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  • Attention, intention and salience in the posterior parietal cortex

    15 August 2018

    Selecting visual targets for saccadic eye movements is a vital step in sensorimotor processing. This selection is made on the basis of target salience: a saccade tends to be made to the most interesting part of the visual field. Both bottom-up and top-down processes have been postulated to contribute to salience, but the exact mechanisms by which some areas of the visual field are rendered to be more interesting than others remain unclear. We propose a computational model to address this issue and investigate its significant features, relating it to neuroanatomy and to aspects of cognitive function.

  • Limits to high-speed simulations of spiking neural networks using general-purpose computers.

    16 August 2018

    To understand how the central nervous system performs computations using recurrent neuronal circuitry, simulations have become an indispensable tool for theoretical neuroscience. To study neuronal circuits and their ability to self-organize, increasing attention has been directed toward synaptic plasticity. In particular spike-timing-dependent plasticity (STDP) creates specific demands for simulations of spiking neural networks. On the one hand a high temporal resolution is required to capture the millisecond timescale of typical STDP windows. On the other hand network simulations have to evolve over hours up to days, to capture the timescale of long-term plasticity. To do this efficiently, fast simulation speed is the crucial ingredient rather than large neuron numbers. Using different medium-sized network models consisting of several thousands of neurons and off-the-shelf hardware, we compare the simulation speed of the simulators: Brian, NEST and Neuron as well as our own simulator Auryn. Our results show that real-time simulations of different plastic network models are possible in parallel simulations in which numerical precision is not a primary concern. Even so, the speed-up margin of parallelism is limited and boosting simulation speeds beyond one tenth of real-time is difficult. By profiling simulation code we show that the run times of typical plastic network simulations encounter a hard boundary. This limit is partly due to latencies in the inter-process communications and thus cannot be overcome by increased parallelism. Overall, these results show that to study plasticity in medium-sized spiking neural networks, adequate simulation tools are readily available which run efficiently on small clusters. However, to run simulations substantially faster than real-time, special hardware is a prerequisite.

  • Synaptic plasticity in neural networks needs homeostasis with a fast rate detector.

    16 August 2018

    Hebbian changes of excitatory synapses are driven by and further enhance correlations between pre- and postsynaptic activities. Hence, Hebbian plasticity forms a positive feedback loop that can lead to instability in simulated neural networks. To keep activity at healthy, low levels, plasticity must therefore incorporate homeostatic control mechanisms. We find in numerical simulations of recurrent networks with a realistic triplet-based spike-timing-dependent plasticity rule (triplet STDP) that homeostasis has to detect rate changes on a timescale of seconds to minutes to keep the activity stable. We confirm this result in a generic mean-field formulation of network activity and homeostatic plasticity. Our results strongly suggest the existence of a homeostatic regulatory mechanism that reacts to firing rate changes on the order of seconds to minutes.

  • Inference of neuronal network spike dynamics and topology from calcium imaging data.

    16 August 2018

    Two-photon calcium imaging enables functional analysis of neuronal circuits by inferring action potential (AP) occurrence ("spike trains") from cellular fluorescence signals. It remains unclear how experimental parameters such as signal-to-noise ratio (SNR) and acquisition rate affect spike inference and whether additional information about network structure can be extracted. Here we present a simulation framework for quantitatively assessing how well spike dynamics and network topology can be inferred from noisy calcium imaging data. For simulated AP-evoked calcium transients in neocortical pyramidal cells, we analyzed the quality of spike inference as a function of SNR and data acquisition rate using a recently introduced peeling algorithm. Given experimentally attainable values of SNR and acquisition rate, neural spike trains could be reconstructed accurately and with up to millisecond precision. We then applied statistical neuronal network models to explore how remaining uncertainties in spike inference affect estimates of network connectivity and topological features of network organization. We define the experimental conditions suitable for inferring whether the network has a scale-free structure and determine how well hub neurons can be identified. Our findings provide a benchmark for future calcium imaging studies that aim to reliably infer neuronal network properties.

  • Entrance channel dependence of quasifission in reactions forming Th220

    16 August 2018

    Mass-angle correlations of binary fragments produced in O16+Pb204, S34+W186, and Ti48,50+Er166,170 reactions have been measured for a range of bombarding energies around their Coulomb barriers. At above-barrier energies, the width of the mass distributions for the fission-like fragments in the Ti50+Er170 reaction are found to be higher than those from the same compound system at similar excitation energies populated via the more mass asymmetric entrance channel reaction S34+W186, which in turn is higher than those for the O16+Pb204 system. The width of the mass distributions of the Ti+Er systems is found to increase with decreasing bombarding energies, in contrast with those of the O16+Pb204 and S34+W186 systems, which show a monotonic reduction in mass widths. This may be associated with the elongated contact configuration at sub-barrier energies. © 2008 The American Physical Society.

  • Determination of the η<sup>'</sup>-nucleus optical potential

    16 August 2018

    The excitation function and momentum distribution of η'mesons have been measured in photon induced reactions on12C in the energy range of 1250-2600 MeV. The experiment was performed with tagged photon beams from the ELSA electron accelerator using the Crystal Barrel and TAPS detectors. The data are compared to model calculations to extract information on the sign and magnitude of the real part of the η'-nucleus potential. Within the model, the comparison indicates an attractive potential of -(37±10(stat)±10(syst))MeV depth at normal nuclear matter density. Since the modulus of this depth is larger than the modulus of the imaginary part of the η'-nucleus potential of -(10±2.5)MeV, determined by transparency ratio measurements, a search for resolved η'-bound states appears promising. © 2013 Elsevier B.V.

  • Experimental constraints on the ω-nucleus real potential

    16 August 2018

    In a search for ω mesic states, the production of ω-mesons in coincidence with forward going protons has been studied in photon induced reactions on12C for incident photon energies of 1250-3100 MeV. Theπ0γ pairs from decays of bound or quasi-free ω-mesons have been measured with the CBELSA/TAPS detector system in coincidence with protons registered in the MiniTAPS forward array. Structures in the total energy distribution of theπ0γ pairs, which would indicate the population and decay of bound ω11B states, are not observed. Theπ0γ cross section of 0.3 nb MeV-1sr-1observed in the bound state energy regime between -100 and 0 MeV may be accounted for by yield leaking into the bound state regime because of the large in-medium width of the ω-meson. A comparison of the measured total energy distribution with calculations suggests the real partV0of the ω11B potential to be small and only weakly attractive withV0(ρ =ρ0) = - 15 ± 35(stat) ± 20(syst) MeV in contrast to some theoretical predictions of attractive potentials with a depth of 100-150 MeV. © 2014 Elsevier B.V.

  • The PANDA GEM-based TPC prototype

    16 August 2018

    We report on the development of a GEM-based TPC detector prototype for the PANDA experiment. The design and requirements of this device will be illustrated, with particular emphasis on the properties of the recently tested GEM-detector, the characterization of the read-out electronics and the development of the tracking software that allows to evaluate the GEM-TPC data. © 2010 Elsevier B.V. All rights reserved.