41 research outputs found
Structural Cycles in Food Webs
Traditionally, food webs have been constructed as structural directed graphs that describe "who eats whom," but it is common to interpret them directly as energy flow diagrams, where predations represent energy transfers from the prey to the predator. It is the aim of this work to point out that food webs are incomplete as energy flow diagrams if they ignore passive flows to detritus (dead organic material), a misconception that is common both in empirical data sets and in assembly models, where detritus often is either ignored or treated as an unlimited energy source. When individuals die, they contribute to the detritus pool, and might be an energy source for other species in the system. This feedback loop is of high importance, since it increases the number of pathways available for energy flow, revealing the significance of indirect effects, and making the functional role of the top predators less clear. These additional energy pathways increase the structural cyclicity of the system (measured in terms of of the dominant eigenvalue (lambda) of the adjacency matrix A). In this work we show the importance of the structural cyclicity by comparing empirical data sets to 5 different assembly models. Of these models: cascade (Cohen & Newman 1985), constant connectance (Martinez 1992), niche (Williams & Martinez 2000), modified niche (original in this work), and cyber-ecosystem (Fath 2004), the two last include detritus feedback. We show that when passive flows to detritus are included, the structural cyclicity is increased both in models and empirical data sets. We also show that there is in an approximately linear relationship with the link density (L/N), defined as the number of links per specie
Cyclic energy pathways in ecological food webs
Standard ecology textbooks typically maintain that nutrients cycle, but energy flows in unidirectional chains. However, here we use a new metric that allows for the identification and quantification of cyclic energy pathways. Some of these important pathways occur due to the contribution of dead organic matter to detrital pools and those organisms that feed on them, reintroducing some of that energy back into the food web. Recognition of these cyclic energy pathways profoundly impacts many aspects of ecology such as trophic levels, control, and the importance of indirect effects. Network analysis, specifically the maximum eigenvalue of the connectance matrix, is used to identify both the presence and strength of these structural cycles
Cosmic Evolution and Primordial Black Hole Evaporation
A cosmological model in which primordial black holes (PBHs) are present in
the cosmic fluid at some instant t=t_0 is investigated. The time t_0 is
naturally identified with the end of the inflationary period. The PBHs are
assumed to be nonrelativistic in the comoving fluid, to have the same mass, and
may be subject to evaporation for t>t_0. Our present work is related to an
earlier paper of Zimdahl and Pavon [Phys. Rev. D {\bf 58}, 103506 (1998)], but
in contradistinction to these authors we assume that the (negative) production
rate of the PBHs is zero. This assumption appears to us to be more simple and
more physical. Consequences of the formalism are worked out. In particular, the
four-divergence of the entropy four-vector in combination with the second law
in thermodynamics show in a clear way how the the case of PBH evaporation
corresponds to a production of entropy. Accretion of radiation onto the black
holes is neglected. We consider both a model where two different sub-fluids
interact, and a model involving one single fluid only. In the latter case an
effective bulk viscosity naturally appears in the formalism.Comment: 18 pages, LaTeX, no figures. Extended discussion of the black hole
evaporation process. Version to appear in Phys. Rev.
Astrocytic Ion Dynamics: Implications for Potassium Buffering and Liquid Flow
We review modeling of astrocyte ion dynamics with a specific focus on the
implications of so-called spatial potassium buffering, where excess potassium
in the extracellular space (ECS) is transported away to prevent pathological
neural spiking. The recently introduced Kirchoff-Nernst-Planck (KNP) scheme for
modeling ion dynamics in astrocytes (and brain tissue in general) is outlined
and used to study such spatial buffering. We next describe how the ion dynamics
of astrocytes may regulate microscopic liquid flow by osmotic effects and how
such microscopic flow can be linked to whole-brain macroscopic flow. We thus
include the key elements in a putative multiscale theory with astrocytes
linking neural activity on a microscopic scale to macroscopic fluid flow.Comment: 27 pages, 7 figure
Light Rays at Optical Black Holes in Moving Media
Light experiences a non-uniformly moving medium as an effective gravitational
field, endowed with an effective metric tensor , being the refractive index and the
four-velocity of the medium. Leonhardt and Piwnicki [Phys. Rev. A {\bf 60},
4301 (1999)] argued that a flowing dielectric fluid of this kind can be used to
generate an 'optical black hole'. In the Leonhardt-Piwnicki model, only a
vortex flow was considered. It was later pointed out by Visser [Phys. Rev.
Lett. {\bf 85}, 5252 (2000)] that in order to form a proper optical black hole
containing an event horizon, it becomes necessary to add an inward radial
velocity component to the vortex flow. In the present paper we undertake this
task: we consider a full spiral flow, consisting of a vortex component plus a
radially infalling component. Light propagates in such a dielectric medium in a
way similar to that occurring around a rotating black hole. We calculate, and
show graphically, the effective potential versus the radial distance from the
vortex singularity, and show that the spiral flow can always capture light in
both a positive, and a negative, inverse impact parameter interval. The
existence of a genuine event horizon is found to depend on the strength of the
radial flow, relative to the strength of the azimuthal flow. A limitation of
our fluid model is that it is nondispersive.Comment: 30 pages, LaTeX, 4 ps figures. Expanded discussion especially in
section 6; 5 new references. Version to appear in Phys. Rev.
Event Timing in Associative Learning: From Biochemical Reaction Dynamics to Behavioural Observations
Associative learning relies on event timing. Fruit flies for example, once trained with an odour that precedes electric shock, subsequently avoid this odour (punishment learning); if, on the other hand the odour follows the shock during training, it is approached later on (relief learning). During training, an odour-induced Ca++ signal and a shock-induced dopaminergic signal converge in the Kenyon cells, synergistically activating a Ca++-calmodulin-sensitive adenylate cyclase, which likely leads to the synaptic plasticity underlying the conditioned avoidance of the odour. In Aplysia, the effect of serotonin on the corresponding adenylate cyclase is bi-directionally modulated by Ca++, depending on the relative timing of the two inputs. Using a computational approach, we quantitatively explore this biochemical property of the adenylate cyclase and show that it can generate the effect of event timing on associative learning. We overcome the shortage of behavioural data in Aplysia and biochemical data in Drosophila by combining findings from both systems
Investigating large-scale brain dynamics using field potential recordings: Analysis and interpretation
New technologies to record electrical activity from the brain on a massive scale offer tremendous opportunities for discovery. Electrical measurements of large-scale brain dynamics, termed field potentials, are especially important to understanding and treating the human brain. Here, our goal is to provide best practices on how field potential recordings (EEG, MEG, ECoG and LFP) can be analyzed to identify large-scale brain dynamics, and to highlight critical issues and limitations of interpretation in current work. We focus our discussion of analyses around the broad themes of activation, correlation, communication and coding. We provide best-practice recommendations for the analyses and interpretations using a forward model and an inverse model. The forward model describes how field potentials are generated by the activity of populations of neurons. The inverse model describes how to infer the activity of populations of neurons from field potential recordings. A recurring theme is the challenge of understanding how field potentials reflect neuronal population activity given the complexity of the underlying brain systems
25th Annual Computational Neuroscience Meeting: CNS-2016
Abstracts of the 25th Annual Computational Neuroscience
Meeting: CNS-2016
Seogwipo City, Jeju-do, South Korea. 2â7 July 201
Modelling human choices: MADeM and decisionâmaking
Research supported by FAPESP 2015/50122-0 and DFG-GRTK 1740/2. RP and AR are also part of the Research, Innovation and Dissemination Center for Neuromathematics FAPESP grant (2013/07699-0). RP is supported by a FAPESP scholarship (2013/25667-8). ACR is partially supported by a CNPq fellowship (grant 306251/2014-0)