417 research outputs found
Neuron as a reward-modulated combinatorial switch and a model of learning behavior
This paper proposes a neuronal circuitry layout and synaptic plasticity
principles that allow the (pyramidal) neuron to act as a "combinatorial
switch". Namely, the neuron learns to be more prone to generate spikes given
those combinations of firing input neurons for which a previous spiking of the
neuron had been followed by a positive global reward signal. The reward signal
may be mediated by certain modulatory hormones or neurotransmitters, e.g., the
dopamine. More generally, a trial-and-error learning paradigm is suggested in
which a global reward signal triggers long-term enhancement or weakening of a
neuron's spiking response to the preceding neuronal input firing pattern. Thus,
rewards provide a feedback pathway that informs neurons whether their spiking
was beneficial or detrimental for a particular input combination. The neuron's
ability to discern specific combinations of firing input neurons is achieved
through a random or predetermined spatial distribution of input synapses on
dendrites that creates synaptic clusters that represent various permutations of
input neurons. The corresponding dendritic segments, or the enclosed individual
spines, are capable of being particularly excited, due to local sigmoidal
thresholding involving voltage-gated channel conductances, if the segment's
excitatory and absence of inhibitory inputs are temporally coincident. Such
nonlinear excitation corresponds to a particular firing combination of input
neurons, and it is posited that the excitation strength encodes the
combinatorial memory and is regulated by long-term plasticity mechanisms. It is
also suggested that the spine calcium influx that may result from the
spatiotemporal synaptic input coincidence may cause the spine head actin
filaments to undergo mechanical (muscle-like) contraction, with the ensuing
cytoskeletal deformation transmitted to the axon initial segment where it
may...Comment: Version 5: added computer code in the ancillary files sectio
An Operating Principle of the Cerebral Cortex, and a Cellular Mechanism for Attentional Trial-and-Error Pattern Learning and Useful Classification Extraction
A feature of the brains of intelligent animals is the ability to learn to
respond to an ensemble of active neuronal inputs with a behaviorally
appropriate ensemble of active neuronal outputs. Previously, a hypothesis was
proposed on how this mechanism is implemented at the cellular level within the
neocortical pyramidal neuron: the apical tuft or perisomatic inputs initiate
"guess" neuron firings, while the basal dendrites identify input patterns based
on excited synaptic clusters, with the cluster excitation strength adjusted
based on reward feedback. This simple mechanism allows neurons to learn to
classify their inputs in a surprisingly intelligent manner. Here, we revise and
extend this hypothesis. We modify synaptic plasticity rules to align with
behavioral time scale synaptic plasticity (BTSP) observed in hippocampal area
CA1, making the framework more biophysically and behaviorally plausible. The
neurons for the guess firings are selected in a voluntary manner via feedback
connections to apical tufts in the neocortical layer 1, leading to dendritic
Ca2+ spikes with burst firing, which are postulated to be neural correlates of
attentional, aware processing. Once learned, the neuronal input classification
is executed without voluntary or conscious control, enabling hierarchical
incremental learning of classifications that is effective in our inherently
classifiable world. In addition to voluntary, we propose that pyramidal neuron
burst firing can be involuntary, also initiated via apical tuft inputs, drawing
attention towards important cues such as novelty and noxious stimuli. We
classify the excitations of neocortical pyramidal neurons into four categories
based on their excitation pathway: attentional versus automatic and
voluntary/acquired versus involuntary. Additionally, we hypothesize that
dendrites within pyramidal neuron minicolumn bundles are coupled via
depolarization...Comment: 20 pages, 13 figure
Multi-scale space-variant FRep cellular structures
Existing mesh and voxel based modeling methods encounter difficulties when dealing with objects containing cellular structures
on several scale levels and varying their parameters in space. We describe an alternative approach based on using real functions evaluated procedurally at any given point. This allows for modeling fully parameterized, nested and multi-scale cellular
structures with dynamic variations in geometric and cellular properties. The geometry of a base unit cell is defined using Function Representation (FRep) based primitives and operations. The unit cell is then replicated in space using periodic
space mappings such as sawtooth and triangle waves. While being replicated, the unit cell can vary its geometry and topology due
to the use of dynamic parameterization. We illustrate this approach by several examples of microstructure generation within a given volume or
along a given surface. We also outline some methods for direct rendering and fabrication not involving auxiliary mesh and voxel
representations
Feature based volumes for implicit intersections.
The automatic generation of volumes bounding the intersection of two implicit surfaces (isosurfaces of real functions of 3D point coordinates) or feature based volumes (FBV) is presented. Such FBVs are defined by constructive operations, function normalization and offsetting. By applying various offset operations to the intersection of two surfaces, we can obtain variations in the shape of an FBV. The resulting volume can be used as a boundary for blending operations applied to two corresponding volumes, and also for visualization of feature curves and modeling of surface based structures including microstructures
A Metapopulation Model for Chikungunya Including Populations Mobility on a Large-Scale Network
In this work we study the influence of populations mobility on the spread of
a vector-borne disease. We focus on the chikungunya epidemic event that
occurred in 2005-2006 on the R\'eunion Island, Indian Ocean, France, and
validate our models with real epidemic data from the event. We propose a
metapopulation model to represent both a high-resolution patch model of the
island with realistic population densities and also mobility models for humans
(based on real-motion data) and mosquitoes. In this metapopulation network, two
models are coupled: one for the dynamics of the mosquito population and one for
the transmission of the disease. A high-resolution numerical model is created
out from real geographical, demographical and mobility data. The Island is
modeled with an 18 000-nodes metapopulation network. Numerical results show the
impact of the geographical environment and populations' mobility on the spread
of the disease. The model is finally validated against real epidemic data from
the R\'eunion event.Comment: Accepted in Journal of Theoretical biolog
The electro-disintegration of few body systems revisited
Recent studies of the electro-disintegration of the few body systems at JLab
have revived the field. Not only recoil momentum distributions have been
determined in a single shot. But also they confirm that the diagrammatic
approach, which I developed 25 years ago, is relevant to analyze them, provided
that the Nucleon-Nucleon scattering amplitude, determined in the same energy
range, is used. They provide us with a solid starting point to address the
issue of the propagation of exotic components of hadrons in nuclear matterComment: 6 pages,7 figure
BSP-fields: An Exact Representation of Polygonal Objects by Differentiable Scalar Fields Based on Binary Space Partitioning
The problem considered in this work is to find a dimension independent algorithm for the generation of signed scalar fields exactly representing polygonal objects and satisfying the following requirements: the defining real function takes zero value exactly at the polygonal object boundary; no extra zero-value isosurfaces should be generated; C1 continuity of the function in the entire domain. The proposed algorithms are based on the binary space partitioning (BSP) of the object by the planes passing through the polygonal faces and are independent of the object genus, the number of disjoint components, and holes in the initial polygonal mesh. Several extensions to the basic algorithm are proposed to satisfy the selected optimization criteria. The generated BSP-fields allow for applying techniques of the function-based modeling to already existing legacy objects from CAD and computer animation areas, which is illustrated by several examples
Shape conforming volumetric interpolation with interior distances
Source based heterogeneous modelling is a powerful way of defining gradient materials within a volume. The current solutions do not take into account the topology of the object and can provide counter intuitive results for complex objects. This paper presents a method to interpolate material properties and attributes based on the accessibility of the points with respect to the material features defined by the user. Our method requires the nonoverlapping source features with constant material to interpolate gradient materials, by using Voronoi diagrams on interior distances. It leads to intuitive material properties across the shape regardless of its topology or complexity. We show how the shape conforming field is defined inside the volume and can be extended outside the volume to create a valid operator for a heterogeneous modelling system dealing with scalar fields. The presented method is computationally efficient and has several applications, such as material property interpolation and shape aware procedural micro structures
- …