Showing papers by "Brown University published in 1991"

Efficiently computing static single assignment form and the control dependence graph

Abstract: In optimizing compilers, data structure choices directly influence the power and efficiency of practical program optimization. A poor choice of data structure can inhibit optimization or slow compilation to the point that advanced optimization features become undesirable. Recently, static single assignment form and the control dependence graph have been proposed to represent data flow and control flow properties of programs. Each of these previously unrelated techniques lends efficiency and power to a useful class of program optimizations. Although both of these structures are attractive, the difficulty of their construction and their potential size have discouraged their use. We present new algorithms that efficiently compute these data structures for arbitrary control flow graphs. The algorithms use {\em dominance frontiers}, a new concept that may have other applications. We also give analytical and experimental evidence that all of these data structures are usually linear in the size of the original program. This paper thus presents strong evidence that these structures can be of practical use in optimization.

Conceptualization and Rationale for Consensus Definitions of Terms in Major Depressive Disorder: Remission, Recovery, Relapse, and Recurrence

Abstract: • In 1988, the MacArthur Foundation Research Network on the Psychobiology of Depression convened a task force to examine the ways in which change points in the course of depressive illness had been described and the extent to which inconsistency in these descriptions might be impeding research on this disorder. We found considerable inconsistency across and even within research reports and concluded that research on depressive illness would be well served by greater consistency in the definition change points in the course of illness. We propose an internally consistent, empirically defined conceptual scheme for the terms remission, recovery , relapse , and recurrence . In addition, we propose tentative operational criteria for each term. Finally, we discuss ways to assess the usefulness of such operational criteria through reanalysis of existing data and the design and conduct of new experiments.

Estimation of planar curves, surfaces, and nonplanar space curves defined by implicit equations with applications to edge and range image segmentation

Abstract: The author addresses the problem of parametric representation and estimation of complex planar curves in 2-D surfaces in 3-D, and nonplanar space curves in 3-D. Curves and surfaces can be defined either parametrically or implicitly, with the latter representation used here. A planar curve is the set of zeros of a smooth function of two variables x-y, a surface is the set of zeros of a smooth function of three variables x-y-z, and a space curve is the intersection of two surfaces, which are the set of zeros of two linearly independent smooth functions of three variables x-y-z For example, the surface of a complex object in 3-D can be represented as a subset of a single implicit surface, with similar results for planar and space curves. It is shown how this unified representation can be used for object recognition, object position estimation, and segmentation of objects into meaningful subobjects, that is, the detection of 'interest regions' that are more complex than high curvature regions and, hence, more useful as features for object recognition. >

Convergence of approximation schemes for fully nonlinear second order equations

Abstract: The convergence of a wide class of approximation schemes to the viscosity solution of fully nonlinear second-order elliptic or parabolic, possibly degenerate, partial differential equations is studied. It is proved that any monotone, stable, and consistent scheme converges (to the correct solution), provided that there exists a comparison principle for the limiting equation. Several examples are given where the result applies. >

Reshaping the cortical motor map by unmasking latent intracortical connections

Abstract: The primary motor cortex (MI) contains a map organized so that contralateral limb or facial movements are elicited by electrical stimulation within separate medial to lateral MI regions. Within hours of a peripheral nerve transection in adult rats, movements represented in neighboring MI areas are evoked from the cortical territory of the affected body part. One potential mechanism for reorganization is that adjacent cortical regions expand when preexisting lateral excitatory connections are unmasked by decreased intracortical inhibition. During pharmacological blockade of cortical inhibition in one part of the MI representation, movements of neighboring representations were evoked by stimulation in adjacent MI areas. These results suggest that intracortical connections form a substrate for reorganization of cortical maps and that inhibitory circuits are critically placed to maintain or readjust the form of cortical motor representations.

Family of crack-tip fields characterized by a triaxiality parameter—I. Structure of fields

Abstract: Central to the J-based fracture mechanics approach is the existence of a HRR near-tip field which dominates the actual field over size scales comparable to those over which the micro-separation processes are active. There is now general agreement that the applicability of the J-approach is limited to so-called high-constraint crack geometries. We review the J-annulus concept and then develop the idea of a J-Q annulus. Within the J-Q annulus, the full range of high- and low-triaxiality fields are shown to be members of a family of solutions parameterized by Q when distances are measured in terms of J σ 0 , where σ0 is the yield stress. The stress distribution and the maximum stress depend on Q alone while J sets the size scale over which large stresses and strains develop. Full-field solutions show that the Q-family of fields exists near the crack tip of different crack geometries at large-scale yielding. The Q-family provides a framework for quantifying the evolution of constraint as plastic flow progresses from small-scale yielding to fully yielded conditions, and the limiting (steady-state) constraint when it exist. The Q value of a crack geometry can be used to rank its constraint, thus giving a precise meaning to the term crack-tip constraints, a term which is widely used in the fracture literature but has heretofore been unquantified. A two-parameter fracture mechanics approach for tensile mode crack tip states in which the fracture toughness and the resistance curve depend on Q, i. JC(Q) and JR(Δa, Q), is proposed.

Hypertext : the convergence of contemporary critical theory and technology

Abstract: George Landow's "Hypertext" brought together the worlds of literary theory and computer technology to explore the implications of giving readers instant, easy access to a virtual library of sources as well as unprecedented control of what and how they read. In hypermedia, Landow saw a strikingly literal embodiment of many major points of contemporary literary theory, particularly Derrida's idea of "de-centring" and Barthe's conception of the "readerly" versus "writerly" text. In this second edition, Landow shifts the focus from Intermedia to Microcosm, Storyspace and the World Wide Web. He offers new, specific information about kinds of hypertext, different modes of linking, attitudes towards technology, and the proliferation of pornography on the Internet. He also comments extensively on the rhetoric and style of writing in and with hypermedia.

Transforming growth factors beta 1 and alpha in chronic liver disease : effects of interferon alfa therapy

Abstract: Background. Cirrhosis is a diffuse process of hepatic fibrosis and regenerative nodule formation of unknown pathogenesis. Transforming growth factor (TGF) β1 induces the production of extracellular matrix proteins by liver cells and has been implicated in the pathogenesis of hepatic fibrosis in laboratory animals. TGFα is a hepatocyte mitogen that participates in liver regeneration. Methods. Using Northern blot analysis, we studied the expression of TGFβ1 messenger RNA (mRNA) in liver specimens from 42 patients with chronic hepatitis and cirrhosis and 12 subjects with either normal or fatty livers. The results were correlated with measurements of procollagen Type I mRNA in liver tissue, procollagen Type III peptide in serum, and the degree of histologie injury. We also investigated whether TGFα mRNA would be detectable in biopsy specimens of livers with proliferative activity. Results. TGFβ1 mRNA expression correlated closely with the expression of procollagen Type I mRNA (r = 0.94) and serum pro...

Intrinsic oscillations of neocortex generated by layer 5 pyramidal neurons

Abstract: Rhythmic activity in the neocortex varies with different behavioral and pathological states and in some cases may encode sensory information. However, the neural mechanisms of these oscillations are largely unknown. Many pyramidal neurons in layer 5 of the neocortex showed prolonged, 5- to 12-hertz rhythmic firing patterns at threshold. Rhythmic firing was due to intrinsic membrane properties, sodium conductances were essential for rhythmicity, and calcium-dependent conductances strongly modified rhythmicity. Isolated slices of neocortex generated epochs of 4- to 10-hertz synchronized activity when N-methyl-D-aspartate receptor-mediated channels were facilitated. Layer 5 was both necessary and sufficient to produce these synchronized oscillations. Thus, synaptic networks of intrinsically rhythmic neurons in layer 5 may generate or promote certain synchronized oscillations of the neocortex.

Self-efficacy in weight management.

Abstract: Self-efficacy is an important mediating mechanism in advancing understanding of the treatment of obesity. This study developed and validated the Weight Efficacy Life-Style Questionnaire (WEL), improving on previous studies by the use of clinical populations, cross-validation of the initial factor analysis, exploration of the best fitting theoretical model of self-efficacy, and examination of change in treatment. The resulting 20-item WEL consists of five situational factors: Negative Emotions, Availability, Social Pressure, Physical Discomfort, and Positive Activities. A hierarchical model was found to provide the best fit to the data. Results from two separate clinical treatment studies (total N = 382) show that the WEL is sensitive to changes in global scores as well as to a subset of the five situational factor scores. Treatment programs may be incomplete if they change only a subset of the situational dimensions of self-efficacy. Theoretical and clinical implications are discussed.

Planning and Control

Abstract: "Planning and Control" explores planning and control by reformulating the two areas in a common control framework, developing the corresponding techniques side-by-side, and identifying opportunities for integrating their ideas and methods. This book is organized around the central roles of prediction, observation, and computation control. The first three chapters deal with predictive models of physical systems based on temporal logic and the differential calculus. Chapter 4 and 5 present some basic concepts in planning and control, including controllability, observability, stability, feedback control, task reduction, conditional plans, and the relationship between goals and preferences. Chapters 6 and 7 consider issues of uncertainty, covering state estimation and the Kalman filter, stochastic dynamic programming, probabilistic modeling, and graph-based decision models. The remaining chapters investigate selected topics in time-critical decision making, adaptive control, and hybrid control architectures. Throughout, the reader is led to consider critical tradeoffs involving the accuracy of prediction, the availability of information from observation, and the costs and benefits of computation in dynamic environments. This book is useful to researchers in artificial intelligence and control theory, and others concerned with the design of complex applications in robotics, automated manufacturing, and time-critical decision support.

Forcing mechanisms of the Indian Ocean monsoon

Abstract: Sediments in the Arabian Sea provide biological, biogeochemical and lithogenic evidence of past changes in the Indian Ocean summer monsoon winds. For the past 350,000 years, this system has been externally forced by cyclical changes in solar radiation, and internally phase-locked to the transport of latent heat from the southern subtropical Indian Ocean to the Tibetan Plateau. In contrast to the results of general circulation models, these geological data suggest that the climate change associated with variability in global ice volume is not a primary factor in determining the strength and timing of the monsoon winds.

Psychological impairment in the wake of disaster: the disaster-psychopathology relationship.

Abstract: The present review examines the relationship between disaster occurrence and psychopathology outcome for 52 studies that used quantitative measures of such a relationship. Descriptive and inferential techniques were used to examine relationships among four sets of variables: (a) the characteristics of the victim population, (b) the characteristics of the disaster, (c) study methodology, and (d) the type of psychopathology. A small but consistently positive relationship between disasters and psychopathology was found. The distribution of effect-size estimates was significantly heterogeneous, and this heterogeneity was partially accounted for by methodological characteristics of the research. When controlling for methodology, victim and disaster characteristics also contributed variance to the disaster-psychopathology relationship. Implications for future research are outlined in view of these results.

Blue‐green injection laser diodes in (Zn,Cd)Se/ZnSe quantum wells

Abstract: Laser diode action in the blue‐green has been observed from (Zn,Cd)Se quantum wells within ZnSe/Zn(S,Se) p‐n heterojunctions up to 250 K. Operation is reported for two different configurations for which the GaAs substrate serves either as the n‐ or p‐type injecting contact. In pulsed operation, output powers exceeding 0.6 W have been measured in devices prepared on both n‐type and p‐type GaAs epitaxial buffer layers and substrates.

Self-Management of Chronic Disease by Older Adults: A Review and Questions for Research

Abstract: This article summarizes the literature describing the at-home management of and psychosocial coping with five chronic diseases (heart disease, asthma, chronic obstructive pulmonary disease, arthritis, and diabetes) by the general population of adults. It also reviews the literature describing self-management of these chronic diseases by older adults. Conclusions drawn subsequent to the review are (a) that there are strong commonalities in the essential nature of tasks that exist across disease entities, (b) that the context for self-management of disease by the ill elderly is likely to differ somewhat from the context for other age groups. Questions for future research are posed.

Adult survivors of childhood sexual abuse and subsequent risk of HIV infection.

Abstract: BACKGROUND: Epidemiologic description of long-term adverse health effects of childhood sexual abuse is lacking, despite estimates that perhaps 30 percent of adults have experienced sexual assault in childhood. METHODS: In an adult cohort enrolled to investigate causes of transmission of human immunodeficiency virus, we identified current behaviors affecting risk of infection that were associated with a history of early sexual abuse. One hundred and eighty-six individuals provided information on the occurrence of abuse and subsequent sexual and drug using activities. RESULTS: Approximately half of the women and one-fifth of the men reported a history of rape during childhood or adulthood. Twenty-eight percent of the women and 15 percent of the men recalled that they had been sexually assaulted during childhood. People who reported childhood rape compared with people who did not were four times more likely to be working as prostitutes (90 percent confidence interval = 2.0, 8.0). Women were nearly three times more likely to become pregnant before the age of 18 (90% CI = 1.6, 4.1). Men who reported a history of sexual abuse had a twofold increase in prevalence of HIV infection relative to unabused men (90% CI = 1.0, 3.9). CONCLUSIONS: The disturbing prevalence of early sexual abuse and its possible health-related consequences call for prompt and routine investigation of sexual abuse histories. Identification of sexual victimization may be an important component for management of risk factors for human immunodeficiency virus. Language: en

Sculpting: an interactive volumetric modeling technique

Abstract: We present a new interactive modeling technique based on the notion of sculpting a solid material. A sculpting tool is controlled by a 3D input device and the material is represented by voxel data; the tool acts by modifying the values in the voxel array, much as a "paint" program's "paintbrush" modifies bitmap values. The voxel data is converted to a polygonal surface using a "marching-cubes" algorithm; since the modifications to the voxel data are local, we accelerate this computation by an incremental algorithm and accelerate the display by using a special data structure for determining which polygons must be redrawn in a particular screen region. We provide a variety of tools: one that cuts away material, one that adds material, a "sandpaper" tool, a "heat gun," etc. The technique provides an intuitive direct interaction, as if the user were working with clay or wax. The models created are free-form and may have complex topology; however, they are not precise, so the technique is appropriate for modeling a boulder or a tooth but not for modeling a crankshaft.

Deformation of metal-matrix composites with continuous fibers: geometrical effects of fiber distribution and shape

Abstract: Numerical results are presented on the effects of fiber distribution and of fiber cross-sectional geometry on the deformation of a metal-matrix composite reinforced with continuous fibers. The geometrical variables analyzed include several periodic and random distributions of uni-directional cylindrical fibers with circular, hexagonal and square shapes. The model system chosen for the finite element analysis is a 6061-O aluminum alloy reinforced with boron fibers. It is shown that, for a reinforcement level of 46 vol.% boron fibers, tensile and shear deformation in orientations transverse to the fiber axes are markedly influenced by both the packing of fibers and their cross-sectional shape, although the deformation of the composite is insensitive to such geometrical effects for tensile loading in the direction of fiber alignment. The effect of fiber distribution on the constitutive response is relatively stronger than that of fiber shape. The geometrical effects of fiber distribution on deformation are significantly less pronounced for the 6061 aluminum-matrix composite with 20 vol.% boron fibers. Details of the evolution of plastic strains and hydrostatic stresses in the constrained matrix during far-field loading are discussed, and the influence of such constraint on the geometry dependence of deformation is examined. The scientific and practical implications of the results are also pinpointed.

Structural Image Restoration through Deformable Templates

Abstract: Prior knowledge on the space of possible images is given in the form of a function or template in some domain. The set of all possible true images is assumed to be formed by a composition of that function with continuous mappings of the domain into itself. A prior Gaussian distribution is given on the set of continuous mappings. The observed image is assumed to be a degradation of the true image with additive noise. Given the observed image, a posterior distribution is then obtained and has the form of a nonlinear perturbation of the Gaussian measure on the space of mappings. We present simulations of the posterior distribution that lead to structural reconstructions of the true image in the sense that it enables us to determine landmarks and other characteristic features of the image, as well as to spot pathologies in it. Moreover, we show that the reconstruction algorithm is relatively robust when the images are degraded by noise that is not necessarily additive.

When trees collide: an approximation algorithm for the generalized Steiner problem on networks

Abstract: We give the first approximation algorithm for the {\em generalized network Steiner tree problem}, a problem in network design. An instance consists of a network with link-costs and, for each pair ${i,j}$ of nodes, an edge-connectivity requirement. The goal is to find a minimum-cost network using the available links and satisfying the requirements. Our algorithm outputs a solution whose cost is within $ 2 \log R $ of optimal, where $R$ is the highest requirement value. In the course of proving the performance guarantee, we prove a combinatorial min-max approximate equality relating minimum-cost networks to maximum packings of certain kinds of cuts. As a consequence of the proof of this theorem, we obtain an approximation algorithm for optimally packing these cuts; we show that this algorithm has application to estimating the reliability of a probabilistic network.

Inverse perspective mapping simplifies optical flow computation and obstacle detection

Abstract: We present a scheme for obstacle detection from optical flow which is based on strategies of biological information processing. Optical flow is established by a local "voting" (non-maximum suppression) over the outputs of correlation-type motion detectors similar to those found in the fly visual system. The computational theory of obstacle detection is discussed in terms of space-variances of the motion field. An efficient mechanism for the detection of disturbances in the expected motion field is based on "inverse perspective mapping", i.e., a coordinate transform or retinotopic mapping applied to the image. It turns out that besides obstacle detection, inverse perspective mapping has additional advantages for regularizing optical flow algorithms. Psychophysical evidence for body-scaled obstacle detection and related neurophysiological results are discussed.

Photoacoustic monopole radiation in one, two, and three dimensions.

Abstract: We show that the photoacoustic pressure in one, two, and three dimensions can be found as mappings of the optical deposition of heat in space for short optical radiation pulses. In addition, we find the photoacoustic pressure to be proportional to the zeroth derivative in one dimension, the one-half derivative in two dimensions, and the first derivative in three dimensions of the optical radiation intensity for long pulses. Experiments with fluid layers, cylinders, and droplets give ultrasonic wave forms that are in general agreement with the theorems.

An analysis of the effects of matrix void growth on deformation and ductility in metal-ceramic composites

Abstract: Deformation and failure of metal-matrix composites, by the nucleation and growth of voids within the ductile matrix, are studied numerically and experimentally. The matrix material is modelled as an elastic-viscoplastic ductile porous solid to characterize the evolution of damage from void formation. The material systems chosen for parametric analyses and for quantitative comparisons between numerical analyses and experiments are aluminum alloys discontinuously reinforced with SiC. The brittle reinforcement phase, in the form of spheres, particulates with sharp corners, or cylindrical whiskers, is modelled as elastic or rigid, with the interfaces between the ductile matrix and the brittle reinforcement assumed to be perfectly bonded. The overall constitutive response of the composite and the evolution of matrix failure are analyzed using finite element models within the context of axisymmetric and plane strain formulations. Detailed parametric analyses of the effects of (i) reinforcement shape, (ii) reinforcement volume fraction, (iii) mechanical properties of the matrix, (iv) nucleation strain and volume fraction of void-nucleating particles, and (v) reinforcement distribution on the overall deformation and ductility of the composite are discussed. The numerical predictions of yield strength, strain hardening exponent and ductility for the composites with different volume fractions of SiC particulates are also compared with experimental measurements.