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May 2, 2016

Structure and Stability in Canonical Cortical Computations

Date: Thursday, May 5th, 2016

Speaker:  Yashar Ahmadian,UO, Institute of Neuroscience, Departments of Biology and Mathematics

Abstract:

The cerebral cortex, or the gray matter, is evolutionarily the newest part of the brain, and underlies most of our intelligent behavior. After an introduction to biological neural networks and theoretical approaches to studying their dynamics, I will present my work on the dynamics of local neural circuits in the cortex. The vast majority of cortical neurons are of the excitatory type and they are highly interconnected: a typical neuron receives thousands of excitatory inputs and in turn excites other neurons. How does such a network prevent runaway activity despite this strong positive feedback? Single-neuronal biology provides one stabilizing mechanism: neurons cannot activate at indefinitely high rates for biophysical reasons.  But this still leaves open the following question: can cortical networks self-organize into a stable state with moderate activity, without relying on single-neuronal saturation? I will show that fast feedback from the minority of inhibitory neurons is generically sufficient to dynamically stabilize cortical networks even when single-neuronal nonlinearities are of the expansive, non-saturating type.
I will then explore the computational consequences of this collective inhibitory stabilization, and show that it accounts for a wide range of “contextual modulation” effects (modulations of responses of neurons to their preferred stimuli by contextual stimuli). Contextual modulation is a ubiquitous and canonical brain computation, and is an early manifestation of the global integration of sensory information that underlies higher level perception and object recognition.
I will also discuss some of the transient and time-dependent properties of such networks, and how our theory accounts for modulations of cortical oscillations and correlated ‘noise’ in the visual cortex. Time allowing, I will briefly mention my work using random matrix theory and large-N expansions to study the interplay of disordered and structured network connectivity in the dynamics of neural networks.

Host: Raghu Parthasarathy

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Colloquium Archive

April 26, 2016

Tools for Ultrafast Spectroscopy

Date: Thursday, April 28th, 2016

Speaker:  Dr. Matthew Kelley, Senior Scientist Newport Corporation

Abstract:

In this talk I will introduce Newport’s Technology and Application Center and touch briefly on most of our products and activities. I will spend most of the time discussing two products that I am responsible for: Transient Absorption Spectroscopy (TAS) and Time-Resolved Fluorescence Spectroscopy (TRFLS). I will introduce each spectroscopy and give background on the operation and instruments that go into each product. Then I will discuss the advantage of each type of spectroscopy and cover some representative applications.

Host: Bryan Boggs

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Colloquium Archive

April 15, 2016

Impact Response of Granular Materials: From the Origin of the Universe to Catastrophic Asteroid Strikes

Date: Thursday, April 21st, 2016

Speaker: Xiang Cheng, University of Minnesota

Abstract:

Granular materials are large conglomerations of discrete macroscopic particles. Examples include seeds, sand, coals, powder of pharmacy, etc. Though simple, they show unique properties different from other familiar forms of matter. The unusual behaviors of granular materials are clearly illustrated in various impact processes, where the impact-induced fast deformation of granular materials leads to emergent flow patterns revealing distinctive granular physics. Here, we explored the impact response of granular materials in two specific experiments:
First, we investigated impact cratering in granular media induced by the strike of liquid drops—a ubiquitous phenomenon relevant to many important environmental, agricultural and industrial processes. Surprisingly, we found that granular impact cratering by liquid drops follows the same energy scaling and reproduces the same crater morphology as that of asteroid impact craters. Inspired by this similarity, we develop a simple model that quantitatively describes various features of liquid-drop imprints in granular media. Our study sheds light on the mechanisms governing raindrop impacts on granular surfaces and reveals an interesting analogy between familiar phenomena of raining and catastrophic asteroid strikes.
Second, we performed the granular analog to “water bell” experiments. When a wide jet of granular material impacts on a fixed cylindrical target, it deforms into a sharply-defined sheet or cone with a shape mimicking a liquid of zero surface tension. The jets’ particulate nature appears when the number of particles in the beam cross-section is decreased: the emerging structures broaden, gradually disintegrating into diffuse sprays. The experiment reveals a universal fluid structure arising from the collision of discrete particles, which has a counterpart in the behavior of quark-gluon plasmas created by colliding heavy ions at the Relativistic Heavy Ion Colliders.

Host: Eric Corwin

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Colloquium Archive

 

April 11, 2016

Observation of Gravitational Waves from a Binary Black Hole Merger

Date: Thursday, April 14th, 2016

Speaker: Raymond Frey, University of Oregon

Abstract:

On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) simultaneously observed a transient gravitational-wave signal due to the inspiral and merger of a binary black hole system at a distance of more than one billion light-years. The signal was very clearly detected, allowing the LIGO collaboration to claim the first direct detection of gravitational waves, the ripples in spacetime predicted 100 years ago by Einstein.

This was also the first indication that black holes in this range of mass — about 30 times the mass of the sun – exist in nature. In this talk, I will discuss how the detection was made, the tests of Einstein’s theory of general relativity, the implications for astrophysics, and the future of gravitational-wave astronomy.

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Colloquium Archive

 

April 4, 2016

Communication Shapes Collective Information Encoding of Chemosensing

Date: Thursday, April 7th, 2016

Speaker: Bo Sun, Oregon State University

Abstract:

Chemosensing is a major task assigned to a cell and is critical to its survival, differentiation, and other functions such as motility. Since the seminal work of Howard Berg and Edward Purcell, physicists have been trying to crack the code of cell chemosensing. While most of studies have been focusing on single cell level chemosensing and response, we have initiated projects to understand how a group of communicating cells encode information differently. In particular, we study the calcium dynamics of fibroblast cells in response to extracellular ATP, a scenario resembles the processes during inflammation and wound healing. We find there are two collective coding mechanisms of a monolayer of fibroblast cells: correlations and inter-spike-interval. We have further characterized the robustness of each coding mechanism with respect to attack by cancer cells.

Host: Eric Corwin

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Colloquium Archive

March 29, 2016

Biologically Inspired Soft Matter Devices in Robotics

Date: Thursday, March 31st, 2016

Speaker: Yigit Menguc, Oregon State University

Abstract:

Incredible biological mechanisms have emerged through evolution, and can provide a wellspring of inspiration for engineers. One promising area emerging from biological inspiration is the design of devices and robots made of compliant materials, as part of a larger field of research in “soft robotics.” In this talk, the topics of designing soft, biologically inspired mechanisms will be presented in two case studies: controllable adhesives and soft wearable sensors. Additionally, the talk will cover the methods of fabricating soft devices through 3D printing, soft lithography, and laser micromachining. Surfaces covered in arrays of micro-fibers, inspired by the toes of a gecko, rely on compliance to repeatedly and controllably adhere to almost any surface while simultaneously shedding dirt. Sensors made of liquid metal encapsulated in rubber as soft as skin can track motion of the human body while naturally moving with its kinematics. However, these exciting soft mechanisms have certain challenges. The biological mechanisms that serve as a source of inspirations are made of materials that are vastly more compliant than the metal and plastic that engineers and roboticists normally use. To imitate and improve on nature’s design, we must create mechanisms with materials like fabric and rubber. It is difficult to characterize these hyperelastic, viscoelastic, and generally nonlinear materials, and it is difficult to integrate them into traditional fabrication techniques, but the development of such soft robotic devices promises to bring robots more and more into our daily lives.

Host: Eric Corwin

Time: 4:00-5:00pm
Location: 100 Willamette Hall

 

Colloquium Archive

 

March 18, 2016

Topology and Localization with Long-range Interactions

Special Physics/ITS Colloquium, Tuesday, March 29th, 2016

Speaker: Chris Laumann, University of Washington

Abstract:

The trapping of ultracold atomic gases has opened new windows into the quantum phases of neutral particles. A remarkable amount of pretty physics – Bose condensation, Cooper paired superfluidity and Mott localization – has been observed. Even more remarkably this has been done with interactions, which are typically short-ranged and weak.

Recent progress in the control of long-range interacting systems — polar molecules, Rydberg atoms and trapped ions — allows a completely different piece of physics to enter the game. In this talk, I will review the general setting and then give two examples of how long-range interactions can lead to new physics inaccessible in existing systems.

First, I will describe the adiabatic preparation of topologically ordered phases in driven, dipolar-interacting spin systems. Second, in the presence of strong disorder, I will show that these systems can directly explore physics beyond that of single particle Anderson localization and will comment on recent experiments. Finally, time permitting, I will combine these stories and describe how localization can protect topological order far out-of-equilibrium.

Time: 4:00-5:00pm
Location: 331 Klamath Hall

Host: Dietrich Belitz

Colloquium Archive

 

March 4, 2016

Searching for Ultra-High Energy Cosmic Rays with Smartphones

Physics Colloquium, Thursday, March 10th, 2016

Speaker: Daniel Whiteson, University of California, Irvine

Abstract:

We propose a novel approach for observing cosmic rays at ultra-high energy (10^18 eV) by repurposing the existing network of smartphones as a ground detector array. Extensive air showers generated by cosmic rays produce muons and high-energy photons, which can be detected by the CMOS sensors of smartphone cameras. The small size and low efficiency of each sensor is compensated by the large number of active phones. We show that if user adoption targets are met, such a network will have significant observing power at the highest energies.

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Host: Michael Raymer

Colloquium Archive

 

February 26, 2016

Universality in Soft Active Matter

Physics Colloquium, Thursday, March 3rd, 2016

Speaker: Chiu Fan Lee, Imperial College, London

Abstract:

Biology systems operate in the far from equilibrium regime and one defining feature of living organisms is their motility. In the hydrodynamic limit, a system of motile organisms may be viewed as a form of active matter, which has been shown to exhibit behaviour analogous to that found in equilibrium systems, such as phase separation in the case of motility-induced aggregation, and critical phase transition in incompressible active fluids. In this talk, I will use the concept of universality to categorise some of the emergent behaviour observed in active matter. Specifically, I will show that i) the coarsening kinetics of motility-induced phase separation belongs to the Lifshitz-Slyozov-Wagner universality class [1]; ii) the order-disorder phase transition in incompressible polar active fluids (IPAF) constitutes a novel universality class [2], and iii)  the behaviour of IPAF in the ordered phase in 2D belongs to the Kardar-Parisi-Zhang universality class [3].

References:

[1]          C. F. Lee, “Interface stability, interface fluctuations, and the Gibbs-Thomson relation in motility-induced phase separations,” arXiv: 1503.08674, 2015.

[2]          L. Chen, J. Toner, and C. F. Lee, “Critical phenomenon of the order-disorder transition in incompressible active fluids,” New Journal of Phyics, 17, 042002, 2015.

[3]          L. Chen, C. F. Lee, and J. Toner, “Birds, magnets, soap, and sandblasting: surprising connections to incompressible polar active fluids in 2D,” arXiv:1601.01924, 2016.

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Host: Dietrich Belitz

Colloquium Archive

 

 

February 19, 2016

Complexity of Multi-photon Interferometry

Physics Colloquium, Thursday, February 25th, 2016

Speaker: Hubert de Guise, Lakehead University

Abstract:

The simulation of many fully indistinguishable photons interfering at the output of a random interferometer with many more channels than photons supplies a link between computational complexity theory and linear optics.  This link is the basic ingredient of the BosonSampling problem, an idea originating with Aaronson and Arkhipov in 2011.

This colloquium will discuss some simple aspects of multi-photon interferometry, with emphasis on partial distinguishability of photons and its consequences on and connections with computationally complex functions of the scattering matrix.

Some work reported in this colloquium has been done in collaboration with colleagues from Calgary, Vienna and Singapore, and others.

Time: 4:00-5:00pm
Location: 100 Willamette Hall

Host: Michael Raymer

Colloquium Archive

 

 

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