Decorrelation of neural-network activity by inhibitory feedback

Correlations in spike-train ensembles can seriously impair the encoding of information by their spatio-temporal structure. An inevitable source of correlation in finite neural networks is common presynaptic input to pairs of neurons. Recent theoretical and experimental studies demonstrate that spike correlations in recurrent neural networks are considerably smaller than expected based on the amount of shared presynaptic input. By means of a linear network model and simulations of networks of leaky integrate-and-fire neurons, we show that shared-input correlations are efficiently suppressed by inhibitory feedback. To elucidate the effect of feedback, we compare the responses of the intact recurrent network and systems where the statistics of the feedback channel is perturbed. The suppression of spike-train correlations and population-rate fluctuations by inhibitory feedback can be observed both in purely inhibitory and in excitatory-inhibitory networks. The effect is fully understood by a linear theory and becomes already apparent at the macroscopic level of the population averaged activity. At the microscopic level, shared-input correlations are suppressed by spike-train correlations: In purely inhibitory networks, they are canceled by negative spike-train correlations. In excitatory-inhibitory networks, spike-train correlations are typically positive. Here, the suppression of input correlations is not a result of the mere existence of correlations between excitatory (E) and inhibitory (I) neurons, but a consequence of a particular structure of correlations among the three possible pairings (EE, EI, II)

Assessment of quadratic nonlinear cardiorespiratory couplings during tilt table test by means of real wavelet biphase

In this paper a method for assessment of Quadratic Phase Coupling (QPC) between respiration and Heart Rate Variability (HRV) is presented. Methods: First, a method for QPC detection is proposed named Real Wavelet Biphase (RWB). Then, a method for QPC quantification is proposed based on the Normalized Wavelet Biamplitude (NWB). A simulation study has been conducted to test the reliability of RWB to identify QPC, even in the presence of constant delays between interacting oscillations, and to discriminate it from Quadratic Phase Uncoupling. Significant QPC was assessed based on surrogate data analysis. Then, quadratic cardiorespiratory couplings were studied during a tilt table test protocol of 17 young healthy subjects. Results: Simulation study showed that RWB is able to detect even weak QPC with delays in the range of 0 - 2 s, which are usual in the Autonomic Nervous System (ANS) control of heart rate. Results from the database revealed a significant reduction (p<0.05) of NWB between respiration and both low and high frequencies of HRV in head-up tilt position compared to early supine. Conclusion: The proposed technique detects and quantifies robustly QPC and is able to track the coupling between respiration and various HRV components during ANS changes. Significance: The proposed method can help to assess alternations of nonlinear cardiorespiratory interactions related to ANS dysfunction and physiological regulation of HRV in cardiovascular diseases

Stability Analysis of Fixed-Point Digital Filters using Computer Generated Lyapunov Functions- Part I: Direct Form and Coupled Form Filters

We demonstrate the applicability of the constructive stability algorithm of Brayton and Tong in the stability analysis of fixed-point digital filters. In the present paper, we consider direct form and coupled form filters while in a companion paper we treat wave digital filters and lattice filters. We compare our results with existing ones which deal with either the global asymptotic stability of digital filters or with existence (resp., nonexistence) of limit cycles in digital filters. Several of the present results are new while some of the present results constitute improvements over existing results. In a few cases, the present results are more conservative than existing results. It is emphasized that whereas the existing results are obtained by several diverse methods, the present results are determined by one unified approach

Intermittent ERK oscillations downstream of FGF in mouse embryonic stem cells

Signal transduction networks generate characteristic dynamic activities to process extracellular signals and guide cell fate decisions such as to divide or differentiate. The differentiation of pluripotent cells is controlled by FGF/ERK signaling. However, only a few studies have addressed the dynamic activity of the FGF/ERK signaling network in pluripotent cells at high time resolution. Here, we use live cell sensors in wild-type and Fgf4-mutant mouse embryonic stem cells to measure dynamic ERK activity in single cells, for defined ligand concentrations and differentiation states. These sensors reveal pulses of ERK activity. Pulsing patterns are heterogeneous between individual cells. Consecutive pulse sequences occur more frequently than expected from simple stochastic models. Sequences become more prevalent with higher ligand concentration, but are rarer in more differentiated cells. Our results suggest that FGF/ERK signaling operates in the vicinity of a transition point between oscillatory and non-oscillatory dynamics in embryonic stem cells. The resulting heterogeneous dynamic signaling activities add a new dimension to cellular heterogeneity that may be linked to divergent fate decisions in stem cell cultures.Fil: Raina, Dhruv. Institut Max Planck fur Molekulare Physiologie; AlemaniaFil: Fabris, Fiorella. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigación en Biomedicina de Buenos Aires - Instituto Partner de la Sociedad Max Planck; ArgentinaFil: Morelli, Luis Guillermo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigación en Biomedicina de Buenos Aires - Instituto Partner de la Sociedad Max Planck; Argentina. Institut Max Planck fur Molekulare Physiologie; AlemaniaFil: Schroter, Christian. Institut Max Planck fur Molekulare Physiologie; Alemani

A novel framework for high-quality voice source analysis and synthesis

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The analysis, parameterization and modeling of voice source estimates obtained via inverse filtering of recorded speech are some of the most challenging areas of speech processing owing to the fact humans produce a wide range of voice source realizations and that the voice source estimates commonly contain artifacts due to the non-linear time-varying source-filter coupling. Currently, the most widely adopted representation of voice source signal is Liljencrants-Fant's (LF) model which was developed in late 1985. Due to the overly simplistic interpretation of voice source dynamics, LF model can not represent the fine temporal structure of glottal flow derivative realizations nor can it carry the sufficient spectral richness to facilitate a truly natural sounding speech synthesis. In this thesis we have introduced Characteristic Glottal Pulse Waveform Parameterization and Modeling (CGPWPM) which constitutes an entirely novel framework for voice source analysis, parameterization and reconstruction. In comparative evaluation of CGPWPM and LF model we have demonstrated that the proposed method is able to preserve higher levels of speaker dependant information from the voice source estimates and realize a more natural sounding speech synthesis. In general, we have shown that CGPWPM-based speech synthesis rates highly on the scale of absolute perceptual acceptability and that speech signals are faithfully reconstructed on consistent basis, across speakers, gender. We have applied CGPWPM to voice quality profiling and text-independent voice quality conversion method. The proposed voice conversion method is able to achieve the desired perceptual effects and the modified speech remained as natural sounding and intelligible as natural speech. In this thesis, we have also developed an optimal wavelet thresholding strategy for voice source signals which is able to suppress aspiration noise and still retain both the slow and the rapid variations in the voice source estimate

Lag, lock, sync, slip: the many 'phases' of coupled flagella

This is the final version of the article. Available from the Royal Society via the DOI in this recordIn a multitude of life's processes, cilia and flagella are found indispensable. Recently, the biflagellated chlorophyte alga Chlamydomonas has become a model organism for the study of ciliary motility and synchronization. Here, we use high-speed, high-resolution imaging of single pipette-held cells to quantify the rich dynamics exhibited by their flagella. Underlying this variability in behaviour are biological dissimilarities between the two flagella—termed cis and trans, with respect to a unique eyespot. With emphasis on the wild-type, we derive limit cycles and phase parametrizations for self-sustained flagellar oscillations from digitally tracked flagellar waveforms. Characterizing interflagellar phase synchrony via a simple model of coupled oscillators with noise, we find that during the canonical swimming breaststroke the cis flagellum is consistently phase-lagged relative to, while remaining robustly phase-locked with, the trans flagellum. Transient loss of synchrony, or phase slippage, may be triggered stochastically, in which the trans flagellum transitions to a second mode of beating with attenuated beat envelope and increased frequency. Further, exploiting this alga's ability for flagellar regeneration, we mechanically induced removal of one or the other flagellum of the same cell to reveal a striking disparity between the beatings of the cis and trans flagella, in isolation. These results are evaluated in the context of the dynamic coordination of Chlamydomonas flagella.Financial support is acknowledged from the EPSRC, ERC Advanced Investigator Grant 247333, and a Senior Investigator Award from the Wellcome Trust (R.E.G.)

Modeling and Simulation of Non-Equilibrium Effects in Modern Semiconductor Nanostructures

In laser physics, for the construction of new devices as well as the command and optimization of established configurations, it is crucial to properly understand the interplay between the different physical effects underlying the laser operation. In many cases, it may be rather difficult to experimentally access relevant key parameters. Furthermore, a decent understanding of the operation principles may be required already in the design stadium before the fabrication of the first prototype. Hence, an appropriate laser model is highly desirable since it allows for the simulation and analysis of important aspects of the laser performance. Modeling and simulation may help to represent or characterize, understand or analyze, asses or solve research problems encountered in experiments and verify assumptions made in theoretical investigations.The operation of modern microcavity lasers is governed by a complicated interplay of a variety of interaction processes. The systematic modeling of such devices therefore poses a significant challenge to the formulation of a physics driven laser theory based on the description of microscopic processes rather than on phenomenological approaches that mimic empirical observations. Especially, this applies to the modeling of high power VECSEL applications in extreme parameter regions well above the threshold. Since the emission characteristics of the laser systems is strongly dependent on the actual geometry of the setup, on the material parameters of both the dielectric structure and the gain region, and on the actual excitation state of the optically active material, the simulation of the experimentally observed features poses a highly non-trivial problem. While in many situations, it is sufficient to assume that the high carrier densities and the related fast carrier-carrier scattering effects in lasers lead to a sufficiently fast thermalization of the carrier system, under high power VECSEL operation conditions, such an assumption is usually not satisfied. In these systems, the carrier distribution in the valence and conduction bands may deviate significantly from Fermi-Dirac functions provided by the quasi-equilibrium conditions of a thermalized carrier system as the stimulated emission tends to burn kinetic holes into the carrier distributions leading to gain modifications which affect the emission characteristics of the device. A theory appropriate for the simulation of non-equilibrium laser performance, therefore, must be able to closely track the quantum kinetic carrier dynamics in the optically active material. This implies that for the simulation of the VECSEL performance a model has to be used that allows for the monitoring of the carrier dynamics on a microscopic time scale (i.e. a few femtoseconds) for macroscopic time periods (i.e. several nanoseconds) corresponding to the build up of stable laser oscillations - all the while being computationally thus feasible that even parametric studies do not become impractical.In our approach, the system consisting of semiconductor gain medium and laser field is described within the context of semi-classical laser theory by the Maxwell-semiconductor-Bloch equations (MSBE). Many-body Coulomb effects are included at the level of the screened Hartree–Fock approximation, and the effective relaxation rate approximation is used to account for the effects of carrier–carrier and carrier–phonon collisions. Here, we follow this approach to analyze the dominant non-equilibrium effects in the multimode operation of VECSEL devices under high excitation conditions. To test our model and to study the relevance of non-equilibrium effects, we pick the examples (i) of a traditional device configuration that exhibits dual-mode emission and (ii) of ultrashort pulse generation via modelocking in a VECSEL For the two-color operation, a model study is presented demonstrating that the non-linear laser theory on the basis of the MSBE is fully adequate for the description of high power applications where non-equilibrium effects gain increased importance. Altogether, our microscopic simulations give the first numerical verification that dynamically, stable two-wavelength oscillation of a semiconductor laser can occur when the mode coupling between two wavelengths is weak. Using the same model approach, we further carry out the first microscopic simulations of modelocking in a simple VECSEL configuration. Thus, we present a preliminary analysis of ultrafast mode-locking in order to expose the role of hot carrier distributions in establishing this feature

Large Eddy Simulation and Analysis of Shear Flows in Complex Geometries

In the present work, large eddy simulation is used to numerically investigate two types of shear flows in complex geometries, (i) a novel momentum driven countercurrent shear flow in dump geometry and (ii) a film cooling flow (inclined jet in crossflow). Verification of subgrid scale model is done through comparisons with measurements for a turbulent flow over back step, present cases of counter current shear and film cooling flow. In the first part, a three dimensional stability analysis is conducted for countercurrent shear flow using Dynamic mode decomposition and spectral analysis. Kelvin-Helmholtz is identified as primary instability mechanism and observed as global mode at a specific parameter. Mechanism of global mode synchronization over distinct spatial location is studied. In the second part, the flow physics of film cooling flows is analysed. The origin, evolution of various coherent flow structures and their role in film cooling heat transfer is studied based on detailed flow visualization. Further, the contribution of coherent structures in film cooling heat transfer and mixing is studied through modal analysis. Low frequency modes are found to have large contribution in cooling surface adiabatic temperature fluctuation while high frequency modes play larger role in bulk mixing. Finally, a new contoured crater shape is developed and shown to have improved performance at shallow depth compared to earlier designs

Design and implementation of a frequency synthesizer for an IEEE 802.15.4/Zigbee transceiver

The frequency synthesizer, which performs the main role of carrier generation for the down-conversion/up-conversion operations, is a key building block in radio transceiver front-ends. The design of a synthesizer for a 2.4 GHz IEEE 802.15.4/Zigbee transceiver forms the core of this work. This thesis provides a step-by-step procedure for the design of a frequency synthesizer in a transceiver environment, from the mapping of standard-specifications to its integrated circuit implementation in a CMOS technology. The results show that careful system level planning leads to high-performance realizations of the synthesizer. A strategy of using different supply voltages to enhance the performance of each building block is discussed. A section is presented on layout and board level issues, especially for radio-frequency systems, and their effect on synthesizer performance. The synthesizer consumes 15.5 mW and meets the specifications of the 2.4 GHz IEEE 802.15.4/Zigbee standard. It is capable of 5 GHz operation with a VCO sensitivity of 135 MHz/V and a tuning range of 700 MHz. It can be seen that the adopted methodology can be used for the design of high-performance frequency synthesizers for any narrow-band wireless standard