Emergence of slow-switching assemblies in structured neuronal networks

Unraveling the interplay between connectivity and spatio-temporal dynamics in neuronal networks is a key step to advance our understanding of neuronal information processing. Here we investigate how particular features of network connectivity underpin the propensity of neural networks to generate slow-switching assembly (SSA) dynamics, i.e., sustained epochs of increased firing within assemblies of neurons which transition slowly between different assemblies throughout the network. We show that the emergence of SSA activity is linked to spectral properties of the asymmetric synaptic weight matrix. In particular, the leading eigenvalues that dictate the slow dynamics exhibit a gap with respect to the bulk of the spectrum, and the associated Schur vectors exhibit a measure of block-localization on groups of neurons, thus resulting in coherent dynamical activity on those groups. Through simple rate models, we gain analytical understanding of the origin and importance of the spectral gap, and use these insights to develop new network topologies with alternative connectivity paradigms which also display SSA activity. Specifically, SSA dynamics involving excitatory and inhibitory neurons can be achieved by modifying the connectivity patterns between both types of neurons. We also show that SSA activity can occur at multiple timescales reflecting a hierarchy in the connectivity, and demonstrate the emergence of SSA in small-world like networks. Our work provides a step towards understanding how network structure (uncovered through advancements in neuroanatomy and connectomics) can impact on spatio-temporal neural activity and constrain the resulting dynamics.Comment: The first two authors contributed equally -- 18 pages, including supplementary material, 10 Figures + 2 SI Figure

Revealing cell assemblies at multiple levels of granularity

Background: Current neuronal monitoring techniques, such as calcium imaging and multi-electrode arrays, enable recordings of spiking activity from hundreds of neurons simultaneously. Of primary importance in systems neuroscience is the identification of cell assemblies: groups of neurons that cooperate in some form within the recorded population. New method: We introduce a simple, integrated framework for the detection of cell-assemblies from spiking data without a priori assumptions about the size or number of groups present. We define a biophysically-inspired measure to extract a directed functional connectivity matrix between both excitatory and inhibitory neurons based on their spiking history. The resulting network representation is analyzed using the Markov Stability framework, a graph theoretical method for community detection across scales, to reveal groups of neurons that are significantly related in the recorded time-series at different levels of granularity. Results and comparison with existing methods: Using synthetic spike-trains, including simulated data from leaky-integrate-and-fire networks, our method is able to identify important patterns in the data such as hierarchical structure that are missed by other standard methods. We further apply the method to experimental data from retinal ganglion cells of mouse and salamander, in which we identify cell-groups that correspond to known functional types, and to hippocampal recordings from rats exploring a linear track, where we detect place cells with high fidelity. Conclusions: We present a versatile method to detect neural assemblies in spiking data applicable across a spectrum of relevant scales that contributes to understanding spatio-temporal information gathered from systems neuroscience experiments

Long-term Health-related Quality of Life Outcomes Following Radical Cystectomy

Objective To evaluate the long-term (>5 years) health-related quality of life (HRQOL) outcomes following radical cystectomy, comparing Indiana pouch (IP), neobladder (NB), and ileal conduit (IC). Materials and Methods The departmental radical cystectomy database was queried to identify patients who underwent radical cystectomy and urinary diversion for bladder cancer between 1991 and 2009 and had not died. Three hundred patients were identified and sent the validated Bladder Cancer Index instrument. Results A total of 128 (43%) patients completed the survey. When adjusted for gender, age at surgery, surgeon, and time since surgery, IC and IP patients had significantly better urinary function than NB patients (P = .0013). Sexual bother was less in NB than IP (P = .0387). Among men ≥65 years of age, IC patients had significantly better urinary function (P = .0376) than NB patients (91.6 vs 49.4, respectively). Among men <65 years of age, IC and IP patients (76.0 and 82.8, respectively) had significantly better urinary function than NB patients (50.7) (P = .0199). Among women greater than 65 years, bowel bother was significantly better (P = .0095) for IC patients than IP patients (44.8 vs 69.5, respectively). Conclusion Urinary diversion type after radical cystectomy affects HRQOL differently in long-term survivors. Age and gender at surgery influenced HRQOL based on diversion procedure. Urinary function but not urinary bother was significantly better in IC and IP compared to NB diversions. Prospective longitudinal studies using validated HRQOL tools will further help guide preoperative diversion choice decisions between patient and surgeon

The Kepler Pixel Response Function

Kepler seeks to detect sequences of transits of Earth-size exoplanets orbiting Solar-like stars. Such transit signals are on the order of 100 ppm. The high photometric precision demanded by Kepler requires detailed knowledge of how the Kepler pixels respond to starlight during a nominal observation. This information is provided by the Kepler pixel response function (PRF), defined as the composite of Kepler's optical point spread function, integrated spacecraft pointing jitter during a nominal cadence and other systematic effects. To provide sub-pixel resolution, the PRF is represented as a piecewise-continuous polynomial on a sub-pixel mesh. This continuous representation allows the prediction of a star's flux value on any pixel given the star's pixel position. The advantages and difficulties of this polynomial representation are discussed, including characterization of spatial variation in the PRF and the smoothing of discontinuities between sub-pixel polynomial patches. On-orbit super-resolution measurements of the PRF across the Kepler field of view are described. Two uses of the PRF are presented: the selection of pixels for each star that maximizes the photometric signal to noise ratio for that star, and PRF-fitted centroids which provide robust and accurate stellar positions on the CCD, primarily used for attitude and plate scale tracking. Good knowledge of the PRF has been a critical component for the successful collection of high-precision photometry by Kepler.Comment: 10 pages, 5 figures, accepted by ApJ Letters. Version accepted for publication

Passive water control at the surface of a superhydrophobic lichen

Some lichens have a super-hydrophobic upper surface, which repels water drops, keeping the surface dry but probably preventing water uptake. Spore ejection requires water and is most efficient just after rainfall. This study was carried out to investigate how super-hydrophobic lichens manage water uptake and repellence at their fruiting bodies, or podetia. Drops of water were placed onto separate podetia of Cladonia chlorophaea and observed using optical microscopy and cryo-scanning-electron microscopy (cryo-SEM) techniques to determine the structure of podetia and to visualise their interaction with water droplets. SEM and optical microscopy studies revealed that the surface of the podetia was constructed in a three-level structural hierarchy. By cryo-SEM of water-glycerol droplets placed on the upper part of the podetium, pinning of the droplet to specific, hydrophilic spots (pycnidia/apothecia) was observed. The results suggest a mechanism for water uptake, which is highly sophisticated, using surface wettability to generate a passive response to different types of precipitation in a manner similar to the Namib Desert beetle. This mechanism is likely to be found in other organisms as it offers passive but selective water control

A Large Catalog of Homogeneous Ultra-Violet/Optical GRB Afterglows: Temporal and Spectral Evolution

We present the second Swift Ultra-Violet/Optical Telescope (UVOT) gamma-ray burst (GRB) afterglow catalog, greatly expanding on the first Swift UVOT GRB afterglow catalog. The second catalog is constructed from a database containing over 120,000 independent UVOT observations of 538 GRBs first detected by Swift, the High Energy Transient Explorer 2 (HETE2), the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), the Interplanetary Network (IPN), Fermi, and Astro-rivelatore Gamma a Immagini Leggero (AGILE). The catalog covers GRBs discovered from 2005 Jan 17 to 2010 Dec 25. Using photometric information in three UV bands, three optical bands, and a `white' or open filter, the data are optimally co-added to maximize the number of detections and normalized to one band to provide a detailed light curve. The catalog provides positional, temporal, and photometric information for each burst, as well as Swift Burst Alert Telescope (BAT) and X-Ray Telescope (XRT) GRB parameters. Temporal slopes are provided for each UVOT filter. The temporal slope per filter of almost half the GRBs are fit with a single power-law, but one to three breaks are required in the remaining bursts. Morphological comparisons with the X-ray reveal that approximately 75% of the UVOT light curves are similar to one of the four morphologies identified by Evans et al. (2009). The remaining approximately 25% have a newly identified morphology. For many bursts, redshift and extinction corrected UV/optical spectral slopes are also provided at 2000, 20,000, and 200,000 seconds.Comment: 44 pages, 14 figures, to be published in Astrophysical Journal Supplementa

The Political Consequences of Terrorism: Terror Events, Casualties, and Government Duration

How does transnational terrorism affect the stability of parliamentary governments? Does terrorism cause some governments to fail prematurely and/or does it enhance the probability that some governments will stay in office longer than they otherwise would? Using a duration model on a sample of 18 advanced parliamentary democracies between the late-1960s and 2003, we find that terrorism exacerbates the likelihood of government failure for some governments but not others. Our principal finding is that right-oriented governments are able to keep their hold on power more than left-wing governments when confronted with transnational terrorism. Keywords: terrorism, government duration, partsianship, casualties What are the political consequences of transnational terror attacks? 1 While research demonstrates that transnational terror attacks have economic consequences such as reducing growth (Gaibulloev and Sandler 2008), we know less about how terror attacks affect domestic politics in democratic states. If terror attacks affect domestic politics, do these attacks affect all democratic states and governments equally? To address one aspect of this broader question, we ask how terror events and their resulting casualties influence the durability of parliamentary governments. Specifically, we examine whether terror attacks lead to *The authors wish to thank Daina Chiba and Joseph Young for his helpful comments and suggestions. We also thank Jonathan Martin and Tyson Meredith for their research assistance. Replication materials and robustness checks can be found on web.missouri.edu/~williamslaro/. 1 While we do not question that both transnational and domestic terrorism have economic and political domestic consequences, the causes, goals, and responses to each type of terrorism are likely to be different. As Enders, Sandler, and Gaibulloev (2011) suggest, transnational terrorism is likely to emerge from broader grievances in other countries (see Savun and Phillips (2009) about states&apos; foreign policies and terrorism), which leads to terror attacks aimed at drawing attention to national issues beyond the borders of the attacked country attempting to force policy changes desired by the terrorists. In contrast, they suggest that domestic terrorism emerges either from separatists, individuals trying to overthrow the government, or issue-specific causes such as social justice or environmental degradation (that is, the Earth Liberation Front). Furthermore, responses to transnational and domestic terrorism by the government and the public may differ. Transnational terrorism requires a large increase in defense spending and defense efforts while domestic terrorism is usually dealt with through the police and requires very little international collaboration. Given these differences, we focus on shocks from transnational terrorism

AMiBA Wideband Analog Correlator

A wideband analog correlator has been constructed for the Yuan-Tseh Lee Array for Microwave Background Anisotropy. Lag correlators using analog multipliers provide large bandwidth and moderate frequency resolution. Broadband IF distribution, backend signal processing and control are described. Operating conditions for optimum sensitivity and linearity are discussed. From observations, a large effective bandwidth of around 10 GHz has been shown to provide sufficient sensitivity for detecting cosmic microwave background variations.Comment: 28 pages, 23 figures, ApJ in press