On statistics, computation and scalability

How should statistical procedures be designed so as to be scalable computationally to the massive datasets that are increasingly the norm? When coupled with the requirement that an answer to an inferential question be delivered within a certain time budget, this question has significant repercussions for the field of statistics. With the goal of identifying "time-data tradeoffs," we investigate some of the statistical consequences of computational perspectives on scability, in particular divide-and-conquer methodology and hierarchies of convex relaxations.Comment: Published in at http://dx.doi.org/10.3150/12-BEJSP17 the Bernoulli (http://isi.cbs.nl/bernoulli/) by the International Statistical Institute/Bernoulli Society (http://isi.cbs.nl/BS/bshome.htm

Tourism Space: An Attempt at a Fresh Look

In this article, the author is trying to answer the fundamental question: what is present-day tourism space like at a time of highly increasing flows of people or even a shift from the space of a ‘place’ to the space of a ‘flow’? The article puts special stress on how to define the current unique multi-functional space. The author attempts to define tourism space as a new entity, founded on poly-functionality (i.e. different functions and use of the same space both at the same time and in different seasons), multi-scale (overlapping of tourism spaces depending on the scale concerned), multi-layer, as well as the multi-motivation of its creators and users, or even multi-relativity

Long-range coupling of prefrontal cortex and visual (MT) or polysensory (STP) cortical areas in motion perception

To investigate how, where and when moving auditory cues interact with the perception of object-motion during self-motion, we conducted psychophysical, MEG, and fMRI experiments in which the subjects viewed nine textured objects during simulated forward self-motion. On each trial, one object was randomly assigned its own looming motion within the scene. Subjects reported which of four labeled objects had independent motion within the scene in two conditions: (1) visual information only and (2) with additional moving- auditory cue. In MEG, comparison of the two conditions showed: (i) MT activity is similar across conditions, (ii) late after the stimulus presentation there is additional activity in the auditory cue condition ventral to MT, (iii) with the auditory cue, the right auditory cortex (AC) shows early activity together with STS, (iv) these two activities have different time courses and the STS signals occur later in the epoch together with frontal activity in the right hemisphere, (v) for the visual-only condition activity in PPC (posterior parietal cortex) is stronger than in the auditory-cue condition. fMRI conducted for visual-only condition reveals activations in a network of parietal and frontal areas and in MT. In addition, Dynamic Granger Causality analysis showed for auditory cues a strong connection of the AC with STP but not with MT suggesting binding of visual and auditory information at STP. Also, while in the visual-only condition PFC is connected with MT, in the auditory-cue condition PFC is connected to STP (superior temporal polysensory) area. These results indicate that PFC allocates attention to the “object” as a whole, in STP to a moving visual-auditory object, and in MT to a moving visual object.Accepted manuscrip

Experimental and modeling study of collagen scaffolds with the effects of crosslinking and fiber alignment

Collagen type I scaffolds are commonly used due to its abundance, biocompatibility, and ubiquity. Most applications require the scaffolds to operate under mechanical stresses. Therefore understanding and being able to control the structural-functional integrity of collagen scaffolds becomes crucial. Using a combined experimental and modeling approach, we studied the structure and function of Type I collagen gel with the effects of spatial fiber alignment and crosslinking. Aligned collagen scaffolds were created through the flow of magnetic particles enmeshed in collagen fibrils to mimic the anisotropy seen in native tissue. Inter- and intra- molecular crosslinking was modified chemically with Genipin to further improve the stiffness of collagen scaffolds. The anisotropic mechanical properties of collagen scaffolds were characterized using a planar biaxial tensile tester and parallel plate rheometer. The tangent stiffness from biaxial tensile test is two to three orders of magnitude higher than the storage moduli from rheological measurements. The biphasic nature of collagen gel was discussed and used to explain the mechanical behavior of collagen scaffolds under different types of mechanical tests. An anisotropic hyperelastic constitutive model was used to capture the characteristics of the stress-strain behavior exhibited by collagen scaffolds

Programming with C++ concepts

AbstractThis paper explores the definition, applications, and limitations of concepts and concept maps in C++, with a focus on library composition. We also compare and contrast concepts to adaptation mechanisms in other languages.Efficient, non-intrusive adaptation mechanisms are essential when adapting data structures to a library’s API. Development with reusable components is a widely practiced method of building software. Components vary in form, ranging from source code to non-modifiable binary libraries. The Concepts language features, slated to appear in the next version of C++, have been designed with such compositions in mind, promising an improved ability to create generic, non-intrusive, efficient, and identity-preserving adapters.We report on two cases of data structure adaptation between different libraries, and illustrate best practices and idioms. First, we adapt GUI widgets from several libraries, with differing APIs, for use with a generic layout engine. We further develop this example to describe the run-time concept idiom, extending the applicability of concepts to domains where run-time polymorphism is required. Second, we compose an image processing library and a graph algorithm library, by making use of a transparent adaptation layer, enabling the efficient application of graph algorithms to the image processing domain. We use the adaptation layer to realize a few key algorithms, and report little or no performance degradation

Working Paper: Measuring polycentricity via network flows, spatial interaction, and percolation

Polycentricity is most commonly measured by location-based metrics (e.g. employment density or total number of workers, above a threshold, used to count the number of centres). While these metrics are good indicators of location ‘centricity’, the results are sensitive to threshold-choice. We consider here the alternate idea that a centre’s status depends on which other locations it is con- nected to in terms of trip inflows and outflows: this is inherently a network rather than a location idea. A set of flow and network-based centricity metrics for measuring metropolitan area poly- centricity using Journey-To-Work (JTW) data are presented: (a) trip-based, (b) density-based, and, (c) accessibility-based. Using these measures, polycentricity is computed and rank-centricity distributions are plotted to test whether these distributions follow Zipf-like or Chirstaller-like distributions. Further, a percolation theory framework is proposed for the full origin-destination (OD) matrix, where trip flows are used as a thresholding parameter to count the number of sub-centres. It is found that trip flows prove to be an effective measure to count and hierarchically organise metropolitan area sub-centres, and provide one way of dealing with the arbitrariness of defining a threshold on numbers of employed persons, employment density, or centricities to count sub-centres. These measures demonstrated on data from the Greater Sydney region show that the trip flow-based threshold and network centricities help to characterize polycentricity more robustly than the traditional number or density-based thresholds alone and provide unexpected insights into the connections between land use, transport, and urban structure