Building connectomes using diffusion MRI: why, how and but

Why has diffusion MRI become a principal modality for mapping connectomes in vivo? How do different image acquisition parameters, fiber tracking algorithms and other methodological choices affect connectome estimation? What are the main factors that dictate the success and failure of connectome reconstruction? These are some of the key questions that we aim to address in this review. We provide an overview of the key methods that can be used to estimate the nodes and edges of macroscale connectomes, and we discuss open problems and inherent limitations. We argue that diffusion MRI-based connectome mapping methods are still in their infancy and caution against blind application of deep white matter tractography due to the challenges inherent to connectome reconstruction. We review a number of studies that provide evidence of useful microstructural and network properties that can be extracted in various independent and biologically-relevant contexts. Finally, we highlight some of the key deficiencies of current macroscale connectome mapping methodologies and motivate future developments

Testing for Alpha in Linear Factor Pricing Models with a Large Number of Securities

This paper proposes a novel test of zero pricing errors for the linear factor pricing model when the number of securities, N, can be large relative to the time dimension, T, of the return series. The test is based on Student t tests of individual securities and has a number of advantages over the existing standardised Wald type tests. It allows for non-Gaussianity and general forms of weakly cross correlated errors. It does not require estimation of an invertible error covariance matrix, it is much faster to implement, and is valid even if N is much larger than T. Monte Carlo evidence shows that the proposed test performs remarkably well even when T = 60 and N = 5;000. The test is applied to monthly returns on securities in the S&P 500 at the end of each month in real time, using rolling windows of size 60. Statistically significant evidence against Sharpe-Lintner CAPM and Fama-French three factor models are found mainly during the recent financial crisis. Also we find a significant negative correlation between a twelve-months moving average p-values of the test and excess returns of long/short equity strategies (relative to the return on S&P 500) over the period November 1994 to June 2015, suggesting that abnormal profits are earned during episodes of market inefficiencies

Delaying sexual debut as a strategy for reducing HIV epidemic in Kenya

The study purpose was to determine the association between sexual debut and HIV sero-status, and factors contributing to a positive HIV sero-status. Retrospective cross-sectional data from the Kenya Demographic and Health Survey-2003 were used. Data on women aged 15-49 (n=3,273) and men aged 15-54 (n=2,917) accepting HIV testing were retained from three datasets. Stata version 10.1 was used for analyses, p<0.05. Nine percent of women and 5% of men tested positive for HIV, of whom 46% and 49%, p<0.001 were aged 16-21 respectively. After adjusting for confounding, women and men who had sexual debut aged 16-21 were 2.31 (95% CI: 1.52-3.51), p<0.001 and 1.83 (95% CI: 1.07-3.13), p<0.05 times more likely to test positive for HIV compared to those who never had sex respectively. Early sexual debut continues to be a major risk factor for acquiring HIV infection later in life, suggesting earlier interventions could have a major impact towards fighting the disease. (Afr J Reprod Health 2013; 17[2]: 46-57).Le but de l&apos;étude était de déterminer l&apos;association entre l&apos;activité sexuelle et situation sérologique du VIH et les facteurs qui contribuent à un effet positif de la situation sérologique du VIH. Nous nous sommes servis des données transversales rétrospectives de l&apos;Enquête démographique et de santé au Kenya -2003. Les données sur les femmes âgées de 15-49 ans (n = 3273) et les hommes âges de 15-54 ans (n = 2917) qui ont accepte le dépistage du VIH ont été retenus à partir de trois ensembles de données. La version Stata 10.1 a été utilisé pour les analyses, p <0,05. Les analyses ont révélé la présence du VIH chez neuf pour cent des femmes et 5% des hommes, dont 46% et 49%, p <0,001 étaient âgés de 16 à 21 ans respectivement. Après l’ajustement pour les facteurs confondants, les femmes et les hommes qui ont eu leurs premiers rapports sexuels a l’âge de 16 à 21 ans étaient 2,31 (IC à 95%: 1,52 à 3,51), p <0,001 et 1,83 (IC à 95%: 1.7 à 3.13), p <0,05 fois plus susceptibles d’avoir le résultat positif dans les analyses pour déterminer la présence du VIH par rapport à ceux qui n&apos;ont jamais eu des rapports sexuels, respectivement. L&apos;initiation au rapport sexuel précoce continue d&apos;être un facteur de risque majeur de contracter l&apos;infection du VIH plus tard dans la vie, ce qui suggère que les interventions antérieures pourraient avoir un impact majeur vers la lutte contre la maladie. (Afr J Reprod Health 2013; 17[2]: 46-57)

Distinguishing Electron Diffusion and Extraction in Methylammonium Lead Iodide

Charge diffusion and extraction are crucial steps in the operation of solar cells. Here we show that time-resolved photoluminescence can be used to study electron diffusion in hybrid perovskite films and subsequent transfer to the adjacent electron extraction layer. As diffusion and transfer to the extraction layer are consecutive processes, they can be hard to distinguish, but by exciting from each side of the sample we can separate them and identify which process limits charge extraction. We find that the introduction of a fullerene monolayer between the methylammonium lead iodide (MAPbI3) and the electron-transporting SnO2 layers greatly increases the electron transfer velocity between them to the extent that electron diffusion limits the rate of electron extraction. Our results suggest that increasing the electron diffusion coefficient in MAPbI3 would further enhance the electron extraction rate, which could result in more efficient n–i–p type solar cells