Showing papers by "University of California, Irvine published in 2004"

Finding scientific topics

Abstract: A first step in identifying the content of a document is determining which topics that document addresses. We describe a generative model for documents, introduced by Blei, Ng, and Jordan [Blei, D. M., Ng, A. Y. & Jordan, M. I. (2003) J. Machine Learn. Res. 3, 993-1022], in which each document is generated by choosing a distribution over topics and then choosing each word in the document from a topic selected according to this distribution. We then present a Markov chain Monte Carlo algorithm for inference in this model. We use this algorithm to analyze abstracts from PNAS by using Bayesian model selection to establish the number of topics. We show that the extracted topics capture meaningful structure in the data, consistent with the class designations provided by the authors of the articles, and outline further applications of this analysis, including identifying “hot topics” by examining temporal dynamics and tagging abstracts to illustrate semantic content.

IntCal04 terrestrial radiocarbon age calibration, 0-26 cal kyr BP

Abstract: A new calibration curve for the conversion of radiocarbon ages to calibrated (cal) ages has been constructed and internationally ratified to replace IntCal98, which extended from 0-24 cal kyr BP (Before Present, 0 cal BP = AD 1950). The new calibration data set for terrestrial samples extends from 0-26 cal kyr BP, but with much higher resolution beyond 11.4 cal kyr BP than IntCal98. Dendrochronologically-dated tree-ring samples cover the period from 0-12.4 cal kyr BP. Beyond the end of the tree rings, data from marine records (corals and foraminifera) are converted to the atmospheric equivalent with a site-specific marine reservoir correction to provide terrestrial calibration from 12.4-26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a coherent statistical approach based on a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve (Buck and Blackwell, this issue). The tree-ring data sets, sources of uncertainty, and regional offsets are discussed here. The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed in brief, but details are presented in Hughen et al. (this issue a). We do not make a recommendation for calibration beyond 26 cal kyr BP at this time; however, potential calibration data sets are compared in another paper (van der Plicht et al., this issue).

Review: information technology and organizational performance: an integrative model of it business value

Abstract: Despite the importance to researchers, managers, and policy makers of how information technology (IT) contributes to organizational performance, there is uncertainty and debate about what we know and don't know. A review of the literature reveals that studies examining the association between information technology and organizational performance are divergent in how they conceptualize key constructs and their interrelationships. We develop a model of IT business value based on the resource-based view of the firm that integrates the various strands of research into a single framework. We apply the integrative model to synthesize what is known about IT business value and guide future research by developing propositions and suggesting a research agenda. A principal finding is that IT is valuable, but the extent and dimensions are dependent upon internal and external factors, including complementary organizational resources of the firm and its trading partners, as well as the competitive and macro environment. Our analysis provides a blueprint to guide future research and facilitate knowledge accumulation and creation concerning the organizational performance impacts of information technology.

Of mice and not men: differences between mouse and human immunology

Abstract: Mice are the experimental tool of choice for the majority of immunologists and the study of their immune responses has yielded tremendous insight into the workings of the human immune system. However, as 65 million years of evolution might suggest, there are significant differences. Here we outline known discrepancies in both innate and adaptive immunity, including: balance of leukocyte subsets, defensins, Toll receptors, inducible NO synthase, the NK inhibitory receptor families Ly49 and KIR, FcR, Ig subsets, the B cell (BLNK, Btk, and lambda5) and T cell (ZAP70 and common gamma-chain) signaling pathway components, Thy-1, gammadelta T cells, cytokines and cytokine receptors, Th1/Th2 differentiation, costimulatory molecule expression and function, Ag-presenting function of endothelial cells, and chemokine and chemokine receptor expression. We also provide examples, such as multiple sclerosis and delayed-type hypersensitivity, where complex multicomponent processes differ. Such differences should be taken into account when using mice as preclinical models of human disease.

The antecedents, consequences and mediating role of organizational ambidexterity

Abstract: We investigated contextual organizational ambidexterity, defined as the capacity to simultaneously achieve alignment and adaptability at a business-unit level. Building on the leadership and organization context literatures, we argue that a context characterized by a combination of stretch, discipline, support, and trust facilitates contextual ambidexterity. Further, ambidexterity mediates the relationship between these contextual features and performance. Data collected from 4,195 individuals in 41 business units supported our hypotheses. A recurring theme in a variety of organizational literatures is that successful organizations in a dynamic environment are ambidextrous—aligned and efficient in their management of today’s business demands, while also adaptive enough to changes in the environment that they will still be around tomorrow (Duncan, 1976; Tushman & O’Reilly, 1996). The simple idea behind the value of ambidexterity is that the demands on an organization in its task environment are always to some degree in conflict (for instance, investment in current versus future projects, differentiation versus low-cost production), so there are always trade-offs to be made. Although these trade-offs can never entirely be eliminated, the most successful organizations reconcile them to a large degree, and in so doing enhance their long-term competitiveness. Authors have typically viewed ambidexterity in structural terms. According to Duncan (1976), who first used the term, organizations manage trade-offs between conflicting demands by putting in place “dual structures,” so that certain business units—or groups within business units—focus on alignment, while others focus on adaptation (Duncan, 1976).

The amygdala modulates the consolidation of memories of emotionally arousing experiences

Abstract: Converging findings of animal and human studies provide compelling evidence that the amygdala is critically involved in enabling us to acquire and retain lasting memories of emotional experiences. This review focuses primarily on the findings of research investigating the role of the amygdala in modulating the consolidation of long-term memories. Considerable evidence from animal studies investigating the effects of posttraining systemic or intra-amygdala infusions of hormones and drugs, as well as selective lesions of specific amygdala nuclei, indicates that (a) the amygdala mediates the memory-modulating effects of adrenal stress hormones and several classes of neurotransmitters; (b) the effects are selectively mediated by the basolateral complex of the amygdala (BLA); (c) the influences involve interactions of several neuromodulatory systems within the BLA that converge in influencing noradrenergic and muscarinic cholinergic activation; (d) the BLA modulates memory consolidation via efferents to other brain regions, including the caudate nucleus, nucleus accumbens, and cortex; and (e) the BLA modulates the consolidation of memory of many different kinds of information. The findings of human brain imaging studies are consistent with those of animal studies in suggesting that activation of the amygdala influences the consolidation of long-term memory; the degree of activation of the amygdala by emotional arousal during encoding of emotionally arousing material (either pleasant or unpleasant) correlates highly with subsequent recall. The activation of neuromodulatory systems affecting the BLA and its projections to other brain regions involved in processing different kinds of information plays a key role in enabling emotionally significant experiences to be well remembered.

Impact of the Metabolic Syndrome on Mortality From Coronary Heart Disease, Cardiovascular Disease, and All Causes in United States Adults

Abstract: Background— Mortality resulting from coronary heart disease (CHD), cardiovascular disease (CVD), and all causes in persons with diabetes and pre-existing CVD is high; however, these risks compared with those with metabolic syndrome (MetS) are unclear. We examined the impact of MetS on CHD, CVD, and overall mortality among US adults. Methods and Results— In a prospective cohort study, 6255 subjects 30 to 75 years of age (54% female) (representative of 64 million adults in the United States) from the Second National Health and Nutrition Examination Survey were followed for a mean±SD of 13.3±3.8 years. MetS was defined by modified National Cholesterol Education Program criteria. From sample-weighted multivariable Cox proportional-hazards regression, compared with those with neither MetS nor prior CVD, age-, gender-, and risk factor–adjusted hazard ratios (HRs) for CHD mortality were 2.02 (95% CI, 1.42 to 2.89) for those with MetS and 4.19 (95% CI, 3.04 to 5.79) for those with pre-existing CVD. For CVD mortal...

Targeting recovery: Priorities of the spinal cord-injured population

Abstract: In the United States alone, there are more than 200,000 individuals living with a chronic spinal cord injury (SCI). Healthcare for these individuals creates a significant economic burden for the country, not to mention the physiological, psychological, and social suffering these people endure everyday. Regaining partial function can lead to greater independence, thereby improving quality of life. To ascertain what functions are most important to the SCI population, in regard to enhancing quality of life, a novel survey was performed in which subjects were asked to rank seven functions in order of importance to their quality of life. The survey was distributed via email, postal mail, the internet, interview, and word of mouth to the SCI community at large. A total of 681 responses were completed. Regaining arm and hand function was most important to quadriplegics, while regaining sexual function was the highest priority for paraplegics. Improving bladder and bowel function was of shared importance to both injury groups. A longitudinal analysis revealed only slight differences between individuals injured 3 years. The majority of participants indicated that exercise was important to functional recovery, yet more than half either did not have access to exercise or did not have access to a trained therapist to oversee that exercise. In order to improve the relevance of research in this area, the concerns of the SCI population must be better known and taken into account. This approach is consistent with and emphasized by the new NIH roadmap to discovery.

What we talk about when we talk about context

Abstract: The emergence of ubiquitous computing as a new design paradigm poses significant challenges for human-computer interaction (HCI) and interaction design. Traditionally, HCI has taken place within a constrained and well-understood domain of experience—single users sitting at desks and interacting with conventionally-designed computers employing screens, keyboards and mice for interaction. New opportunities have engendered considerable interest in “context-aware computing”—computational systems that can sense and respond to aspects of the settings in which they are used. However, considerable confusion surrounds the notion of “context”—what it means, what it includes and what role it plays in interactive systems. This paper suggests that the representational stance implied by conventional interpretations of “context” misinterprets the role of context in everyday human activity, and proposes an alternative model that suggests different directions for design.

The author-topic model for authors and documents

Abstract: We introduce the author-topic model, a generative model for documents that extends Latent Dirichlet Allocation (LDA; Blei, Ng, & Jordan, 2003) to include authorship information. Each author is associated with a multinomial distribution over topics and each topic is associated with a multinomial distribution over words. A document with multiple authors is modeled as a distribution over topics that is a mixture of the distributions associated with the authors. We apply the model to a collection of 1,700 NIPS conference papers and 160,000 CiteSeer abstracts. Exact inference is intractable for these datasets and we use Gibbs sampling to estimate the topic and author distributions. We compare the performance with two other generative models for documents, which are special cases of the author-topic model: LDA (a topic model) and a simple author model in which each author is associated with a distribution over words rather than a distribution over topics. We show topics recovered by the author-topic model, and demonstrate applications to computing similarity between authors and entropy of author output.

Alternative states and positive feedbacks in restoration ecology

Abstract: There is increasing interest in developing better predictive tools and a broader conceptual framework to guide the restoration of degraded land. Traditionally, restoration efforts have focused on re-establishing historical disturbance regimes or abiotic conditions, relying on successional processes to guide the recovery of biotic communities. However, strong feedbacks between biotic factors and the physical environment can alter the efficacy of these successional-based management efforts. Recent experimental work indicates that some degraded systems are resilient to traditional restoration efforts owing to constraints such as changes in landscape connectivity and organization, loss of native species pools, shifts in species dominance, trophic interactions and/or invasion by exotics, and concomitant effects on biogeochemical processes. Models of alternative ecosystem states that incorporate system thresholds and feedbacks are now being applied to the dynamics of recovery in degraded systems and are suggesting ways in which restoration can identify, prioritize and address these constraints.

Marine04 marine radiocarbon age calibration, 0-26 cal kyr BP

Abstract: New radiocarbon calibration curves, IntCal04 and Marine04, have been constructed and internationally rati- fied to replace the terrestrial and marine components of IntCal98. The new calibration data sets extend an additional 2000 yr, from 0-26 cal kyr BP (Before Present, 0 cal BP = AD 1950), and provide much higher resolution, greater precision, and more detailed structure than IntCal98. For the Marine04 curve, dendrochronologically-dated tree-ring samples, converted with a box diffusion model to marine mixed-layer ages, cover the period from 0-10.5 cal kyr BP. Beyond 10.5 cal kyr BP, high-res- olution marine data become available from foraminifera in varved sediments and U/Th-dated corals. The marine records are corrected with site-specific 14C reservoir age information to provide a single global marine mixed-layer calibration from 10.5-26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve (Buck and Blackwell, this issue). The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed here. The tree-ring data sets, sources of uncertainty, and regional offsets are presented in detail in a companion paper by Reimer et al. (this issue). ABSTRACT. New radiocarbon calibration curves, IntCal04 and Marine04, have been constructed and internationally rati- fied to replace the terrestrial and marine components of IntCal98. The new calibration data sets extend an additional 2000 yr, from 0-26 cal kyr BP (Before Present, 0 cal BP = AD 1950), and provide much higher resolution, greater precision, and more detailed structure than IntCal98. For the Marine04 curve, dendrochronologically-dated tree-ring samples, converted with a box diffusion model to marine mixed-layer ages, cover the period from 0-10.5 cal kyr BP. Beyond 10.5 cal kyr BP, high-res- olution marine data become available from foraminifera in varved sediments and U/Th-dated corals. The marine records are corrected with site-specific 14C reservoir age information to provide a single global marine mixed-layer calibration from 10.5-26.0 cal kyr BP. A substantial enhancement relative to IntCal98 is the introduction of a random walk model, which takes into account the uncertainty in both the calendar age and the 14C age to calculate the underlying calibration curve (Buck and Blackwell, this issue). The marine data sets and calibration curve for marine samples from the surface mixed layer (Marine04) are discussed here. The tree-ring data sets, sources of uncertainty, and regional offsets are presented in detail in a companion paper by Reimer et al. (this issue).

Progressive Nitrogen Limitation of Ecosystem Responses to Rising Atmospheric Carbon Dioxide

Abstract: A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that centers on the concept of progressive N limitation (PNL) for studying the interactions between C and N in terrestrial ecosystems. In PNL, available soil N becomes increasingly limiting as C and N are sequestered in long-lived plant biomass and soil organic matter. Our analysis focuses on the role of PNL in regulating ecosystem responses to rising atmospheric carbon dioxide concentration, but the concept applies to any perturbation that initially causes C and N to accumulate in organic forms. This article examines conditions under which PNL may or may not constrain net primary production and C sequestration in terrestrial ecosystems. While the PNL-centered framework has the potential to explain diverse experimental...

Dopamine in drug abuse and addiction: results from imaging studies and treatment implications

Abstract: The involvement of dopamine in drug reinforcement is well recognized but its role in drug addiction is much less clear. Imaging studies have shown that the reinforcing effects of drugs of abuse in humans are contingent upon large and fast increases in dopamine that mimic but exceed in the intensity and duration those induced by dopamine cell firing to environmental events. In addition, imaging studies have also documented a role of dopamine in motivation, which appears to be encoded both by fast as well as smooth DA increases. Since dopamine cells fire in response to salient stimuli, the supraphysiological activation by drugs is likely to be experienced as highly salient (driving attention, arousal conditioned learning and motivation) and may also reset the thresholds required for environmental events to activate dopamine cells. Indeed, imaging studies have shown that in drug-addicted subjects, dopamine function is markedly disrupted (decreases in dopamine release and in dopamine D2 receptors in striatum) and this is associated with reduced activity of the orbitofrontal cortex (neuroanatomical region involved with salience attribution and motivation and implicated in compulsive behaviors) and the cingulate gyrus (neuroanatomical region involved with inhibitory control and attention and implicated in impulsivity). However, when addicted subjects are exposed to drug-related stimuli, these hypoactive regions become hyperactive in proportion to the expressed desire for the drug. We postulate that decreased dopamine function in addicted subjects results in decreased sensitivity to nondrug-related stimuli (including natural reinforcers) and disrupts frontal inhibition, both of which contribute to compulsive drug intake and impaired inhibitory control. These findings suggest new strategies for pharmacological and behavioral treatments, which focus on enhancing DA function and restoring brain circuits disrupted by chronic drug use to help motivate the addicted subject in activities that provide alternative sources of reinforcement, counteract conditioned responses, enhance their ability to control their drive to take drugs and interfere with their compulsive administration.

Pollen limitation of plant reproduction: Ecological and evolutionary causes and consequences

Abstract: Determining whether seed production is pollen limited has been an area of intensive empirical study over the last two decades. Yet current evidence does not allow satisfactory assessment of the causes or consequences of pollen limitation. Here, we critically evaluate existing theory and issues concerning pollen limitation. Our main conclusion is that a change in approach is needed to determine whether pollen limitation reflects random fluctuations around a pollen–resource equilibrium, an adaptation to stochastic pollination environments, or a chronic syndrome caused by an environmental perturbation. We formalize and extend D. Haig and M. Westoby's conceptual model, and illustrate its use in guiding research on the evolutionary consequences of pollen limitation, i.e., whether plants evolve or have evolved to ameliorate pollen limitation. This synthesis also reveals that we are only beginning to understand when and how pollen limitation at the plant level translates into effects on plant population dynamics...

Real-time evolution using the density matrix renormalization group.

Abstract: We describe an extension to the density matrix renormalization group method incorporating real-time evolution. Its application to transport problems in systems out of equilibrium and frequency dependent correlation functions is discussed and illustrated in several examples. We simulate a scattering process in a spin chain which generates a spatially nonlocal entangled wave function.

A meta‐analysis of mycorrhizal responses to nitrogen, phosphorus, and atmospheric CO2 in field studies

Abstract: Summary • Numerous field studies have measured mycorrhizal dynamics under additions of nitrogen (N), phosphorus (P), or atmospheric CO 2 to test the hypothesis that plants should invest in mycorrhizal fungi when soil nutrients are limiting. • Here meta-analyses were used to integrate nutrient responses across independent field-based studies. Responses were compared between ecto- and arbuscular mycorrhizal fungi, and among fertilizer types, methods of measurement, biomes, and lead investigators. Relationships between degree of response and study length, fertilization rates, total amounts of nutrients applied, and numbers of replicates were also tested. • Across studies, mycorrhizal abundance decreased 15% under N fertilization and 32% under P fertilization. Elevated CO 2 elicited a 47% increase. Nitrogen effects varied significantly among studies, and P effects varied significantly among lead investigators. Most other factors did not affect mycorrhizal responses. • These results support the plant investment hypothesis, and suggest that global standing stocks of mycorrhizal fungi may increase substantially under elevated CO 2 but decline moderately under P additions. Effects of N deposition may be difficult to predict for individual ecosystems, with a slightly negative influence overall.

Revised Nomenclature for Avian Telencephalon and Some Related Brainstem Nuclei

Abstract: The standard nomenclature that has been used for many telencephalic and related brainstem structures in birds is based on flawed assumptions of homology to mammals. In particular, the outdated terminology implies that most of the avian telencephalon is a hypertrophied basal ganglia, when it is now clear that most of the avian telencephalon is neurochemically, hodologically, and functionally comparable to the mammalian neocortex, claustrum, and pallial amygdala (all of which derive from the pallial sector of the developing telencephalon). Recognizing that this promotes misunderstanding of the functional organization of avian brains and their evolutionary relationship to mammalian brains, avian brain specialists began discussions to rectify this problem, culminating in the Avian Brain Nomenclature Forum held at Duke University in July 2002, which approved a new terminology for avian telencephalon and some allied brainstem cell groups. Details of this new terminology are presented here, as is a rationale for each name change and evidence for any homologies implied by the new names. Revisions for the brainstem focused on vocal control, catecholaminergic, cholinergic, and basal ganglia-related nuclei. For example, the Forum recognized that the hypoglossal nucleus had been incorrectly identified as the nucleus intermedius in the Karten and Hodos (1967) pigeon brain atlas, and what was identified as the hypoglossal nucleus in that atlas should instead be called the supraspinal nucleus. The locus ceruleus of this and other avian atlases was noted to consist of a caudal noradrenergic part homologous to the mammalian locus coeruleus and a rostral region corresponding to the mammalian A8 dopaminergic cell group. The midbrain dopaminergic cell group in birds known as the nucleus tegmenti pedunculopontinus pars compacta was recognized as homologous to the mammalian substantia nigra pars compacta and was renamed accordingly; a group of gamma-aminobutyric acid (GABA)ergic neurons at the lateral edge of this region was identified as homologous to the mammalian substantia nigra pars reticulata and was also renamed accordingly. A field of cholinergic neurons in the rostral avian hindbrain was named the nucleus pedunculopontinus tegmenti, whereas the anterior nucleus of the ansa lenticularis in the avian diencephalon was renamed the subthalamic nucleus, both for their evident mammalian homologues. For the basal (i.e., subpallial) telencephalon, the actual parts of the basal ganglia were given names reflecting their now evident homologues. For example, the lobus parolfactorius and paleostriatum augmentatum were acknowledged to make up the dorsal subdivision of the striatal part of the basal ganglia and were renamed as the medial and lateral striatum. The paleostriatum primitivum was recognized as homologous to the mammalian globus pallidus and renamed as such. Additionally, the rostroventral part of what was called the lobus parolfactorius was acknowledged as comparable to the mammalian nucleus accumbens, which, together with the olfactory tubercle, was noted to be part of the ventral striatum in birds. A ventral pallidum, a basal cholinergic cell group, and medial and lateral bed nuclei of the stria terminalis were also recognized. The dorsal (i.e., pallial) telencephalic regions that had been erroneously named to reflect presumed homology to striatal parts of mammalian basal ganglia were renamed as part of the pallium, using prefixes that retain most established abbreviations, to maintain continuity with the outdated nomenclature. We concluded, however, that one-to-one (i.e., discrete) homologies with mammals are still uncertain for most of the telencephalic pallium in birds and thus the new pallial terminology is largely devoid of assumptions of one-to-one homologies with mammals. The sectors of the hyperstriatum composing the Wulst (i.e., the hyperstriatum accessorium intermedium, and dorsale), the hyperstriatum ventrale, the neostriatum, and the archistriatum have been renamed (respectively) the hyperpallium (hypertrophied pallium), the mesopallium (middle pallium), the nidopallium (nest pallium), and the arcopallium (arched pallium). The posterior part of the archistriatum has been renamed the posterior pallial amygdala, the nucleus taeniae recognized as part of the avian amygdala, and a region inferior to the posterior paleostriatum primitivum included as a subpallial part of the avian amygdala. The names of some of the laminae and fiber tracts were also changed to reflect current understanding of the location of pallial and subpallial sectors of the avian telencephalon. Notably, the lamina medularis dorsalis has been renamed the pallial-subpallial lamina. We urge all to use this new terminology, because we believe it will promote better communication among neuroscientists. Further information is available at http://avianbrain.org

Do Actions Speak Louder Than Words? The Response of Asset Prices to Monetary Policy Actions and Statements

Abstract: We investigate the effects of U.S. monetary policy on asset prices using a high-frequency event-study analysis. We test whether these effects are adequately captured by a single factor-changes in the federal funds rate target - and find that they are not. Instead, we find that two factors are required. These factors have a structural interpretation as a "current federal funds rate target" factor and a "future path of policy" factor, with the latter closely associated with Federal Open Market Committee statements.We measure the effects of these two factors on bond yields and stock prices using a new intraday data set going back to 1990. According to our estimates, both monetary policy actions and statements have important but differing effects on asset prices, with statements having a much greater impact on longer-term Treasury yields.

The ADP/ATP translocator is not essential for the mitochondrial permeability transition pore

Abstract: A sudden increase in permeability of the inner mitochondrial membrane, the so-called mitochondrial permeability transition, is a common feature of apoptosis and is mediated by the mitochondrial permeability transition pore (mtPTP). It is thought that the mtPTP is a protein complex formed by the voltage-dependent anion channel, members of the pro- and anti-apoptotic BAX-BCL2 protein family, cyclophilin D, and the adenine nucleotide (ADP/ATP) translocators (ANTs)1,2. The latter exchange mitochondrial ATP for cytosolic ADP and have been implicated in cell death. To investigate the role of the ANTs in the mtPTP, we genetically inactivated the two isoforms of ANT3,4,5 in mouse liver and analysed mtPTP activation in isolated mitochondria and the induction of cell death in hepatocytes. Mitochondria lacking ANT could still be induced to undergo permeability transition, resulting in release of cytochrome c. However, more Ca2+ than usual was required to activate the mtPTP, and the pore could no longer be regulated by ANT ligands. Moreover, hepatocytes without ANT remained competent to respond to various initiators of cell death. Therefore, ANTs are non-essential structural components of the mtPTP, although they do contribute to its regulation.

Why we blog

Abstract: Bloggers are driven to document their lives, provide commentary and opinions, express deeply felt emotions, articulate ideas through writing, and form and maintain community forums.

A fast sweeping method for Eikonal equations

Abstract: In this paper a fast sweeping method for computing the numerical solution of Eikonal equations on a rectangular grid is presented. The method is an iterative method which uses upwind difference for discretization and uses Gauss-Seidel iterations with alternating sweeping ordering to solve the discretized system. The crucial idea is that each sweeping ordering follows a family of characteristics of the corresponding Eikonal equation in a certain direction simultaneously. The method has an optimal complexity of O(N) for N grid points and is extremely simple to implement in any number of dimensions. Monotonicity and stability properties of the fast sweeping algorithm are proven. Convergence and error estimates of the algorithm for computing the distance function is studied in detail. It is shown that 2 n Gauss-Seidel iterations is enough for the distance function in n dimensions. An estimation of the number of iterations for general Eikonal equations is also studied. Numerical examples are used to verify the analysis.

Predictions and tests of climate‐based hypotheses of broad‐scale variation in taxonomic richness

Abstract: Broad-scale variation in taxonomic richness is strongly correlated with climate. Many mechanisms have been hypothesized to explain these patterns; however, testable predictions that would distinguish among them have rarely been derived. Here, we examine several prominent hypotheses for climate–richness relationships, deriving and testing predictions based on their hypothesized mechanisms. The ‘energy–richness hypothesis’ (also called the ‘more individuals hypothesis’) postulates that more productive areas have more individuals and therefore more species. More productive areas do often have more species, but extant data are not consistent with the expected causal relationship from energy to numbers of individuals to numbers of species. We reject the energy–richness hypothesis in its standard form and consider some proposed modifications. The ‘physiological tolerance hypothesis’ postulates that richness varies according to the tolerances of individual species for different sets of climatic conditions. This hypothesis predicts that more combinations of physiological parameters can survive under warm and wet than cold or dry conditions. Data are qualitatively consistent with this prediction, but are inconsistent with the prediction that species should fill climatically suitable areas. Finally, the ‘speciation rate hypothesis’ postulates that speciation rates should vary with climate, due either to faster evolutionary rates or stronger biotic interactions increasing the opportunity for evolutionary diversification in some regions. The biotic interactions mechanism also has the potential to amplify shallower, underlying gradients in richness. Tests of speciation rate hypotheses are few (to date), and their results are mixed.

Principles of brain evolution

Abstract: Part 1: Evolutionary Neuroscience: This Book's Scope And Ambition The Book's Scope and Major Themes Part 2: A Historyof Comparative Neurobiology The Birth of Comparative Neuroanatomy Darwin's Time: The Owen-Huxley Debate The Era of Comparative Cytoarchitectonics Comparative Hodology and Histochemistry The Rise of Neurocladistics The Rejuvenation of Comparative Neuroembryology Conclusions Part 3: Conservation In Vertebrate Brains A "Who's Who" of Vertebrates Comparing Adult Brains Adult Brain Regions Adult Cell Types Neuron-typical Molecules Comparing Embryonic Brains The Neuromeric Model Criticisms of the Neuromeric Model Mapping Embryos onto Adults Conclusion Part 4: Evolutionary Changes In Overall Brain Size Changes in Relative Brain Size Mechanisms of Brain-body Scaling Functional Correlates of Relative Brain Size Changes in Absolute Brain Size Constraints and Compromises Conclusions Part 5: Evolutionary Changes In Brain Region Size Concerted versus Mosaic Evolution Concerted Evolution Mosaic Evolution Toward a Synthesis Functional Correlates of Brain Region Size The Principle of Proper Mass Absolute Size and Functional Capacity Proportional Size and Influence Relative Size and Adaptation Synthesis: The Avian Hippocampus Conclusions Part 6: Evolutionary Changes In Brain Region Structure Homology and Novelty Phylogenetic Conversion: Lamination Phylogenetic Proliferation: Segregation Phylogenetic Proliferation: Addition Conclusions Part 7: Evolution Of Neuronal Connectivity Epigenetic Population Matching and Cascades The Parcellation Hypothesis Connectional invasion and Its Consequences General Principles of Network Design Synthesis and Conclusions Part 8: What's Special About Mammal Brains? Early Mammals and their Brains The Phylogenetic History of Neocortex Beyond the Neocortex Conclusion Part 9: What's Special About Human Brains? Primate Behavior and Overall Brain Size Evolutionary Changes in Primate Brain Organization Hominin Behavior and Overall Brain Size Evolutionary Changes in Hominin Brain Organization Conclusions Part 10: What's Special About Human Brains? Explanatory Strategies in Evolutionary Neuroscience Steps Toward Synthesis Absolute and Relative Brain Size Conclusion

Polyamines and cancer: old molecules, new understanding

Abstract: The amino-acid-derived polyamines have long been associated with cell growth and cancer, and specific oncogenes and tumour-suppressor genes regulate polyamine metabolism. Inhibition of polyamine synthesis has proven to be generally ineffective as an anticancer strategy in clinical trials, but it is a potent cancer chemoprevention strategy in preclinical studies. Clinical trials, with well-defined goals, are now underway to evaluate the chemopreventive efficacy of inhibitors of polyamine synthesis in a range of tissues.

Friends or Strangers? Firm-Specific Uncertainty, Market Uncertainty, and Network Partner Selection

Abstract: In this study, we address the topic of interorganizational network change by exploring factors that affect the choice of alliance and interlock partners. While many studies have been devoted to investigating various factors driving network partner choice, there is also an interesting and unexplored tension in this body of work. On the one hand, much work emphasizes change in social structure--showing that firms expand networks by forming new relationships with new partners. At the same time, other scholars emphasize stability of social structure--showing that firms tend to choose past exchange partners. We seek to reconcile this tension by proposing that firms form new relationships with new partners as a form of exploration, and form additional relationships with existing partners as a form of exploitation (March 1991). Further, whether exploration or exploitation is chosen depends on the type of uncertainty that firms are facing: whether it is firm-specific or market-level uncertainty. We test our hypotheses using data on both interlock and alliance networks for the 300 largest U.S. firms during the 1988-1993 period. The results provide some evidence that whether networks are stable or changing depends on the type of uncertainty experienced by firms.

Mild cognitive impairment can be distinguished from Alzheimer disease and normal aging for clinical trials.

Abstract: Background Mild cognitive impairment (MCI) represents a transitional state between the cognitive changes of normal aging and very early dementia and is becoming increasingly recognized as a risk factor for Alzheimer disease (AD). The Memory Impairment Study (MIS) is a multicenter clinical trial in patients with MCI designed to evaluate whether vitamin E or donepezil is effective at delaying the time to a clinical diagnosis of AD. Objective To describe the baseline characteristics of patients with MCI recruited for the MIS and compare them with those of elderly controls and patients with AD in another clinical trial. Design Descriptive and comparative study of patients with MCI participating in a multicenter clinical trial. Setting Memory disorder centers in the United States and Canada. Patients A total of 769 patients with MCI, 107 cognitively normal elderly controls, 122 patients with very mild AD (Clinical Dementia Rating [CDR] 0.5), and 183 patients with mild AD (CDR 1.0) were evaluated. Patients in the MIS met operational criteria for amnestic MCI. Controls were recruited in parallel with the MCI group, underwent the same assessments, and had a CDR of 0. Main Outcome Measures Clinical, neuropsychologic, functional, neuroimaging, and genetic measures. Results Mean ± SD Alzheimer's Disease Assessment Scale–Cognitive Subscale scores were 5.6 ± 3.3 for controls, 11.3 ± 4.4 for patients with MCI, 18.0 ± 6.2 for the AD CDR 0.5 group, and 25.2 ± 8.8 for the AD CDR 1.0 group. Compared with controls, patients with MCI were most impaired on memory tasks, with less severe impairments in other cognitive domains. Patients with MCI were more likely than controls but less likely than patients with AD to carry the apolipoprotein E ϵ4 allele. Patients with MCI had hippocampal volumes that were intermediate between those of controls and patients with AD. Conclusions Patients with MCI had a predominant memory impairment with relative sparing of other cognitive domains and were intermediate between clinically normal individuals and patients with AD on cognitive and functional ratings. These results demonstrate the successful implementation of operational criteria for this unique group of at-risk patients in a multicenter clinical trial.