FIGSS Seminar Winter Semester 2016

The interdisciplinary FIGSS Seminar is held in English and provides an opportunity for students and post-docs of FIAS to report on the status of their research.

Location: FIAS, Ruth-Moufang-Str. 1, 60438 Frankfurt am Main
FIAS Lounge
Time: Monday, 12:30

 

 

17.10.2016

Franziska Matthäus

 

31.10.2016
Kai Ueltzhoeffer

Stochastic Dynamics underlying Cognitive Stability and Flexibility

Cognitive stability and flexibility are core functions in the successful pursuit of behavioral goals. While cognitive stability allows us to follow behavioral plans in an environment full of rich, distracting stimuli, cognitive flexibility enables us to adapt to relevant changes in environmental demands. We developed a simple, fast behavioral experiment that allowed us to quantify both of these abilities in individual subjects in terms of their ability to switch between two different tasks when instructed to (flexibility) and in terms of their ability to ignore salient distracting stimuli (stability).
On a biological level, these functions require the stable maintenance and flexible manipulation of representations encoding the currently relevant behavioral strategies in so-called “working memory”. These representations have to be active, in the sense that they can be read out and modified quickly, to guide and adapt behavior. This is why we developed a model of neural circuits underlying the individual behavior of subjects in this task using attractor network models, where information can be stored in stable states of neural activity. The very same basic architecture cannot only be used to store information in the stable states of the system, but also to convert continuous input signals to categorical decisions by biased transitions between such stable states. This allowed us to create a simple network implementing the working memory maintenance of the currently relevant task rule and the integration of this rule representation with individual stimuli into discrete behavioral decisions. We could fit the individual behavioral statistics of 20 subjects performing our task. Furthermore, to test the biological relevance of our model, we used it to predict the individual time courses of energy consumption of a hypothetical brain network implementing the working memory part of model. Using functional MRI data as a proxy for neural energy consumption, we found a brain network closely matching these predictions, which consisted of two regions that were known to be consistently activated by task switching and inhibition experiments, but whose computational role was still under debate. Furthermore, we could use our model to quantify the individual stability of these working memory representations of the currently relevant task rule. We found that the energy consumption of a well characterized working memory updating network correlated with the individual stability of these representations, hinting at a mechanistic role of stochastic neural dynamics for stable and flexible behavior.

07.11.2016

 

Lukas Klimmasch

Why do we perceive the world in 3-D?

Stereoscopic vision is our visual system’s ability to extract information about the three-dimensional world from two two-dimensional images. The horizontal separation of the two eyes leads to the generation of two slightly different views of the visual scene. The differences between the two images are called binocular disparities. The brain uses these to extract depth information and, therefore, faces the challenge of matching individual features in the left eye’s image with the correct ones in the right eye’s image. In healthy humans, disparities are used to generate movements of both eyes that lead to the fixation of the same object.

We present a biologically plausible model for the autonomous learning and self-calibration of such vergence eye movements. While granting insights into the processes happening in healthy brains, it provides the basis for studying common visual processing and eye disorders like strabismus and amblyopia, as well as possible treatments.

14.11.2016

Max Murakami

What is a neuron?


Neuroscience is one of FIAS' fields of expertise. It's the study of the nervous system, which comprises a multitude of nerve cells, so called neurons. Grasping the basic properties and functions of neurons allows us to appreciate the challenges and advances of current neuroscience.
I will introduce the neuron by reviewing past developments of neuroscience and presenting the basic structure and function of a typical neuron. The talk will mainly target non-neuroscientists.

21.11.2016

Oliver Pfante

Volatility Inference and Return Dependencies in Stochastic Volatility Models

Stochastic volatility models (SVM) are the major tool in academia and finance 
to model daily returns of stocks. In these models returns are normally
distributed with a time-dependent standard deviation also called volatility. SVM 
successfully capture many of the stylised facts observed in financial data: fat tails,
volatility-clustering, and leverage. However, calibrating SVM on financial data has
turned out tricky because only returns of stock prices are directly observable but 
not their variance or volatility, respectively.
Even though calibration has been widely recognised as the main issue in the 
applications of SVM it has been an unsolved problem to quantify precisely the
amount of information returns deliver about their underlying volatility. Our group
tackled this problem in terms of Shannon’s Information theory and revealed a surprising
connection between the difficulty of calibration and a century old problem in 
thermodynamics already stated by Boltzmann: what is the convergence rate of a
perturbed thermodynamical system toward its equilibrium? Mathematics has made 
a tremendous progress in solving this problem during the last fifteen years and proved 
exponentially quick convergence.
It turns out that SVM are inherently hard to calibrate for the same reason non-equlibrium
thermodynamical states are barely observable: they rapidly converge toward their equilibrium.

28.11.2016

Stephan Endres

Chiral symmetry and beyond - what electromagnetic probes from heavy-ion collisions tell about the fundamental properties of matter

The investigation of heavy-ion collisions at accelerator facilities aims for the understanding of the properties of matter under extreme conditions, comparable to those which prevailed in the early universe or which can be found in the dense cores of neutron stars. On the macroscopic scale, goal of the experimental and theoretical studies is to determine the phase diagram of strongly interacting matter, while on the microscopic level one is interested in the individual particle's features, as for example the generation of its mass. In my talk I will show that macroscopic and microscopic in-medium phenomena are tightly connected to specific symmetry properties of nature, their breaking and restoration. I will give a historical and conceptual introduction to symmetries in physics and illustrate the connection to other fields of research. Further I will discuss why electromagnetic observables are good probes for the medium properties (and the according symmetries) governed by the strong interaction.

05.12.2016

LongGang Pang

Identifying QCD transition using deep learning

Supervised learning with a deep convolutional neural network is used to identify the QCD equation of state (EoS) employed in relativistic hydrodynamic simulations of heavy-ion collisions. The final-state particle spectra ρ(pT , Φ) provide directly accessible information from experiments. High-level correlations of ρ(pT , Φ) learned by the neural network act as an “EoS-meter”, effective in detecting the nature of the QCD transition. The EoS-meter is model independent and insensitive to other sim- ulation input. Thus it provides a formidable direct connection of heavy-ion collision observable with the bulk properties of QCD.

12.12.2016

Sven Köppel

Probing star collisions with Exascale computers


Conservation laws written in sets of partial differential equations are
a powerful mathematical tool to describe all kind of phenomena from
nature up to society. Solving such sets of equations numerically is a
long-standing pillar of scientific computing. However, for the next
generation of computers ("Exascale computing") which we expect to see in
around three years, current methods gradually fail. I want to present
modern concepts to tackle this limit. We apply these schemes to the set
of 58 coupled first order differential equations which represent
Einstein‘s theory of General Relativity in a split of "space and time".
We evolve spacetimes of black holes and neutron stars with this method
to study their dynamics and properties by measuring the ejected and
recently experimentally detected gravitational waves.

19.12.2016

Christmas-Special

TBA

16.01.2017

Gabriele Inghirami

Magneto-hydrodynamical numerical simulations of heavy-ion collisions.


In this talk, first I will briefly explain why heavy ion collisions are an important tool to study the physics of strong interaction. Then, I will give a basic introduction to their modeling based on relativistic hydrodynamics and I will explain why taking into account also the effects due to the magnetic fields may provide significant additional insights. Next, an overview of preliminary recent results based on magneto-hydrodynamical numerical simulations of heavy-ion collisions will follow. A quick glimpse of the plans for the future will conclude the seminar.

30.01.2017

Antonia Hufnagl

TBA

06.02.217

Andrej Ilner

In-medium effects of the K* resonance in heavy-ion collisions

 

One of the most interesting questions in physics is how the universe was created. According to the Big Bang Theory, the known universe

was expanded from a very hot and dense state of matter. In order to recreate and to study matter at such extreme conditions one would need to artificially create a medium with a very high energy density.

For decades heavy-ion collisions at such facilities like the LHC at Geneva, RHIC in Brookhaven and in the future also at FAIR in Darmstadt are used to create  a medium of high temperature and energy density where the protons and neutrons split into their subparticles, the quarks and gluons, which under normal conditions cannot be observed.

A new phase of matter is then created which is called the Quark-Gluon-Plasma (QGP). We use the strange K* vector meson resonances, particles that are very short-lived and not seen in normal matter, in order to study the properties of the QGP as well as the dense hadronic medium.

We present our results and discuss how they relate to experimental data obtained in heavy-ion collisions at RHIC and LHC energies.

 

 

 

 

 


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