6,933 research outputs found
The recovery of microwave scattering parameters from scatterometric measurements with special application to the sea
As part of an effort to demonstrate the value of the microwave scatterometer as a remote sea wind sensor, the interaction between an arbitrarily polarized scatterometer antenna and a noncoherent distributive target was derived and applied to develop a measuring technique to recover all the scattering parameters. The results are helpful for specifying antenna polarization properties for accurate retrieval of the parameters not only for the sea but also for other distributive scenes
Exploration Laboratory Analysis - ARC
The Exploration Laboratory Analysis (ELA) project supports the Exploration Medical Capability (ExMC) risk, Risk of Inability to Adequately Treat an Ill or Injured Crew Member, and ExMC Gap 4.05: Lack of minimally invasive in-flight laboratory capabilities with limited consumables required for diagnosing identified Exploration Medical Conditions. To mitigate this risk, the availability of inflight laboratory analysis instrumentation has been identified as an essential capability in future exploration missions. Mission architecture poses constraints on equipment and procedures that will be available to treat evidence-based medical conditions according to the Space Medicine Exploration Medical Conditions List (SMEMCL). The SMEMCL provided diagnosis and treatment for the evidence-based medical conditions and hence, a basis for developing ELA functional requirements
Toward RADSCAT measurements over the sea and their interpretation
Investigations into several areas which are essential to the execution and interpretation of suborbital observations by composite radiometer - scatterometer sensor (RADSCAT) are reported. Experiments and theory were developed to demonstrate the remote anemometric capability of the sensor over the sea through various weather conditions. It is shown that weather situations found in extra tropical cyclones are useful for demonstrating the all weather capability of the composite sensor. The large scale fluctuations of the wind over the sea dictate the observational coverage required to correlate measurements with the mean surface wind speed. Various theoretical investigations were performed to establish a premise for the joint interpretation of the experiment data. The effects of clouds and rains on downward radiometric observations over the sea were computed. A method of predicting atmospheric attenuation from joint observations is developed. In other theoretical efforts, the emission and scattering characteristics of the sea were derived. Composite surface theories with coherent and noncoherent assumptions were employed
Dynamics of Neural Networks with Continuous Attractors
We investigate the dynamics of continuous attractor neural networks (CANNs).
Due to the translational invariance of their neuronal interactions, CANNs can
hold a continuous family of stationary states. We systematically explore how
their neutral stability facilitates the tracking performance of a CANN, which
is believed to have wide applications in brain functions. We develop a
perturbative approach that utilizes the dominant movement of the network
stationary states in the state space. We quantify the distortions of the bump
shape during tracking, and study their effects on the tracking performance.
Results are obtained on the maximum speed for a moving stimulus to be
trackable, and the reaction time to catch up an abrupt change in stimulus.Comment: 6 pages, 7 figures with 4 caption
Guidance of ganciclovir therapy with pp65 antigenemia in cytomegalovirus-free recipients of livers from seropositive donors
A Moving Bump in a Continuous Manifold: A Comprehensive Study of the Tracking Dynamics of Continuous Attractor Neural Networks
Understanding how the dynamics of a neural network is shaped by the network
structure, and consequently how the network structure facilitates the functions
implemented by the neural system, is at the core of using mathematical models
to elucidate brain functions. This study investigates the tracking dynamics of
continuous attractor neural networks (CANNs). Due to the translational
invariance of neuronal recurrent interactions, CANNs can hold a continuous
family of stationary states. They form a continuous manifold in which the
neural system is neutrally stable. We systematically explore how this property
facilitates the tracking performance of a CANN, which is believed to have clear
correspondence with brain functions. By using the wave functions of the quantum
harmonic oscillator as the basis, we demonstrate how the dynamics of a CANN is
decomposed into different motion modes, corresponding to distortions in the
amplitude, position, width or skewness of the network state. We then develop a
perturbative approach that utilizes the dominating movement of the network's
stationary states in the state space. This method allows us to approximate the
network dynamics up to an arbitrary accuracy depending on the order of
perturbation used. We quantify the distortions of a Gaussian bump during
tracking, and study their effects on the tracking performance. Results are
obtained on the maximum speed for a moving stimulus to be trackable and the
reaction time for the network to catch up with an abrupt change in the
stimulus.Comment: 43 pages, 10 figure
Richardson's pair diffusion and the stagnation point structure of turbulence
DNS and laboratory experiments show that the spatial distribution of
straining stagnation points in homogeneous isotropic 3D turbulence has a
fractal structure with dimension D_s = 2. In Kinematic Simulations the time
exponent gamma in Richardson's law and the fractal dimension D_s are related by
gamma = 6/D_s. The Richardson constant is found to be an increasing function of
the number of straining stagnation points in agreement with pair duffusion
occuring in bursts when pairs meet such points in the flow.Comment: 4 pages; Submitted to Phys. Rev. Let
The current status of hepatic transplantation at the University of Pittsburgh.
Tacrolimus is a more potent and satisfactory immunosuppressant than CyA for combination therapy with prednisone. In randomized trials comparing the 2 drugs, the ability of tacrolimus to rescue intractably rejecting grafts on the competing CyA arm allowed equalization of patient and graft survival on both arms when the intent-to-treat analytic methodology was applied. The ability of tacrolimus to systematically rescue the treatment failures of CyA suggested, as a matter of common sense, that it is the preferred baseline drug for hepatic transplantation. This conclusion was supported by analysis of secondary end points, including the ability to prevent rejection. Hepatic-intestinal, multivisceral and isolated intestinal transplantation became feasible on a practical basis only after the advent of tacrolimus. Nevertheless, better management strategies must be devised before intestinal transplantation, alone or with other abdominal viscera, will meet its potential. One such strategy is based on the discovery of the presence of previously unsuspected, low-level donor leukocyte chimerism in long-surviving allograft recipients. We believe that this chimerism is the essential explanation for the feasibility of organ transplantation and a link to the acquired neonatal tolerance demonstrated by Billingham, Brent and Medawar (32). The hematolymphopoietic chimerism in organ recipients explains why weaning to a drug-free state in selected long-term survivors is frequently feasible and particularly if the allograft is a liver. Weaning should never be attempted without a stepwise protocol and careful monitoring of graft function. Recognition of the natural chimerism that develops after whole organ transplantation has led to efforts to augment it with perioperative donor BM infusion. This procedure has been shown to be free of significant complications (including GVHD) in all kinds of whole organ recipients, including those given intestine. The prospects of clinical xenotransplantation must be evaluated in the same context of chimerism as that delineated for allotransplantation with the discovery of spontaneous chimerism. Before addressing chimerism-related questions in xenotransplantation, the additional barrier of the complement activation syndromes that cause hyperacute rejection will have to be surmounted. Although measures to effectively transplant xenografts have so far eluded us, the availability of the more potent drug, tacrolimus, and recognition of the seminal basis of allograft (or xenograft) acceptance via chimerism has inserted an element of reality into the largely wishful thinking that has been evident in discussions about the future of xenotransplantation
Dynamical Synapses Enhance Neural Information Processing: Gracefulness, Accuracy and Mobility
Experimental data have revealed that neuronal connection efficacy exhibits
two forms of short-term plasticity, namely, short-term depression (STD) and
short-term facilitation (STF). They have time constants residing between fast
neural signaling and rapid learning, and may serve as substrates for neural
systems manipulating temporal information on relevant time scales. The present
study investigates the impact of STD and STF on the dynamics of continuous
attractor neural networks (CANNs) and their potential roles in neural
information processing. We find that STD endows the network with slow-decaying
plateau behaviors-the network that is initially being stimulated to an active
state decays to a silent state very slowly on the time scale of STD rather than
on the time scale of neural signaling. This provides a mechanism for neural
systems to hold sensory memory easily and shut off persistent activities
gracefully. With STF, we find that the network can hold a memory trace of
external inputs in the facilitated neuronal interactions, which provides a way
to stabilize the network response to noisy inputs, leading to improved accuracy
in population decoding. Furthermore, we find that STD increases the mobility of
the network states. The increased mobility enhances the tracking performance of
the network in response to time-varying stimuli, leading to anticipative neural
responses. In general, we find that STD and STP tend to have opposite effects
on network dynamics and complementary computational advantages, suggesting that
the brain may employ a strategy of weighting them differentially depending on
the computational purpose.Comment: 40 pages, 17 figure
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