17 research outputs found
Structure and Dynamics of a Phase-Separating Active Colloidal Fluid
We examine a minimal model for an active colloidal fluid in the form of
self-propelled Brownian hard spheres that interact purely through excluded
volume. Despite the absence of an aligning interaction, this system shows the
signature behaviors of an active fluid, including anomalous number fluctuations
and phase separation behavior. Using simulations and analytic modeling, we
quantify the phase diagram and separation kinetics. The dense phase is a unique
material that we call an active hexatic, which exhibits the structural
signatures of a crystalline solid near the crystal-hexatic transition point,
but the rheological and transport properties associated with a viscoelastic
fluid.Comment: 5 pages, 5 figure
EyeRIS: A General-Purpose System for Eye Movement Contingent Display Control
In experimental studies of visual performance, the need often emerges to modify the stimulus according to the eye movements perfonncd by the subject. The methodology of Eye Movement-Contingent Display (EMCD) enables accurate control of the position and motion of the stimulus on the retina. EMCD procedures have been used successfully in many areas of vision science, including studies of visual attention, eye movements, and physiological characterization of neuronal response properties. Unfortunately, the difficulty of real-time programming and the unavailability of flexible and economical systems that can be easily adapted to the diversity of experimental needs and laboratory setups have prevented the widespread use of EMCD control. This paper describes EyeRIS, a general-purpose system for performing EMCD experiments on a Windows computer. Based on a digital signal processor with analog and digital interfaces, this integrated hardware and software system is responsible for sampling and processing oculomotor signals and subject responses and modifying the stimulus displayed on a CRT according to the gaze-contingent procedure specified by the experimenter. EyeRIS is designed to update the stimulus within a delay of 10 ms. To thoroughly evaluate EyeRIS' perforltlancc, this study (a) examines the response of the system in a number of EMCD procedures and computational benchmarking tests, (b) compares the accuracy of implementation of one particular EMCD procedure, retinal stabilization, to that produced by a standard tool used for this task, and (c) examines EyeRIS' performance in one of the many EMCD procedures that cannot be executed by means of any other currently available device.National Institute of Health (EY15732-01
Mutation-Selection Balance: Ancestry, Load, and Maximum Principle
We show how concepts from statistical physics, such as order parameter,
thermodynamic limit, and quantum phase transition, translate into biological
concepts in mutation-selection models for sequence evolution and can be used
there. The article takes a biological point of view within a population
genetics framework, but contains an appendix for physicists, which makes this
correspondence clear. We analyze the equilibrium behavior of deterministic
haploid mutation-selection models. Both the forward and the time-reversed
evolution processes are considered. The stationary state of the latter is
called the ancestral distribution, which turns out as a key for the study of
mutation-selection balance. We find that it determines the sensitivity of the
equilibrium mean fitness to changes in the fitness values and discuss
implications for the evolution of mutational robustness. We further show that
the difference between the ancestral and the population mean fitness, termed
mutational loss, provides a measure for the sensitivity of the equilibrium mean
fitness to changes in the mutation rate. For a class of models in which the
number of mutations in an individual is taken as the trait value, and fitness
is a function of the trait, we use the ancestor formulation to derive a simple
maximum principle, from which the mean and variance of fitness and the trait
may be derived; the results are exact for a number of limiting cases, and
otherwise yield approximations which are accurate for a wide range of
parameters. These results are applied to (error) threshold phenomena caused by
the interplay of selection and mutation. They lead to a clarification of
concepts, as well as criteria for the existence of thresholds.Comment: 54 pages, 15 figures; to appear in Theor. Pop. Biol. 61 or 62 (2002