Showing papers by "University of Massachusetts Amherst published in 1997"

A comparison of goodness-of-fit tests for the logistic regression model.

Abstract: Recent work has shown that there may be disadvantages in the use of the chi-square-like goodness-of-fit tests for the logistic regression model proposed by Hosmer and Lemeshow that use fixed groups of the estimated probabilities. A particular concern with these grouping strategies based on estimated probabilities, fitted values, is that groups may contain subjects with widely different values of the covariates. It is possible to demonstrate situations where one set of fixed groups shows the model fits while the test rejects fit using a different set of fixed groups. We compare the performance by simulation of these tests to tests based on smoothed residuals proposed by le Cessie and Van Houwelingen and Royston, a score test for an extended logistic regression model proposed by Stukel, the Pearson chi-square and the unweighted residual sum-of-squares. These simulations demonstrate that all but one of Royston's tests have the correct size. An examination of the performance of the tests when the correct model has a quadratic term but a model containing only the linear term has been fit shows that the Pearson chi-square, the unweighted sum-of-squares, the Hosmer-Lemeshow decile of risk, the smoothed residual sum-of-squares and Stukel's score test, have power exceeding 50 per cent to detect moderate departures from linearity when the sample size is 100 and have power over 90 per cent for these same alternatives for samples of size 500. All tests had no power when the correct model had an interaction between a dichotomous and continuous covariate but only the continuous covariate model was fit. Power to detect an incorrectly specified link was poor for samples of size 100. For samples of size 500 Stukel's score test had the best power but it only exceeded 50 per cent to detect an asymmetric link function. The power of the unweighted sum-of-squares test to detect an incorrectly specified link function was slightly less than Stukel's score test. We illustrate the tests within the context of a model for factors associated with low birth weight.

Controlling Polymer-Surface Interactions with Random Copolymer Brushes

Abstract: A simple technique for precisely controlling the interfacial energies and wetting behavior of polymers in contact with solid surfaces is described. End-functionalized statistical random copolymers of styrene and methylmethacrylate were synthesized, with the styrene fraction f varying from 0 to 1, and were end-grafted onto silicon substrates to create random copolymer brushes about 5 nanometers thick. For f < 0.7, polystyrene (PS) films (20 nanometers thick) rapidly dewet from the brushes when heated well above the glass transition temperature. The contact angle of the resulting polymer droplets increased monotonically with decreasing f . Similar behavior was observed for poly(methylmethacrylate) (PMMA) films but with an opposite dependence on f . The interfacial energies of the random copolymer brushes with PS and PMMA were equal when f was about 0.6. Thus, precise control of the relative surface affinities of PS and PMMA was possible, demonstrating a way to manipulate polymer-surface interactions.

Shape quantization and recognition with randomized trees

Abstract: We explore a new approach to shape recognition based on a virtually infinite family of binary features (queries) of the image data, designed to accommodate prior information about shape invariance and regularity. Each query corresponds to a spatial arrangement of several local topographic codes (or tags), which are in themselves too primitive and common to be informative about shape. All the discriminating power derives from relative angles and distances among the tags. The important attributes of the queries are a natural partial ordering corresponding to increasing structure and complexity; semi-invariance, meaning that most shapes of a given class will answer the same way to two queries that are successive in the ordering; and stability, since the queries are not based on distinguished points and substructures. No classifier based on the full feature set can be evaluated, and it is impossible to determine a priori which arrangements are informative. Our approach is to select informative features and build tree classifiers at the same time by inductive learning. In effect, each tree provides an approximation to the full posterior where the features chosen depend on the branch that is traversed. Due to the number and nature of the queries, standard decision tree construction based on a fixed-length feature vector is not feasible. Instead we entertain only a small random sample of queries at each node, constrain their complexity to increase with tree depth, and grow multiple trees. The terminal nodes are labeled by estimates of the corresponding posterior distribution over shape classes. An image is classified by sending it down every tree and aggregating the resulting distributions. The method is applied to classifying handwritten digits and synthetic linear and nonlinear deformations of three hundred L AT E X symbols. Stateof-the-art error rates are achieved on the National Institute of Standards and Technology database of digits. The principal goal of the experiments on L AT E X symbols is to analyze invariance, generalization error and related issues, and a comparison with artificial neural networks methods is presented in this context.

Design of millimeter wave microstrip reflectarrays

Abstract: This paper discusses the theoretical modeling and practical design of millimeter wave reflectarrays using microstrip patch elements of variable size. A full-wave treatment of plane wave reflection from a uniform infinite array of microstrip patches is described and used to generate the required patch-design data and to calculate the radiation patterns of the reflectarray. The critical parameters of millimeter wave reflectarray design, such as aperture efficiency, phase errors, losses, and bandwidth are also discussed. Several reflectarray feeding techniques are described, and measurements from four reflectarray design examples at 28 and 77 GHz are presented.

Assessing the predictive validity of two methods of measuring self-image congruence

Abstract: The predictive validity of two measurement methods of self-image congruence—traditional versus new—were compared in six studies involving different consumer populations, products, consumption settings, and dependent variables (brand preference, preference for product form, consumer satisfaction/dissatisfaction, brand attitude, and program choice). The traditional method is based on tapping the subject’s perception of product-user image and the subject’s perception of his/her self-image along a predetermined set of image attributes and adding the self-congruity scores across all image dimensions. Three problems were identified and discussed in relation to the traditional method: (1) the use of discrepancy scores, (2) the possible use of irrelevant images, and (3) the use of the compensatory decision rule. The new method is based on tapping the psychological experience of self-congruity directly and globally. The findings demonstrated the predictive validity of the new method over and beyond the traditional method.

Reliable multicast transport protocol (RMTP)

Abstract: This paper presents the design, implementation, and performance of a reliable multicast transport protocol (RMTP). The RMTP is based on a hierarchical structure in which receivers are grouped into local regions or domains and in each domain there is a special receiver called a designated receiver (DR) which is responsible for sending acknowledgments periodically to the sender, for processing acknowledgment from receivers in its domain, and for retransmitting lost packets to the corresponding receivers. Since lost packets are recovered by local retransmissions as opposed to retransmissions from the original sender, end-to-end latency is significantly reduced, and the overall throughput is improved as well. Also, since only the DRs send their acknowledgments to the sender, instead of all receivers sending their acknowledgments to the sender, a single acknowledgment is generated per local region, and this prevents acknowledgment implosion. Receivers in RMTP send their acknowledgments to the DRs periodically, thereby simplifying error recovery. In addition, lost packets are recovered by selective repeat retransmissions, leading to improved throughput at the cost of minimal additional buffering at the receivers. This paper also describes the implementation of RMTP and its performance on the Internet.

Middle Management’s Strategic Influence and Organizational Performance†

Abstract: This study investigated relationships between middle managers’ formal position, their strategic influence and organizational performance. Among the 259 middle managers represented in the study, managers with formal positions in boundary–spanning sub–units reported higher levels of strategic influence activity than others. At the organizational level of analysis, the study found that firm performance was associated with more uniform levels of downward strategic influence, and more varied levels of upward influence among middle management cohorts. The findings suggest that middle managers’ strategic influence arises from their ability to mediate between internal and external selection environments. In addition, positive effects on organizational performance appear to depend on: (1) whether the overall pattern of upward influence is conducive to shifts in the network centrality of individual managers; and (2) whether the pattern of downward influence is consistent with an appropriate balance between the organization’s need for control and flexibility.

Climatic Change at High Elevation Sites: An Overview

Abstract: This paper provides an overview of climatic changes that have been observed during the past century at certain high-elevation sites, and changes in a more distant past documented by a variety of climate-sensitive environmental indicators, such as tree-rings and alpine glaciers, that serve as a measure of the natural variability of climate in mountains over longer time scales. Detailed studies such as those found in this special issue of Climatic Change , as well as those noted in this review, for the mountain regions of the world, advance our understanding in a variety of ways. They are not only helpful to characterize present and past climatological features in the mountainous zones, but they also provide useful information to the climate modeling community. Because of the expected refinements in the physical parameterizations of climate models in coming years, and the probable increase in the spatial resolution of GCMs, the use of appropriate data from high elevation sites will become of increasing importance for model initialization, verification, and intercomparison purposes. The necessity of accurate projections of climate change is paramount to assessing the likely impacts of climate change on mountain biodiversity, hydrology and cryosphere, and on the numerous economic activities which take place in these regions.

Rheology of Polymers Near Liquid-Solid Transitions

Abstract: Polymeric materials near the liquid-solid transition (LST) exhibit a very distinct relaxation pattern. The reference point for analyzing these patterns is the instant of LST at which relaxation becomes self-similar over wide ranges of the relaxation time. The universality of this transition and its consequences have been explored extensively during the past decade. This study will present an overview of rheological implications inherent in liquid-solid transitions of polymers. The LST can be most reliably detected in a dynamic mechanical experiment in which the frequency independence of the loss tangent marks the LST. A wide variety of rheological observations of materials in the vicinity of an LST are discussed with respect to their universality. It is shown that polymer chemistry, molecular weight, stoichiometry, temperature, inhomogeneities, etc. greatly influence the material behavior near the LST. However, the characteristic self-similar relaxation is shown by all investigated materials, independent of the nature of the LST (e.g., both, physically and chemically crosslinking polymers). Several theories predict chemical and rheological properties in the vicinity of an LST. They are briefly discussed and compared with experimental results. A variety of applications for polymers near LST are presented that either already exist or can be envisioned. The self-similar relaxation behavior which results in a power law relaxation spectrum and modulus is not restricted to materials near LST. Different classes of polymers are described that also show power law relaxation behavior. What makes the self-similar relaxation specific for materials at LST is its occurrence at long times with the longest relaxation time diverging to infinity.

Hostile and Benevolent Sexism: Measuring Ambivalent Sexist Attitudes Toward Women

Abstract: A theory of sexism as ambivalence, not just hostility, toward women is presented. Ambivalent Sexism Theory distinguishes between hostile and “benevolent” sexism (each addresses issues of power, gender differentiation, and sexuality). Benevolent sexism encompasses subjectively positive (for the sexist) attitudes toward women in traditional roles: protective paternalism, idealization of women, and desire for intimate relations. Hostile sexism encompasses the negative equivalents on each dimension: dominative paternalism, derogatory beliefs, and heterosexual hostility. Both forms of sexism serve to justify and maintain patriarchy and traditional gender roles. The validity of a measure of these constructs, the Ambivalent Sexism Inventory (ASI), is reviewed. Comparisons are offered between the ASI and other measures of sexist attitudes (e.g., the AWS), with suggestions for the proper domains of different scales.

Professional as Agent: Knowledge Asymmetry in Agency Exchange

Abstract: In this article I examine those business exchanges in which firms hire professional service organizations and give them limited decisionmaking authority to perform knowledge-intensive tasks. I frame such exchanges within agency theory perspective and invoke the extant literature on professions to delineate several attributes that make principal-professional exchanges intrinsically distinct from others. such as owner-manager agency. In doing so. I question and complement same key assumptions in agency theory and also discuss explicitly how the study of principal-professional exchanges helps highlight important considerations not addressed in the mainstream theory. I then present an expanded framework that integrates agency theory and the literature on the professions and present several propositions to outline four types of restraints on potential opportunistic behavior of professional agents; (1) self-control, (2) community control, (3) bureaucratic control, and (4) client control. The article ends with t...

Improved query performance with variant indexes

Abstract: The read-mostly environment of data warehousing makes it possible to use more complex indexes to speed up queries than in situations where concurrent updates are present. The current paper presents a short review of current indexing technology, including row-set representation by Bitmaps, and then introduces two approaches we call Bit-Sliced indexing and Projection indexing. A Projection index materializes all values of a column in RID order, and a Bit-Sliced index essentially takes an orthogonal bit-by-bit view of the same data. While some of these concepts started with the MODEL 204 product, and both Bit-Sliced and Projection indexing are now fully realized in Sybase IQ, this is the first rigorous examination of such indexing capabilities in the literature. We compare algorithms that become feasible with these variant index types against algorithms using more conventional indexes. The analysis demonstrates important performance advantages for variant indexes in some types of SQL aggregation, predicate evaluation, and grouping. The paper concludes by introducing a new method whereby multi-dimensional group-by queries, reminiscent of OLAP/Datacube queries but with more flexibility, can be very efficiently performed.

Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period

Abstract: Contribution of Southern Ocean surface-water stratification to low atmospheric CO 2 concentrations during the last glacial period

Gender differences in social desirability and social approval bias in dietary self-report.

Abstract: Social desirability (the tendency to respond in such a way as to avoid criticism) and social approval (the tendency to seek praise) are two prominent response set biases evident in answers on structured questionnaires. These biases were tested by comparing nutrient intakes as estimated from a single 24-hour diet recall interview (24 HR) and a 7-day dietary recall (7DDR). Data were collected as part of the Worcester Area Trial for Counseling in Hyperlipidemia, a randomized, physician-delivered nutrition intervention trial for hypercholesterolemic patients conducted in Worcester, Massachusetts, from 1991 to 1995. Of the 1,278 total study subjects, 759 had complete data for analysis. Men overestimated their fat and energy intakes on the 7DDR as compared with the 24HR according to social approval: One unit increase in the social approval score was associated with an overestimate of 21.5 kcal/day in total energy intake and 1.2 g/day in total fat intake. Women, however, underestimated their dietary intakes on the 7DDR relative to the 24HR according to social desirability: One unit increase in the social desirability score was associated with an underestimate of 19.2 kcal/day in energy intake and 0.8 g/day in total fat. The results from the present study indicate that social desirability and social approval biases appear to vary by gender. Such biases may lead to misclassification of dietary exposure estimates resulting in a distortion in the perceived relation between health-related outcomes and exposure to specific foods or nutrients. Because these biases may vary according to the perceived demands of research subjects, it is important that they be assessed in a variety of potential research study populations.

Vasopressin/Serotonin Interactions in the Anterior Hypothalamus Control Aggressive Behavior in Golden Hamsters

Abstract: Studies in several species of rodents show that arginine vasopressin (AVP) acting through a V1A receptor facilitates offensive aggression, i.e., the initiation of attacks and bites, whereas serotonin (5-HT) acting through a 5-HT1B receptor inhibits aggressive responding. One area of the CNS that seems critical for the organization of aggressive behavior is the basolateral hypothalamus, particularly the anterior hypothalamic region. The present studies examine the neuroanatomical and neurochemical interaction between AVP and 5-HT at the level of the anterior hypothalamus (AH) in the control of offensive aggression in Syrian golden hamsters. First, specific V1A and 5-HT1Bbinding sites in the AH are shown by in vitro receptor autoradiography. The binding for each neurotransmitter colocalizes with a dense field of immunoreactive AVP and 5-HT fibers and putative terminals. Putative 5-HT synapses on AVP neurons in the area of the AH are identified by double-staining immunocytochemistry and laser scanning confocal microscopy. These morphological data predispose a functional interaction between AVP and 5-HT at the level of the AH. When tested for offensive aggression in a resident/intruder paradigm, resident hamsters treated with fluoxetine, a selective 5-HT reuptake inhibitor, have significantly longer latencies to bite and bite fewer times than vehicle-treated controls. Conversely, AVP microinjections into the AH significantly shorten the latency to bite and increase biting attacks. The action of microinjected AVP to increase offensive aggression is blocked by the pretreatment of hamsters with fluoxetine. These data suggest that 5-HT inhibits fighting, in part, by antagonizing the aggression-promoting action of the AVP system.

Runt Homology Domain Proteins in Osteoblast Differentiation: AML3/CBFA1 Is a Major Component of a Bone-Specific Complex

Abstract: The AML/CBFA family of runt homology domain (rhd) transcription factors regulates expression of mammalian genes of the hematopoietic lineage. AML1, AML2 and AML3 are the three AML genes identified to date which influence myeloid cell growth and differentiation. Recently AML-related proteins were identified in an osteoblast-specific promoter binding complex that functionally modulates bone-restricted transcription of the osteocalcin gene. In the present study we demonstrate that in primary rat osteoblasts AML-3 is the AML family member present in the osteoblast-specific complex. Antibody specific for AML-3 completely supershifts this complex, in contrast to antibodies with specificity for AML-1 or AML-2, AML-3 is present as a single 5.4 kb transcript in bone tissues. To establish the functional involvement of AML factors in osteoblast differentiation, we pursued antisense strategies to alter expression of rhd genes. Treatment of osteoblast cultures with rhd antisense oligonucleotides significantly decreased three parameters which are linked to differentiation of normal diploid osteoblasts: the representation of alkaline phosphatase-positive cells, osteocalcin production, and the formation of mineralized nodules. Our findings indicate that AML-3 is a key transcription factor in bone cells and that the activity of rhd proteins is required for completion of osteoblast differentiation.

Parity-based loss recovery for reliable multicast transmission

Abstract: We investigate how FEC (Forward Error Correction) can be combined with ARQ (Automatic Repeat Request) to achieve scalable reliable multicast transmission. We consider the two scenarios where FEC is introduced as a transparent layer underneath a reliable multicast layer that uses ARQ, and where FEC and ARQ are both integrated into a single layer that uses the retransmission of parity data to recover from the loss of original data packets.To evaluate the performance improvements due to FEC, we consider different types of loss behaviors (spatially or temporally correlated loss, homogeneous or heterogeneous loss) and loss rates for up to 106 receivers. Our results show that introducing FEC as a layer below ARQ can improve multicast transmission efficiency and scalability and that there are substantial additional improvements when the two are integrated.

On optimal call admission control in cellular networks

Abstract: Two important Quality-of-Service (QoS) measures for current cellular networks are the fractions of new and handoff “calls” that are blocked due to unavailability of “channels” (radio and/or computing resources). Based on these QoS measures, we derive optimal admission control policies for three problems: minimizing a linear objective function of the new and handoff call blocking probabilities (MINOBJ), minimizing the new call blocking probability with a hard constraint on the handoff call blocking probability (MINBLOCK) and minimizing the number of channels with hard constraints on both of the blocking probabilities (MINC). We show that the well-known Guard Channel policy is optimal for the MINOBJ problem, while a new Fractional Guard Channel policy is optimal for the MINBLOCK and MINC problems. The Guard Channel policy reserves a set of channels for handoff calls while the Fractional Guard Channel policy effectively reserves a non-integral number of guard channels for handoff calls by rejecting new calls with some probability that depends on the current channel occupancy. It is also shown that the Fractional policy results in significant savings (20-50\%) in the new call blocking probability for the MINBLOCK problem and provides some, though small, gains over the Guard Channel policy for the MINC problem. Further, we also develop computationally inexpensive algorithms for the determination of the parameters for the optimal policies.

Signaling by phosphoinositide-3,4,5-trisphosphate through proteins containing pleckstrin and Sec7 homology domains.

Abstract: Signal transmission by many cell surface receptors results in the activation of phosphoinositide (PI) 3-kinases that phosphorylate the 3′ position of polyphosphoinositides. From a screen for mouse proteins that bind phosphoinositides, the protein GRP1 was identified. GRP1 binds phosphatidylinositol-3,4,5-trisphosphate [PtdIns(3,4,5)P3] through a pleckstrin homology (PH) domain and displays a region of high sequence similarity to the yeast Sec7 protein. The PH domain of the closely related protein cytohesin-1, which, through its Sec7 homology domain, regulates integrin β2 and catalyzes guanine nucleotide exchange of the small guanine nucleotide-binding protein ARF1, was also found to specifically bind PtdIns(3,4,5)P3. GRP1 and cytohesin-1 appear to connect receptor-activated PI 3-kinase signaling pathways with proteins that mediate biological responses such as cell adhesion and membrane trafficking.

Attention-deficit/hyperactivity disorder, self-regulation, and time: toward a more comprehensive theory.

Abstract: This article describes the current clinical view of attention-deficit/hyperactivity disorder (ADHD) and suggests a model of prefrontal lobe executive functions that explains better than current theories the cognitive and behavioral deficits associated with ADHD. The model shows how behavioral inhibition is related to and necessary for the proficient performance of four executive functions that subserve self-regulation: nonverbal working memory; the internalization of speech; the self-regulation of affect, motivation, and arousal; and reconstitution. These functions permit the construction, execution, and control of behavior by internally represented information, which removes behavior from control by the immediate context and brings it under the control of time. ADHD disrupts this process and returns control of behavior to the temporal now. A blindness to past, future, and time more generally, as well as an inability to direct behavior toward the future and to sustain it are among many of the deficits predicted by this model for persons with ADHD.

Microbial Fe(III) reduction in subsurface environments

Abstract: Until recently, nonenzymatic processes were generally considered to account for much of the Fe(III) reduction in subsurface environments. However, it is now clear that enzymatic Fe(III) reduction catalyzed by microorganisms which conserve energy to support growth by completely oxidizing organic compounds to carbon dioxide accounts for most of the Fe(III) reduction. Microbial Fe(III) reduction in deep pristine aquifers releases dissolved inorganic carbon into groundwater which may increase aquifer porosity. The Fe(II) released into the groundwater is an important groundwater quality problem in many aquifers. Microbial oxidation of organic contaminants coupled to Fe(III) reduction removes significant amounts of pollutants from many contaminated aquifers. Fe(III) reduction and hence contaminant removal can be accelerated in aquifer sediments with the addition of Fe(III) chelators or humic substances. Both of these amendments alleviate the need for Fe(III) reducers to come into direct physical contact with Fe(III) oxides in order to reduce them. Some Fe(III)-reducing microorganisms can reduce contaminant metals and metalloids such as uranium, technetium, cobalt, chromium and selenium. This metabolism may be useful for remediation of metal-contaminated subsurface environments. Fe(III) reducers and some of the insoluble Fe(II) products of Fe(III) reduction can reducively dechlorinate chlorinated contaminants. Magnetite that is similar to that produced by known Fe(III)-reducing microorganisms has been recovered at depths as great as 6.7 km on Earth and has been observed in a Martian meteorite. Thus, microbial oxidation of organic matter coupled to the reduction of Fe(III) to Fe(II) appears to be a important process in a variety of subsurface environments.

Decision Tree Induction Based on Efficient Tree Restructuring

Abstract: The ability to restructure a decision tree efficiently enables a variety of approaches to decision tree induction that would otherwise be prohibitively expensive. Two such approaches are described here, one being incremental tree induction (ITI), and the other being non-incremental tree induction using a measure of tree quality instead of test quality (DMTI). These approaches and several variants offer new computational and classifier characteristics that lend themselves to particular applications.

The Opacity of the Lyα Forest and Implications for Ωb and the Ionizing Background

Abstract: We have measured the distribution function of the flux decrement D = 1 - e-τ caused by Lyα forest absorption from intervening gas in the lines of sight to high-redshift QSOs from a sample of seven high-resolution QSO spectra obtained with the Keck telescope. The observed flux decrement distribution function (FDDF) is compared with the FDDF from two simulations of the Lyα forest: a ΛCDM model (with Ω = 0.4, Λ = 0.6), computed with the Eulerian code of Cen & Ostriker, and a standard cold dark matter (SCDM) model (with Ω = 1), computed with the smoothed particle hydrodynamics code of Hernquist et al. Good agreement is obtained between the shapes of the simulated and observed FDDFs for both simulations after fitting only one free parameter, which controls the mean flux decrement. The difference between the predicted FDDFs from the two simulations is small, and we show that it arises mostly from a different temperature in the low-density gas (caused by different assumptions that were made about the reionization history in the two simulations), rather than differences between the two cosmological models or numerical effects in the two codes, which use very different computational methods. A measurement of the parameter μΩ -->2b h -->3/Γ (where Γ is the H I ionization rate due to the ionizing background) is obtained by requiring the mean flux decrement in the simulations to agree with the observed one. Estimating the lower limit Γ > 7 × 10-13 s-1 from the abundance of known QSOs, we derive a lower limit on the baryonic matter density, Ωbh2 > 0.021 (0.017) for the ΛCDM (SCDM) model. The difference between the lower limits inferred from the two models is again due to different temperatures in the low-density gas. We give general analytical arguments for why this lower limit is unlikely to be reduced for any other models of structure formation by gravitational collapse that can explain the observed Lyα forest. When combined with constraints from big bang nucleosynthesis, the large Ωb we infer is inconsistent with some recent D/H determinations (Rugers & Hogan), favoring a low deuterium abundance as reported by Tytler, Fan & Burles. Adopting a fixed Ωb, the measurement of μ(z) allows a determination of the evolution of the ionizing radiation field with redshift. Our models predict an intensity that is approximately constant with redshift, which is in agreement with the assumption that the ionizing background is produced by known quasars for z < 3, but requires additional sources of ionizing photons at higher redshift given the observed rapid decline of the quasar abundance.

Phrasal translation and query expansion techniques for cross-language information retrieval

Abstract: Dictionary methods for cross-language information retrieval give performance below that for mono-lingual retrieval. Failure to translate multi-term phrases has been shown to be one of the factors responsible for the errors associated with dictionary methods. First, we study the importance of phrasal translation for this approach. Second, we explore the role of phrases in query expansion via local context analysis and local feedback and show how they can be used to significantly reduce the error associated with automatic dictionary translation.

Pollen Tube Growth and the Intracellular Cytosolic Calcium Gradient Oscillate in Phase while Extracellular Calcium Influx Is Delayed.

Abstract: Ratio images of cytosolic Ca2+ (Ca2+i) in growing, fura-2-dextran-loaded Lilium longiflorum pollen tubes taken at 3- to 5-sec intervals showed that the tip-focused [Ca2+]i gradient oscillates with the same period as growth. Similarly, measurement of the extracellular inward current, using a noninvasive ion-selective vibrating probe, indicated that the tip-directed extracellular Ca2+ (Ca2+o) current also oscillates with the same period as growth. Cross-correlation analysis revealed that whereas the [Ca2+]i gradient oscillates in phase with growth, the influx of Ca2+o lags by ~11 sec. Ion influx thus appears to follow growth, with the effect that the rate of growth at a given point determines the magnitude of the ion influx ~11 sec later. To explain the phase delay in the extracellular inward current, there must be a storage of Ca2+ for which we consider two possibilities: either the inward current represents the refilling of intracellular stores (capacitative calcium entry), or it represents the binding of the ion within the cell wall domain.