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Search Results: 1 - 10 of 32306 matches for " Peter Klaver "
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Simultaneous EEG-fMRI during a Working Memory Task: Modulations in Low and High Frequency Bands
Lars Michels,Kerstin Bucher,Rafael Lüchinger,Peter Klaver,Ernst Martin,Daniel Jeanmonod,Daniel Brandeis
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0010298
Abstract: EEG studies of working memory (WM) have demonstrated load dependent frequency band modulations. FMRI studies have localized load modulated activity to the dorsolateral prefrontal cortex (DLPFC), medial prefrontal cortex (MPFC), and posterior parietal cortex (PPC). Recently, an EEG-fMRI study found that low frequency band (theta and alpha) activity negatively correlated with the BOLD signal during the retention phase of a WM task. However, the coupling of higher (beta and gamma) frequencies with the BOLD signal during WM is unknown.
A Current Induced Transition in atomic-sized contacts of metallic Alloys
Jan W. T. Heemskerk,Yves Noat,David J. Bakker,Jan M. van Ruitenbeek,Barend J. Thijsse,Peter Klaver
Physics , 2002, DOI: 10.1103/PhysRevB.67.115416
Abstract: We have measured conductance histograms of atomic point contacts made from the noble-transition metal alloys CuNi, AgPd, and AuPt for a concentration ratio of 1:1. For all alloys these histograms at low bias voltage (below 300 mV) resemble those of the noble metals whereas at high bias (above 300 mV) they resemble those of the transition metals. We interpret this effect as a change in the composition of the point contact with bias voltage. We discuss possible explanations in terms of electromigration and differential diffusion induced by current heating.
Measuring Semileptonic Asymmetries in LHCb
Suzanne Klaver,for the LHCb Collaboration
Physics , 2015,
Abstract: The $C\!P$-violating flavour-specific asymmetry in neutral $b$ mesons provides a method for testing the Standard Model. The measurements from the D0 experiment yield values of this asymmetry that disagree with the Standard Model at a level of 3.6 $\sigma$. This contribution discusses the latest LHCb measurements in this sector both from $B^0$ mesons ($a_{\mathrm{sl}}^d$) and $B^0_s$ mesons ($a_{\mathrm{sl}}^s$). Using their 2011 dataset, corresponding to an integrated luminosity of 1.0 $\mathrm{fb}^{-1}$ obtained in 2011, LHCb measured a value of $a_{\mathrm{sl}}^s = (-0.06 \pm 0.50_{\text{stat}} \pm 0.36_{\text{syst}}) \%$. Combining the 2011 and 2012 datasets, with an integrated luminosity of 3 $\mathrm{fb}^{-1}$, LHCb measured $a_{\mathrm{sl}}^d = (-0.02 \pm 0.19_{\text{stat}} \pm 0.30_{\text{syst}}) \%$. These are the most precise measurements of the parameters $a_{\mathrm{sl}}^s$ and $a_{\mathrm{sl}}^d$ to date. Plans for an updated result for $a_{\mathrm{sl}}^s$ using the full 3 $\mathrm{fb}^{-1}$ dataset are discussed. This will include new methods to determine detection asymmetries which are the dominating systematic uncertainty of the 2011 measurement.
Frontal GABA Levels Change during Working Memory
Lars Michels, Ernst Martin, Peter Klaver, Richard Edden, Fernando Zelaya, David J. Lythgoe, Rafael Lüchinger, Daniel Brandeis, Ruth L. O’Gorman
PLOS ONE , 2012, DOI: 10.1371/journal.pone.0031933
Abstract: Functional neuroimaging metrics are thought to reflect changes in neurotransmitter flux, but changes in neurotransmitter levels have not been demonstrated in humans during a cognitive task, and the relationship between neurotransmitter dynamics and hemodynamic activity during cognition has not yet been established. We evaluate the concentration of the major inhibitory (GABA) and excitatory (glutamate + glutamine: Glx) neurotransmitters and the cerebral perfusion at rest and during a prolonged delayed match-to-sample working memory task. Resting GABA levels in the dorsolateral prefrontal cortex correlated positively with the resting perfusion and inversely with the change in perfusion during the task. Further, only GABA increased significantly during the first working memory run and then decreased continuously across subsequent task runs. The decrease of GABA over time was paralleled by a trend towards decreased reaction times and higher task accuracy. These results demonstrate a link between neurotransmitter dynamics and hemodynamic activity during working memory, indicating that functional neuroimaging metrics depend on the balance of excitation and inhibition required for cognitive processing.
Surveillance as an Option for the Treatment of Small Renal Masses
S. Klaver,S. Joniau,H. Van Poppel
Advances in Urology , 2008, DOI: 10.1155/2008/705958
Abstract: Objectives. To review the natural history and biological potential of small renal masses in order to evaluate surveillance as a treatment option. Methods. Literature search of Medline and additional references from non-Medline-indexed publications concerning surveillance of small renal masses. Results. The natural history and biological potential of small renal masses can still not be unambiguously predicted at present. There seems to be no clear correlation between tumour size and presence of benign histology. The majority of small renal masses grow and the majority are cancer, but one cannot safely assume that a lack of growth on serial CT scans is the confirmation of absence of malignancy. Needle core biopsies could be used to help in decision making. They show a high accuracy for histopathological tumour type but are less accurate in evaluating Fuhrman grade. Conclusions. At present, surveillance of small renal masses should only be considered in elderly and/or infirm patients with competing health risks, in those with a limited life expectancy, and in those for whom minimal invasive treatment or surgery is not an option. In all other patients, active surveillance should only be considered in the context of a study protocol. Long-term, prospective studies are needed to provide a more accurate assessment of the natural history and metastastic potential of small renal masses.
Presenilins and the γ-secretase: still a complex problem
David H Small, David W Klaver, Lisa Foa
Molecular Brain , 2010, DOI: 10.1186/1756-6606-3-7
Abstract: Alzheimer's disease (AD) is the most common cause of dementia in the elderly. Typically 5-10% of the population over the age of 65 have dementia, and of these cases, a large percentage have AD [1]. AD is characterised by the presence of proteinaceous deposits in the brain [2]. The extracellular amyloid deposits, which are found in the neuropil (amyloid plaques) and in association with small-medium size cerebral blood vessels (cerebral amyloid angiopathy), are composed of a 4 kDa polypeptide known as the amyloid-β protein (Aβ) which is derived by proteolytic cleavage from a much larger amyloid-β precursor protein (APP) [3]. Aβ displays a spontaneous ability to aggregate into oligomers and larger fibrillar structures, and it is generally thought that the accumulation of oligomeric Aβ is chiefly responsible for the neurodegeneration that occurs in AD [4].For the generation of Aβ, APP is first cleaved on the N-terminal side of the Aβ sequence by the β-site APP cleaving enzyme-1 (BACE1), a transmembrane aspartyl protease [3]. The resulting 99-amino acid residue C-terminal fragment (C99) is then cleaved by the γ-secretase to yield Aβ and a C-terminal APP intracellular domain (AICD) fragment (Fig. 1). The function of the AICD fragment is unclear, although it is thought to have a role in intracellular signalling. For example, AICD may be involved in the regulation of gene transcription, synaptic plasticity and cytoskeletal dynamics [5].The major form of Aβ possesses 40 amino-acid residues (Aβ1-40). However, other minor species are also produced which vary in the C-terminal sequence. Production of a longer 42-residue species (Aβ1-42) is thought to be intimately associated with AD pathogenesis [6]. Aβ1-42 aggregates more readily than Aβ1-40, and increased production of Aβ1-42 may seed aggregation of Aβ1-40 or other Aβ species [4].Approximately 5% of all AD cases are autosomal dominant [7]. Soon after the complete APP sequence was cloned in 1987 [8], it became clear that at le
Defect and solute properties in dilute Fe-Cr-Ni austenitic alloys from first principles
T. P. C. Klaver,D. J. Hepburn,G. J. Ackland
Physics , 2011, DOI: 10.1103/PhysRevB.85.174111
Abstract: We present results of an extensive set of first-principles density functional theory calculations of point defect formation, binding and clustering energies in austenitic Fe with dilute concentrations of Cr and Ni solutes.
First Principles Calculations of Defects in Unstable Crystals: Austenitic Iron
G. J. Ackland,T. P. C. Klaver,D. J. Hepburn
Physics , 2011,
Abstract: First principles calculations have given a new insight into the energies of point defects in many different materials, information which cannot be readily obtained from experiment. Most such calculation are done at zero Kelvin, with the assumption that finite temperature effects on defect energies and barriers are small. In some materials, however, the stable crystal structre of interest is mechanically unstable at 0K. In such cases, alternate approaches are needed. Here we present results of first principles calculations of austenitic iron using the VASP code. We determine an appropriate reference state for collinear magnetism to be the antiferromagnetic double-layer (AFM-d) which is both stable and lower in energy than other possible models for the low temperature limit of paramagnetic fcc iron. We then consider the energetics of dissolving typical alloying impurities (Ni, Cr) in the materials, and their interaction with point defects typical of the irradiated environment. We show that using standard methods there is a very strong dependence of calculated defect formation energies on the reference state chosen. Furthermore, there is a correlation between local free volume magnetism and energetics. The effect of substitutional Ni and Cr on defect properties is weak, rarely more than tenths of eV, so it is unlikely that small amounts of Ni and Cr will have a significant effect on the radiation damage in austenitic iron at high temperatures.
A Vectorial Capacity Product to Monitor Changing Malaria Transmission Potential in Epidemic Regions of Africa
Pietro Ceccato,Christelle Vancutsem,Robert Klaver,James Rowland,Stephen J. Connor
Journal of Tropical Medicine , 2012, DOI: 10.1155/2012/595948
Abstract: Rainfall and temperature are two of the major factors triggering malaria epidemics in warm semi-arid (desert-fringe) and high altitude (highland-fringe) epidemic risk areas. The ability of the mosquitoes to transmit Plasmodium spp. is dependent upon a series of biological features generally referred to as vectorial capacity. In this study, the vectorial capacity model (VCAP) was expanded to include the influence of rainfall and temperature variables on malaria transmission potential. Data from two remote sensing products were used to monitor rainfall and temperature and were integrated into the VCAP model. The expanded model was tested in Eritrea and Madagascar to check the viability of the approach. The analysis of VCAP in relation to rainfall, temperature and malaria incidence data in these regions shows that the expanded VCAP correctly tracks the risk of malaria both in regions where rainfall is the limiting factor and in regions where temperature is the limiting factor. The VCAP maps are currently offered as an experimental resource for testing within Malaria Early Warning applications in epidemic prone regions of sub-Saharan Africa. User feedback is currently being collected in preparation for further evaluation and refinement of the VCAP model. 1. Introduction Malaria is a major public health threat to the African continent and its control is critical to achieving the Millennium Development Goals in this region [1]. Although considerable progress has been made to reduce the malaria burden in sub-Saharan Africa by introducing control measures such as the provision of insecticide-treated mosquito nets, indoor residual spraying, and easier access to effective antimalarial drugs [2], malaria epidemics continue to occur. Periodic epidemics of malaria are a major public health problem for many sub-Saharan African countries. Populations in epidemic-prone areas have a poorly developed immunity to malaria and the disease remains life threatening to all age groups [3]. The impact of epidemics could be minimized through prediction, improved prevention through timely vector control, and deployment of appropriate control measures. The implementation of a Malaria Early Warning System enables regional health ministries to focus on epidemiological surveillance and be better prepared to take necessary actions. Rainfall and temperature anomalies are two of the major environmental factors triggering epidemics in warm semi-arid and altitude areas. Increases in epidemics often occur in these regions after excessive rains or increases in temperature [4, 5]. The ability
A Method of Drusen Measurement Based on the Geometry of Fundus Reflectance
R Theodore Smith, Takayuki Nagasaki, Janet R Sparrow, Irene Barbazetto, Caroline CW Klaver, Jackie K Chan
BioMedical Engineering OnLine , 2003, DOI: 10.1186/1475-925x-2-10
Abstract: An interactive semi-automated procedure was developed to level the macular background reflectance for the purpose of morphometric analysis of drusen. 12 color fundus photographs of patients with age-related macular degeneration and drusen were analyzed. After digitizing the photographs, the underlying background pattern in the green channel was leveled by an algorithm based on the elliptically concentric geometry of the reflectance in the normal macula: the gray scale values of all structures within defined elliptical boundaries were raised sequentially until a uniform background was obtained. Segmentation of drusen and area measurements in the central and middle subfields (1000 μm and 3000 μm diameters) were performed by uniform thresholds. Two observers using this interactive semi-automated software measured each image digitally. The mean digital measurements were compared to independent stereo fundus gradings by two expert graders (stereo Grader 1 estimated the drusen percentage in each of the 24 regions as falling into one of four standard broad ranges; stereo Grader 2 estimated drusen percentages in 1% to 5% intervals).The mean digital area measurements had a median standard deviation of 1.9%. The mean digital area measurements agreed with stereo Grader 1 in 22/24 cases. The 95% limits of agreement between the mean digital area measurements and the more precise stereo gradings of Grader 2 were -6.4 % to +6.8 % in the central subfield and -6.0 % to +4.5 % in the middle subfield. The mean absolute differences between the digital and stereo gradings 2 were 2.8 +/- 3.4% in the central subfield and 2.2 +/- 2.7% in the middle subfield.Semi-automated, supervised drusen measurements may be done reproducibly and accurately with adaptations of commercial software. This technique for macular image analysis has potential for use in clinical research.Color fundus photographs have been routinely employed for diagnostic purposes for many years, and fundus photo gradings are c
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