Helmut Schmidt

Massey University, Palmerston North City, Manawatu-Wanganui, New Zealand

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Publications (5)8.01 Total impact

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    ABSTRACT: Translationally invariant integro-differential equations are a common choice of model in neuroscience for describing the coarse-grained dynamics of cortical tissue. Here we analyse the propagation of travelling wavefronts in models of neural media that incorporate some form of modulation or randomness such that translational invariance is broken. We begin with a study of neural architectures in which there is a periodic modulation of the neuronal connections. Recent techniques from two-scale convergence analysis are used to construct a homogenized model in the limit that the spatial modulation is rapid compared with the scale of the long range connections. For the special case that the neuronal firing rate is a Heaviside we calculate the speed of a travelling homogenized front exactly and find how the wave speed changes as a function of the amplitude of the modulation. For this special case we further show how to obtain more accurate results about wave speed and the conditions for propagation failure by using an interface dynamics approach that circumvents the requirement of fast modulation. Next we turn our attention to forms of disorder that arise via the variation of firing rate properties across the tissue. To model this we draw parameters of the firing rate function from a distribution with prescribed spatial correlations and analyse the corresponding fluctuations in the wave speed. Finally we consider generalisations of the model to incorporate adaptation and stochastic forcing and show how recent numerical techniques developed for stochastic partial differential equations can be used to determine the wave speed by minimising the L 2 norm of a travelling disordered activity profile against a fixed test function.
    Discrete and Continuous Dynamical Systems 08/2012; 32(8). · 1.01 Impact Factor
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    Stephen Coombes, Helmut Schmidt, Ingo Bojak
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    ABSTRACT: Neural field models describe the coarse-grained activity of populations of interacting neurons. Because of the laminar structure of real cortical tissue they are often studied in two spatial dimensions, where they are well known to generate rich patterns of spatiotemporal activity. Such patterns have been interpreted in a variety of contexts ranging from the understanding of visual hallucinations to the generation of electroencephalographic signals. Typical patterns include localized solutions in the form of traveling spots, as well as intricate labyrinthine structures. These patterns are naturally defined by the interface between low and high states of neural activity. Here we derive the equations of motion for such interfaces and show, for a Heaviside firing rate, that the normal velocity of an interface is given in terms of a non-local Biot-Savart type interaction over the boundaries of the high activity regions. This exact, but dimensionally reduced, system of equations is solved numerically and shown to be in excellent agreement with the full nonlinear integral equation defining the neural field. We develop a linear stability analysis for the interface dynamics that allows us to understand the mechanisms of pattern formation that arise from instabilities of spots, rings, stripes and fronts. We further show how to analyze neural field models with linear adaptation currents, and determine the conditions for the dynamic instability of spots that can give rise to breathers and traveling waves.
    Journal of mathematical neuroscience. 05/2012; 2(1):9.
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    Helmut Schmidt, Ingo Bojak, Stephen Coombes
    BMC Neuroscience 01/2010; · 3.00 Impact Factor
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    Stephen Coombes, Helmut Schmidt
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    ABSTRACT: Many tissue level models of neural networks are written in the language of nonlinear integro-differential equations. Analytical solutions have only been obtained for the special case that the nonlinearity is a Heaviside function. Thus the pursuit of even approximate solutions to such models is of interest to the broad mathematical neuroscience community. Here we develop one such scheme, for stationary and travelling wave solutions, that can deal with a certain class of smoothed Heaviside functions. The distribution that smoothes the Heaviside is viewed as a fundamental object, and all expressions describing the scheme are constructed in terms of integrals over this distribution. The comparison of our scheme and results from direct numerical simulations is used to highlight the very good levels of approximation that can be achieved by iterating the process only a small number of times.
    Discrete and Continuous Dynamical Systems 01/2010; · 1.01 Impact Factor
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    Helmut Schmidt, Stephen Coombes
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    ABSTRACT: or> Meeting abstracts - A single PDF containing all abstracts in this Supplement is available
    BMC Neuroscience 01/2009; · 3.00 Impact Factor

Publication Stats

14 Citations
8.01 Total Impact Points

Institutions

  • 2012
    • Massey University
      • Institute of Information and Mathematical Sciences
      Palmerston North City, Manawatu-Wanganui, New Zealand
  • 2009–2010
    • University of Nottingham
      • School of Mathematical Sciences
      Nottigham, England, United Kingdom