording multi-unit neural activity in feline peripheral nerves.
Animals↗
Action potential classification with dual channel intrafascicular electrodes.
Using recordings of peripheral nerve activity made with carbon fiber intrafascicular electrodes, we compared the performance of three different recording techniques (single channel, differential, and dual channel) and four different unit classification methods (linear discriminant analysis, template matching, a novel time amplitude windowing technique, and neural networks) in terms of errors in waveform classification and artifact rejection. Dual channel recording provided uniformly superior unit separability, neural networks gave the lowest classification error rates, and template matching had the best artifact rejection performance.
Action Potentials↗
Analysis of single-unit firing patterns in multi-unit intrafascicular recordings.
A system for extracting single-unit activity patterns from multi-unit neural recordings was tested using real and simulated neural data. The system provided reliable estimates of firing frequency for individual units in simulated multi-unit data and allowed reliable determinations of the responses of individual cutaneous mechanoreceptor units to 'natural' stimuli such as brushing or pressing on the skin. An implementation of the system, which operated online and in real time, was used to obtain estimates of multiple, single-unit responses from multi-unit intrafascicular electrode recordings. The pattern of activity across the population of units in a given recording gave a reliable indication of the type of stimulus that had evoked the activity. It was concluded that this system, used in combination with intrafascicular peripheral nerve recordings, could be used to provide online, real-time information about peripheral stimuli.
Action Potentials↗
Mobility performance with a pixelized vision system.
A visual prosthesis, based on electrical stimulation of the visual cortex, has been suggested as a means for partially restoring functional vision in the blind. The prosthesis would create a pixelized visual sense consisting of punctate spots of light (phosphenes). The present study investigated the feasibility of achieving visually-guided mobility with such a visual sense. Psychophysical experiments were conducted on normally sighted human subjects, who were required to walk through a maze which included a series of obstacles, while their visual input was restricted to information from a pixelized vision simulator. Walking speed and number of body contacts with obstacles and walls were measured as a function of pixel number, pixel spacing, object minification, and field of view. The results indicate that a 25 x 25 array of pixels distributed within the foveal visual area could provide useful visually guided mobility in environments not requiring a high degree of pattern recognition.
Blindness↗
Separation of action potentials in multiunit intrafascicular recordings.
Classification of action potentials in multiunit recordings was based on the use of various types of features to uniquely characterize action potentials from different cells. We compared classification results obtained using three types of descriptive features: digitized data points, amplitude and duration (time domain) parameters, and fast Fourier transform (FFT) coefficients. Digitized data points used as descriptive features provided good classification success and required minimal computation. Time-domain features gave comparable results but required more computation. FFT coefficients were less effective than the other features. As the signal-to-noise ratio of the recordings increased, smaller differences in feature values could be discriminated.
Action Potentials↗
Reading speed with a pixelized vision system.
A visual prosthesis based on electrical stimulation of the visual cortex with an array of penetrating electrodes is expected to produce pixelized visual images consisting of punctate spots of light (phosphenes). We measured reading speed in subjects viewing text with optically simulated phosphene fields in order to obtain estimates of the following design parameters for such an electrode array: pixel number, pixel spacing, and visual-field size. Comparisons were made between scanning the text with eye movements and scanning the text with head movements. The results indicate that a 25 x 25 array of pixels representing four letters of text projected on a foveal visual field of 1.7 degrees is sufficient to provide reading rates near 170 words/min with scrolled text and near 100 words/min with fixed text.
Humans↗
A silicon-based, three-dimensional neural interface: manufacturing processes for an intracortical electrode array.
A method has been developed for the manufacture of a "three-dimensional" electrode array geometry for chronic intracortical stimulation. This silicon based array consists of a 4.2 x 4.2 x 0.12 mm thick monocrystalline substrate, from which project 100 conductive, silicon needles sharpened to facilitate cortical penetration. Each needle is electrically isolated from the other needles, and is about 0.09 mm thick at its base and 1.5 mm long. The sharpened end of each needle is coated with platinum to facilitate charge transfer into neural tissue. The following manufacturing processes were used to create this array. 1) Thermomigration of 100 aluminum pads through an n-type silicon block. This creates trails of highly conductive p+ silicon isolated from each other by opposing pn junctions. 2) A combination of mechanical and chemical micromachining which creates individual penetrating needles of the p+ silicon trails. 3) Metal deposition to create active electrode areas and electrical contact pads. 4) Array encapsulation with polyimide. The geometrical, mechanical, and electrical properties of these arrays should make them well suited as interfaces to cortical tissue.
Animals↗
Information contained in sensory nerve recordings made with intrafascicular electrodes.
Multiunit recordings were made in anesthetized cats with chronically implanted intrafascicular electrodes over a period of six months. Neural signals recorded with these electrodes consisted of activity in sensory fibers innervating a variety of cutaneous mechanoreceptors. Mechanical stimuli were used to selectively activate individual nerve fibers, and the receptive field and receptor type were identified for each unit. Over a period of six months, there was a net shift in the recorded population, but the electrodes continued to provide a representative sample of the activity in the fascicle as a whole. The total number of units from which activity could be recorded remained roughly constant with time, and individual units persisted in the recordings for up to six months. These results indicate that intrafascicular electrodes could be used to sample information carried by individual somatosensory fibers on a long term basis.
Animals↗
An intrafascicular electrode for recording of action potentials in peripheral nerves.
We are developing a new type of bipolar recording electrode intended for implantation within individual fascicles of mammalian peripheral nerves. In the experiments reported here we used electrodes fabricated from 25 microns diameter Pt wire, 50 microns 90% Pt-10% Ir wire and 7 microns carbon fibers. The electrodes were implanted in the sciatic nerves of rats and in the ulnar nerves of cats. The signal-to-noise ratio of recorded activity induced by nonnoxious mechanical stimulation of the skin and joints was studied as a function of the type of electrode material used, the amount of insulation removed from the recording zone, and the longitudinal separation of the recording zones of bipolar electrode pairs. Both acute and short term (two day) chronic experiments were performed. The results indicate that a bipolar electrode made from Teflon-insulated, 25 microns diameter, 90% Pt-10% Ir wire, having a 1-2 mm long recording zone, can be used for recording of peripheral nerve activity when implanted with one wire inside the fascicle and the other lead level with the first lead, but outside the fascicle. No insulating cuff needs to be placed around the nerve trunk.
Action Potentials↗
A chronic intracortical electrode array: preliminary results.
Two sets of electrode arrays made of either 25- or 50-microns-diameter Teflon-insulated platinum-iridium wire and Teflon have been developed for chronic intracortical electrical stimulation. Cortical histological studies were performed following acute and chronic implantation in cats. While some neural damage resulted from the implantations of either array configuration, a unique set of problems was associated with each diameter wire. Arrays with 50-microns electrodes and lead wires tended to maintain interelectrode spacing upon implantation, but the percutaneous leads retained residual stress which made array implantation difficult. Arrays with 25-microns electrodes and lead wires suffered from changes in interelectrode spacing upon implantation, but were much easier to manipulate during surgery. Both array configurations demonstrated some movement after implantation. It is concluded that a chronic intracortical stimulating electrode array of this geometry should have the following properties: 1) the penetrating electrodes and supporting substrate must be stiff (to maintain interelectrode spacings upon implantation), and 2) the percutaneous leads must be extremely flexible (to avoid array movement after implantation).
Animals↗
Inhibition of regeneration: the ultrastructure of reactive astrocytes and abortive axon terminals in the transition zone of the dorsal root.
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Animals↗
Axonotmesis as treatment for neurotmesis.
The effect of proximal nerve crush on regeneration after unrepaired nerve transection was studied in feline cutaneous nerves. Delays between the initial transection and the subsequent nerve crush varied between 0 (immediate crush) and 13 months. A survival time of 6 to 12 months after the crush was allowed for regeneration to complete. Data from these animals was compared with data from animals in which the nerve was simply transected. Four measures of regeneration success were used to assess the degree of functional recovery: (i) the number of fibers crossing the neuroma and regenerating down the distal stump; (ii) the conduction velocity and diameter of individual fibers in the distal stump relative to their conduction velocity and diameter in the proximal stump; (iii) the number of fibers reinnervating cutaneous mechanoreceptors; and (iv) the number of cutaneous type I mechanoreceptors present after regeneration. As a whole, the data showed a statistically significant increase in the number of fibers entering the distal stump and in the number of fibers reinnervating the skin when the nerve was crushed 6 months after transection compared with nerves that were only transected. There was also an increase in the size and conduction velocity of fibers in the distal stump of nerves that had been crushed 4 to 6 months after transection.
Animals↗
Rapid displacements of the skin lack clear positional information.
Rapid skin indentations can produce strong tap-like sensations that contain little information about skin indentation depth. Although slower stimuli produce weaker sensations, subjects can more accurately identify how the position of the skin surface changes with respect to the deeper tissues. This dissociation between intensity and depth information suggests that intensity and depth are served by different neural circuits.
Humans↗
The neural signal for the intensity of a tactile stimulus.
The effect of indenting the skin at different rates on the perceived intensity of the stimulus was studied by indenting the skin of the fingertip with two triangular waveforms, given as a pair. The subjects were asked to judge which member of the pair was more intense. Perceived intensity was found to increase both with the depth and the speed of the indentation. In contrast, changes in the rate of skin indentation had little influence on perceived skin indentation depth. This suggests that intensity and depth are different attributes of tactile sensibility. Since the skin is viscous, a rapid indentation is more forceful than a slow indentation of the same depth, raising the possibility that perceived intensity is related to stimulus force. Even though intensity judgments were more closely correlated with the force of a stimulus than with the indentation it produced, a rapidly increasing force was felt as more intense than one that increased more slowly but attained the same final magnitude. When mechanoreceptors in the palmar aspect of the monkey's hand were excited with triangular stimuli like those used in the psychophysical studies, their discharge frequency increased with the rate of skin indentation. However, the receptors were distinctly more rate sensitive than the human judgments of stimulus intensity, suggesting that impulse summation in the central nervous system summates (integrates in the mathematical sense) the receptor input so as to enhance, relatively, the perceived intensity of the slower stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)
Adult↗
A quantitative evaluation of suture and tubulization nerve repair techniques.
The radial and sural nerves in cats were transected and either left unrepaired, repaired with 11-0 epineurial sutures, sutured and treated with topical application of Kenalog, or tubulated with hypoantigenic collagen. One year later the nerves were examined electrophysiologically, using single unit recording techniques, and histologically, by light microscopy. The results indicate that regeneration through the neuroma was comparable in all four groups, but that reinnervation of sensory structures in the skin was most successful in the tubulated nerves.
Animals↗
The neural signal for skin indentation depth. II. Steady indentations.
The glabrous skin of the monkey's hand was stimulated with a waveform that indented the skin at a rate of 0.4 mm/sec, held the skin steadily or nearly steadily indented for 12 sec or longer, and then retracted back to the starting position. Recordings were made of activity in single afferent fibers in response to these stimuli. The average discharge frequency of 21 slowly adapting mechanoreceptors declined 38% during the first 12 sec of a steady indentation when the amplitude of the displacement was 0.65 mm and 36% when the displacement was 1.3 mm. When the plateau was not steady but the indentation depth gradually decreased by 15% during the 12-sec plateau period, the average decline was 47% for the 0.65-mm indentation and 46% for the 1.3-mm stimulus. When the indentation depth gradually increased by 15% during the 12-sec plateau, the discharge declined an average of 26% during the 0.65-mm indentation and 22% during the 1.3-mm displacement. To determine the effect of receptor adaptation on the perception of skin indentation depth, 13 human subjects had the skin of their fingertips indented 1 mm with similar trapezoidal waveform and were asked whether the indentation depth increased or decreased during the plateau portion of the stimulus. Ten of the 13 subjects thought that the indentation depth was increasing when the plateau was steady. The method of limits was then used to determine how much the stimulus had to change for the subject to feel the depth during the plateau as unchanging; i.e., a "perceptual zero."(ABSTRACT TRUNCATED AT 250 WORDS)
Adolescent↗