A quantitative approach to modeling mammalian myelinated nervefibers for electrical prosthesis design

Presents an upgraded cable model of mammalian myelinated nerve fibers in an extracellularly applied field. The kinetics of the nodes is based upon voltage clamp data in rat motor fibers at 37°C (J.R. Schwartz and G. Eikhof, 1987), while the resting membrane potential is computed with the Goldman equation. The resulting spike shape, conduction velocity, strength/duration behavior, and absolute and relative refractory period are in good quantitative agreement with published experimental data in mammals at normal body temperature and at 20°C. Results at intermediate temperatures however, suggest that the widely used concept of a constant Q₁₀ for the rate constants is invalid. In addition, the model generates realistic abortive spikes towards the end of the absolute refractory period and it can describe the consequences of repetitive firing. The results stress the advantages of a multiple nonlinear node model even if only time aspects of nerve behavior are under study. It turned out, that the model presented here describes in vivo neural properties relevant for electrical prosthesis design better than previous models in literature     

Large-scale production of polypropylenimine dendrimers

The synthesis and characterization of a new series of polypropylenimine dendrimers is reported. Using a repetition of the sequence of a Michael addition to a primary amine, followed by a heterogeneously catalyzed hydrogenation, ultra-pure polypropylenimine macromolecules with molecular weights up to 6912 are synthesized. The reaction sequence allows the preparation of these dendrimers at very large scales, whilst the availability of a simple purification in the sequence affords ultra-pure samples. The polypropylenimine dendrimers are fully characterized; apart from the first 0.5 generation they are all oils, possess a Tg in the range from −90 to −40 °C, are unexpectedly stable, and their intrinsic viscosity drops after generation 4.     

A central spectrum theory of binaural processing. The binaural edge pitch revisited

Recently, Klein and Hartmann [J. Acoust. Soc. Am. 70, 51-61 (1981)] investigated a new dichotic pitch, called the binaural edge pitch (BEP). They used computer-generated periodic noise signals to generate BEP. In the study presented here, the BEP is investigated in order to evaluate the predictions of the central spectrum theory with regard to this stimulus. Pitch-matching experiments using a nonperiodic BEP stimulus, produced by means of a modulation technique, led to the conclusion that the strongest pitch sensation in the BEP has the character of a weak fluctuating pure tones in noise, which corresponds to a frequency, equal or almost equal to the frequency of the phase transition. This result fits in with the predictions of the central spectrum theory, which, for instance, does not need the assumption of central lateral inhibition for explaining this pitch. Furthermore, it is shown that this theory can also predict the results obtained in lateralization measurements and BMLD measurements using BEP stimuli as well as related stimuli. The results are compared with the data obtained by Klein and Hartmann.     

Dependence of spin-asymmetry in the Shubnikov-de Haas resistance on back-gating, illumination and uniaxial stress

We present measurements of the Shubnikov-de Haas (SdH) resistivity on GaAs/AlGaAs heterojunctions. We compare measurements in which in the same sample the electron density was increased to the same value either by applying a back-gate voltage or illumination. We conclude that both the overlap of the electron wavefunctions in the two-dimensional electron gas with positive and negative scattering centers and the number of positive and negative scatterers determine the asymmetry in the SdH-peaks. The asymmetry in the SdH-peaks depends on the electron density because the scattering on remote and background impurities have a different electron density dependence. After illumination the neutralization rate of the acceptors in the GaAs determines whether the asymmetry increases or decreases. We show that the asymmetry of the SdH-peaks can also be modified by applying uniaxial stress.     

Assessing the placement of a cochlear electrode array by multidimensional scaling

Correct placement of the electrode is crucial for cochlear implantation (CI) surgery. It determines the access to the auditory nerve and subsequent hearing performance. Here, we propose an objective measures tool that can partially verify the electrode position. The intracochlear spread of the electrical fields is measured and analyzed by means of multidimensional scaling resulting in an intuitive visual representation. The user can then detect major issues, such as electrode foldover or ossification. Other implantation issues, such as electrode migration into the scala vestibuli, may not significantly alter the electrical conduction pattern and remain undetected. Still, as the measurement is quick and readily available, it may be a valuable intraoperative verification tool.     

Improving the accuracy of the boundary element method by the use of second-order interpolation functions

The boundary element method (BEM) is a widely used method to solve biomedical electromagnetic volume conduction problems. The commonly used formulation of this method uses constant interpolation functions for the potential and flat triangular surface elements. Linear interpolation for the potential on a flat triangular mesh turned out to yield a better accuracy. In this paper, we introduce quadratic interpolation functions for the potential and quadratically curved surface elements, resulting from second-order spatial interpolation. Theoretically, this results in an accuracy that is inversely proportional to the third power of element size. The method is tested on a four concentric sphere geometry, representative for electroencephalogram modeling, and compared to previous solutions of this problem in literature. In addition, a cylindrical test configuration is used. We conclude that the use of quadratic interpolation functions for the potential and of quadratically curved surface elements in BEM results in a significant increase in accuracy and in some cases even a reduction of the computation time with the same number of nodes involved in the calculations.