Latest documents http://publikationen.stub.uni-frankfurt.de/index/index/ Mon, 05 Aug 2013 13:19:17 +0200 Mon, 05 Aug 2013 13:19:17 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/29499 Understanding causal relationships, or effective connectivity, between parts of the brain is of utmost importance because a large part of the brain’s activity is thought to be internally generated and, hence, quantifying stimulus response relationships alone does not fully describe brain dynamics. Past efforts to determine effective connectivity mostly relied on model based approaches such as Granger causality or dynamic causal modeling. Transfer entropy (TE) is an alternative measure of effective connectivity based on information theory. TE does not require a model of the interaction and is inherently non-linear. We investigated the applicability of TE as a metric in a test for effective connectivity to electrophysiological data based on simulations and magnetoencephalography (MEG) recordings in a simple motor task. In particular, we demonstrate that TE improved the detectability of effective connectivity for non-linear interactions, and for sensor level MEG signals where linear methods are hampered by signal-cross-talk due to volume conduction. Raul Vicente; Michael Wibral; Michael Lindner; Gordon Pipa article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/29499 Wed, 08 May 2013 13:19:17 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/29528 Background: After induction of DNA double strand breaks (DSBs), the DNA damage response (DDR) is activated. One of the earliest events in DDR is the phosphorylation of serine 139 on the histone variant H2AX (gH2AX) catalyzed by phosphatidylinositol 3-kinases-related kinases. Despite being extensively studied, H2AX distribution[1] across the genome and gH2AX spreading around DSBs sites[2] in the context of different chromatin compaction states or transcription are yet to be fully elucidated. Materials and methods: gH2AX was induced in human hepatocellular carcinoma cells (HepG2) by exposure to 10 Gy X-rays (250 kV, 16 mA). Samples were incubated 0.5, 3 or 24 hours post irradiation to investigate early, intermediate and late stages of DDR, respectively. Chromatin immunoprecipitation was performed to select H2AX, H3 and gH2AX-enriched chromatin fractions. Chromatin-associated DNA was then sequenced by Illumina ChIP-Seq platform. HepG2 gene expression and histone modification (H3K36me3, H3K9me3) ChIP-Seq profiles were retrieved from Gene Expression Omnibus (accession numbers GSE30240 and GSE26386, respectively). Results: First, we combined G/C usage, gene content, gene expression or histone modification profiles (H3K36me3, H3K9me3) to define genomic compartments characterized by different chromatin compaction states or transcriptional activity. Next, we investigated H3, H2AX and gH2AX distributions in such defined compartments before and after exposure to ionizing radiation (IR) to study DNA repair kinetics during DDR. Our sequencing results indicate that H2AX distribution followed H3 occupancy and, thus, the nucleosome pattern. The highest H2AX and H3 enrichment was observed in transcriptionally active compartments (euchromatin) while the lowest was found in low G/C and gene-poor compartments (heterochromatin). Under physiological conditions, the body of highly and moderately transcribed genes was devoid of gH2AX, despite presenting high H2AX levels. gH2AX accumulation was observed in 5’ or 3’ flanking regions, instead. The same genes showed a prompt gH2AX accumulation during the early stage of DDR which then decreased over time as DDR proceeded. Finally, during the late stage of DDR the residual gH2AX signal was entirely retained in heterochromatic compartments. At this stage, euchromatic compartments were completely devoid of gH2AX despite presenting high levels of non-phosphorylated H2AX. Conclusions: We show that gH2AX distribution ultimately depends on H2AX occupancy, the latter following H3 occupancy and, thus, nucleosome pattern. Both H2AX and H3 levels were higher in actively transcribed compartments. However, gH2AX levels were remarkably low over the body of actively transcribed genes suggesting that transcription levels antagonize gH2AX spreading. Moreover, repair processes did not take place uniformly across the genome; rather, DNA repair was affected by genomic location and transcriptional activity. We propose that higher H2AX density in euchromaticcompartments results in high relative gH2AXconcentration soon after the activation of DDR, thus favoring the recruitment of the DNA repair machinery to those compartments. When the damage is repaired and gH2AX is removed, its residual fraction is retained in the heterochromatic compartments which are then targeted and repaired at later times. Francesco Natale; Alexander Rapp; Wei Yu; Marco Durante; Gisela Taucher-Scholz; Maria Cristina Cardoso conferenceobject http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/29528 Fri, 26 Apr 2013 08:27:31 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/29809 Current theories of the pathophysiology of schizophrenia have focused on abnormal temporal coordination of neural activity. Oscillations in the gamma-band range (>25 Hz) are of particular interest as they establish synchronization with great precision in local cortical networks. However, the contribution of high gamma (>60 Hz) oscillations toward the pathophysiology is less established. To address this issue, we recorded magnetoencephalographic (MEG) data from 16 medicated patients with chronic schizophrenia and 16 controls during the perception of Mooney faces. MEG data were analysed in the 25–150 Hz frequency range. Patients showed elevated reaction times and reduced detection rates during the perception of upright Mooney faces while responses to inverted stimuli were intact. Impaired processing of Mooney faces in schizophrenia patients was accompanied by a pronounced reduction in spectral power between 60–120 Hz (effect size: d = 1.26) which was correlated with disorganized symptoms (r = −0.72). Our findings demonstrate that deficits in high gamma-band oscillations as measured by MEG are a sensitive marker for aberrant cortical functioning in schizophrenia, suggesting an important aspect of the pathophysiology of the disorder. Christine Grützner; Michael Wibral; Limin Sun; Davide Rivolta; Wolf Singer; Konrad Maurer; Peter J. Uhlhaas article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/29809 Fri, 19 Apr 2013 15:06:20 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/29148 The way we perceive the visual world depends crucially on the state of the observer. In the present study we show that what we are holding in working memory (WM) can bias the way we perceive ambiguous structure from motion stimuli. Holding in memory the percept of an unambiguously rotating sphere influenced the perceived direction of motion of an ambiguously rotating sphere presented shortly thereafter. In particular, we found a systematic difference between congruent dominance periods where the perceived direction of the ambiguous stimulus corresponded to the direction of the unambiguous one and incongruent dominance periods. Congruent dominance periods were more frequent when participants memorized the speed of the unambiguous sphere for delayed discrimination than when they performed an immediate judgment on a change in its speed. The analysis of dominance time-course showed that a sustained tendency to perceive the same direction of motion as the prior stimulus emerged only in the WM condition, whereas in the attention condition perceptual dominance dropped to chance levels at the end of the trial. The results are explained in terms of a direct involvement of early visual areas in the active representation of visual motion in WM. Lisa Scocchia; Matteo Valsecchi; Karl R. Gegenfurtner; Jochen Triesch article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/29148 Thu, 21 Mar 2013 13:41:36 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/25663 In complex networks such as gene networks, traffic systems or brain circuits it is important to understand how long it takes for the different parts of the network to effectively influence one another. In the brain, for example, axonal delays between brain areas can amount to several tens of milliseconds, adding an intrinsic component to any timing-based processing of information. Inferring neural interaction delays is thus needed to interpret the information transfer revealed by any analysis of directed interactions across brain structures. However, a robust estimation of interaction delays from neural activity faces several challenges if modeling assumptions on interaction mechanisms are wrong or cannot be made. Here, we propose a robust estimator for neuronal interaction delays rooted in an information-theoretic framework, which allows a model-free exploration of interactions. In particular, we extend transfer entropy to account for delayed source-target interactions, while crucially retaining the conditioning on the embedded target state at the immediately previous time step. We prove that this particular extension is indeed guaranteed to identify interaction delays between two coupled systems and is the only relevant option in keeping with Wiener’s principle of causality. We demonstrate the performance of our approach in detecting interaction delays on finite data by numerical simulations of stochastic and deterministic processes, as well as on local field potential recordings. We also show the ability of the extended transfer entropy to detect the presence of multiple delays, as well as feedback loops. While evaluated on neuroscience data, we expect the estimator to be useful in other fields dealing with network dynamics. Michael Wibral; Nicolae Pampu; Viola Priesemann; Felix Siebenhühner; Hannes Seiwert; Michael Lindner; Joseph T. Lizier; Raul Vicente article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/25663 Wed, 13 Mar 2013 08:19:07 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/28938 Radiation damage following the ionising radiation of tissue has different scenarios and mechanisms depending on the projectiles or radiation modality. We investigate the radiation damage effects due to shock waves produced by ions. We analyse the strength of the shock wave capable of directly producing DNA strand breaks and, depending on the ion's linear energy transfer, estimate the radius from the ion's path, within which DNA damage by the shock wave mechanism is dominant. At much smaller values of linear energy transfer, the shock waves turn out to be instrumental in propagating reactive species formed close to the ion's path to large distances, successfully competing with diffusion. Eugene Surdutovich; Alexander V. Yakubovich; Andrey V. Solov'yov article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/28938 Mon, 18 Feb 2013 15:51:02 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/28354 The information processing abilities of neural circuits arise from their synaptic connection patterns. Understanding the laws governing these connectivity patterns is essential for understanding brain function. The overall distribution of synaptic strengths of local excitatory connections in cortex and hippocampus is long-tailed, exhibiting a small number of synaptic connections of very large efficacy. At the same time, new synaptic connections are constantly being created and individual synaptic connection strengths show substantial fluctuations across time. It remains unclear through what mechanisms these properties of neural circuits arise and how they contribute to learning and memory. In this study we show that fundamental characteristics of excitatory synaptic connections in cortex and hippocampus can be explained as a consequence of self-organization in a recurrent network combining spike-timing-dependent plasticity (STDP), structural plasticity and different forms of homeostatic plasticity. In the network, associative synaptic plasticity in the form of STDP induces a rich-get-richer dynamics among synapses, while homeostatic mechanisms induce competition. Under distinctly different initial conditions, the ensuing self-organization produces long-tailed synaptic strength distributions matching experimental findings. We show that this self-organization can take place with a purely additive STDP mechanism and that multiplicative weight dynamics emerge as a consequence of network interactions. The observed patterns of fluctuation of synaptic strengths, including elimination and generation of synaptic connections and long-term persistence of strong connections, are consistent with the dynamics of dendritic spines found in rat hippocampus. Beyond this, the model predicts an approximately power-law scaling of the lifetimes of newly established synaptic connection strengths during development. Our results suggest that the combined action of multiple forms of neuronal plasticity plays an essential role in the formation and maintenance of cortical circuits. Pengsheng Zheng; Christos Dimitrakakis; Jochen Triesch article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/28354 Mon, 21 Jan 2013 12:45:47 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27319 In recent years, Hagedorn states have been used to explain the equilibrium and transport properties of a hadron gas close to the QCD critical temperature. These massive resonances are shown to lower h/s to near the AdS/CFT limit close to the phase transition. A comparison of the Hagedorn model to recent lattice results is made and it is found that the hadrons can reach chemical equilibrium almost immediately, well before the chemical freeze-out temperatures found in thermal fits for a hadron gas without Hagedorn states. Carsten Greiner; Jacquelyn Noronha-Hostler; Jorge Noronha conferenceobject http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27319 Mon, 17 Dec 2012 16:38:29 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27787 Perception is an active inferential process in which prior knowledge is combined with sensory input, the result of which determines the contents of awareness. Accordingly, previous experience is known to help the brain “decide” what to perceive. However, a critical aspect that has not been addressed is that previous experience can exert 2 opposing effects on perception: An attractive effect, sensitizing the brain to perceive the same again (hysteresis), or a repulsive effect, making it more likely to perceive something else (adaptation). We used functional magnetic resonance imaging and modeling to elucidate how the brain entertains these 2 opposing processes, and what determines the direction of such experience-dependent perceptual effects. We found that although affecting our perception concurrently, hysteresis and adaptation map into distinct cortical networks: a widespread network of higher-order visual and fronto-parietal areas was involved in perceptual stabilization, while adaptation was confined to early visual areas. This areal and hierarchical segregation may explain how the brain maintains the balance between exploiting redundancies and staying sensitive to new information. We provide a Bayesian model that accounts for the coexistence of hysteresis and adaptation by separating their causes into 2 distinct terms: Hysteresis alters the prior, whereas adaptation changes the sensory evidence (the likelihood function). Caspar Martin Schwiedrzik; Christian C. Ruff; Andreea Lazar; Frauke C. Leitner; Wolf Singer; Lucia Melloni article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27787 Mon, 17 Dec 2012 09:17:02 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27436 We report on our implementation of LatticeQCD applications using OpenCL. We focus on the general concept and on distributing different parts on hybrid systems, consisting of both CPUs (Central Processing Units) and GPUs (Graphic Processing Units). Owe Philipsen; Christopher Pinke; Christian Schäfer; Lars Zeidlewicz; Matthias Bach conferenceobject http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27436 Fri, 14 Dec 2012 15:04:24 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27776 The biological effects of energetic heavy ions are attracting increasing interest for their applications in cancer therapy and protection against space radiation. The cascade of events leading to cell death or late effects starts from stochastic energy deposition on the nanometer scale and the corresponding lesions in biological molecules, primarily DNA. We have developed experimental techniques to visualize DNA nanolesions induced by heavy ions. Nanolesions appear in cells as “streaks” which can be visualized by using different DNA repair markers. We have studied the kinetics of repair of these “streaks” also with respect to the chromatin conformation. Initial steps in the modeling of the energy deposition patterns at the micrometer and nanometer scale were made with MCHIT and TRAX models, respectively. Marcus Bleicher; Lucas Burigo; Marco Durante; Maren Herrlitz; Michael Krämer; Igor Mishustin; Iris Müller; Francesco Natale; Igor Pshenichnov; Stefan Schramm; Gisela Taucher-Scholz; Cathrin Wälzlein article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27776 Thu, 13 Dec 2012 16:28:23 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27310 The results of the microscopic transport calculations of -nucleus interactions within a GiBUU model are presented. The dominating mechanism of hyperon production is the strangeness exchange processes → γπ and → ΞK. The calculated rapidity spectra of Ξ hyperons are significantly shifted to forward rapidities with respect to the spectra of S = −1 hyperons. We argue that this shift should be a sensitive test for the possible exotic mechanisms of -nucleus annihilation. The production of the double Λ-hypernuclei by Ξ− interaction with a secondary target is calculated. Alexei B. Larionov; Theodoros Gaitanos; Horst Lenske; Ulrich Mosel conferenceobject http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27310 Thu, 13 Dec 2012 13:24:21 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27458 periodical http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27458 Wed, 21 Nov 2012 15:49:45 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27591 periodicalpart http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27591 Wed, 21 Nov 2012 15:45:19 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27588 In the year 2010 the Frankfurt Institute for Advanced Studies has successfully continued to follow its agenda to pursue theoretical research in the natural sciences. As stipulated in its charter, FIAS closely collaborates with extramural research institutions, like the Max Planck Institute for Brain Research in Frankfurt and the GSI Helmholtz Center for Heavy Ion Research, Darmstadt and with research groups at the science departments of Goethe University. The institute also engages in the training of young researchers and the education of doctoral students. This Annual Report documents how these goals have been pursued in the year 2010. Notable events in the scientific life of the Institute will be presented, e.g., teaching activities in the framework of the Frankfurt International Graduate School for Science (FIGSS), colloquium schedules, conferences organized by FIAS, and a full bibliography of publications by authors affiliated with FIAS. The main part of the Report consists of short one-page summaries describing the scientific progress reached in individual research projects in the year 2010... periodicalpart http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27588 Wed, 21 Nov 2012 15:42:55 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27587 In this Annual Report we present some of the ongoing activities of FIAS and of the associated graduate school, the “Frankfurt International Graduate School for Science” (FIGSS) in the year 2009. The main part of the Report consists of a collection of short reports describing the research projects of scientists working at or associated with FIAS. periodicalpart http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27587 Wed, 21 Nov 2012 15:38:28 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27497 In the juvenile brain, the synaptic architecture of the visual cortex remains in a state of flux for months after the natural onset of vision and the initial emergence of feature selectivity in visual cortical neurons. It is an attractive hypothesis that visual cortical architecture is shaped during this extended period of juvenile plasticity by the coordinated optimization of multiple visual cortical maps such as orientation preference (OP), ocular dominance (OD), spatial frequency, or direction preference. In part (I) of this study we introduced a class of analytically tractable coordinated optimization models and solved representative examples, in which a spatially complex organization of the OP map is induced by interactions between the maps. We found that these solutions near symmetry breaking threshold predict a highly ordered map layout. Here we examine the time course of the convergence towards attractor states and optima of these models. In particular, we determine the timescales on which map optimization takes place and how these timescales can be compared to those of visual cortical development and plasticity. We also assess whether our models exhibit biologically more realistic, spatially irregular solutions at a finite distance from threshold, when the spatial periodicities of the two maps are detuned and when considering more than 2 feature dimensions. We show that, although maps typically undergo substantial rearrangement, no other solutions than pinwheel crystals and stripes dominate in the emerging layouts. Pinwheel crystallization takes place on a rather short timescale and can also occur for detuned wavelengths of different maps. Our numerical results thus support the view that neither minimal energy states nor intermediate transient states of our coordinated optimization models successfully explain the architecture of the visual cortex. We discuss several alternative scenarios that may improve the agreement between model solutions and biological observations. Lars Reichl; Dominik Heide; Siegrid Löwel; Justin C. Crowley; Matthias Kaschube; Fred Wolf article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27497 Fri, 09 Nov 2012 10:06:56 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27498 In the primary visual cortex of primates and carnivores, functional architecture can be characterized by maps of various stimulus features such as orientation preference (OP), ocular dominance (OD), and spatial frequency. It is a long-standing question in theoretical neuroscience whether the observed maps should be interpreted as optima of a specific energy functional that summarizes the design principles of cortical functional architecture. A rigorous evaluation of this optimization hypothesis is particularly demanded by recent evidence that the functional architecture of orientation columns precisely follows species invariant quantitative laws. Because it would be desirable to infer the form of such an optimization principle from the biological data, the optimization approach to explain cortical functional architecture raises the following questions: i) What are the genuine ground states of candidate energy functionals and how can they be calculated with precision and rigor? ii) How do differences in candidate optimization principles impact on the predicted map structure and conversely what can be learned about a hypothetical underlying optimization principle from observations on map structure? iii) Is there a way to analyze the coordinated organization of cortical maps predicted by optimization principles in general? To answer these questions we developed a general dynamical systems approach to the combined optimization of visual cortical maps of OP and another scalar feature such as OD or spatial frequency preference. From basic symmetry assumptions we obtain a comprehensive phenomenological classification of possible inter-map coupling energies and examine representative examples. We show that each individual coupling energy leads to a different class of OP solutions with different correlations among the maps such that inferences about the optimization principle from map layout appear viable. We systematically assess whether quantitative laws resembling experimental observations can result from the coordinated optimization of orientation columns with other feature maps. Lars Reichl; Dominik Heide; Siegrid Löwel; Justin C. Crowley; Matthias Kaschube; Fred Wolf article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27498 Fri, 09 Nov 2012 09:59:40 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/26972 At nonzero temperature, it is expected that QCD undergoes a phase transition to a deconfined, chirally symmetric phase, the Quark-Gluon Plasma (QGP). I review what we expect theoretically about this possible transition, and what we have learned from heavy ion experiments at RHIC. I argue that while there are unambiguous signals for qualitatively new behavior at RHIC, versus experiments at lower energies, that in detail, no simple theoretical model can explain all salient features of the data. Robert D. Pisarski conferenceobject http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/26972 Fri, 02 Nov 2012 10:09:57 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/26815 I discuss the physics of non-Abelian plasmas which are locally anisotropic in momentum space. Such momentum-space anisotropies are generated by the rapid longitudinal expansion of the matter created in the first 1 fm/c of an ultrarelativistic heavy ion collision. In contrast to locally isotropic plasmas anisotropic plasmas have a spectrum of soft unstable modes which are characterized by exponential growth of transverse chromo-magnetic/-electric fields at short times. This instability is the QCD analogue of the Weibel instability of QED. Parametrically the chromo-Weibel instability provides the fastest method for generation of soft background fields and dominates the short-time dynamics of the system. The existence of the chromo-Weibel instability has been proven using diagrammatic methods, transport theory, and numerical solution of classical Yang-Mills fields. I review the results obtained from each of these methods and discuss the numerical techniques which are being used to determine the late-time behavior of plasmas subject to a chromo-Weibel instability. Michael Strickland article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/26815 Thu, 01 Nov 2012 11:02:55 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27018 The simultaneous recording of the activity of many neurons poses challenges for multivariate data analysis. Here, we propose a general scheme of reconstruction of the functional network from spike train recordings. Effective, causal interactions are estimated by fitting generalized linear models on the neural responses, incorporating effects of the neurons’ self-history, of input from other neurons in the recorded network and of modulation by an external stimulus. The coupling terms arising from synaptic input can be transformed by thresholding into a binary connectivity matrix which is directed. Each link between two neurons represents a causal influence from one neuron to the other, given the observation of all other neurons from the population. The resulting graph is analyzed with respect to small-world and scale-free properties using quantitative measures for directed networks. Such graph-theoretic analyses have been performed on many complex dynamic networks, including the connectivity structure between different brain areas. Only few studies have attempted to look at the structure of cortical neural networks on the level of individual neurons. Here, using multi-electrode recordings from the visual system of the awake monkey, we find that cortical networks lack scale-free behavior, but show a small, but significant small-world structure. Assuming a simple distance-dependent probabilistic wiring between neurons, we find that this connectivity structure can account for all of the networks’ observed small-world-ness. Moreover, for multi-electrode recordings the sampling of neurons is not uniform across the population. We show that the small-world-ness obtained by such a localized sub-sampling overestimates the strength of the true small-world structure of the network. This bias is likely to be present in all previous experiments based on multi-electrode recordings. Felipe Gerhard; Gordon Pipa; Bruss Lima; Sergio Neuenschwander; Wulfram Gerstner article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27018 Tue, 30 Oct 2012 08:39:56 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27358 Mitochondria form a dynamic tubular reticulum within eukaryotic cells. Currently, quantitative understanding of its morphological characteristics is largely absent, despite major progress in deciphering the molecular fission and fusion machineries shaping its structure. Here we address the principles of formation and the large-scale organization of the cell-wide network of mitochondria. On the basis of experimentally determined structural features we establish the tip-to-tip and tip-to-side fission and fusion events as dominant reactions in the motility of this organelle. Subsequently, we introduce a graph-based model of the chondriome able to encompass its inherent variability in a single framework. Using both mean-field deterministic and explicit stochastic mathematical methods we establish a relationship between the chondriome structural network characteristics and underlying kinetic rate parameters. The computational analysis indicates that mitochondrial networks exhibit a percolation threshold. Intrinsic morphological instability of the mitochondrial reticulum resulting from its vicinity to the percolation transition is proposed as a novel mechanism that can be utilized by cells for optimizing their functional competence via dynamic remodeling of the chondriome. The detailed size distribution of the network components predicted by the dynamic graph representation introduces a relationship between chondriome characteristics and cell function. It forms a basis for understanding the architecture of mitochondria as a cell-wide but inhomogeneous organelle. Analysis of the reticulum adaptive configuration offers a direct clarification for its impact on numerous physiological processes strongly dependent on mitochondrial dynamics and organization, such as efficiency of cellular metabolism, tissue differentiation and aging. Valerii M. Sukhorukov; Daniel Dikov; Andreas S. Reichert; Michael Meyer-Hermann article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27358 Mon, 29 Oct 2012 17:08:35 +0100 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27265 During natural vision, primates perform frequent saccadic eye movements, allowing only a narrow time window for processing the visual information at each location. Individual neurons may contribute only with a few spikes to the visual processing during each fixation, suggesting precise spike timing as a relevant mechanism for information processing. We recently found in V1 of monkeys freely viewing natural images, that fixation-related spike synchronization occurs at the early phase of the rate response after fixation-onset, suggesting a specific role of the first response spikes in V1. Here, we show that there are strong local field potential (LFP) modulations locked to the onset of saccades, which continue into the successive fixation periods. Visually induced spikes, in particular the first spikes after the onset of a fixation, are locked to a specific epoch of the LFP modulation. We suggest that the modulation of neural excitability, which is reflected by the saccade-related LFP changes, serves as a corollary signal enabling precise timing of spikes in V1 and thereby providing a mechanism for spike synchronization. Junji Ito; Pedro Maldonado; Wolf Singer; Sonja Grün article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27265 Thu, 18 Oct 2012 16:45:12 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27267 Following the discovery of context-dependent synchronization of oscillatory neuronal responses in the visual system, the role of neural synchrony in cortical networks has been expanded to provide a general mechanism for the coordination of distributed neural activity patterns. In the current paper, we present an update of the status of this hypothesis through summarizing recent results from our laboratory that suggest important new insights regarding the mechanisms, function and relevance of this phenomenon. In the first part, we present recent results derived from animal experiments and mathematical simulations that provide novel explanations and mechanisms for zero and nero-zero phase lag synchronization. In the second part, we shall discuss the role of neural synchrony for expectancy during perceptual organization and its role in conscious experience. This will be followed by evidence that indicates that in addition to supporting conscious cognition, neural synchrony is abnormal in major brain disorders, such as schizophrenia and autism spectrum disorders. We conclude this paper with suggestions for further research as well as with critical issues that need to be addressed in future studies. Peter J. Uhlhaas; Gordon Pipa; Bruss Lima; Lucia Melloni; Sergio Neuenschwander; Danko Nikolić; Wolf Singer article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/27267 Thu, 18 Oct 2012 12:59:10 +0200 http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/26810 In binocular rivalry, presentation of different images to the separate eyes leads to conscious perception alternating between the two possible interpretations every few seconds. During perceptual transitions, a stimulus emerging into dominance can spread in a wave-like manner across the visual field. These traveling waves of rivalry dominance have been successfully related to the cortical magnification properties and functional activity of early visual areas, including the primary visual cortex (V1). Curiously however, these traveling waves undergo a delay when passing from one hemifield to another. In the current study, we used diffusion tensor imaging (DTI) to investigate whether the strength of interhemispheric connections between the left and right visual cortex might be related to the delay of traveling waves across hemifields. We measured the delay in traveling wave times (ΔTWT) in 19 participants and repeated this test 6 weeks later to evaluate the reliability of our behavioral measures. We found large interindividual variability but also good test–retest reliability for individual measures of ΔTWT. Using DTI in connection with fiber tractography, we identified parts of the corpus callosum connecting functionally defined visual areas V1–V3. We found that individual differences in ΔTWT was reliably predicted by the diffusion properties of transcallosal fibers connecting left and right V1, but observed no such effect for neighboring transcallosal visual fibers connecting V2 and V3. Our results demonstrate that the anatomical characteristics of topographically specific transcallosal connections predict the individual delay of interhemispheric traveling waves, providing further evidence that V1 is an important site for neural processes underlying binocular rivalry. Erhan Genç; Johanna Bergmann; Frank Tong; Randolph Blake; Wolf Singer; Axel Kohler article http://publikationen.stub.uni-frankfurt.de/frontdoor/index/index/docId/26810 Thu, 18 Oct 2012 10:09:58 +0200