Current distance relations for fiber stimulation with pointsources

The following results are based on computer simulations and on activating function analysis. In the near field, denervated muscle fibers as well as nerve fibers with a sealed ending are easier to stimulate in the central region than with electrodes close to the end. When electrode-fiber distance is increased, the electrode location for optimal stimulation efficacy shifts from a central position to a region beyond the fiber end for cathodic stimulation and to a position above the terminating part of the fiber for anodic currents. The phenomenon becomes more pronounced with increasing distance between the electrode and fiber axis, because in the far field, the current-distance relation changes from quadratic to cubic, whereas stimulation at the fiber end obeys a rather constant quadratic law.     

Improving myoelectric pattern recognition robustness to electrode shift by changing interelectrode distance and electrode configuration

Pattern recognition of myoelectric signals for prosthesis control has been extensively studied in research settings and is close to clinical implementation. These systems are capable of intuitively controlling the next generation of dexterous prosthetic hands. However, pattern recognition systems perform poorly in the presence of electrode shift, defined as movement of surface electrodes with respect to the underlying muscles. This paper focused on investigating the optimal interelectrode distance, channel configuration, and electromyography feature sets for myoelectric pattern recognition in the presence of electrode shift. Increasing interelectrode distance from 2 to 4 cm improved pattern recognition system performance in terms of classification error and controllability (p < 0.01). Additionally, for a constant number of channels, an electrode configuration that included electrodes oriented both longitudinally and perpendicularly with respect to muscle fibers improved robustness in the presence of electrode shift (p < 0.05). We investigated the effect of the number of recording channels with and without electrode shift and found that four to six channels were sufficient for pattern recognition control. Finally, we investigated different feature sets for pattern recognition control using a linear discriminant analysis classifier and found that an autoregressive set significantly (p < 0.01) reduced sensitivity to electrode shift compared to a traditional time-domain feature set.     

利用单电极不对称双向脉冲刺激实现哺乳动物有髓神经纤维选择性兴奋的仿真研究

本研究的目的是要从理论上探讨利用单电极双向脉冲刺激实现哺乳动物神经纤维选择性刺激,(即当刺激一束神经时,不兴奋粗神经而兴奋细神经)的可能性.双向脉冲刺激可以降低刺激脉冲对神经纤维产生的电化学损伤.为研究哺乳动物有髓神经纤维的电特性,建立了一个基于简单的无穷大、各向同性的容积导体模型的仿真系统.利用该仿真系统,采用"不对称但电荷平衡"的双向脉冲刺激,计算了神经纤维的兴奋和阻断阈值与纤维直径、纤维-电极间距离的关系.结果表明:在距电极一定距离内采用该双向脉冲刺激模式确实可以实现哺乳动物有髓神经纤维的选择性兴奋.     

Proposal of a new method for narrowing and moving the stimulated region of cochlear implants: animal experiment and numerical analysis

We have proposed the tripolar electrode stimulation method (TESM) for narrowing the stimulation region and continuously moving the stimulation site for cochlear implants. The TESM stimulates the auditory nerve array using three adjacent electrodes which are selected among the electrodes of an electrode array within the lymphatic fluid. Current is emitted from each of the two lateral electrodes and received by the central electrode. The current received by the central electrode is made equal to the sum of the currents emitted from the lateral electrodes. In this paper, we evaluate whether or not TESM works according to a theory which is based on numerical analysis using an electrical equivalent circuit model of the auditory nerve fibers. In this simulation, the sums of the excited model fibers are compared to the compound action potentials (CAP's) which we obtained through animal experiments. To identify the main parameter while maintaining the amplitude of the CAP (the sum of the fired fibers), we assumed the presence of some parameters from the radial current density profile. In the case of the width value among the parameters being kept constant, the amplitude of the CAP was almost constant; thus, the number of the fired fibers was also almost constant. The width value equals the distance between the points at which the profile of the radial current density of the electrode array and the line of the radial threshold current density of the electrode array intersect. It is possible to determine the measure of the stimulation region or site by controlling the width value and the ratios of the currents emitted from the lateral electrodes. As a result, we succeeded in narrowing the stimulation region by controlling the sum of the currents emitted from the two lateral electrodes. Also we succeeded in continuously moving the stimulation site by modifying the currents emitted from the two lateral electrodes.     

A new technique for transmission of signals from implantabletransducers

To reduce space requirements for implant electronics in in vivo telemetry applications, the purpose of this project was to develop and test a new data transmission method that utilizes the ionic properties of bodily fluids as the transmission medium. Motivated by an interest in using the new method to transmit information from a sensor which measures tension in anterior cruciate ligament (ACL) grafts, a sine wave was injected into a cadaver leg using platinum electrodes implanted into the lateral femoral epicondyle. The signal was detected by electromyogram (EMG) surface electrodes. The effect of transmission frequency, the current injected, interelectrode separation, distance of the electrodes from the joint line, and the surface of electrode placement on the signal attenuation was studied. The logarithmic relation between attenuation and frequency was constant from 2 kHz until 10 kHz. For frequencies above 10 kHz, the attenuation increased linearly at the rate of 1 dB/octave. Attenuation was inversely sensitive to both current and interelectrode separation with larger separations and currents giving less attenuation. Attenuation was significantly less for the lateral thigh surface than for the anterior surface and increased with increasing distance from the joint line for both surfaces. For the application of interest here, suitable values of transmission variables to avoid the possible negative consequences of injecting current into living tissue are a current of 3 mA injected at a frequency of 37 kHz. The values of reception variables for minimum attenuation are wide interelectrode separation (5 cm) with the electrodes placed 5 cm proximal of the joint line on the lateral surface of the thigh. With the exception of the surface which is application dependent, these values of the reception variables should also be appropriate for other applications     

On Modeling the Single Motor Unit Action Potential

A model has been proposed for the generation of single motor unit potentials routinely observed in the clinical EMG examination of the normal biceps brachii muscle. A dipole representation was chosen for the single fiber activity. The motor unit was constructed from a uniform random array of single fibers. Motor unit potentials generated by this array have been observed at various distances both inside and outside the array. The effects of single fiber dipole axial dispersions on the potentials observed at increasing distances from the array have also been investigated. Motor unit potentials generated by the model have been compared with existing data from multielectrode studies in the biceps brachii.     

Selective activation of small motor axons by quasitrapezoidalcurrent pulses

We have found a method to activate electrically smaller nerve fibers without activating larger fibers in the same nerve trunk. The method takes advantage of the fact that action potentials are blocked with less membrane hyperpolarization in larger fibers than in smaller fibers. In our nerve stimulation system, quasitrapezoidal-shaped current pulses were delivered through a tripolar cuff electrode to effect differential block by membrane hyperpolarization. The quasitrapezoidal-shaped pulses with a square leading edge, a 350 microsecond(s) plateau, and an exponential trailing phase ensured the block of propagating action potentials and prevented the occurrence of anodal break excitation. The tripolar cuff electrode design restricted current flow inside the cuff and thus eliminated the undesired nerve stimulation due to a "virtual cathode." Experiments were performed on 13 cats. The cuff electrode was placed on the medial gastrocnemius nerve. Both compound and single fiber action potentials were recorded from L7 ventral root filaments. The results demonstrated that larger alpha motor axons could be blocked at lower current levels than smaller alpha motor axons, and that all alpha fibers could be blocked at lower current levels than gamma fibers. A statistical analysis indicated that the blocking threshold was correlated with the axonal conduction velocity or fiber diameter. This method could be used in physiological experiments and neural prostheses to achieve a small-to-large recruitment order in motor or sensory systems.     

Determination of nerve conduction velocity distribution fromsampled compound action potential signals

The sampled compound action potential (CAP) data sequence was expressed as the circular convolution of the delay sequence and the sampled single fiber action potential (SFAP) data sequence. An algorithm, based on Hirose's method (1986) was then developed to separate the delay sequence from the sampled CAP data sequence, and the nerve conduction velocity distribution (NCVD) was consequently calculated from the delay sequence. The NCVD was found to be the product of the amplitude of the SFAP and the number of fibers. Simulations show that the estimated results were in good agreement with the calculated results. Experiments were performed on ten sciatic nerves from five bullfrogs (Rana pipens) using two independent variables: interelectrode distance and stimulus current strength. The results estimated from CAP's recorded under each condition reflect the corresponding feature of NCVD of the condition. The advantage of the technique is to provide detailed information about both slow and fast conducting fibers. This technique also offers the possibility to directly calculate the nerve fiber diameter distribution from the sampled CAP data sequences     

Evaluation of the cable model for electrical stimulation of unmyelinated nerve fibers

The cable model, used to calculate the membrane potential of an unmyelinated nerve fiber due to electrical stimulation, is reexamined under passive steady-state conditions. The validity of two of the assumptions of the cable model are evaluated, namely that the membrane potential be a function of the axial coordinate only and that the extracellular potential due to the presence of the nerve fiber be negligible. The membrane potential calculated from the passive steady-state cable model is compared with the membrane potential obtained from an analytical three-dimensional (3-D) volume conductor model of a nerve fiber. It is shown that for very small electrode-fiber distances (of only a few fiber radii), both assumptions are violated and the two models give quite different results. Over a wide range of the electrode-fiber distance (about 0.1 mm to 1 cm), both assumptions are fulfilled and the two models give approximately the same results. For very large distances (more than 10 cm, independent of fiber diameter) only the second assumption is satisfied, but a modification of the activating function of the cable model allows to calculate the membrane potential in agreement with the 3-D model.     

Noise characteristics and statistics of picosecond Stokes pulsesgenerated in optical fibers through stimulated Raman scattering

The growth of the Stokes pulse from spontaneous noise during stimulated Raman scattering of picosecond pump pulses in optical fibers, is investigated by using a Langevin-noise term in the coupled nonlinear Schrodinger equations, which include pump depletion, group-velocity mismatch, fiber dispersion, and self- and cross-phase modulation. The model makes use of the actual Raman-gain spectrum of optical fibers. Numerical simulations are used to examine the average behavior of the Stokes pulse, and shot-to-shot fluctuations that are likely to occur in practice. It is shown that the Raman-induced energy transfer is significantly affected by group-velocity dispersion for pump-pulse widths shorter than 5 ps. Examination of the average temporal width shows that the Stokes pulse is initially as wide as the pump pulse, undergoes a gain induced compression and then rebroadens for distances longer than a walk-off length. The effect of varying pump and fiber parameters is to change the minimum value of the Stokes-pulse width, and the distance at which the minimum occurs. The shot-to-shot energy and pulse-width fluctuations initially increase before being reduced at fiber lengths longer than the walk-off length. The primary effect of dispersive and nonlinear effects is to change the distance beyond which fluctuations decrease     

Bipolar stimulation of a three-dimensional bidomain incorporatingrotational anisotropy

A bidomain model of cardiac tissue was used to examine the effect of transmural fiber rotation during bipolar stimulation in three-dimensional (3-D) myocardium. A 3-D tissue block with unequal anisotropy and two types of fiber rotation (none and moderate) was stimulated along and across fibers via bipolar electrodes on the epicardial surface, and the resulting steady-state interstitial (Φ _ϵ) and transmembrane (V_m) potentials were computed. Results demonstrate that the presence of rotated fibers does not change the amount of tissue polarized by the point surface stimuli, but does cause changes in the orientation of Φ_ϵ, and V_m in the depth of the tissue, away from the epicardium. Further analysis revealed a relationship between the Laplacian of Φ _ϵ, regions of virtual electrodes, and fiber orientation that was dependent upon adequacy of spatial sampling and the interstitial anisotropy. These findings help to understand the role of fiber architecture during extracellular stimulation of cardiac muscle     

Characterization of signals and noise rejection with bipolarlongitudinal intrafascicular electrodes

Longitudinal intrafascicular electrodes (LIFEs) are fine electrodes threaded into the extracellular space between axons in peripheral nerves or spinal roots. The authors are developing these electrodes for application in functional electrical stimulation and in basic physiology. An area of concern in chronic recording application of LIFEs is the possibility of electromyogram and other external noise sources masking the recorded neural signals. The authors characterized neural signals recorded by LIFEs and confirmed by three independent methods that increasing interelectrode spacing for bipolar LIFEs increases signal amplitude. The spectrum of neural signal from bipolar and monopolar LIFE lies between 300 Hz and 10 kHz. The amplitude of the spectrum increases with increasing interelectrode spacing, although the distribution is not affected. Single unit analysis of LIFE recordings show that they record selectively from units closest to the electrode active site. Units with conduction velocities ranging from 50-120 m/s were identified. Extraneural noise, as stimulus artifact or electromyogram, is much reduced with bipolar LIFE recording, as compared to monopolar recordings. Relative improvement in neural signal to extraneural noise increases with interelectrode spacing up to about 2 mm. Since there is no further improvement beyond 2 mm, the authors conclude that the preferred interelectrode spacing for bipolar LIFEs is 2 mm     

Optimal 40 Gb/s modulation formats for spectrally efficient long-haul DWDM systems

Three modulation formats are compared by numerical simulation of highly dense (75-GHz-spaced for 40 Gb/s channel), long-haul (600-1800 km) wavelength division multiplexed systems with three fiber types. Nonreturn-to-zero (NRZ) format, being the most spectrally compact and the simplest in transmitter and receiver configuration of the three, seems to be capable enough at shorter transmission distances than 1000 km regardless of fiber type. Carrier-suppressed return-to-zero (RZ) format, being the most tolerant to the self-phase modulation effect, showed better performance with fibers having larger chromatic dispersion. However, its transmission distance with low dispersion fibers is severely limited by the four-wave mixing effect. Bit-synchronous intensity modulated differential phase shift keying (IM-DPSK) format seems to be the best choice for a transmission distance beyond 1000 km because of its superior tolerance to optical noise and fiber nonlinear effects regardless of fiber types, despite slightly more complex transmitter and receiver configurations.     

Analysis of Models for External Stimulation of Axons

Extracellular electrodes produce electrical fields at the outside of nerve fibers. Discretization of the axon's length coordinate allows simulation of the excitation by a system of differential equations in time, and difference equations in space. For myelinated fibers this segmentation is naturally given by the nodes of Ranvier, whereas unmyelinated axons can be segmented arbitrarily. In both cases the equations are similar and can be treated in parallel. The activity of the axon depends on the second space derivative of the extracellular medium. The activating function is discussed for monopolar electrodes but the principle can be extended to arbitrary configurations of electrodes.     

A novel electrode array for diameter-dependent control of axonal excitability: a simulation study

Electrical extracellular stimulation of peripheral nerve activates the large-diameter motor fibers before the small ones, a recruitment order opposite the physiological recruitment of myelinated motor fibers during voluntary muscle contraction. Current methods to solve this problem require a long-duration stimulus pulse which could lead to electrode corrosion and nerve damage. The hypothesis that the excitability of specific diameter fibers can be suppressed by reshaping the profile of extracellular potential along the axon using multiple electrodes is tested using computer simulations in two different volume conductors. Simulations in a homogenous medium with a nine-contact electrode array show that the current excitation threshold (Ith) of large diameter axons (13-17 microm) (0.6-3.0 mA) is higher than that of small-diameter axons (2-7 microm) (0.4-0.7 mA) with 200-microm axon-electrode distance and 10-micros stimulus pulse. The electrode array is also tested in a three-dimensional finite-element model of the sacral root model of dog (ventral root of S3). A single cathode activates large-diameter axons before activating small axons. However, a nine-electrode array activates 50% of small axons while recruiting only 10% of large ones and activates 90% of small axons while recruiting only 50% of large ones. The simulations suggest that the near-physiological recruitment order can be achieved with an electrode array. The diameter selectivity of the electrode array can be controlled by the electrode separation and the method is independent of pulse width.     

Modeling Needle Stimulation of Denervated Muscle Fibers: Voltage–Distance Relations and Fiber Polarization Effects

A finite-element model of the human thigh was coupled with a 1-D compartment model to simulate the excitation of denervated muscle fibers with a needle electrode. For short electrode-fiber distances, the specific characteristics of the needle geometry determined the areas of lowest threshold values. With increasing distance, these areas shifted toward the needle's center of charge. Comparison of the 1-D model with a 3-D fiber model showed that the assumption of rotational symmetry underlying the 1-D model leads to an overestimation of thresholds. For a 40- mum-diameter fiber stimulated with 50 mus pulses at electrode-fiber distances between 50 mum and 1 mm, the 1-D/3-D threshold ratios were between 1.14 and 1.35 for the muscle fiber model, and between 1.11 and 1.17 for Hodgkin-Huxley membrane properties at 20 ^degC. For both membrane models, the deviation was more pronounced for large fiber diameters and short stimulation pulses. Qualitative results of the 1-D model like voltage-distance relations and predictions of spike initiation sites were correct.     

Axon termination conditions for electrical stimulation

The cable model for electrical stimulation near the terminal of a passive fiber is derived for excitation by an arbitrary, time-varying, applied extracellular field. Unless the termination impedance is comparable to that of the mammalian node of Ranvier, the end-conditions require the longitudinal intracellular current at the fiber terminal to be negligibly small. This requirement substantially alters the membrane potential profile from that obtained with a fiber of infinite length. Stimulation near the end of a fiber may result in lower thresholds and may reverse the anodal/cathodal threshold ratio obtained with stimulation in the mid-portion of the fiber. Chronaxie for stimulation near the terminal may be much smaller than at a distance from the terminal and the strength-duration curve may be nonmonotonic. These differences may have significant implications for any application of electrical stimulation where fiber terminations may play a role in the excitatory process     

一种新的优化喇曼掺Er混合光纤放大器结构

对于给定的非线性损伤，对喇曼／掺Er混合光纤放大器(HFA)结构进行优化，即优化光纤喇曼放大器(FRA)与掺Er光纤放大器(EDFA)的增益比和色散补偿光纤(DCF)长度比，得到一种最优HFA结构。分别比较了最优HFA、一般HFA、全FRA和全EDFA用作接收机前置放大器的噪声性能，以及用于多级链路在线放大器时系统所能到达的最大可传输距离。结果表明，最优HFA能提高系统性能。     

A new lensed-fiber configuration employing cascaded GI-fiber chips

A new scheme is proposed for lensed fibers having high coupling efficiency between laser diodes and single-mode fibers with a long working distance. The new lensed fiber consists of a pair of GI-fiber tips having different focusing parameters. The measured net coupling loss between a laser diode operating at a wavelength of 1.3 μm and a single-mode fiber is as low as 1.5 dB. The working distances are around 50 μm, much longer than those of conventional lensed fibers