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Previous CIRCS seminars (2003-2002) |
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Previous Seminars
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October 27, 1998 ABSTRACT: Biologic systems are remarkable for their structural variability and dynamical complexity. One major challenge in biomedical research is to quantitatively describe such complex variability. Recently, the concept of fractal (scale-free) growth and form has suggested novel approaches to quantify and understand morphogenesis and function of nonlinear biologic systems. Basic concepts of fractal and related long-memory processes are very useful for describing important aspects of integrated physiologic functions. Three examples -- heartbeat, respiration, and human walking dynamics -- will be discussed to illustrate how long-range correlations are a common feature in healthy dynamics. I will also present some recent results of applying these quantitative measures for prognostic/diagnostic purposes, such as monitoring patients at high risk of sudden cardiac death. |
October 20, 1998 ABSTRACT: Many neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease, scrapie, mad cow disease,and Huntington's disease are characterized by the presence of ordered protein aggregates, of different composition, in the affected regions of the brain. Genetic studies suggest that aggregation may play a causitive role in all of these diseases. Therefore, it is important to understand its molecular mechanism in order to conceive new therapies. This talk will focus on studies done in the Lansbury laboratory concerning the structure and mechanism of ordered protein aggregation in AD and, more recently PD. |
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Tuesday, May 5, 1998 ABSTRACT: We study thermal ratchets, i.e. particles moving in asymmetric periodic potentials, using a generalized Langevin equation. This scheme allows for a clear distinction of thermal noise, whether "#000000" or "colored", and time-dependent external fields, determinitic or stochastic. We show that a net current arises only if the forcing is done by an external field; that is, thermal fluctuations cannot produce a drift by themselves. Hence, the only necessary condition for rectifying an external field, producing a current, is the asymmetry of the ratchet potential. The use of the generalized Langevin equation gives access to a wide variation of the relevant time scales; we find current inversions for external fields correlated in time scales shorter than the thermal noise. |
Tuesday, May 12, 1998 This talk was rescheduled from its original date of April 21. "Simple physical models of self-organization in cell biology" by Prof. Jorge V. José Director, CIRCS ABSTRACT: In recent years a good number of physicists have been attracted by the present growth of interesting biological problems. The general feeling is that there are a number of outstanding questions where the general training of physicists as problem solvers can make relevant contributions to biology. There are examples of physicists that have in fact achieved success, but in many cases they did so by becoming more biologists than traditional physicists. My collaborators and I recently have also been attracted by the lure of biological problems. Along the way we have learned a few things about the differences between research in the two fields and their communities. We have chosen to specifically look at problems of self-organization in cell biology, and neurobiology, that have some aspects similar but specifically different from what one does in nonequilibrium statistical physics. In my talk I will briefly describe results obtained in collaboration with F. Gibbons and J-F Chauwin of model calculations for the microtubule transport in "motility in vitro assays" and on the formation of the mitotic spindle, of importance in cell divisions.The latter was believed for over a hundred years to depend essentially on the presence of chromosomes and centrosomes (I'll define this concepts in my talk.) Recent in vitro experiments (Heald et al. Nature, Vol 382, 420 (1996)) have shown, however, that mitotic spindles can form around DNA-coated micrometer beads, without chromosomes!. Rather than studying that problem first, as physicists often do, we began studying a simpler one, for which there are also experiments. We have been able to in fact fit the experimental results using our simplified model, and later we have been proposed a model of the mitotic spindle problem. As physicists, we thought that successfully fiting the experimental data was very good. For biologists, however, that is not that clear! I will try to make this talk as self-contained as possible for those that are not familiar with the basic biological facts of this problem. I will conclude with a few comments as to the reception by biologists of the few things we have so far done. |
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May 9th, 2000 Title: "Viscous nonlinear dynamics of elastic filaments: twist, kinks, and drag." by Thomas R. Powers, Division of Engineering and Applied Sciences Harvard University ABSTRACT: As D'Arcy Thompson emphasized (1), growth and form in the biological world are governed by simple physical laws. Using ideas from elasticity theory, fluid dynamics, pattern formation, and geometry, I study the shape and motion of a flexible filament rotating in a viscous fluid. Motivated by recent experiments that show large torsional stress in linear DNA during transcription, I first consider the dynamics of a rod with a single kink. I describe how kinks block speedometer-cable motion and trap torsional stress, and give simple scaling laws for shape and stress vs. rotation rate. I argue that the bent rod should exhibit bistability between extended and folded states at high rotation rate, and present experimental results confirming this expectation. Then I turn to the interplay between speedometer-cable and crankshaft motion in naturally straight rods. At a critical rotation rate, there is a Hopf bifurcation from "twirling" motion, in which the centerline of the rod remains straight, to "whirling" motion, in which the centerline crankshafts about a single axis much more slowly then the speedometer-cable motion of each rod element about the local tangent. The connection to #000000F's theorem relating link, twist and writhe for closed ribbons is explained. |
Thursday, May 4th, 2000 NOTE SPECIAL DAY, JOINT COLLOQUIUM/CIRCS SEMINAR Title: "Modeling of the visual cortex." by Dr. Michael J. Shelley, Courant Institute |
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Tuesday, April 28, 1998 ABSTRACT: We will describe the behavior of the dynamic nonlinear response of a quantum spin glass in an exactly solvable fully connected quantum spherical model with random Gaussian bond distribution. In the quantum critical regime where the microscopic energy scale is set entirely by temperature the nonlinear response is found to be frequency independent and nonsingular. On the contrary, the genuine static nonlinear susceptibility diverges everywhere on the critical boundary with unusual violation of the universal scaling by the double logarithms at the zero-temperature critical point. Implications for experiments on quantum dipolar spin glasses are also noted. |
Tuesday, April 21, 1998
"Turing Pattern Formation in the Chlorine
Dioxide-Iodine-Malonic Acid Reaction-Diffusion System" ABSTRACT: The formation of localized
structures in the chlorine dioxide-idodine-malonic acid (CDIMA) reaction-diffusion
system is investigated numerically using a realistic model of this system.
We analyze the one-dimensional patterns formed along the gradients imposed
by boundary feeds, and study their linear stability to symmetry-breaking
perturbations (the Turing instability) in the plane transverse to these
gradients. We establish that an often-invoked local linear analysis
is inappropriate for predicting the linear stability of these patterns.
Using a fully nonuniform analysis, we explore the pattern formation
as a function of two control parameters, and compare with existing experimental
results. Finally, we present numerical solution of the CDIMA system
in two dimensions. The nature of the transition from one-dimensional
non-symmetry breaking front patterns to symmetry-breaking transverse
spots in thin-strip reactors (continuous or discontinuous) is determined.
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Wednesday, April 15, 1998 ABSTRACT: Oscillations are ubiquitous in neural systems and have been the focus of several recent studies. In particular, fast global oscillations (>30Hz) have been reported together with individual neuron recordings showing irregular spike emission at a low rate. We propose a simple model of this partial synchronization phenomenon which allows to determine its main characteristics. |
Tuesday, April 14, 1998 ABSTRACT: Ultrathin films of metals
quench condensed onto cold substrates have been employed in numerous
investigations of fundamental problems in two dimensional electron physics.
These include the phenomena of weak localization, the superconductor
to insulator transition, and disorder induced electron correlation effects.
Despite their importance, very little is known about their structure
or how their structure forms. Technological challenges have impeded
direct measurements of quench condensed film morphology. I will describe
electron tunneling and transport measurements that reveal how the low
temperature electronic properties of quench condensed films can depend
strongly on their structure and, how this dependence can be used to
infer characteristics of that structure. In addition, I will present
in situ STM measurements of quench condensed film morphology that provide
insight into their growth. |
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Tuesday, April 7, 1998 "The Architecture of Life" ABSTRACT: A living organism represents the ultimate complex adaptive system. Our work focuses on the question of how groups of molecules self-organize to create living cells and tissues with emergent properties, such as the ability to change shape, move, and grow. Most complexity-based approaches focus on nodes, connections, and resultant pattern formation. We have extended this approach by taking into account the importance of architecture, mechanics, and structure in the evolution of biological form. This work has led to the discovery of fundamental design principles that guide self-assembly in natural systems, from the simplest inorganic compounds to the most complex living cells and tissues. These building rules are ased on the use of a particular form of geodesic architecture, known as tensegrity, which causes hierarchical collections of different interacting components to self-organize and mechanically stabilize in three dimensions. Shape and pattern stability emerge through establishment of a force balance between globally acting attractive (tensile) forces and locally acting repulsive (compressive) forces or, in simplest terms, through continuous tension and local compression (tensional integrity, or "tensegrity"). Recent development of a mathematical explanation for the mechanical behavior of living cells and tissues based on tensegrity may provide a useful computational tool for analysis in other complex adaptive systems ranging from protein folding to cosmology. |
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Tuesday, January 13
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Tuesday, March 10, 1998
"Simulation of dynamic phases of vortices in the driven XY model: moving solid and plastic flow" by Dr. Daniel Dominguez Centro Atomico Bariloche, Argentina. Back to top |
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