Showing papers by "Helsinki University of Technology published in 1989"

Realistic conductivity geometry model of the human head for interpretation of neuromagnetic data

Abstract: The computational and practical aspects of a realistically shaped multilayer model for the conductivity geometry of the human head are discussed. A method to handle the numerical difficulties caused by the presence of poorly conducting skull is presented. Using the method, both the potential on the surface of the head and the magnetic field outside the head can be computed accurately. The procedure is tested with the multilayer sphere model, for which analytical expressions are available. The method is then applied to a realistically shaped head model, and it is shown numerically that for the computation of B produced by cerebral current sources, it is sufficient to consider a brain-shaped homogeneous conductor only, since the secondary currents on the outer interfaces give only a negligible contribution to the magnetic field outside the head. Comparisons with the sphere model are included to pinpoint areas where the homogeneous conductor model provides essential improvements in the calculation of the magnetic field outside the head. >

Realization of general nondiffracting beams with computer-generated holograms

Abstract: A new class of solutions to the scalar wave equation was introduced recently that represents transversely localized but totally nondiffracting fields. We show by the method of stationary phase that any of these wave fields can be realized approximately with a laser and a single computer-generated hologram. We briefly discuss various techniques for coding and fabrication of the required hologram and the associated diffraction efficiencies. Using both binary-amplitude and four-level phase holograms, we demonstrate experimentally the formation of arbitrary-order Bessel beams and rotationally nonsymmetric beams.

Self-organizing semantic maps

Abstract: Self-organized formation of topographic maps for abstract data, such as words, is demonstrated in this work The semantic relationships in the data are reflected by their relative distances in the map Two different simulations, both based on a neural network model that implements the algorithm of the selforganizing feature maps, are given For both, an essential, new ingredient is the inclusion of the contexts, in which each symbol appears, into the input data This enables the network to detect the "logical similarity" between words from the statistics of their contexts In the first demonstration, the context simply consists of a set of attribute values that occur in conjunction with the words In the second demonstration, the context is defined by the sequences in which the words occur, without consideration of any associated attributes Simple verbal statements consisting of nouns, verbs, and adverbs have been analyzed in this way Such phrases or clauses involve some of the abstractions that appear in thinking, namely, the most common categories, into which the words are then automatically grouped in both of our simulations We also argue that a similar process may be at work in the brain

Industrial microwave sensors

Abstract: Surveys the different types of microwave sensors and reviews the latest developments reported by European institutes and companies. Attention is given to resonator, transmission, reflection, radar, and radiometer sensors, and to active imaging. >

Self-consistency aspects of dielectric mixing theories

Abstract: Dielectric mixing formulas that relate the effective permittivity of a heterogeneous medium with the properties of the constituent materials are considered. The restriction for this approach is that the sizes of the inclusions in the mixture have to be considerably smaller than the wavelength to avoid scattering effects. The mixing rules are presented for cases where the scatterers can be homogeneous or multilayer spheres. The formula contains a free parameter v, and for different choices of this parameter, many mixing formulae result as special cases of the given mixing rule. Mixing formulas presented in the literature are also reviewed and the results from different formulae are compared, using dry and wet snow as paradigmatic examples. >

SQUID magnetometers for low-frequency applications

Abstract: We present a novel formulation for SQUID operation, which enables us to evaluate and compare the sensitivity and applicability of different devices. SQUID magnetometers for low-frequency applications are analyzed, taking into account the coupling circuits and electronics. We discuss nonhysteretic and hysteretic single-junction rf SQUIDs, but the main emphasis is on the dynamics, sensitivity, and coupling considerations of dc-SQUID magnetometers. A short review of current ideas on thin-film, dc-SQUID design presents the problems in coupling and the basic limits of sensitivity. The fabrication technology of tunnel-junction devices is discussed with emphasis on how it limits critical current densities, specific capacitances of junctions, minimum linewidths, conductor separations, etc. Properties of high-temperature superconductors are evaluated on the basis of recently published results on increased flux creep, low density of current carriers, and problems in fabricating reliable junctions. The optimization of electronics for different types of SQUIDs is presented. Finally, the most important low-frequency applications of SQUIDs in biomagnetism, metrology, geomagnetism, and some physics experiments demonstrate the various possibilities that state-of-the-art SQUIDs can provide.

Positron affinities for elemental metals

Abstract: The relevant quantity in the comparison of the absolute positron energy levels in different materials is the sum of the internal electron and positron chemical potentials, i.e. the sum of the Fermi level and the bottom of the lowest positron band relative to a common, well-defined reference energy. This sum is defined as the positron affinity. The positron affinity reflects the preference of the positron for different components in heterostructures made of different materials and the preference between the host matrix and precipitates in alloys. Moreover, the affinity is closely related to the positron work function and positronium formation potential which are important parameters in the slow-positron-beam experiments. The authors have determined the positron affinity for the alkaline and alkaline-earth metals, 3d-, 4d-, and 5d-transition metal series, and for some metals on the right in the Periodic Table. The diamond structure semiconductors are also considered. The determination is based on the self-consistent electron structure calculations and the subsequent calculation of the positron band structure within the local-density approximation. The trends are studied and interpreted along the different columns and rows of the Periodic Table. The results are also compared with available experiments.

Neuromagnetic steady-state responses to auditory stimuli.

Abstract: Steady‐state magnetic responses to clicks presented at rates between 10 and 70 Hz have been recorded in healthy humans. The responses were highest in amplitude around 40 Hz. This amplitude enhancement is satisfactorily explained by summation of responses evoked by single clicks. The field maps suggest activation of the auditory cortex at all stimulus frequencies. Similar responses were obtained with gated noise bursts and by pauses in a series of clicks. The mean ‘‘apparent latency,’’ determined from the phase lag at rates 30–70 Hz, was 54 ms. The physiological relevance of this quantity is shown to be questionable.

Recording and Interpretation of Cerebral Magnetic Fields

Abstract: Contemporary brain research progresses along two main lines: the microlevel approach explores single neurons and subcellular elements, while macrolevel studies focus on more complex cerebral functions, including behavior. This review presents results obtained mainly in our laboratory by means of an intermediate method, magnetoencephalography (MEG), which reflects cortical activity of neuronal populations at the level fo cytoarchitectonic areas. Because it is completely noninvasive, MEG can be used to study brain functions that are characteristically human.

Selective listening modifies activity of the human auditory cortex.

Abstract: We have studied the effect of selective listening on the neuromagnetic evoked activity of the human auditory cortex In the word categorization experiment the stimuli were 5-letter words, each beginning with /k/ Half of them were targets, ie, names of animals or plants, and half other meaningful Finnish words In the duration discrimination experiment equiprobable tones of 425 ms (targets) or 600 ms duration were presented In both experiments the interstimulus interval (ISI) was 23 s and the stimuli of the two classes were presented randomly Subjects either ignored the stimuli (reading condition) or counted the number of targets (listening condition) The magnetic field over the head was measured with a 7-channel 1st-order SQUID-gradiometer The stimuli evoked a transient response followed by a sustained field The transient response did not differ between the two conditions but the sustained field was significantly larger in the listening than reading condition; the increase began 120–200 ms after stimulus onset and continued for several hundred milliseconds The equivalent source locations of both transient and sustained responses agreed with activation of the supratemporal auditory cortex In the dichotic listening experiment 25-ms square-wave stimuli were presented randomly and equiprobably either to the left or to the right ear at an ISI of 08–1 s, either alone or in presence of a speech masker Counting the stimuli of either ear resulted in differences between responses to relevant and irrelevant sounds The difference began 140–150 ms after stimulus onset and peaked at 200–240 ms During monaural speech masking, N100m was larger for attended than ignored stimuli The results suggest that neural mechanisms underlying direction of attention include modification of the activity of the auditory cortex and that the mechanisms are similar for words and tones

Kinoform Phase Relief Synthesis: A Stochastic Method

Abstract: We introduce a new method of finding periodic phase relief structures, which produce the desired diffraction pattern with high light efficiency (75% to 96%) and low noise (<±1%). Use is made of explicit equations for the angular spectrum of the field immediately behind such structures and of different stochastic nonlinear optimization methods.

Polarizability and Effective Permittivity of Layered and Continuously Inhomogeneous Dielectric Ellipsoids

Abstract: In this paper, the polarizabilities of partially and continuously inhomogeneous dielectric ellipsoids are analyzed. Multilayer inclusions where the boundaries between the layers are confocal ellipsoids, are solved in a static field. Transmission line analogy is applied, resulting in matrices with which the field components and the polarizability can be solved. In case of the continuous permittivity profile, a differential equation is derived for the scattered potential, and from the amplitude of this function, the polarizability results. The limitation in the continuous case is that the permittivity function of the ellipsoid depend only on one coordinate (ξ) in the ellipsoidal coordinate system. The theory is applied to calculating the microwave attenuation of melting hail at the frequency of 1 GHz, where the hydrometeors are modeled as two-layer ellipsoids.

Remarks on Picard-Lindelo¨f iteration: part II

Abstract: The paper discusses Picard-Lindelof iteration for systems of autonomous linear equations on finite intervals, as well as its numerical variants. Most of the discussion is under a model assumption which roughly says that the coupling terms are of moderate size compared with the slow time scales in the problem. It is shown that the speed of convergence is quite independent of the step sizes already for very large time steps. This makes it possible to design strategies in which the mesh gets gradually refined during the iteration in such a way that the iteration error stays essentially on the level of discretization error.

Effect of the structure of external alkoxysilane donors on the polymerization of propene with high-activity ziegler-natta catalysts

Abstract: Propene was polymerized in bulk in the presence of high activity heterogeneous Ziegler-Natta catalyst with the purpose of finding correlations between the structure of an external alkoxysilane donor and the polymer. Nineteen silane compounds (1–8) of the structure RnSi(OR′)4-n, where n = 1–4, R°C6H5, alkyl or H; R′ = C1–3-alkyl, were used as external donors. The effect of Si/Al mole ratio was studied. The effect of the external alkoxysilane donor on the polymerization strongly depends on the number and size of alkoxy groups and the size of hydrocarbon groups attached to the silicon atom. Two major effects were observed: selective deactivation of atactic active centers and the increased production of high-molecular-weight isotactic polypropylene. Evidently, at least one free non-hindered alkoxy group in the complex between alkoxysilane and AlEt3 is required for the selective deactivation. Also, the size of the hydrocarbon group of the donor strongly influences the selectivity of the deactivation.

Chiral Maxwell-Garnett mixing formula

Abstract: The Maxwell-Garnett mixing formula, previously well known for dielectric mixtures, is generalised for the first time to cover chiral materials. The mixing formula expresses the permittivity, permeability and chirality of a two-component mixture with spherical inclusions as functions of the material parameters of the components. The result gives the classical Maxwell-Garnett formula as a special case. The mixing formula can be applied for the design of chiral composites that have attracted keen interest in recent years.

Modelling longitudinal elastic an shrinkage properties of wood

Abstract: A model was developed for estimating elastic and shrinkage properties of a softwood cell wall from the properties of its polymeric constituents: cellulose, hemicellulose and lignin. The theory of composite materials was used. Based on a literature survey, models of latewood, earlywood and compressionwood of a softwood cell wall structure were made. The model takes into account the helical winding of the microfibrils in the cell wall and it estimates the behaviour of a balanced laminated double-cell wall in which rotation is restrained by adjacent cells. The calculated elastic and shrinkage properties were compared with earlier test results and good agreement was found.

Transverse laser-mode structure determination from spatial coherence measurements: Experimental results

Abstract: We demonstrate experimentally the feasibility of a certain coherence-theoretic algorithm for determining the relative modal weights of a laser beam that consists of multiple Hermite-Gaussian transverse modes. A computer-controlled Young's two-pinhole interferometer with a linear CCD array is used for the coherence measurements. The required scaling parameter (beam width) is obtained iteratively by library routines. Numerical simulations on the stability of the algorithm in the presence of experimental noise are also presented.

Incentive strategies and equilibria for dynamic games with delayed information

Abstract: The construction of time-lag incentive strategies for continuous time convex problems is considered. The strategies are affine in the data available and they are represented by means of Stieltjes measures. It is shown how incentive strategies can be used as equilibrium strategies in symmetric games where the decision makers are cooperative.

Fumaric acid production from xylose by immobilized Rhizopus arrhizus cells

Abstract: The production of fumaric acid by immobilized Rhizopus arrhizus TKK 204-1-1a mycelium was optimized in batch fermentations using statistical experimental design and empiric modelling. The maximum fumaric acid concentration was obtained at a xylose concentration of about 6% and a carbon:nitrogen ratio of about 160. In repeated batch fermentations with immobilized cells the highest volumetric productivity of fumaric acid reached was 87 mg/l per hour when the initial xylose concentration was 10%, the C:N ratio 160 and the residence time 1.75 days. The maximum product concentration was 16.4 g/l when the initial xylose concentration was 10%, the C:N ratio 160 and the residence time 10.25 days. The maximum yield from initial xylose (6.47%) was 23.7% with a product concentration of 15.3 g/l and volumetric productivity of 71 mg/l per hour at a residence time of 9 days and a C:N ratio of 188.3. Immobilization could increase the fumaric acid concentration to a level 3.4 times higher than that produced by free cells.