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Recently, two-photon imaging has allowed intravital tracking of lymphocyte migration and cellular interactions during germinal center (GC) reactions. The implications of two-photon measurements obtained by several investigators are currently the subject of controversy. With the help of two mathematical approaches, we reanalyze these data. It is shown that the measured lymphocyte migration frequency between the dark and the light zone is quantitatively explained by persistent random walk of lymphocytes. The cell motility data imply a fast intermixture of cells within the whole GC in approximately 3 h, and this does not allow for maintenance of dark and light zones. The model predicts that chemotaxis is active in GCs to maintain GC zoning and demonstrates that chemotaxis is consistent with two-photon lymphocyte motility data. However, the model also predicts that the chemokine sensitivity is quickly down-regulated. On the basis of these fi ndings, we formulate a novel GC lymphocyte migration model and propose its verifi cation by new two-photon experiments that combine the measurement of B cell migration with that of specifi c chemokine receptor expression levels. In addition, we discuss some statistical limitations for the interpretation of two-photon cell motility measurements in general.
The dissertation deals with the general problem of how the brain can establish correspondences between neural patterns stored in different cortical areas. Although an important capability in many cognitive areas like language understanding, abstract reasoning, or motor control, this thesis concentrates on invariant object recognition as application of correspondence finding. One part of the work presents a correspondence-based, neurally plausible system for face recognition. Other parts address the question of visual information routing over several stages by proposing optimal architectures for such routing ('switchyards') and deriving ontogenetic mechanisms for the growth of switchyards. Finally, the idea of multi-stage routing is united with the object recognition system introduced before, making suggestions of how the so far distinct feature-based and correspondence-based approaches to object recognition could be reconciled.
We argue that Clustering in heavy ion collisions could be the missing element in resolving the socalled HBT puzzle, and briefly discuss the different physical situations where clustering could be present. We then propose a method by which clustering in heavy ion collisions could be detectedin a model-independent way.
We calculate leading-order dilepton yields from a quark-gluon plasma which has a time-dependent anisotropy in momentum space. Such anisotropies can arise during the earliest stages of quark-gluon plasma evolution due to the rapid longitudinal expansion of the created matter. A phenomenological model for the proper time dependence of the parton hard momentum scale, p_hard, and the plasma anisotropy parameter, xi, is proposed. The model describes the transition of the plasma from a 0+1 dimensional collisionally-broadened expansion at early times to a 0+1 dimensional ideal hydrodynamic expansion at late times. We find that high-energy dilepton production is enhanced by pre-equilibrium emission up to 50% at LHC energies, if one assumes an isotropization/thermalization time of 2 fm/c. Given sufficiently precise experimental data this enhancement could be used to determine the plasma isotropization time experimentally.
Synchronisierte Antworten aus der Großhirnrinde : ein Lösungsvorschlag für das Bindungsproblem
(2005)
The interplay of charmonium production and suppression in In+In and Pb+Pb reactions at 158 AGeV and in Au+Au reactions at sqrt(s)=200 GeV is investigated with the HSD transport approach within the hadronic comover model' and the QGP melting scenario'. The results for the J/Psi suppression and the Psi' to J/Psi ratio are compared to the recent data of the NA50, NA60, and PHENIX Collaborations. We find that, at 158 AGeV, the comover absorption model performs better than the scenario of abrupt threshold melting. However, neither interaction with hadrons alone nor simple color screening satisfactory describes the data at sqrt(s)=200 GeV. A deconfined phase is clearly reached at RHIC, but a theory having the relevant degrees of freedom in this regime (strongly interacting quarks/gluons) is needed to study its transport properties.
This thesis contributes to the field of soft matter research and studies the importance of hydrodynamic interactions during free-solution electrophoresis of linear polyelectrolytes by means of coarse-grained molecular dynamics simulations including full electro-hydrodynamic interactions. The center of attention is the specific role of hydrodynamic interactions on the electrophoretic behaviour of charged macromolecules. Points of interest are the dependence of hydrodynamic interactions on the chain length, the chain flexibility and the surrounding counterions, and their combined influence on important observables such as the static chain conformations and the dynamic transport coefficients, i.e., the diffusion and the electrophoretic mobility. These problems are addressed by extensive computer simulations that are quantitatively matched with experimental results. Existing theoretical predictions are carefully examined and are augmented by the observations in this thesis.
The timing of feedback to early visual cortex in the perception of long-range apparent motion
(2008)
When 2 visual stimuli are presented one after another in different locations, they are often perceived as one, but moving object. Feedback from area human motion complex hMT/V5+ to V1 has been hypothesized to play an important role in this illusory perception of motion. We measured event-related responses to illusory motion stimuli of varying apparent motion (AM) content and retinal location using Electroencephalography. Detectable cortical stimulus processing started around 60-ms poststimulus in area V1. This component was insensitive to AM content and sequential stimulus presentation. Sensitivity to AM content was observed starting around 90 ms post the second stimulus of a sequence and most likely originated in area hMT/V5+. This AM sensitive response was insensitive to retinal stimulus position. The stimulus sequence related response started to be sensitive to retinal stimulus position at a longer latency of 110 ms. We interpret our findings as evidence for feedback from area hMT/V5+ or a related motion processing area to early visual cortices (V1, V2, V3).
What is the energy function guiding behavior and learningµ Representationbased approaches like maximum entropy, generative models, sparse coding, or slowness principles can account for unsupervised learning of biologically observed structure in sensory systems from raw sensory data. However, they do not relate to behavior. Behavior-based approaches like reinforcement learning explain animal behavior in well-described situations. However, they rely on high-level representations which they cannot extract from raw sensory data. Combinations of multiple goal functions seems the methodology of choice to understand the complexity of the brain. But what is the set of possible goals. ...
A small-world network has been suggested to be an efficient solution for achieving both modular and global processing-a property highly desirable for brain computations. Here, we investigated functional networks of cortical neurons using correlation analysis to identify functional connectivity. To reconstruct the interaction network, we applied the Ising model based on the principle of maximum entropy. This allowed us to assess the interactions by measuring pairwise correlations and to assess the strength of coupling from the degree of synchrony. Visual responses were recorded in visual cortex of anesthetized cats, simultaneously from up to 24 neurons. First, pairwise correlations captured most of the patterns in the population´s activity and, therefore, provided a reliable basis for the reconstruction of the interaction networks. Second, and most importantly, the resulting networks had small-world properties; the average path lengths were as short as in simulated random networks, but the clustering coefficients were larger. Neurons differed considerably with respect to the number and strength of interactions, suggesting the existence of "hubs" in the network. Notably, there was no evidence for scale-free properties. These results suggest that cortical networks are optimized for the coexistence of local and global computations: feature detection and feature integration or binding.
Poster presentation A central problem in neuroscience is to bridge local synaptic plasticity and the global behavior of a system. It has been shown that Hebbian learning of connections in a feedforward network performs PCA on its inputs [1]. In recurrent Hopfield network with binary units, the Hebbian-learnt patterns form the attractors of the network [2]. Starting from a random recurrent network, Hebbian learning reduces system complexity from chaotic to fixed point [3]. In this paper, we investigate the effect of Hebbian plasticity on the attractors of a continuous dynamical system. In a Hopfield network with binary units, it can be shown that Hebbian learning of an attractor stabilizes it with deepened energy landscape and larger basin of attraction. We are interested in how these properties carry over to continuous dynamical systems. Consider system of the form Math(1) where xi is a real variable, and fi a nondecreasing nonlinear function with range [-1,1]. T is the synaptic matrix, which is assumed to have been learned from orthogonal binary ({1,-1}) patterns ξμ, by the Hebbian rule: Math. Similar to the continuous Hopfield network [4], ξμ are no longer attractors, unless the gains gi are big. Assume that the system settles down to an attractor X*, and undergoes Hebbian plasticity: T´ = T + εX*X*T, where ε > 0 is the learning rate. We study how the attractor dynamics change following this plasticity. We show that, in system (1) under certain general conditions, Hebbian plasticity makes the attractor move towards its corner of the hypercube. Linear stability analysis around the attractor shows that the maximum eigenvalue becomes more negative with learning, indicating a deeper landscape. This in a way improves the system´s ability to retrieve the corresponding stored binary pattern, although the attractor itself is no longer stabilized the way it does in binary Hopfield networks.
In this work the nuclear structure of exotic nuclei and superheavy nuclei is studied in a relativistic framework. In the relativistic mean-field (RMF) approximation, the nucleons interact with each other through the exchange of various effective mesons (scalar, vector, isovector-vector). Ground state properties of exotic nuclei and superheavy nuclei are studied in the RMF theory with the three different parameter sets (ChiM, NL3, NL-Z2). Axial deformation of nuclei within two drip lines are performed with the parameter set (ChiM). The position of drip lines are investigated with three different parameter sets (ChiM, NL3, NL-Z2) and compared with the experimental drip line nuclei. In addition, the structure of hypernuclei are studied and for a certain isotope, hyperon halo nucleus is predicted.
Spatio-temporal dynamics of primary lymphoid follicles during organogenesis and lymphneogenesis
(2007)
Primary lymphoid follicles are structures which are important for adaptive immune responses in mammals. Within the follicles follicular dendritic cells (FDC) are maintained by constant stimuli provided by B cells. It is thought that the FDC are important for immune response. It is of interest to know how lymphoid follicles are regulated in order to understand their role in various autoimmune diseases in which these follicles are created ectopically. With the help of a tissue simulation relying on an agent-based cell model on top of a regular triangulation various scenarios suggested by the available experimental data have been investigated. In order to cope with the complexity in the simulation of immune tissue the regular triangulation has been implemented for the use on parallel computers. The algorithms for kinetic and dynamic regular triangulation have been created newly. Also the cell model underlying the simulation has been designed newly in many aspects. The simulations allowed to identify common factors that regulate the formation of lymphoid follicles normally during organogenesis in development and lymphneogenesis in the course of diseases. The generation of FDC from local stromal populations under the influence of B cell aggregates is shown to be possible with the given experimental parameters. The sequence of the organogenesis and lymphneogenesis can be described with regard to the morphology of the B and T zone. Tests for the stability of the primary lymphoid follicle system constraints the regulation of the B cell efflux. The required lymphatic vessels around the lymphoid follicle are shown to be negatively correlated with the FDC network. Moreover it is shown that the adjacent T zone consisting of its own stromal population and T cells has similar regulation principles. This easily explains the intermediate ring of B cells found around the T zone during development and certain signaling molecule deficiencies. A major result of this thesis is that the generation of FDC needs negative regulation while a number of other possible mechanisms is incompatible with the available experimental data. Moreover the observed microanatomy was brought into a functional relationship with data on the cellular level finally culminating in the proposal of new experiments that shed light on the dynamics of the primary lymphoid follicle. One conclusion is that the FDC directly or indirectly influence the angiogenesis and lymphangiogenesis processes in secondary lymphoid tissues. The work presented here may help to guide experiments with the help of computers in order to reduce the amount of experiments and design them in a way to maximize the amount of information about biological systems.
This work is devoted to the description of mechanisms that might be responsible for avian magnetoreception. Two possible theoretical concepts underlying this phenomenon are formulated and their functionality is proven in realistic geomagnetic fields. It has been suggested that the "magnetic sense" in birds may be mediated by the blue light receptor protein- cryptochrome- which is known to be localized in the retinas of migratory birds. Cryptochromes are a class of photoreceptor signaling proteins that are found in a wide variety of organisms and which primarily perform regulatory functions, such as the entrainment of circadian rhythm in mammals and the inhibition of hypocotyl growth in plants. Recent experiments have shown that the activity of cryptochrome-1 in Arabidopsis thaliana is enhanced by the presence of a weak external magnetic field, confirming the ability of cryptochrome to mediate magnetic field responses. Cryptochrome's signaling is tied to the photoreduction of an internally bound chromophore, flavin adenine dinucleotide (FAD). The spin chemistry of this photoreduction process, which involves electron transfer from a chain of three tryptophans, is modulated by the presence of a magnetic field in an effect known as the radical pair mechanism. Cryptochrome was suggested as a possible magnetoreceptor for the first time in 2000. However, no realistic calculations of the magnetic field effect in cryptochrome were performed. One of the goals of the present thesis is computationally to study the electron spin dynamics in cryptochrome and to show the feasibility of a cryptochrome-based compass in birds. In particular, the activation yield of cryptochrome was studied as a function of an external magnetic field and it was shown that the activation of the protein can be influenced by the geomagnetic field. In the work it has also been proven that cryptochrome provides an inclination compass, which is necessary for bird orientation. The evolution of spin densities as a function of time is also discussed. An alternative mechanism of avian magnetoreception discussed in the thesis is based on the interaction of two iron minerals (magnetite and maghemite) which were only recently found in subcellular compartments within the sensory dendrites of the upper beak of several bird species. The iron minerals in the beak form platelets of crystalline maghemite and assemblies of magnetite nanoparticles (magnetite clusters). The interaction between these particles can be manipulated by an external magnetic field inducing a primary receptor potential via strain-sensitive membrane channels that lead to a certain bird orientation effect. Various properties of the magnetite/maghemite magnetoreceptor system have been considered: the potential energy surface of the magnetite cluster has been calculated and analyzed as a function of the orientation of an external magnetic field; the forces acting on the magnetite cluster were calculated and analyzed; the force differences caused by the change of the direction of external magnetic field were established; the probability of opening the mechanosensitive ion channel was calculated. Finally it has been demonstrated that the iron-mineral based magnetoreceptor provides a polarity magnetic compass. Various conditions at which the magnetoreception process is violated are outlined.
I investigate some of the inert phases in three-flavor, spin-zero color-superconducting quark matter: the CFL phase (the analogue of the B phase in superfluid 3He), the A and A* phases, and the 2SC and sSC phases. I compute the pressure of these phases with and without the neutrality condition. Without the neutrality condition, after the CFL phase the sSC phase is the dominant phase. However, including the neutrality condition, the CFL phase is again the energetically favored phase except for a small region of intermediate densities where the 2SC/A* phase is favored. It is shown that the 2SC phase is identical to the A* phase up to a color rotation. In addition, I calculate the self-energies and the spectral densities of longitudinal and transverse gluons at zero temperature in color-superconducting quark matter in the CFL phase. I find a collective excitation, a plasmon, at energies smaller than two times the gap parameter and momenta smaller than about eight times the gap. The dispersion relation of this mode exhibits a minimum at some nonzero value of momentum, indicating a van Hove singularity.
In this thesis we investigate the role played by gauge fields in providing new observable signatures that can attest to the presence of color superconductivity in neutron stars. We show that thermal gluon fluctuations in color-flavor locked superconductors can substantially increase their critical temperature and also change the order of the transition, which becomes a strong first-order phase transition. Moreover, we explore the effects of strong magnetic fields on the properties of color-flavor locked superconducting matter. We find that both the energy gaps as well as the magnetization are oscillating functions of the magnetic field. Also, it is shown that the magnetization can be so strong that homogeneous quark matter becomes metastable for a range of parameters. This points towards the existence of magnetic domains or other types of magnetic inhomogeneities in the hypothesized quark cores of magnetars. Obviously, our results only apply if the strong magnetic fields observed on the surface of magnetars can be transmitted to their inner core. This can occur if the superconducting protons expected to exist in the outer core form a type-I I superconductor. However, it has been argued that the observed long periodic oscillations in isolated pulsars can only be explained if the outer core is a type-I superconductor rather than type-I I. We show that this is not the only solution for the precession puzzle by demonstrating that the long-term variation in the spin of PSR 1828-11 can be explained in terms of Tkachenko oscillations within superfluid shells.
Background Objects in our environment are often partly occluded, yet we effortlessly perceive them as whole and complete. This phenomenon is called visual amodal completion. Psychophysical investigations suggest that the process of completion starts from a representation of the (visible) physical features of the stimulus and ends with a completed representation of the stimulus. The goal of our study was to investigate both stages of the completion process by localizing both brain regions involved in processing the physical features of the stimulus as well as brain regions representing the completed stimulus. Results Using fMRI adaptation we reveal clearly distinct regions in the visual cortex of humans involved in processing of amodal completion: early visual cortex - presumably V1 - processes the local contour information of the stimulus whereas regions in the inferior temporal cortex represent the completed shape. Furthermore, our data suggest that at the level of inferior temporal cortex information regarding the original local contour information is not preserved but replaced by the representation of the amodally completed percept. Conclusion These findings provide neuroimaging evidence for a multiple step theory of amodal completion and further insights into the neuronal correlates of visual perception.
Der simulierte Säugling : Neuroinformatiker erforschen, wie Babys lernen, Blicke zu verfolgen
(2005)
Background Synchronous neuronal firing has been discussed as a potential neuronal code. For testing first, if synchronous firing exists, second if it is modulated by the behaviour, and third if it is not by chance, a large set of tools has been developed. However, to test whether synchronous neuronal firing is really involved in information processing one needs a direct comparison of the amount of synchronous firing for different factors like experimental or behavioural conditions. To this end we present an extended version of a previously published method NeuroXidence [1], which tests, based on a bi- and multivariate test design, whether the amount of synchronous firing above the chance level is different for different factors.