Biomedical subjects

Antonio Palacios

Publications and source records attributed to Antonio Palacios.

14 recordsLinked to original sources

PubMed2006

Noise-induced intermittent cellular patterns on circular domains.

We study the effects of thermal noise in a stochastic Langevin formulation of a typical example of a pattern-forming system with two-dimensional circular domain. A greater tendency towards dynamic cellular states is observed when the pattern-forming system is subjected to noise, which seems to explain the prevailing behavior of related laboratory experiments. We also report on two-dimensional numerical observations of certain dynamic states, homoclinic intermittent states, which until now, had only been observed in laboratory experiments.

Journal Article↗

PubMed2006

Cooperative dynamics in coupled noisy dynamical systems near a critical point: The dc superconducting quantum interference device as a case study.

Dynamical systems that operate near the onset of coupling-induced oscillations can exhibit enhanced sensitivity to external perturbations under suitable operating parameters. This cooperative behavior and the attendant enhancement in the system response (quantified here via a signal-to-noise ratio at the fundamental of the coupling-induced oscillation frequency) are investigated in this work. As a prototype, we study an array of dc superconducting quantum interference device (SQUID) rings locally coupled, unidirectionally as well as bidirectionally, in a ring configuration; it is well known that each individual SQUID can be biased through a saddle-node bifurcation to oscillatory behavior. We show that biasing the array near the bifurcation point of coupling-induced oscillations can lead to a significant performance enhancement.

Journal Article↗

PubMed2006

Complex behavior in driven unidirectionally coupled overdamped Duffing elements.

It is well known that overdamped unforced dynamical systems do not oscillate. However, well-designed coupling schemes, together with the appropriate choice of initial conditions, can induce oscillations (corresponding to transitions between the stable steady states of each nonlinear element) when a control parameter exceeds a threshold value. In recent publications [A. Bulsara, Phys. Rev. E 70, 036103 (2004); V. In, ibid. 72, 045104 (2005)], we demonstrated this behavior in a specific prototype system, a soft-potential mean-field description of the dynamics in a hysteretic "single-domain" ferromagnetic sample. These oscillations are now finding utility in the detection of very weak "target" magnetic signals, via their effect on the oscillation characteristics--e.g., the frequency and asymmetry of the oscillation wave forms. We explore the underlying dynamics of a related system, coupled bistable "standard quartic" dynamic elements; the system shows similarities to, but also significant differences from, our earlier work. dc as well as time-periodic target signals are considered; the latter are shown to induce complex oscillatory behavior in different regimes of the parameter space. In turn, this behavior can be harnessed to quantify the target signal.

Journal Article↗

PubMed2005

Complex dynamics in unidirectionally coupled overdamped bistable systems subject to a time-periodic external signal.

Recently, we have studied the emergence of oscillatory behavior in overdamped undriven nonlinear dynamic systems subject to carefully crafted coupling schemes and operating conditions [V. In, Phys. Rev. E 68, 045102(R) (2003).] The theoretical ideas have been validated in an experimental setup of N = 3 coupled ferromagnetic cores subject to a dc external magnetic "target" signal; the oscillations (corresponding to the periodic switching of each core between its stable steady states of magnetization) are triggered when the coupling constant crosses a threshold value, with the oscillation frequency exhibiting a characteristic scaling behavior with the "separation" of the coupling constant from its threshold value, as well as with the external signal amplitude. Here, we consider the system response to a time-periodic signal. We demonstrate experimentally that, depending on the signal amplitude and frequency, the response can be either synchronized to the signal frequency or to one-third this frequency. These phenomena afford unique techniques for time-periodic signal detection and characterization for a large class of sensors.

Journal Article↗

PubMed2005

Stable second-order scheme for integrating the Kuramoto-Sivanshinsky equation in polar coordinates using distributed approximating functionals.

We present an algorithm for the time integration of nonlinear partial differential equations. The algorithm uses distributed approximating functionals, which are based on an analytic approximation method, in order to achieve highly accurate spatial derivatives. The time integration is based on a second-order unconditionally A -stable Crank-Nicolson scheme with a Newton solver. We apply the integration scheme to the Kuramoto-Sivanshinsky equation in polar coordinates, which presents a significant computational challenge due to the stiffness introduced by the estimation of the spatial derivatives at the origin. We present several stationary and nonstationary solutions of the Kuramoto-Sivanshinsky equation and compare with previous numerical results as well as patterns observed in the combustion front of a circular burner. The numerical results of the proposed scheme reproduces several patterns--rotating two-cell, three-cell, hopping three-cell, stationary two-three-four- and five-cell, stationary 5/1,6/1,7/1,8/2 two-ring patterns, etc.--observed in physical experiments. The scheme is extremely robust and can produce long-term simulations consisting of several thousand frames. Although applied to a very specific problem, the approach of combining the framework of distributed approximating functionals with a Crank-Nicolson based time integration is generalizable to a large class of problems.

Journal Article↗

PubMed2005

Multifrequency synthesis using two coupled nonlinear oscillator arrays.

We illustrate a scheme that exploits the theory of symmetry-breaking bifurcations for generating a spatio-temporal pattern in which one of two interconnected arrays, each with N Van der Pol oscillators, oscillates at N times the frequency of the other. A bifurcation analysis demonstrates that this type of frequency generation cannot be realized without the mutual interaction between the two arrays. It is also demonstrated that the mechanism for generating these frequencies between the two arrays is different from that of a master-slave interaction, a synchronization effect, or that of subharmonic and ultraharmonic solutions generated by forced systems. This kind of frequency generation scheme can find applications in the developed field of nonlinear antenna and radar systems.

Journal Article↗

PubMed2005

Hopping behavior in the Kuramoto-Sivashinsky equation.

We report the first observations of numerical "hopping" cellular flame patterns found in computer simulations of the Kuramoto-Sivashinsky equation. Hopping states are characterized by nonuniform rotations of a ring of cells, in which individual cells make abrupt changes in their angular positions while they rotate around the ring. Until now, these states have been observed only in experiments but not in truly two-dimensional computer simulations. A modal decomposition analysis of the simulated patterns, via the proper orthogonal decomposition, reveals spatio-temporal behavior in which the overall temporal dynamics is similar to that of equivalent experimental states but the spatial dynamics exhibits a few more features that are not seen in the experiments. Similarities in the temporal behavior and subtle differences in the spatial dynamics between numerical hopping states and their experimental counterparts are discussed in more detail.

Computer Simulation↗

PubMed2004

Emergent oscillations in unidirectionally coupled overdamped bistable systems.

It is well known that overdamped unforced dynamical systems do not oscillate. However, well-designed coupling schemes, together with the appropriate choice of initial conditions, can induce oscillations when a control parameter exceeds a threshold value. In a recent publication [Phys. Rev. E 68, 045102 (2003)]], we demonstrated this behavior in a specific prototype system, a soft-potential mean-field description of the dynamics in a hysteretic "single-domain" ferromagnetic sample. The previous analysis of this work showed that N (odd) unidirectionally coupled elements with cyclic boundary conditions would, in fact, oscillate when a control parameter-in this case the coupling strength-exceeded a critical value. These oscillations are now finding utility in the detection of very weak "target" signals, via their effect on the oscillation characteristics, e.g., the frequency and asymmetry of the oscillation wave forms. In this paper we explore the underlying dynamics of this system. Scaling laws that govern the oscillation frequency in the vicinity of the critical point, as well as the zero-crossing intervals in the presence of a symmetry-breaking target dc signal, are derived; these quantities are germane to signal detection and analysis.

Journal Article↗

PubMed2004

Experimental analysis and visualization of spatiotemporal patterns in spouted fluidized beds.

A numerical characterization based on experimental data of the spouting regime in a two-dimensional fluidized bed is presented. The aspect ratio of the bed allowed for good visualization of the spouting and solids circulation as the spouting jet gas velocity was varied to highlight the visited bifurcation sequence. Digital video sequences were recorded and then preprocessed for numerical analysis. In this paper, the proper orthogonal decomposition (POD) was applied to these data sets in order to identify and separate the dominant spatial features from the temporal evolution of the spouting dynamics. The results indicate that the overall spatiotemporal dynamics can be captured by a few POD eigenfunctions, and that the POD amplitudes can be used to distinguish between varying degrees of spouting.

Journal Article↗

PubMed2004

Task- and location-switching effects on visual attention.

In two experiments, we examined the effects of task and location switching on the accuracy of reporting target characters in an attentional blink (AB) paradigm. Single-character streams were presented at a rate of 100 msec per character in Experiment 1, and successive pairs of characters on either side of fixation were presented in Experiment 2. On each trial, two targets appeared that were either white letters or black digits embedded in a stream of black letter distractors, and they were separated by between zero and five items in the stream (lags 1-6). Experiment 1 showed that report of the first target was least accurate if it immediately preceded the second target and if the two targets were either both letters or both digits (task repetition cost). Report of the second target was least accurate if one or two distractors intervened between the two targets (the U-shaped AB lag effect) and if one target was a letter and the other a digit (task switch cost). Experiment 2 added location uncertainty as a factor and showed similar effects as Experiment 1, with one exception. Lag 1 sparing (the preserved accuracy in reporting the second of two targets if the second immediately follows the first) was completely eliminated when the task required attention switching across locations. Two-way additive effects were found between task switching and location switching in the AB paradigm. These results suggests separate loci for their attentional effects. It is likely that the AB deficit is due mainly to central memory limitations, whereas location-switching costs occur at early visual levels. Task-switching costs occur at an intermediate visual level, since the present task switch involved encoding differences without changes in stimulus-response mapping rules (i.e., the task was character identification for both letters and digits).

Adult↗

PubMed2003

Experimental observation of multifrequency patterns in arrays of coupled nonlinear oscillators.

Frequency-related oscillations in coupled oscillator systems, in which one or more oscillators oscillate at different frequencies than the other oscillators, have been studied using group theoretical methods by Armbruster and Chossat [Phys. Lett. A 254, 269 (1999)] and more recently by Golubitsky and Stewart [in Geometry, Mechanics, and Dynamics, edited by P. Newton, P. Holmes, and A. Weinstein (Springer, New York, 2002), p. 243]. We demonstrate, experimentally, via electronic circuits, the existence of frequency-related oscillations in a network of two arrays of N oscillators, per array, coupled to one another. Under certain conditions, one of the arrays can be induced to oscillate at N times the frequency of the other array. This type of behavior is different from the one observed in a driven system because it is dictated mainly by the symmetry of the coupled system.

Journal Article↗

PubMed2003

Coupling-induced oscillations in overdamped bistable systems.

It is well known that overdamped and unforced dynamical systems do not oscillate. However, well-designed coupling schemes, together with the appropriate choice of initial conditions, can induce oscillations when a control parameter exceeds a threshold value. We demonstrate this effect in a specific system, a soft-potential mean-field description of the dynamics in a (hysteretic) single-domain ferromagnetic sample. Using a specific (unidirectional, with cyclic boundary conditions) coupling scheme, together with nonidentical initial conditions, one can cause the coupled system of N elements (N odd) to oscillate when the coupling coefficient is swept through a critical value. The ensuing oscillations could find utility in the detection of very weak "target" signals, via their effect on the oscillation characteristics.

Journal Article↗

PubMed1999

Modal decomposition of hopping states in cellular flames.

We use Karhunen-Loeve (KL) decomposition of video images from an experiment to analyze a spatiotemporal dynamic state, unique to cellular flames, referred to as a "hopping state." Ordered states of cellular flames on a circular burner consist of one or two concentric rings of luminous cells. The hopping states correspond to the motions of individual cells in a ring sequentially executing abrupt changes in their angular position, while the other cells in the ring remain symmetric and at rest. KL decomposition separates the spatial and temporal characteristics of the hopping motion. The underlying symmetries of the experiment allow us to deduce a set of normal form equations that describe the formation of these states. We find that they result from secondary bifurcations connecting two primary branches of traveling waves. The solutions corresponding to hopping states exist as mixed-mode solutions away from the secondary bifurcations. (c) 1999 American Institute of Physics.

Journal Article↗

PubMed1997

Cellular pattern formation in circular domains.

An analysis of stationary and nonstationary cellular patterns observed in premixed flames on a circular, porous plug burner is presented. A phenomenological model is introduced, that exhibits patterns similar to the experimental states. The primary modes of the model are combinations of Fourier-Bessel functions, whose radial parts have neighboring zeros. This observation explains several features of patterns, such as the existence of concentric rings of cells and the weak coupling between rings. Properties of rotating rings of cells, including the existence of modulated rotations and heteroclinic cycles can be deduced using mode coupling. For nonstationary patterns, the modal decomposition of experimental data can be carried out using the Karhunen-Loeve (KL) analysis. Experimental states are used to demonstrate the possibility of using KL analysis to differentiate between uniform and nonuniform rotations. The methodology can be extended to study more complicated nonstationary patterns. In particular, it is shown how the complexity of "hopping states" can be unraveled through the analysis. (c) 1997 American Institute of Physics.

Journal Article↗