2,949 research outputs found
Hippocampus and Human Disease
This chapter focuses on two disorders in which the role of the hippocampus has been extensively investigated: Alzheimer's disease and temporal lobe epilepsy. Although in Alzheimer's disease the disease eventually results in widespread destruction of the cerebral cortex, the damage in the earliest stages of disease is restricted to the entorhinal cortex and the hippocampus, and the memory impairment that results from this disruption of the hippocampal formation represents one of the common characteristics of early onset Alzheimer's disease. In temporal lobe epilepsy, the pathological damage is often restricted to the hippocampus in the form of hippocampal sclerosis. However, unlike Alzheimer's disease, in which the hippocampal damage is secondary to the underlying pathological process, the hippocampus in temporal lobe epilepsy is not only sensitive to damage by seizure activity but can also act as the substrate for epileptic seizure generation
Exploratory Comparison of Healthcare costs and benefits of the UK’s Covid-19 response with four European countries
Hybrid Mobile Communication Networks for Planetary Exploration
A paper discusses the continuing work of the Mobile Exploration System Project, which has been performing studies toward the design of hybrid communication networks for future exploratory missions to remote planets. A typical network could include stationary radio transceivers on a remote planet, mobile radio transceivers carried by humans and robots on the planet, terrestrial units connected via the Internet to an interplanetary communication system, and radio relay transceivers aboard spacecraft in orbit about the planet. Prior studies have included tests on prototypes of these networks deployed in Arctic and desert regions chosen to approximate environmental conditions on Mars. Starting from the findings of the prior studies, the paper discusses methods of analysis, design, and testing of the hybrid communication networks. It identifies key radio-frequency (RF) and network engineering issues. Notable among these issues is the study of wireless LAN throughput loss due to repeater use, RF signal strength, and network latency variations. Another major issue is that of using RF-link analysis to ensure adequate link margin in the face of statistical variations in signal strengths
Software Architecture of Sensor Data Distribution In Planetary Exploration
Data from mobile and stationary sensors will be vital in planetary surface exploration. The distribution and collection of sensor data in an ad-hoc wireless network presents a challenge. Irregular terrain, mobile nodes, new associations with access points and repeaters with stronger signals as the network reconfigures to adapt to new conditions, signal fade and hardware failures can cause: a) Data errors; b) Out of sequence packets; c) Duplicate packets; and d) Drop out periods (when node is not connected). To mitigate the effects of these impairments, a robust and reliable software architecture must be implemented. This architecture must also be tolerant of communications outages. This paper describes such a robust and reliable software infrastructure that meets the challenges of a distributed ad hoc network in a difficult environment and presents the results of actual field experiments testing the principles and actual code developed
Catastrophic regime shifts in model ecological communities are true phase transitions
Ecosystems often undergo abrupt regime shifts in response to gradual external
changes. These shifts are theoretically understood as a regime switch between
alternative stable states of the ecosystem dynamical response to smooth changes
in external conditions. Usual models introduce nonlinearities in the
macroscopic dynamics of the ecosystem that lead to different stable attractors
among which the shift takes place. Here we propose an alternative explanation
of catastrophic regime shifts based on a recent model that pictures ecological
communities as systems in continuous fluctuation, according to certain
transition probabilities, between different micro-states in the phase space of
viable communities. We introduce a spontaneous extinction rate that accounts
for gradual changes in external conditions, and upon variations on this control
parameter the system undergoes a regime shift with similar features to those
previously reported. Under our microscopic viewpoint we recover the main
results obtained in previous theoretical and empirical work (anomalous
variance, hysteresis cycles, trophic cascades). The model predicts a gradual
loss of species in trophic levels from bottom to top near the transition. But
more importantly, the spectral analysis of the transition probability matrix
allows us to rigorously establish that we are observing the fingerprints, in a
finite size system, of a true phase transition driven by background
extinctions.Comment: 19 pages, 11 figures, revised versio
Catastrophic Phase Transitions and Early Warnings in a Spatial Ecological Model
Gradual changes in exploitation, nutrient loading, etc. produce shifts
between alternative stable states (ASS) in ecosystems which, quite often, are
not smooth but abrupt or catastrophic. Early warnings of such catastrophic
regime shifts are fundamental for designing management protocols for
ecosystems. Here we study the spatial version of a popular ecological model,
involving a logistically growing single species subject to exploitation, which
is known to exhibit ASS. Spatial heterogeneity is introduced by a carrying
capacity parameter varying from cell to cell in a regular lattice. Transport of
biomass among cells is included in the form of diffusion. We investigate
whether different quantities from statistical mechanics -like the variance, the
two-point correlation function and the patchiness- may serve as early warnings
of catastrophic phase transitions between the ASS. In particular, we find that
the patch-size distribution follows a power law when the system is close to the
catastrophic transition. We also provide links between spatial and temporal
indicators and analyze how the interplay between diffusion and spatial
heterogeneity may affect the earliness of each of the observables. We find that
possible remedial procedures, which can be followed after these early signals,
are more effective as the diffusion becomes lower. Finally, we comment on
similarities and differences between these catastrophic shifts and paradigmatic
thermodynamic phase transitions like the liquid-vapour change of state for a
fluid like water
Do High-Velocity Clouds trace the Dark Matter subhalo population?
Within the cosmological concordance model, Cold Dark Matter (CDM) subhalos
form the building blocks which merge hierarchically to more massive galaxies.
Since intergalactic gas is accreted by massive galaxies, observable e.g. as
high- velocity clouds (HVCs) around the Milky Way, with extremely low
metallicities, these can be suggested to represent the baryonic content of
primordial Dark Matter (DM) subhalos. Another possibility of their origin is
that they stem from disrupted satellite galaxies, but in this case, these gas
clouds move unaccompanied by a bound DM structure. Since HVCs are observed with
long gas tails and with irregular substructures, numerical models are performed
aiming at exploring their structure and compare them with observations. If HVCs
are engulfed by DM subhalos, their gas must leave the DM gravitational
potential and reflect this in their dynamics. On the other hand, the evolution
and survival of pure gas models must be tested to distinguish between
DM-dominated and DM-free clouds and to allow conclusions on their origin. The
models demonstrate that purely baryonic HVCs with low masses are disrupted by
ram-pressure stripping and Kelvin-Helmholtz instabilities, while more massive
ones survive, losing their initially spherical shape and develop significant
substructures including cometary elongations in the column density distribution
("head-tail structure"). On the contrary, HVCs with DM subhalos survive with
more than 90% of their gas mass still bound and spherically shaped, approaching
the Galactic disk like bullets. In addition, we find that velocity gradients
along the cometary head-tail structures does not necessarily offer a
possibility to distinguish between DM-dominated and purely gaseous HVCs.
Comparison of models with observations let us conclude that HVCs are not
embedded in a DM substructure and do not trace the cosmological subhalo
population.Comment: Accepted for publication in A&
The past, present and future challenges in epilepsy related and sudden deaths and biobanking.
Awareness and research on epilepsy-related deaths (ERD), in particular Sudden Unexpected Death in Epilepsy (SUDEP), have exponentially increased over the last two decades. Most publications have focused on guidelines that inform clinicians dealing with these deaths, educating patients, potential risk factors and mechanisms. There is a relative paucity of information available for pathologists who conduct these autopsies regarding appropriate post-mortem practice and investigations. As we move from recognizing SUDEP as the most common form of ERD toward in-depth investigations into its causes and prevention, health professionals involved with these autopsies and post-mortem procedure must remain fully informed. Systematizing a more comprehensive and consistent practice of examining these cases will facilitate 1) more precise determination of cause of death, 2) identification of SUDEP for improved epidemiological surveillance (the first step for an intervention study), and 3) bio-banking and cell-based research. This article reviews how pathologists and healthcare professionals have approached ERD, current practices, logistical problems and areas to improve and harmonize. The main neuropathology, cardiac and genetic findings in SUDEP are outlined, providing a framework for best practices, integration of clinical, pathologic and molecular genetic investigations in SUDEP, and ultimately prevention
Dwyer-Kan homotopy theory of enriched categories
We construct a model structure on the category of small categories enriched
over a combinatorial closed symmetric monoidal model category satisfying the
monoid axiom. Weak equivalences are Dwyer-Kan equivalences, i.e. enriched
functors which induce weak equivalences on morphism objects and equivalences of
ordinary categories when we take sets of connected components on morphism
objects.Comment: 39 pages, corrected versio
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