3,159 research outputs found
A Theory of Cortical Neural Processing.
This dissertation puts forth an original theory of cortical neural processing that is unique in its view of the interplay of chaotic and stable oscillatory neurodynamics and is meant to stimulate new ideas in artificial neural network modeling. Our theory is the first to suggest two new purposes for chaotic neurodynamics: (i) as a natural means of representing the uncertainty in the outcome of performed tasks, such as memory retrieval or classification, and (ii) as an automatic way of producing an economic representation of distributed information. We developed new models, to better understand how the cerebral cortex processes information, which led to our theory. Common to these models is a neuron interaction function that alternates between excitatory and inhibitory neighborhoods. Our theory allows characteristics of the input environment to influence the structural development of the cortex. We view low intensity chaotic activity as the a priori uncertain base condition of the cortex, resulting from the interaction of a multitude of stronger potential responses. Data, distinguishing one response from many others, drives bifurcations back toward the direction of less complex (stable) behavior. Stability appears as temporary bubble-like clusters within the boundaries of cortical columns and begins to propagate through frequency sensitive and non-specific neurons. But this is limited by destabilizing long-path connections. An original model of the post-natal development of ocular dominance columns in the striate cortex is presented and compared to autoradiographic images from the literature with good matching results. Finally, experiments are shown to favor computed update order over traditional approaches for better performance of the pattern completion process
Hydrogen in Type Ic Supernovae?
By definition, a Type Ic supernova (SN Ic) does not have conspicuous lines of
hydrogen or helium in its optical spectrum. SNe Ic usually are modelled in
terms of the gravitational collapse of bare carbon-oxygen cores. We consider
the possibility that the spectra of ordinary (SN 1994I-like) SNe Ic have been
misinterpreted, and that SNe Ic eject hydrogen. An absorption feature usually
attributed to a blend of Si II 6355 and C II 6580 may be produced by H-alpha.
If SN 1994I-like SNe Ic eject hydrogen, the possibility that hypernova (SN
1998bw-like) SNe Ic, some of which are associated with gamma-ray bursts, also
eject hydrogen should be considered. The implications of hydrogen for SN Ic
progenitors and explosion models are briefly discussed.Comment: Accepted by PASP. Several significant changes including one
additional figur
Visualising very large phylogenetic trees in three dimensional hyperbolic space
BACKGROUND: Common existing phylogenetic tree visualisation tools are not able to display readable trees with more than a few thousand nodes. These existing methodologies are based in two dimensional space. RESULTS: We introduce the idea of visualising phylogenetic trees in three dimensional hyperbolic space with the Walrus graph visualisation tool and have developed a conversion tool that enables the conversion of standard phylogenetic tree formats to Walrus' format. With Walrus, it becomes possible to visualise and navigate phylogenetic trees with more than 100,000 nodes. CONCLUSION: Walrus enables desktop visualisation of very large phylogenetic trees in 3 dimensional hyperbolic space. This application is potentially useful for visualisation of the tree of life and for functional genomics derivatives, like The Adaptive Evolution Database (TAED)
Enabling III-V-based optoelectronics with low-cost dynamic hydride vapor phase epitaxy
Silicon is the dominant semiconductor in many semiconductor device
applications for a variety of reasons, including both performance and cost.
III-V materials have improved performance compared to silicon, but currently
they are relegated to applications in high-value or niche markets due to the
absence of a low-cost, high-quality production technique. Here we present an
advance in III-V materials synthesis using hydride vapor phase epitaxy that has
the potential to lower III-V semiconductor deposition costs by orders of
magnitude while maintaining the requisite optoelectronic material quality that
enables III-V-based technologies to outperform Si. We demonstrate the impacts
of this advance by addressing the use of III-Vs in terrestrial photovoltaics, a
highly cost-constrained market. The emergence of a low-cost III-V deposition
technique will enable III-V electronic and opto-electronic devices, with all
the benefits that they bring, to permeate throughout modern society.Comment: pre-prin
Device and method for applying internal pressure to a hollow cylinder
PatentA device and method for applying an internal pressure and resultant hoop stress to a hollow cylindrical object, such as a test cylinder, utilizing an expandable cylinder and upper and lower conically shaped rams. The device is inserted into a test cylinder, and when a compressive force is applied to the upper and lower conically shaped rams, the rams move vertically into upper and lower conically shaped cavities of the expandable cylinder. The sloped inner surfaces of the conically shaped upper and lower cavities convert the vertical motion into an outward radial motion, applying an internal pressure and resultant hoop stress to the cylindrical object
Down but not out: properties of the molecular gas in the stripped Virgo Cluster early-type galaxy NGC4526
We present ALMA data on the 3mm continuum emission, CO isotopologues (12CO,
13CO, C18O), and high-density molecular tracers (HCN, HCO+, HNC, HNCO, CS, CN,
and CH3OH) in NGC4526. These data enable a detailed study of the physical
properties of the molecular gas in a longtime resident of the Virgo Cluster;
comparisons to more commonly-studied spiral galaxies offer intriguing hints
into the processing of molecular gas in the cluster environment. Many molecular
line ratios in NGC4526, along with our inferred abundances and CO/H2 conversion
factors, are similar to those found in nearby spirals. One striking exception
is the very low observed 12CO/13CO(1-0) line ratio, , which is
unusually low for spirals though not for Virgo Cluster early-type galaxies. We
carry out radiative transfer modeling of the CO isotopologues with some
archival (2-1) data, and we use Bayesian analysis with Markov chain Monte Carlo
techniques to infer the physical properties of the CO-emitting gas. We find
surprisingly low [12CO/13CO] abundance ratios of and
at radii of 0.4 kpc and 1 kpc. The emission from the
high-density tracers HCN, HCO+, HNC, CS and CN is also relatively bright, and
CN is unusually optically thick in the inner parts of NGC4526. These features
hint that processing in the cluster environment may have removed much of the
galaxy's relatively diffuse, optically thinner molecular gas along with its
atomic gas. Angular momentum transfer to the surrounding intracluster medium
may also have caused contraction of the disk, magnifying radial gradients such
as we find in [13CO/C18O]. More detailed chemical evolution modeling would be
interesting in order to explore whether the unusual [12CO/13CO] abundance ratio
is entirely an environmental effect or whether it also reflects the relatively
old stellar population in this early-type galaxy.Comment: Submitted to ApJ. Comments are welcome
Synthesis and characterization of a series of nickel(II) alkoxide precursors and their utility for Ni(0) nanoparticle production
A series of nickel(ii) aryloxide ([Ni(OAr)2(py)x]) precursors was synthesized from an amide-alcohol exchange using [Ni(NR2)2] in the presence of pyridine (py). The H-OAr selected were the mono- and di-ortho-substituted 2-alkyl phenols: alkyl = methyl (H-oMP), iso-propyl (H-oPP), tert-butyl (H-oBP) and 2,6-di-alkyl phenols (alkyl = di-iso-propyl (H-DIP), di-tert-butyl (H-DBP), di-phenyl (H-DPhP)). The crystalline products were solved as solvated monomers and structurally characterized as [Ni(OAr)2(py)x], where x = 4: OAr = oMP (1), oPP (2); x = 3: OAr = oBP (3), DIP (4); x = 2: OAr = DBP (5), DPhP (6). The excited states (singlet or triplet) and various geometries of 1-6 were identified by experimental UV-vis and verified by computational modeling. Magnetic susceptibility of the representative compound 4 was fit to a Curie Weiss model that yielded a magnetic moment of 4.38(3)μB consistent with a Ni2+ center. Compounds 1 and 6 were selected for decomposition studied under solution precipitation routes since they represent the two extremes of coordination. The particle size and crystalline structure were characterized using transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD). The materials isolated from 1 and 6 were found by TEM to form irregular shape nanomaterials (8-15 nm), which by PXRD were found to be Ni0 hcp (PDF: 01-089-7129) and fcc (PDF: 01-070-0989), respectively
Enantioselective isothiourea-catalysed trans-dihydropyridinone synthesis using saccharin-derived ketimines : scope and limitations
The authors thank Syngenta and the EPSRC (grant code EP/K503162/1) (DGS) for funding. The European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013) ERC Grant Agreement No. 279850 is also acknowledged (CMY). ADS thanks the Royal Society for a Wolfson Research Merit Award. We also thank the EPSRC UK National Mass Spectrometry Facility at Swansea University.The catalytic enantioselective synthesis of a range of trans-dihydropyridinones from aryl-, heteroaryl- and alkenylacetic acids and saccharin-derived ketimines with good to excellent stereocontrol (15 examples, up to >95:5 dr, up to >99:1 er) is reported. After extensive optimisation, HyperBTM proved the optimal isothiourea catalyst for this transformation at −78 °C, giving trans-dihydropyridones with generally excellent levels of diastereo- and enantioselectivity.Publisher PDFPeer reviewe
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