Looking at Dots

Journal of Physics: Conference Series, 2011

We study the superposition of a non-Poisson renewal process with the presence of a superimposed Poisson noise. The non-Poisson renewals mark the passage between meta-stable states in system with self-organization. We propose methods to measure the amount of information due to the two independent processes independently, and we see that a superficial study based on the survival probabilities yield stretched-exponential relaxations. Our method is in fact able to unravel the inverse-power law relaxation of the isolated non-Poisson processes, even when noise is present. We provide examples of this behavior in system of diverse nature, from blinking nano-crystals to weak turbulence. Finally we focus our discussion on events extracted from human electroencephalograms, and we discuss their connection with emerging properties of integrated neural dynamics, i.e. consciousness.

Journal of vision, 2017

We report a novel phenomenon in which long sequences of random dot arrays refreshing at 2.5 Hz lead to persistent illusory percepts of coherent apparent motion. We term this effect illusory apparent motion (IAM). To quantify this illusion, we devised a persistence task in which observers are primed with a particular motion pattern and must indicate when the motion pattern ends. In Experiment 1 (N = 119), we induced translational apparent motion patterns and show that both drifting motion (e.g., up-up-up-up) and rebounding motion (e.g., up-down-up-down) persists throughout many frames of uncorrelated random dots, although rebounding motion tends to persist for longer (a rebounding bias). In Experiment 2 (N = 60), we induced rotational IAM on an annulus-shaped display, and show that the topology of the display (whether the annulus is complete or has a gap) determines whether or not the rebounding bias is present. Based on our findings, we argue that IAM provides a powerful tool to stu...

Vision Research, 1995

Perception of depth difference between two dots is more difficult if additional dots intervene between them. By varying the onset asynchrony (SOA) between the endpoints and one or several intervening dots, we measured the time--course of the process that elevates stereoscopic thresholds. It turned out that adding the intervening dots under these conditions decreased performance to, and often below, the level that was achieved with a total presentation time corresponding to the SOA for intervening dots presented both binocularly and monocularly. This is an indication for an active inhibitory process.

American Journal of Physics, 2012

In 1855, Lord Kelvin's brother, James Thomson, wrote a paper describing “certain curious motions” on liquid surfaces. In the present paper, we describe several curious motions produced in the simplest possible manner: by introducing a droplet of food coloring into a shallow dish of water. These motions include the spontaneous formation of labyrinthine stripes, the periodic pulsation leading to chaotic stretching and folding, and the formation of migrating slugs of coloring. We use this simple experiment to demonstrate that the ...

A quantum leap in solid state physics was the understanding that macroscopic objects can be described using the symmetries of the Hamiltonian. For instance, if translational symmetry is present, the atomic structure of the macroscopic objects is described by the solution of a unit cell. Disorder, for example a random potential landscape, breaks translational symmetry. Other symmetries are untouched by disorder, such as orthogonal or unitary transformations of the Hamiltonian. Physicists in the seventies and eighties of the last century realized that phase transitions even in disordered solids can be described in a scaling ansatz. This scaling is due to the underlying symmetry of the Hamiltonian and is therefore universal. One type of phase transition in disordered solids is the disorder-induced metal-insulator transition. A metallic solid is transformed to an insulator by an increase of disorder. Therefore the initially extended electronic wave function localizes. Disorder also has an impact on classical systems, for example, reaction-diffusion systems. Especially interesting is the effect of disorder and fluctuations in systems where ordering mechanisms occur, as in the case of phase separation of particles. In the last years, trend words as self-assembly, self-organization, and pattern formation were used to describe different kinds of ordering reactions. These are investigated driven by the motivation to design devices using reaction-diffusion dynamics. The Liesegang pattern formation is a prominent example, due to the simple descriptions of pattern distances and widths. Commonly, Liesegang patterns are described by mean-field approximations. In such scenarios, the concentrations of particles are approximated as smooth functions and the reaction is described by differential equations. In nature, fluctuations of the particle concentrations are present due to disorders such as defects, impurities etc. An important progress achieved in the last decade is the understanding that fluctuations do not necessarily destroy pattern formation. In contrary, they are able to enhance or induce pattern formation and order. The publications presented in Part III are the core of this thesis. In Part I the general context of these publications is introduced, while in Part II they are summarized and discussed in relation with each other. Part I is divided into three chapters. In Chapter 1 different examples of compositionally and structural-topologically disordered systems are described, starting with percolation. Realizations of the tight-binding approximation for different types of disorders are introduced, for instance, the Anderson model, a binary alloy system, quantum percolation, and the random phase model. Gels and glasses are mesoscopic examples of structurally disordered systems. Their properties, especially their local structures, are described. At last, the general properties of complex networks are introduced with emphasis on the description of clustering. In Chapter 2 the transport dynamics of particles in disordered systems are discussed. At first the tight-binding model for electron transport in disordered media is derived. Results for applications are presented, for example, for quantum percolation, and for the Anderson model with and without magnetic field. Furthermore it is shown that the tight-binding model is not only valid for quantum wave transport in disordered media, but also for classical waves. Finally reactiondiffusion dynamics are derived and discussed with focus on the influence of disorder. Chapter 3 is devoted to the understanding of Liesegang pattern formation and the localization of electronic wave functions in disordered media. In the context of Liesegang pattern formation vii four important laws are introduced and mean-field predictions are discussed. The section on localization deals with a scaling ansatz to describe the metal-insulator transition. Additionally, a common statistical method is introduced called level spacing statistic. Eventually, numerical and experimental studies dealing with the critical parameters of the metal-insulator transition are discussed. In Part II the publication presented in Part III are summarized and discussed. Chapters 4 to 6 deal with investigations of Liesegang pattern formation in disordered materials, whereas the aim of Chapters 7 to 9 is to understand the affect of a small magnetic field and of topologicalstructural disorder for metal-insulator phase transitions. Chapter 4 is a detailed review of different models on Liesegang pattern formation. In addition, a lattice-gas model to simulate Liesegang patterns is introduced. Different methods to analyze simulation and experimental results are presented. In Chapter 5 the lattice-gas model is used to investigate how disorder and fluctuations effect the pattern formation process. Additionally, effective mean-field descriptions incorporating disorder and fluctuations are derived. In Chapter 6 these results are applied for designing equidistant and more complex patterns. Chapter 7 is the first publication dealing with calculations on localization effects. A prediction for the shift of the metal-insulator transition induced by a small magnetic field is tested in extensive numerical calculation and confirmed. Finally, the topic of Chapters 8 and 9 is the influence of topological disorder on metal-insulator and optical localization transitions. Topological disorder is introduced, using different types of networks and variations of local topological structures in complex networks. At last, the bibliography and the acknowledgments complete this thesis.

Psychonomic Bulletin & Review, 1994

demonstrated that there are five types of periodic dot patterns (or lattices) : oblique, rectangular, centered rectangular, square, sod hexagonal. Gestalt psychologists studied grouping by proximity in rectangular and square dot patterns . In the first part of tie present paper, I (1) describe the geometry of the five types of lattices, and (2) explain why, for the study of perception, centered rectangular lattices must be divided into two classes (centered rectangular and rhombic) . I also show how all lattices can be located in a two-dimensional space . In the second part of the paper, I show how the geometry of these lattices determines their grouping and their multistability . I introduce the notion of degree of instability and explain how to order lattices from most stable to least stable (hexagonal). In the third part of the paper, I explore the effect of replacing the dote in a lattice with less symmetric motifs, thus creating wallpaper pat-terns_ When a dot pattern is turned into a wallpaper pattern, its perceptual organization can be altered radically, overcoming grouping by proximity . I conclude the paper with an introduction to the implications of motif selection and placement for the perception of the ensuing patterns .

Vision Research

We estimated the sensitivity for detecting a row of collinear target elements (usually dots) by measuring the maximum density of randomly positioned noise elements that allowed 75% correct detection of the orientation of alignment (binary choice: horizontal versus vertical) of the target elements. We varied the number of target elements, their mode of generation, and their accuracy of positioning. As reported previously (Moulden (1994) Higher-order processing in the visual system. Ciba Foundation Symposium 184. Chichester: Wiley), target detection improved rapidly until the number of target elements reached about seven, and then improved more slowly beyond this point. However, this break was reduced (and often removed entirely) when the target array was formed by repositioning pre-existing noise elements lying close to the target location, rather than by superimposition of additional target elements onto the noise array. This almost linear slope of improvement, coupled with the obse...

2008

Vision Research, 2000

Phenomenal transparency in random-dot kinematograms is abolished when two motion directions are 'locally-balanced' by pairing limited-lifetime dots at each location . Journal of Neuroscience, 14, 7357 -7366]. Qian et al. also report that locally-paired stimuli appear as directionless flicker when the paired dots differ in their directions by 90°or more. They attribute this to local inhibition between motion detectors more than 45°apart. We investigated perceived motion in such displays, by requiring subjects to make direction and speed judgements with locally-paired stimuli containing two directions 60, 90 or 120°apart. Subjects perceived coherent motion in these displays and made reliable direction judgements, indicating that the two motions are combined rather than interfering destructively. Our results show that the judged motion of locally-paired stimuli is in the vector-average direction of the two components. This vector-averaging rule also applies when the two sets of component dots differ in their velocity. Similarly, speed judgements comply with a vector-averaging rule for a range of speeds as well as for mixed-speed stimuli. These results suggest that the abolition of transparency does not necessarily imply abolition of a global motion percept. The local interaction abolishing transparency is not exclusively inhibitory, at least for directions up to 120°apart, but generates a vector combination of the superimposed motions.