Breaking the Nonsmooth Barrier: A Scalable Parallel Method for Composite Optimization

Due to their simplicity and excellent performance, parallel asynchronous variants of stochastic gradient descent have become popular methods to solve a wide range of large-scale optimization problems on multi-core architectures. Yet, despite their practical success, support for nonsmooth objectives is still lacking, making them unsuitable for many problems of interest in machine learning, such as the Lasso, group Lasso or empirical risk minimization with convex constraints. In this work, we propose and analyze ProxASAGA, a fully asynchronous sparse method inspired by SAGA, a variance reduced incremental gradient algorithm. The proposed method is easy to implement and significantly outperforms the state of the art on several nonsmooth, large-scale problems. We prove that our method achieves a theoretical linear speedup with respect to the sequential version under assumptions on the sparsity of gradients and block-separability of the proximal term. Empirical benchmarks on a multi-core architecture illustrate practical speedups of up to 12x on a 20-core machine.Comment: Appears in Advances in Neural Information Processing Systems 30 (NIPS 2017), 28 page

Long-time behavior of the Stokes-transport system in a channel

The coupling between the transport equation for the density and the Stokes equation is considered in a periodic channel. More precisely, the density is advected by pure transport by a velocity field given by the Stokes equation with source force coming from the gravity due to differences in the density. Dirichlet boundary conditions are taken for the velocity field on the bottom and top of the channel, and periodic conditions in the horizontal variable. We prove that the affine stratified density profile is stable under small perturbations in Sobolev spaces and prove convergence of the density to another limiting stratified density profile for large time with an explicit algebraic decay rate. Moreover, we are able to precisely identify the limiting profile as the decreasing vertical rearrangement of the initial density. Finally, we study boundary layers formation to precisely characterize the long-time behavior beyond the constant limiting profile and enlighten the optimal decay rate.Comment: 69 page

Improved asynchronous parallel optimization analysis for stochastic incremental methods

As datasets continue to increase in size and multi-core computer architectures are developed, asynchronous parallel optimization algorithms become more and more essential to the field of Machine Learning. Unfortunately, conducting the theoretical analysis asynchronous methods is difficult, notably due to the introduction of delay and inconsistency in inherently sequential algorithms. Handling these issues often requires resorting to simplifying but unrealistic assumptions. Through a novel perspective, we revisit and clarify a subtle but important technical issue present in a large fraction of the recent convergence rate proofs for asynchronous parallel optimization algorithms, and propose a simplification of the recently introduced "perturbed iterate" framework that resolves it. We demonstrate the usefulness of our new framework by analyzing three distinct asynchronous parallel incremental optimization algorithms: Hogwild (asynchronous SGD), KROMAGNON (asynchronous SVRG) and ASAGA, a novel asynchronous parallel version of the incremental gradient algorithm SAGA that enjoys fast linear convergence rates. We are able to both remove problematic assumptions and obtain better theoretical results. Notably, we prove that ASAGA and KROMAGNON can obtain a theoretical linear speedup on multi-core systems even without sparsity assumptions. We present results of an implementation on a 40-core architecture illustrating the practical speedups as well as the hardware overhead. Finally, we investigate the overlap constant, an ill-understood but central quantity for the theoretical analysis of asynchronous parallel algorithms. We find that it encompasses much more complexity than suggested in previous work, and often is order-of-magnitude bigger than traditionally thought.Comment: 67 pages, published in JMLR, can be found online at http://jmlr.org/papers/v19/17-650.html. arXiv admin note: substantial text overlap with arXiv:1606.0480

Morphosyntactic development and severe parental neglect in 4-year-old French-speaking children : ELLAN Study

Language is the most frequently compromised area of development in English-speaking neglected children, particularly the morphosyntactic component of language. This is very worrisome given its central role in academic success and social participation. No previous study has examined the morphosyntactic skills of French-speaking neglected children, despite the morphological richness of French. This study aimed to fill this gap. Forty-four neglected (mean age ¼ 48.32 months, SD ¼ 0.45) and 92 non-neglected (mean age ¼ 48.07 months, SD ¼ 0.24) French-speaking children participated. Measures of morphosyntactic skills were derived from a sample of spontaneous language collected during standardized semistructured play and analyzed using Systematic Analysis of Language Transcripts software (2012) . Four morphosyntactic indicators were compared using analyses of variance and Kolmogorov–Smirnov tests: the mean length of utterances (MLU), verbal inflections, word-level errors, and omission errors. The results indicate that 25.6% of the neglected children presented clinically significant morphosyntactic difficulties, as evidenced by a significantly shorter MLU (M ¼ 5.60, SD ¼ 1.13; M ¼ 6.90, SD ¼ 1.30), fewer verbal inflections, and more frequent word omission errors compared to their non-neglected peers. The results confirm that French-speaking neglected children present many morphosyntactic difficulties. This study argues for sustained speech–language services for these children

Evaluation of q-ball metrics for assessing the integrity of the injured spinal cord

International audienceAssessment of spinal cord integrity following injury is crucial for evaluating the potential for functional rehabilitation [1]. Previous studies showed the benefits of diffusion tensor imaging (DTI) for the non-invasive characterization of the healthy and injured spinal cord [2]. However, biases related to the incapability of DTI to represent complex diffusion profiles suggested the use of less constraining techniques. Recently, we demonstrated that q-ball imaging (QBI) is capable of partly solving fiber crossing information in the intact spinal cord [3]. In this study, we extended the application of QBI in a model of cat partial spinal cord injury and we compared various QBI quantitative metrics to the ones used in DTI. We also proposed an original QBI-based metric to quantify the homogeneity of diffusion directions

Macroscopic-Imaging Technique for Subsurface Quantification of Near-Infrared Markers During Surgery

Obtaining accurate quantitative information on the concentration and distribution of fluorescent markers lying at a depth below the surface of optically turbid media, such as tissue, is a significant challenge. Here, we introduce a fluorescence reconstruction technique based on a diffusion light transport model that can be used during surgery, including guiding resection of brain tumors, for depth-resolved quantitative imaging of near-infrared fluorescent markers. Hyperspectral fluorescence images are used to compute a topographic map of the fluorophore distribution, which yields structural and optical constraints for a three-dimensional subsequent hyperspectral diffuse fluorescence reconstruction algorithm. Using the model fluorophore Alexa Fluor 647 and brain-like tissue phantoms, the technique yielded estimates of fluorophore concentration within ±25% of the true value to depths of 5 to 9 mm, depending on the concentration. The approach is practical for integration into a neurosurgical fluorescence microscope and has potential to further extend fluorescence-guided resection using objective and quantified metrics of the presence of residual tumor tissue

Normative Indicators of Language Development in Québec French at 54, 60, and 66 Months of Age: Results of the ELLAN Study

Cet article vise à présenter des indicateurs normatifs du développement du vocabulaire réceptif et expressif, de la phonologie et de la morphosyntaxe expressives chez des enfants québécois unilingues francophones âgés de 54, 60 et 66 mois. Ces indicateurs sont basés sur les résultats obtenus par 99 enfants recrutés à l’âge de 36 mois (± 1 semaine; M = 36,1 mois; É-T = 0,2) et suivis jusqu’à l’âge de 66 mois. Les données ont été recueillies lors de trois visites à domicile réalisées à six mois d’intervalle, à l’aide d’outils fréquemment utilisés par les orthophonistes dans leur pratique clinique et valides sur le plan psychométrique. Une technique statistique de rééchantillonnage utilisant l’intervalle de confiance à 95 % du 10e rang centile a permis de déterminer les scores reflétant la présence de difficultés pour chaque mesure de langage chez les enfants et de former trois regroupements de scores pour identifier les enfants en difficulté, ceux se situant dans une zone d’incertitude et ceux ayant un développement typique. Les résultats confirment une progression significative des habiletés langagières mesurées entre l’âge de 54 et 66 mois. Ils suggèrent également que les mesures utilisées sont suffisamment sensibles pour détecter cette évolution chez les enfants, justifiant ainsi leur pertinence clinique. L’interprétation des normes issues des outils originaux est discutée à la lumière des résultats obtenus. Les données de la présente étude contribuent à l’accroissement du corpus de connaissances sur les indicateurs normatifs du développement du langage en français québécois et, en ce sens, constituent des points de repère indispensables pour le travail clinique en orthophonie et la recherche.The objective of this study is to present normative indicators of the development of receptive and expressive vocabulary as well as phonological and morphosyntactic components of expressive language among unilingual francophone Québec children aged 3 to 4 years. These indicators are based on the results obtained by 99 children recruited at precisely 3 years of age (M = 36.1 months, SD = 0.2). The data were collected during three separate visits conducted 6 months apart, using psychometrically valid tools frequently used by speech-language pathologists in their clinical practice. A statistical resampling technique using the 95% confidence interval of the 10th percentile on each language measure led to the categorization of children into three groups, namely children presenting difficulties, those in a zone of uncertainty, and those presenting typical development. The results for each measure confirm a significant increase in children’s language skills between the ages of 3 and 4 years. They suggest that the measures used are sensitive enough to detect changes in language skills of children aged 36, 42, and 48 months, thus confirming their clinical relevance. Interpretations of the norms of the original tools are discussed in relation to the current indicators. The normative data provided in this study add to a body of knowledge which serve as essential benchmarks for clinical work and research

Indicateurs normatifs du développement du langage en français québécois à 36, 42 et 48 mois : résultats du projet ELLAN

L’objectif de cette étude est de présenter des indicateurs normatifs du développement du vocabulaire réceptif et expressif ainsi que des composantes phonologique et morphosyntaxique du langage expressif chez des enfants québécois unilingues francophones âgés de 3 à 4 ans. Ces indicateurs sont basés sur les résultats obtenus par 99 enfants recrutés à l’âge de 3 ans précisément (M = 36,1 mois; É-T = 0,2). Les données ont été collectées lors de trois visites distinctes réalisées à six mois d’intervalle, à l’aide d’outils valides sur le plan psychométrique et fréquemment privilégiés par les orthophonistes dans leur pratique clinique. Une technique statistique de rééchantillonage utilisant l’intervalle de confiance à 95% du 10e rang centile aux différentes mesures obtenues a permis de regrouper les enfants en difficulté, ceux se situant dans une zone d’incertitude et ceux ayant un développement typique. Les résultats à chacune des mesures confirment une progression significative des compétences langagières des enfants entre l’âge de 3 et 4 ans. Ils suggèrent que les mesures utilisées sont suffisamment sensibles pour détecter l’évolution des habiletés langagières des enfants âgés de 36, 42 et 48 mois, confirmant ainsi leur pertinence clinique. L’interprétation des normes des outils originaux est discutée à la lumière des scores obtenus. Les données normatives de la présente étude s’ajoutent à un corpus de connaissances qui constitue des points de repères indispensables pour le travail clinique et la recherche

Highly Variable Taxa-specific Coral Bleaching Responses to Thermal Stresses

Complex histories of chronic and acute sea surface temperature (SST) stresses are expected to trigger taxon- and location-specific responses that will ultimately lead to novel coral communities. The 2016 El Niño-Southern Oscillation provided an opportunity to examine large- scale and recent environmental histories on emerging patterns in 226 coral communities distrib- uted across 12 countries from East Africa to Fiji. Six main coral communities were identified that largely varied across a gradient of Acropora to massive Porites dominance. Bleaching intensity was taxon-specific and was associated with complex interactions among the 20 environmental variables that we examined. Coral community structure was better aligned with the historical temperature patterns between 1985 and 2015 than the 2016 extreme temperature event. Addi- tionally, bleaching responses observed during 2016 differed from historical reports during past warm years. Consequently, coral communities present in 2016 are likely to have been reorganized by both long-term community change and acclimation mechanisms. For example, less disturbed sites with cooler baseline temperatures, higher mean historical SST background variability, and infrequent extreme warm temperature stresses were associated with Acropora-dominated communities, while more disturbed sites with lower historical SST background variability and frequent acute warm stress were dominated by stress-resistant massive Porites corals. Overall, the combination of taxon-specific responses, community-level reorganization over time, geographic variation, and multiple environmental stressors suggest complex responses and a diversity of future coral communities that can help contextualize management priorities and activities

Toward Improved Diagnosis Accuracy and Treatment of Children, Adolescents, and Young Adults With Ependymoma: The International SIOP Ependymoma II Protocol

Background: The clinical management of ependymoma in childhood and adolescence is complex and the clinicobiopathological correlates of outcome remain poorly understood. This international SIOP Ependymoma II (SIOP EPII) trial aims to improve the outcome of patients with ependymoma. Methods: SIOP EPII includes any patient <22 years at diagnosis with ependymoma, stratified by age, tumor location, and outcome of the initial surgery. Centralized pathology and imaging is required for diagnosis confirmation. SIOP EPII included three randomized studies according to age, postoperative residue, and suitability to receive radiotherapy. Patients ineligible for interventional strata are followed-up in an observational study. The staging phase aims to determine if central neurosurgical and radiological postoperative MRI reviews increase the resection rate. Patients ≥12 months with (i) no residual disease are randomly assigned in a phase III trial to evaluate the efficacy of post-radiation 16-week chemotherapy (VEC + CDDP) on PFS (stratum I); (ii) centrally confirmed measurable inoperable residual disease are allocated to randomized frontline chemotherapy phase II study (VEC vs. VEC + high-dose methotrexate) and considered for a second-look surgery (stratum II). If second-look surgery is not feasible or tumor residuum remains, patients receive 8 Gy-boost radiotherapy after conformal radiotherapy (phase I). (iii) Patients < 12 months (18 months in the UK) or not eligible to receive radiotherapy are randomized in a phase II study to receive chemotherapy (alternated myelosuppressive and nonmyelosuppressive chemotherapy), with or without valproate (stratum III). To overcome the limitations encountered in the preliminary conclusions of the ACNS-0831 study, a SIOP EPII dedicated on-study amendment has been planned to definitively conclude the relevance of maintenance chemotherapy in stratum I. Secondary outcomes include overall survival, quality of life, neuropsychological and neuroendocrine outcomes, safety, and identification of key prognostic biomarkers (BIOMECA). Clinical Trial Registration: ClinicalTrials.gov, identifier: NCT02265770