Showing papers by "California Institute of Technology published in 1992"
15 Aug 1992
TL;DR: As part of a series of evaluated sets, rate constants and photochemical cross sections compiled by the NASA Panel for Data Evaluation are provided in this article, with particular emphasis on the ozone layer and its possible perturbation by anthropogenic and natural phenomena.
Abstract: As part of a series of evaluated sets, rate constants and photochemical cross sections compiled by the NASA Panel for Data Evaluation are provided. The primary application of the data is in the modeling of stratospheric processes, with particular emphasis on the ozone layer and its possible perturbation by anthropogenic and natural phenomena. Copies of this evaluation are available from the Jet Propulsion Laboratory.
3,218 citations
TL;DR: A quantitative and practical Bayesian framework is described for learning of mappings in feedforward networks that automatically embodies "Occam's razor," penalizing overflexible and overcomplex models.
Abstract: A quantitative and practical Bayesian framework is described for learning of mappings in feedforward networks. The framework makes possible (1) objective comparisons between solutions using alternative network architectures, (2) objective stopping rules for network pruning or growing procedures, (3) objective choice of magnitude and type of weight decay terms or additive regularizers (for penalizing large weights, etc.), (4) a measure of the effective number of well-determined parameters in a model, (5) quantified estimates of the error bars on network parameters and on network output, and (6) objective comparisons with alternative learning and interpolation models such as splines and radial basis functions. The Bayesian "evidence" automatically embodies "Occam's razor," penalizing overflexible and overcomplex models. The Bayesian approach helps detect poor underlying assumptions in learning models. For learning models well matched to a problem, a good correlation between generalization ability and the Bayesian evidence is obtained.
2,906 citations
TL;DR: It is found that there is decorrelation increasing with time but that digital terrain model generation remains feasible and such a technique could provide a global digital terrain map.
Abstract: A radar interferometric technique for topographic mapping of surfaces, implemented utilizing a single synthetic aperture radar (SAR) system in a nearly repeating orbit, is discussed. The authors characterize the various sources contributing to the echo correlation statistics, and isolate the term which most closely describes surficial change. They then examine the application of this approach to topographic mapping of vegetated surfaces which may be expected to possess varying backscatter over time. It is found that there is decorrelation increasing with time but that digital terrain model generation remains feasible. The authors present such a map of a forested area in Oregon which also includes some nearly unvegetated lava flows. Such a technique could provide a global digital terrain map. >
2,167 citations
TL;DR: The goal of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Project is to detect and study astrophysical gravitational waves and use data from them for research in physics and astronomy.
Abstract: The goal of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Project is to detect and study astrophysical gravitational waves and use data from them for research in physics and astronomy. LIGO will support studies concerning the nature and nonlinear dynamics of gravity, the structures of black holes, and the equation of state of nuclear matter. It will also measure the masses, birth rates, collisions, and distributions of black holes and neutron stars in the universe and probe the cores of supernovae and the very early universe. The technology for LIGO has been developed during the past 20 years. Construction will begin in 1992, and under the present schedule, LIGO's gravitational-wave searches will begin in 1998.
2,032 citations
TL;DR: Because of high cloning efficiency, easy manipulation of the cloned DNA, and stable maintenance of inserted DNA, the BAC system may facilitate construction of DNA libraries of complex genomes with fuller representation and subsequent rapid analysis of complex genomic structure.
Abstract: A bacterial cloning system for mapping and analysis of complex genomes has been developed. The BAC system (for bacterial artificial chromosome) is based on Escherichia coli and its single-copy plasmid F factor. It is capable of maintaining human genomic DNA fragments of greater than 300 kilobase pairs. Individual clones of human DNA appear to be maintained with a high degree of structural stability in the host, even after 100 generations of serial growth. Because of high cloning efficiency, easy manipulation of the cloned DNA, and stable maintenance of inserted DNA, the BAC system may facilitate construction of DNA libraries of complex genomes with fuller representation and subsequent rapid analysis of complex genomic structure.
1,735 citations
Book•
01 Jan 1992TL;DR: The second edition of a popular book on combinatorics as discussed by the authors is a comprehensive guide to the whole of the subject, dealing in a unified manner with, for example, graph theory, extremal problems, designs, colorings and codes.
Abstract: This is the second edition of a popular book on combinatorics, a subject dealing with ways of arranging and distributing objects, and which involves ideas from geometry, algebra and analysis. The breadth of the theory is matched by that of its applications, which include topics as diverse as codes, circuit design and algorithm complexity. It has thus become essential for workers in many scientific fields to have some familiarity with the subject. The authors have tried to be as comprehensive as possible, dealing in a unified manner with, for example, graph theory, extremal problems, designs, colorings and codes. The depth and breadth of the coverage make the book a unique guide to the whole of the subject. The book is ideal for courses on combinatorical mathematics at the advanced undergraduate or beginning graduate level. Working mathematicians and scientists will also find it a valuable introduction and reference.
1,678 citations
TL;DR: It is demonstrated that LEAFY is the Arabidopsis homolog of the FLORICAULA gene, which controls floral meristem identity in the distantly related species Antirrhinum majus.
1,408 citations
TL;DR: Within a Bayesian learning framework, objective functions are discussed that measure the expected informativeness of candidate measurements that depend on the assumption that the hypothesis space is correct.
Abstract: Learning can be made more efficient if we can actively select particularly salient data points. Within a Bayesian learning framework, objective functions are discussed that measure the expected informativeness of candidate measurements. Three alternative specifications of what we want to gain information about lead to three different criteria for data selection. All these criteria depend on the assumption that the hypothesis space is correct, which may prove to be their main weakness.
1,316 citations
TL;DR: A prominent, brain-specific protein (PSD-95) enriched in the postsynaptic density fraction from rat brain is highly similar to the Drosophila lethal(1)discs-large-1 (dlg) tumor suppressor protein, suggesting molecular mechanisms critical for growth control in developing organisms may also regulate synapse formation, stabilization, or function in the adult brain.
1,178 citations
TL;DR: The primate visual system contains dozens of distinct areas in the cerebral cortex and several major subcortical structures that are extensively interconnected in a distributed hierarchical network that contains several intertwined processing streams.
Abstract: The primate visual system contains dozens of distinct areas in the cerebral cortex and several major subcortical structures. These subdivisions are extensively interconnected in a distributed hierarchical network that contains several intertwined processing streams. A number of strategies are used for efficient information processing within this hierarchy. These include linear and nonlinear filtering, passage through information bottlenecks, and coordinated use of multiple types of information. In addition, dynamic regulation of information flow within and between visual areas may provide the computational flexibility needed for the visual system to perform a broad spectrum of tasks accurately and at high resolution.
1,151 citations
TL;DR: Responses from neurons in area V1 of the alert macaque monkey to textured patterns modeled after stimuli used in psychophysical experiments of pop- out are consistent with a possible functional role of V1 cells in the mediation of perceptual pop-out and in the segregation of texture borders.
Abstract: 1. We recorded responses from neurons in area V1 of the alert macaque monkey to textured patterns modeled after stimuli used in psychophysical experiments of pop-out. Neuronal responses to a single...
TL;DR: In this paper, single zircons from two Early Cambrian volcanic horizons have been analyzed using the SHRIMP ion microprobe, revealing the presence of both detrital or xenocrystic grains as well as areas within grains that have lost radiogenic Pb.
Abstract: Single zircons from two Early Cambrian volcanic horizons have been analysed using the SHRIMP ion microprobe. Full details of the analytical procedures and data reduction are given. Zircons from tuff within the Lie de Vin Formation, near Tiout, Morocco, show little spread in U-Pb age and have a mean value of 521 ± 7 Ma (2σ). Those from a bentonite within unit 5 of the Meishucun section near Kunming, southern China, show relatively dispersed U-Pb ages, revealing the presence of both detrital or xenocrystic grains as well as areas within grains that have lost radiogenic Pb. The main population has a mean age of 525 ± 7 Ma, but a mean 207 Pb/ 206 Pb age of 539 ± 34 Ma which is a maximum estimate for the bentonite age. These results conflict with previous Rb-Sr whole rock ages of c. 580 Ma for overlying Cambrian shales at Meishucun, and c. 570 Ma for Atdabanian shales from the E. Yangtse Gorges area.
TL;DR: The Einstein-Podolsky-Rosen paradox is demonstrated experimentally for dynamical variables having a continuous spectrum, where the continuous optical amplitudes of a signal beam are inferred in turn from those of a spatially separated but strongly correlated idler beam generated by nondegenerate parametric amplification.
Abstract: The Einstein-Podolsky-Rosen paradox is demonstrated experimentally for dynamical variables having a continuous spectrum. As opposed to previous work with discrete spin or polarization variables, the continuous optical amplitudes of a signal beam are inferred in turn from those of a spatially separated but strongly correlated idler beam generated by nondegenerate parametric amplification. The uncertainty product for the variances of these inferences is observed to be 0.70±0.01, which is below the limit of unity required for the demonstration of the paradox.
TL;DR: An investigation of the spectral response of a small collection of two-state atoms strongly coupled to the field of a high-finesse optical resonator finds a coupling-induced normal-mode splitting even for one intracavity atom, representing a direct spectroscopic measurement of the so-called vacuum Rabi splitting for the atom-cavity system.
Abstract: An investigation of the spectral response of a small collection of two-state atoms strongly coupled to the field of a high-finesse optical resonator is described for mean number N¯≤10 atoms. For weak excitation, a coupling-induced normal-mode splitting is observed even for one intracavity atom, representing a direct spectroscopic measurement of the so-called vacuum Rabi splitting for the atom-cavity system.
TL;DR: Nominally anhydrous minerals constitute a significant reservoir for mantle hydrogen, possibly accommodating all water in the depleted mantle and providing a possible mechanism to recycle water from Earth's surface into the deep mantle.
Abstract: Most minerals of Earth's upper mantle contain small amounts of hydrogen, structurally bound as hydroxyl (OH). The OH concentration in each mineral species is variable, in some cases reflecting the geological environment of mineral formation. Of the major mantle minerals, pyroxenes are the most hydrous, typically containing ∼200 to 500 parts per million H_2O by weight, and probably dominate the water budget and hydrogen geochemistry of mantle rocks that do not contain a hydrous phase. Garnets and olivines commonly contain ∼1 to 50 parts per million. Nominally anhydrous minerals constitute a significant reservoir for mantle hydrogen, possibly accommodating all water in the depleted mantle and providing a possible mechanism to recycle water from Earth's surface into the deep mantle.
TL;DR: This analysis indicates that single mammalian neural crest cells are multipotent, able to generate at least neurons and Schwann cells like their avian counterparts, and generate multipotent progeny, indicating that they are capable of self-renewal and therefore are stem cells.
TL;DR: Cloned the AP3 gene from Arabidopsis based on its homology to the homeotic flower gene deficiens from the distantly related plant Antirrhinum majus and genetic mapping analysis prove that the DEFA homolog is AP3.
TL;DR: In this paper, an Ru(II) complex with a strained olefin was used to produce a carbene species that polymerizes norbornene in organic media both in the absence and presence of protic/aqueous solvents.
Abstract: During the past two decades, intense research efforts have enabled an in-depth understanding of the olefin metathesis reaction as catalyzed by early transition metal complexes. In contrast, the nature of the intermediates and the reaction mechanism for group VIII transition metal metathesis catalysts remain elusive. Such knowledge is important in view of the promise group VIII metals show in polymerizing a wide variety of functionalized cyclic olefins in protic solvents. Highly active late transition metal catalysts should also open the way to the metathesis of functionalized acyclic olefins. Previous studies in our group have focused on the chemistry of highly active, functional-group-tolerant catalysts prepared from aquoruthenium(II) olefin complexes. In these systems, characterization of the catalytic intermediates is difficult due to their very low concentrations and high activity in the reaction mixtures. Although it is reasonable to assume that the active species are ruthenacyclobutanes and ruthenium carbenes (ruthenaolefins), the oxidation state and ligation of these intermediates are not known. Furthermore, the discrete ruthenium carbene complexes that have been isolated to date do not exhibit both metathesis activity and stability to protic/aqueous solvents. We report here the reaction of an Ru(II) complex with a strained olefin to produce a carbene species that polymerizes norbornene in organic media both in the absence and presence of protic/aqueous solvents. In both solvent systems, a stable propagating carbene complex can be observed throughout the course of the polymerization, as has been previously found with titanium, tantalum, tungsten, molybdenum, and ruthenium complexes.
01 Apr 1992
TL;DR: In this paper, the authors derived the signal statistics, an optimal estimator of the interferometric phase, and the expression necessary to calculate the height error budget of the InSAR system.
Abstract: A derivation of the signal statistics, an optimal estimator of the interferometric phase, and the expression necessary to calculate the height-error budget are presented. These expressions are used to derive methods of optimizing the parameters of the interferometric synthetic aperture radar system (InSAR), and are then employed in a specific design example for a system to perform high-resolution global topographic mapping with a one-year mission lifetime, subject to current technological constraints. A Monte Carlo simulation of this InSAR system is performed to evaluate its performance for realistic topography. The results indicate that this system has the potential to satisfy the stringent accuracy and resolution requirements for geophysical use of global topographic data.
TL;DR: It is demonstrated that the Bayesian framework for model comparison described for regression models in MacKay (1992a,b) can also be applied to classification problems and an information-based data selection criterion is derived and demonstrated within this framework.
Abstract: Three Bayesian ideas are presented for supervised adaptive classifiers. First, it is argued that the output of a classifier should be obtained by marginalizing over the posterior distribution of the parameters; a simple approximation to this integral is proposed and demonstrated. This involves a "moderation" of the most probable classifier's outputs, and yields improved performance. Second, it is demonstrated that the Bayesian framework for model comparison described for regression models in MacKay (1992a,b) can also be applied to classification problems. This framework successfully chooses the magnitude of weight decay terms, and ranks solutions found using different numbers of hidden units. Third, an information-based data selection criterion is derived and demonstrated within this framework.
TL;DR: In this article, it is argued that a photoionizing background of the magnitude suggested by applying the Gunn-Peterson constraint to high-redshift quasars, would strongly suppress the cooling of a hydrogen-helium plasma and so inhibit the formation of dwarf galaxies.
Abstract: In hierarchical clustering theories, some sort of feedback mechanism is required to prevent most of the baryonic material collapsing into subgalactic objects at high redshifts. It is argued that a photoionizing background of the magnitude suggested by applying the Gunn-Peterson constraint to high-redshift quasars, would strongly suppress the cooling of a hydrogen-helium plasma and so inhibit the formation of dwarf galaxies. The effectiveness of this mechanism depends on the spectrum of the photoionizing radiation and so galaxy formation could depend on the proximity of protogalactic perturbations to unusual sources of hard photons such as luminous quasars
TL;DR: Neither the FeMo-cofactor nor the P-clusters are exposed to the surface, suggesting that substrate entry, electron transfer, and product release must involve a carefully regulated sequence of interactions between the MoFe-protein and Fe-protein of nitrogenase.
Abstract: Structural models for the nitrogenase FeMo-cofactor and P-clusters are proposed based on crystallographic analysis of the nitrogenase molybdenum-iron (MoFe)-protein from Azotobacter vinelandii at 2.7 angstrom resolution. Each center consists of two bridged clusters; the FeMo-cofactor has 4Fe:3S and 1Mo:3Fe:3S clusters bridged by three non-protein ligands, and the P-clusters contain two 4Fe:4S clusters bridged by two cysteine thiol ligands. Six of the seven Fe sites in the FeMo-cofactor appear to have trigonal coordination geometry, including one ligand provided by a bridging group. The remaining Fe site has tetrahedral geometry and is liganded to the side chain of Cys alpha 275. The Mo site exhibits approximate octahedral coordination geometry and is liganded by three sulfurs in the cofactor, two oxygens from homocitrate, and the imidazole side chain of His alpha 442. The P-clusters are liganded by six cysteine thiol groups, two which bridge the two clusters, alpha 88 and beta 95, and four which singly coordinate the remaining Fe sites, alpha 62, alpha 154, beta 70, and beta 153. The side chain of Ser beta 188 may also coordinate one iron. The polypeptide folds of the homologous alpha and beta subunits surrounding the P-clusters are approximately related by a twofold rotation that may be utilized in the binding interactions between the MoFe-protein and the nitrogenase Fe-protein. Neither the FeMo-cofactor nor the P-clusters are exposed to the surface, suggesting that substrate entry, electron transfer, and product release must involve a carefully regulated sequence of interactions between the MoFe-protein and Fe-protein of nitrogenase.
TL;DR: The receptor system for CNTF is surprisingly unlike those used by the nerve growth factor family of neurotrophic factors, but is instead related to those usedBy a subclass of hematopoietic cytokines.
TL;DR: Fully rotationally resolved spectra of three isotopic species of 1:1 clusters of benzene with water were fit to yield moments of inertia that demonstrate unambiguously that water is positioned above the benzene plane in nearly free internal rotation with both hydrogen atoms pointing toward the π cloud.
Abstract: Fully rotationally resolved spectra of three isotopic species of 1:1 clusters of benzene with water (H_2O, D_2O, and HDO) were fit to yield moments of inertia that demonstrate unambiguously that water is positioned above the benzene plane in nearly free internal rotation with both hydrogen atoms pointing toward the π cloud. Ab initio calculations (MP2 level of electron correlation and 6-31 G** basis set with basis set superposition error corrections) predict a binding energy D_e ≳ 1.78 kilocalories per mole. In both the experimental and theoretical structures, water is situated nearly 1 angstrom within the van der Waals contacts of the monomers, a clear manifestation of hydrogen bond formation in this simple model of aqueous-π electron interactions.
TL;DR: In this paper, the current theoretical and observational understanding of the role of the hydrological cycle in the climate system and its intimate connection to the energy cycle is evaluated, and the main advances expected in modeling and observations in the coming decade.
Abstract: The current theoretical and observational understanding of the roles of the hydrological cycle in the climate system and its intimate connection to the energy cycle is evaluated. An attempt is made to show why the hydrological cycle has emerged as the central element in studies of climate change and to anticipate the main advances expected in modeling and observations in the coming decade.
TL;DR: In this article, the authors show that photorefractive media can support a new type of spatial soliton, in which the diffraction is balanced by the self-scattering (two-wave mixing) of the beam spatial frequency components.
Abstract: We show that photorefractive media can support a new type of spatial soliton, in which the diffraction is balanced by the self-scattering (two-wave mixing) of the beam spatial frequency components. This photorefractive soliton possessses some unique properties, such as independence of the absolute light intensity, and can experience absorption (or gain) with no change in its transverse structure.
TL;DR: The crystal structure of the nitrogenase Fe-protein from Azotobacter vinelandii has been determined and it appears that interactions between the nucleotide and cluster sites must be indirectly coupled by allosteric changes occurring at the subunit interface.
Abstract: The nitrogenase enzyme system catalyzes the ATP (adenosine triphosphate)-dependent reduction of dinitrogen to ammonia during the process of nitrogen fixation. Nitrogenase consists of two proteins: the iron (Fe)-protein, which couples hydrolysis of ATP to electron transfer, and the molybdenum-iron (MoFe)-protein, which contains the dinitrogen binding site. In order to address the role of ATP in nitrogen fixation, the crystal structure of the nitrogenase Fe-protein from Azotobacter vinelandii has been determined at 2.9 angstrom (A) resolution. Fe-protein is a dimer of two identical subunits that coordinate a single 4Fe:4S cluster. Each subunit folds as a single alpha/beta type domain, which together symmetrically ligate the surface exposed 4Fe:4S cluster through two cysteines from each subunit. A single bound ADP (adenosine diphosphate) molecule is located in the interface region between the two subunits. Because the phosphate groups of this nucleotide are approximately 20 A from the 4Fe:4S cluster, it is unlikely that ATP hydrolysis and electron transfer are directly coupled. Instead, it appears that interactions between the nucleotide and cluster sites must be indirectly coupled by allosteric changes occurring at the subunit interface. The coupling between protein conformation and nucleotide hydrolysis in Fe-protein exhibits general similarities to the H-Ras p21 and recA proteins that have been recently characterized structurally. The Fe-protein structure may be relevant to the functioning of other biochemical energy-transducing systems containing two nucleotide-binding sites, including membrane transport proteins.
01 Jan 1992
TL;DR: The Bayesian framework for model comparison and regularisation is demonstrated by studying interpolation and classification problems modelled with both linear and non-linear models, and it is shown that the careful incorporation of error bar information into a classifier's predictions yields improved performance.
Abstract: The Bayesian framework for model comparison and regularisation is demonstrated by studying interpolation and classification problems modelled with both linear and non-linear models. This framework quantitatively embodies 'Occam's razor'. Over-complex and under-regularised models are automatically inferred to be less probable, even though their flexibility allows them to fit the data better.
When applied to 'neural networks', the Bayesian framework makes possible (1) objective comparison of solutions using alternative network architectures; (2) objective stopping rules for network pruning or growing procedures; (3) objective choice of type of weight decay terms (or regularisers); (4) on-line techniques for optimising weight decay (or regularisation constant) magnitude; (5) a measure of the effective number of well-determined parameters in a model; (6) quantified estimates of the error bars on network parameters and on network output. In the case of classification models, it is shown that the careful incorporation of error bar information into a classifier's predictions yields improved performance.
Comparisons of the inferences of the Bayesian framework with more traditional cross-validation methods help detect poor underlying assumptions in learning models.
The relationship of the Bayesian learning framework to 'active learning' is examined. Objective functions are discussed which measure the expected informativeness of candidate data measurements, in the context of both interpolation and classification problems.
The concepts and methods described in this thesis are quite general and will be applicable to other data modelling problems whether they involve regression, classification or density estimation.
TL;DR: In this paper, a Lagrangian trajectory model simulating the formation, transport and deposition of secondary organic aerosol is developed and applied to the Los Angeles area, for the air pollution episode of 27-28 August 1987.
TL;DR: Biogenic magnetite in the human brain may account for high-field saturation effects observed in the T1 and T2 values of magnetic resonance imaging and, perhaps, for a variety of biological effects of low-frequency magnetic fields.
Abstract: Although the mineral magnetite (Fe3O4) is precipitated biochemically by bacteria, protists, and a variety of animals, it has not been documented previously in human tissue. Using an ultrasensitive superconducting magnetometer in a clean-lab environment, we have detected the presence of ferromagnetic material in a variety of tissues from the human brain. Magnetic particle extracts from solubilized brain tissues examined with high-resolution transmission electron microscopy, electron diffraction, and elemental analyses identify minerals in the magnetite-maghemite family, with many of the crystal morphologies and structures resembling strongly those precipitated by magnetotactic bacteria and fish. These magnetic and high-resolution transmission electron microscopy measurements imply the presence of a minimum of 5 million single-domain crystals per gram for most tissues in the brain and greater than 100 million crystals per gram for pia and dura. Magnetic property data indicate the crystals are in clumps of between 50 and 100 particles. Biogenic magnetite in the human brain may account for high-field saturation effects observed in the T1 and T2 values of magnetic resonance imaging and, perhaps, for a variety of biological effects of low-frequency magnetic fields.