Effects of synaptic synchrony on the neuronal input-output relationship

The firing rate of individual neurons depends on the firing frequency of their distributed synaptic inputs, with linear and nonlinear relations subserving different computational functions. This letter explores the relationship between the degree of synchrony among excitatory synapses and the linearity of the response using detailed compartmental models of cortical pyramidal cells. Synchronous input resulted in a linear input-output relationship, while asynchronous stimulation yielded sub- and supraproportional outputs at low and high frequencies, respectively. The dependence of input-output linearity on synchrony was sigmoidal and considerably robust with respect to dendritic location, stimulus irregularity, and alteration of active and synaptic properties. Moreover, synchrony affected firing rate differently at lower and higher input frequencies. A reduced integrate-and-fire model suggested a mechanism explaining these results based on spatiotemporal integration, with fundamental implications relating synchrony to memory encoding.     

The impact of Arabic morphological segmentation on broad-coverage English-to-Arabic statistical machine translation

Morphologically rich languages pose a challenge for statistical machine translation (SMT). This challenge is magnified when translating into a morphologically rich language. In this work we address this challenge in the framework of a broad-coverage English-to-Arabic phrase based statistical machine translation (PBSMT). We explore the largest-to-date set of Arabic segmentation schemes ranging from full word form to fully segmented forms and examine the effects on system performance. Our results show a difference of 2.31 BLEU points averaged over all test sets between the best and worst segmentation schemes indicating that the choice of the segmentation scheme has a significant effect on the performance of an English-to-Arabic PBSMT system in a large data scenario. We show that a simple segmentation scheme can perform as well as the best and more complicated segmentation scheme. An in-depth analysis on the effect of segmentation choices on the components of a PBSMT system reveals that text fragmentation has a negative effect on the perplexity of the language models and that aggressive segmentation can significantly increase the size of the phrase table and the uncertainty in choosing the candidate translation phrases during decoding. An investigation conducted on the output of the different systems, reveals the complementary nature of the output and the great potential in combining them.     

Reaction‐Bonded Boron Carbide/Magnesium–Silicon Composites

Reaction‐bonded boron carbide was manufactured by infiltrating porous boron carbide preforms at 1273 K with a Mg‐Si eutectic alloy. The resulting composite material consists, in addition to the original B₄C, of SiC, Mg₂Si, and a Mg‐rich complex boride/carbide Mg_x(Al,Si)_y(B,C)_z phase. The composites display high hardness (1700 HV), Young's modulus (356 MPa) and a moderate bending strength (230 MPa). The ballistic efficiency (of about 6.7), as determined by the depth of penetration method, is much higher than that of alumina and similar to that of silicon‐infiltrated reaction‐bonded composites.     

Investigation of Slow-Propagation Tungsten Delay Mixtures

The burning characteristics and reliability of slow-propagation tungsten delay mixtures (characteristic time 38–42 s/in) were studied experimentally by using various mixture compositions and tungsten particle sizes. Thermal analysis showed that potassium perchlorate is vital to initiate the reaction. Gravimetric analysis indicated that during the reaction overall mass loss of the delay mixtures did not exceed 5% compared to their initial mass. A mixture containing a tungsten powder of 6–8 μm was found to have the lowest effective activation energy, facilitating good ignition and burning zone propagation. The results of this study were successfully used to design a tiny delay which exhibited reliable ignition, burning and propagation characteristics under extreme conditions.     

The Unreasonable Effectiveness of Data

Problems that involve interacting with humans, such as natural language understanding, have not proven to be solvable by concise, neat formulas like F = ma. Instead, the best approach appears to be to embrace the complexity of the domain and address it by harnessing the power of data: if other humans engage in the tasks and generate large amounts of unlabeled, noisy data, new algorithms can be used to build high-quality models from the data.     

Analysis of bandgap characteristics of two-dimensional periodic structures by using the source-model technique

We introduce a solution based on the source-model technique for periodic structures for the problem of electromagnetic scattering by a two-dimensional photonic bandgap crystal slab illuminated by a transverse-magnetic plane wave. The proposed technique takes advantage of the periodicity of the slab by solving the problem within the unit cell of the periodic structure. The results imply the existence of a frequency bandgap and provide a valuable insight into the relationship between the dimensions of a finite periodic structure and its frequency bandgap characteristics. A comparison shows a discrepancy between the frequency bandgap obtained for a very thick slab and the bandgap obtained by solving the corresponding two-dimensionally infinite periodic structure. The final part of the paper is devoted to explaining in detail this apparent discrepancy.     

Schema mediation for large-scale semantic data sharing

Intuitively, data management and data integration tools should be well suited for exchanging information in a semantically meaningful way. Unfortunately, they suffer from two significant problems: they typically require a common and comprehensive schema design before they can be used to store or share information, and they are difficult to extend because schema evolution is heavyweight and may break backward compatibility. As a result, many large-scale data sharing tasks are more easily facilitated by non-database-oriented tools that have little support for semantics.The goal of the peer data management system (PDMS) is to address this need: we propose the use of a decentralized, easily extensible data management architecture in which any user can contribute new data, schema information, or even mappings between other peers schemas. PDMSs represent a natural step beyond data integration systems, replacing their single logical schema with an interlinked collection of semantic mappings between peers individual schemas.This paper considers the problem of schema mediation in a PDMS. Our first contribution is a flexible language for mediating between peer schemas that extends known data integration formalisms to our more complex architecture. We precisely characterize the complexity of query answering for our language. Next, we describe a reformulation algorithm for our language that generalizes both global-as-view and local-as-view query answering algorithms. Then we describe several methods for optimizing the reformulation algorithm and an initial set of experiments studying its performance. Finally, we define and consider several global problems in managing semantic mappings in a PDMS.Received: 16 December 2002, Accepted: 14 April 2003, Published online: 12 December 2003Edited by: V. Atluri     

Collision efficiency distribution of a bacterial suspension flowing through porous media and implications for field-scale transport

The collision efficiency (alpha) distribution of a bacterial population was determined using multiple packed-bed columns of varying lengths and analyzing the bacteria clean-bed breakthrough concentrations using a distributed colloid filtration theory. This technique allows the alpha distribution to be determined independently from other effects that can cause non-exponential deposition, including detachment and blocking. It was found that multiple probability density functions (PDF's) could accurately replicate the experimental data. Regardless of which PDF was used, a distributed alpha resulted in significantly greater predicted field-scale transport than when using a single alpha. However, there were wide variations in the predicted field-scale transport between the different distributions, suggesting that lab-scale experiments may not be readily utilized to determine the specific PDF that best represents alpha at the field scale. Finally, blocking was observed in the column effluent curves, underscoring the fact that if non-clean-bed processes occur then an approach such as that utilized in the current study may be used to separate the non-clean-bed and clean-bed processes when determining the collision efficiency distribution.