Minimax bounds for sparse PCA with noisy high-dimensional data

We study the problem of estimating the leading eigenvectors of a high-dimensional population covariance matrix based on independent Gaussian observations. We establish a lower bound on the minimax risk of estimators under the $l_2$ loss, in the joint limit as dimension and sample size increase to infinity, under various models of sparsity for the population eigenvectors. The lower bound on the risk points to the existence of different regimes of sparsity of the eigenvectors. We also propose a new method for estimating the eigenvectors by a two-stage coordinate selection scheme.Comment: 1 figur

Stable isotope geochemistry of a late Quaternary coral reef sequence, New Guinea : application of high resolution data to palaeoclimatology

Using physiographic evidence, geologists have endeavoured for a long time to employ fossil coral reefs as ancient sea-level gauges. A further step in this direction has been the production of a numerical chronology of sea-level changes from reefs, through radiometric dating of corals and molluscs that preserve their original mineralogy. Recent oxygen and carbon isotope abundance studies in reef material from a sequence of uplifted late Quaternary coral reefs on the Huon Peninsula, New Guinea, reported in this study, now provide a record of changes in ocean chemistry and temperature. The modern and ancient coral reef environments are isotopically monitored by the giant clam Tridacna gigas which is associated with the coral reefs. 18o;16o measurements in the exoskeleton of the giant clams constitute a record of combined temperature and shifts in the oceanic oxygen isotope composition induced by Pleistocene ice volume changes. Since variations in ice volumes are established by an independent line of evidence from the same site (i.e. stratigraphic sea-level data), the temperature 18 factor alone can be isolated from the 80 record. Apart from detecting a rapid ocean temperature fluctuation late during the last interglacial phase, the palaeotemperature estimates suggest near-interglacial conditions following each of the early Wisconsinan glacial stages, and general decline of the ocean temperature late during the last ice-age stages. The isotopic data also indicate a slow recovery of the ocean temperature to climatic optimum levels during the mid-Holocene. The fluctuations in surface ocean temperature estimated from the coral reef record conflict in magnitude with the palaeotemperature record obtained from deep-sea cores in the west equatorial Pacific. However, they support the cooling at sea-level inferred from the Late Pleistocene lowering of the snow and tree line in the New Guinea Highlands reported by Bowler et al.(1976) Climatic variations during the last 5 years at high latitudes, indirectly inferred from the Huon Peninsula sea-level data, when coupled with tropical ocean temperatures recorded by the u18o data, provide the essential definition of a sequence of events bearing on ice-age initiation, evolution and termination, and allow a critical evaluation of various models of ice ages. Recorded ∂13c changes in the coral reefs from New Guinea indicate variations in the carbon cycle over the last 5 years. In addition, the 18 distinct positive correlation observed between ∂13c and ∂18o records implies a coupling between sea water carbon cycle and sea-level (or climate) itself. It is concluded that only the formation and destruction of a reservoir of organic-rich shelf sediment caused by climatically-induced sea-level oscillations (Niitsuma-Broecker model) are consistent with the data presented. Marked skeletal growth bands in Tridacna allow detailed investigation of seasonal variations in modern and past marine environments by isotopic measurements. In addition to identifying the last interglacial stage (reef VIIa) as probably the wettest period, the variations observed in seasonal isotopic range indicate that important changes occurred in the oceanic and atmospheric circulation late during the last ice-age stages, implied by reduced seasonality. The separation of local and regional dynamic effects from longer-term global changes is an important problem in the coral-reef palaeoclimatic record. To verify the conclusions from the Huon Peninsula reefs, it is vital to extend the same analytical techniques to other reef sites in the tropical Pacific

Excitation of the Ganymede Ultraviolet Aurora

We analyze the ultraviolet aurorae observed on Ganymede by means of the Hubble Space Telescope and compare them to similar phenomena on Earth. We find that the tenuous nature of Ganymede's atmosphere precludes excitation of the aurora by high-energy electrons and requires a local acceleration mechanism. We propose the following as plausible mechanisms for generating both the continuous background emission and the intense auroral bright spots

Coronary sinus ostial occlusion during retrograde delivery of cardioplegic solution significantly improves cardioplegic distribution and efficacy

AbstractThis study documents the gross flow characteristics and capillary distribution of cardioplegic solution delivered retrogradely with the coronary sinus open versus closed. Methods : Five explanted human hearts from transplant recipients were used as experimental models. Hearts served as their own controls and received two doses of warm blood cardioplegic solution, each containing colored microspheres. The first dose was delivered through a retroperfusion catheter with the coronary sinus open and the second dose was delivered with the sinus occluded. Capillary flow was measured at twelve ventricular sites and gross flow was measured by examining coronary sinus regurgitation, thebesian vein drainage, and aortic effluent (nutrient flow). Results : Coronary sinus ostial occlusion allowed for a significant decrease in total cardioplegic flow (1.74 ± 0.40 ml/gm versus 1.06 ± 0.32 ml/gm; p < 0.05) to occur while maintaining an identical intracoronary sinus pressure. Ostial occlusion also resulted in an increase in the ratio of nutrient flow/total cardioplegic flow from 32.3% ± 15.1% to 61.3% ± 7.9% (p < 0.05). A statistically significant improvement in capillary flow was found at the midventricular level in the posterior intraventricular septum and posterolateral right ventricular free wall. This improvement was also documented for the intraventricular septum and right ventricle at the level of the apex. Conclusion : Coronary sinus occlusion during retrograde cardioplegia significantly improves cardioplegic delivery to the right ventricle and posterior intraventricular septum. Furthermore, the technique affords a significant improvement in nutrient cardioplegic flow while reducing the overall volume of cardioplegic solution administered. ( J T HORAC C ARDIOVASC S URG 1995; 109: 941-7

Resource Assessment of the In-Place and Potentially Recoverable Deep Natural Gas Resource of the Onshore Interior Salt Basins, North Central and Northeastern Gulf of Mexico Progress Report

The principal research effort for the first half of Year 3 of the project has been resource assessment. Emphasis has been on estimating the total volume of hydrocarbons generated and the potential amount of this resource that is classified as deep (&gt;15,000 ft) gas in the North Louisiana Salt Basin, the Mississippi Interior Salt Basin, the Manila Subbasin and the Conecuh Subbasin. The amount of this resource that has been expelled, migrated and entrapped is also the focus of the first half of Year 3 of this study

Blockade of the extracellular signal-regulated kinase pathway by U0126 attenuates neuronal damage following circulatory arrest

AbstractObjectivesThe extracellular signal-regulated kinase pathway of the mitogen-activated protein kinase signal transduction cascade has been implicated in the neuronal and endothelial dysfunction witnessed following cerebral ischemia-reperfusion injury. Extracellular signal-regulated kinase is activated by mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2. We evaluated the ability of a mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2–specific inhibitor (U0126) to block extracellular signal-regulated kinase activation and mitigate ischemic neuronal damage in a model of deep hypothermic circulatory arrest.MethodsPiglets underwent normal flow cardiopulmonary bypass (control, n = 4), deep hypothermic circulatory arrest (n = 6), and deep hypothermic circulatory arrest with U0126 (n = 5) at 20°C for 60 minutes. The deep hypothermic circulatory arrest with U0126 group was given 200 μg/kg of U0126 45 minutes prior to initiation of bypass followed by 100 μg/kg at reperfusion. Following 24 hours of post–cardiopulmonary bypass recovery, brains were harvested. Eleven distinct cortical regions were evaluated for neuronal damage using hematoxylin and eosin staining. A section of ischemic cortex was further evaluated by immunohistochemistry with rabbit polyclonal antibody against phosphorylated extracellular signal-regulated kinase 1/2.ResultsThe deep hypothermic circulatory arrest and deep hypothermic circulatory arrest with U0126 groups displayed diffuse ischemic changes. However, the deep hypothermic circulatory arrest with U0126 group possessed significantly lower neuronal damage scores in the right frontal watershed zone of cerebral cortex, basal ganglia, and thalamus (P ≤ .05) and an overall trend toward neuroprotection versus the deep hypothermic circulatory arrest group. This neuroprotection was accompanied by nearly complete blockade of phosphorylated extracellular signal-regulated kinase in the cerebral vascular endothelium.ConclusionsIn this experimental model of deep hypothermic circulatory arrest, U0126 blocked extracellular signal-regulated kinase activation and provided a significant neuroprotective effect. These results support targeting of the extracellular signal-regulated kinase pathway for inhibition as a novel therapeutic approach to mitigate neuronal damage following deep hypothermic circulatory arrest

Syncytiotrophoblast Microvesicles Released from Pre-Eclampsia Placentae Exhibit Increased Tissue Factor Activity

Background: Pre-eclampsia is a complication of pregnancy associated with activation of coagulation. It is caused by the placenta, which sheds increased amounts of syncytiotrophoblast microvesicles (STBM) into the maternal circulation. We hypothesized that STBM could contribute to the haemostatic activation observed in pre-eclampsia. Methodology/Principal Findings: STBM were collected by perfusion of the maternal side of placentae from healthy pregnant women and women with pre-eclampsia at caesarean section. Calibrated automated thrombography was used to assess thrombin generation triggered by STBM-borne tissue factor in platelet poor plasma (PPP). No thrombin was detected in PPP alone but the addition of STBM initiated thrombin generation in 14/16 cases. Pre-eclampsia STBM significantly shortened the lag time (LagT, P = 0.01) and time to peak thrombin generation (TTP, P = 0.005) when compared to normal STBM. Blockade of tissue factor eliminated thrombin generation, while inhibition of tissue factor pathway inhibitor significantly shortened LagT (p = 0.01) and TTP (P,0.0001), with a concomitant increase in endogenous thrombin potential. Conclusions/Significance: STBM triggered thrombin generation in normal plasma in a tissue factor dependent manner, indicating that TF activity is expressed by STBM. This is more pronounced in STBM shed from pre-eclampsia placentae. As more STBM are shed in pre-eclampsia these observations give insight into the disordered haemostasis observed in thi