843 research outputs found
A computer simulation of oscillatory behavior in primary visual cortex
Periodic variations in correlated cellular activity have been observed in many regions of the cerebral cortex. The recent discovery of stimulus-dependent, spatially-coherent oscillations in primary visual cortex of the cat has led to suggestions of neural information encoding schemes based on phase and/or frequency variation. To explore the mechanisms underlying this behavior and their possible functional consequences, we have developed a realistic neural model, based on structural features of visual cortex, which replicates observed oscillatory phenomena. In the model, this oscillatory behavior emerges directly from the structure of the cortical network and the properties of its intrinsic neurons; however, phase coherence is shown to be an average phenomenon seen only when measurements are made over multiple trials. Because average coherence does not ensure synchrony of firing over the course of single stimuli, oscillatory phase may not be a robust strategy for directly encoding stimulus-specific information. Instead, the phase and frequency of cortical oscillations may reflect the coordination of general computational processes within and between cortical areas. Under this interpretation, coherence emerges as a result of horizontal interactions that could be involved in the formation of receptive field properties
Computer Simulation of Oscillatory Behavior in Cerebral Cortical Networks
It has been known for many years that specific regions of the working
cerebral cortex display periodic variations in correlated cellular
activity. While the olfactory system has been the focus of much of
this work, similar behavior has recently been observed in primary
visual cortex. We have developed models of both the olfactory
and visual cortex which replicate the observed oscillatory properties
of these networks. Using these models we have examined the
dependence of oscillatory behavior on single cell properties and network
architectures. We discuss the idea that the oscillatory events
recorded from cerebral cortex may be intrinsic to the architecture
of cerebral cortex as a whole, and that these rhythmic patterns
may be important in coordinating neuronal activity during sensory
processing
A Computer Simulation of Olfactory Cortex with Functional Implications for Storage and Retrieval of Olfactory Information
Based on anatomical and physiological data, we have developed a computer simulation of piriform
(olfactory) cortex which is capable of reproducing spatial and temporal patterns of actual
cortical activity under a variety of conditions. Using a simple Hebb-type learning rule in conjunction
with the cortical dynamics which emerge from the anatomical and physiological organization
of the model, the simulations are capable of establishing cortical representations for different
input patterns. The basis of these representations lies in the interaction of sparsely distributed,
highly divergent/convergent interconnections between modeled neurons. We have shown that
different representations can be stored with minimal interference. and that following learning
these representations are resistant to input degradation, allowing reconstruction of a representation
following only a partial presentation of an original training stimulus. Further, we have
demonstrated that the degree of overlap of cortical representations for different stimuli can
also be modulated. For instance similar input patterns can be induced to generate distinct cortical
representations (discrimination). while dissimilar inputs can be induced to generate overlapping
representations (accommodation). Both features are presumably important in classifying olfactory
stimuli
GENESIS: A System for Simulating Neural Networks
We have developed a graphically oriented, general purpose simulation system to facilitate the modeling of neural networks.
The simulator is implemented under UNIX and X-windows and is
designed to support simulations at many levels of detail.
Specifically, it is intended for use in both applied network
modeling and in the simulation of detailed, realistic, biologically-based
models. Examples of current models developed under this
system include mammalian olfactory bulb and cortex, invertebrate
central pattern generators, as well as more abstract connectionist
simulations
How many clones need to be sequenced from a single forensic or ancient DNA sample in order to determine a reliable consensus sequence?
Forensic and ancient DNA (aDNA) extracts are mixtures of endogenous aDNA, existing in more or less damaged state, and contaminant DNA. To obtain the true aDNA sequence, it is not sufficient to generate a single direct sequence of the mixture, even where the authentic aDNA is the most abundant (e.g. 25% or more) in the component mixture. Only bacterial cloning can elucidate the components of this mixture. We calculate the number of clones that need to be sampled (for various mixture ratios) in order to be confident (at various levels of confidence) to have identified the major component. We demonstrate that to be >95% confident of identifying the most abundant sequence present at 70% in the ancient sample, 20 clones must be sampled. We make recommendations and offer a free-access web-based program, which constructs the most reliable consensus sequence from the user's input clone sequences and analyses the confidence limits for each nucleotide position and for the whole consensus sequence. Accepted authentication methods must be employed in order to assess the authenticity and endogeneity of the resulting consensus sequences (e.g. quantification and replication by another laboratory, blind testing, amelogenin sex versus morphological sex, the effective use of controls, etc.) and determine whether they are indeed aDNA
Accurate Determination of Phenotypic Information from Historic Thoroughbred Horses by Single Base Extension
Historic DNA have the potential to identify phenotypic information otherwise invisible in the historical, archaeological and palaeontological record. In order to determine whether a single nucleotide polymorphism typing protocol based on single based extension (SNaPshot™) could produce reliable phenotypic data from historic samples, we genotyped three coat colour markers for a sample of historic Thoroughbred horses for which both phenotypic and correct geotypic information were known from pedigree information in the General Stud Book. Experimental results were consistent with the pedigrees in all cases. Thus we demonstrate that historic DNA techniques can produce reliable phenotypic information from museum specimens.© 2010 Campana et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
A search of CO emission lines in blazars: the low molecular gas content of BL Lac objects compared to quasars
BL Lacertae (Lac) objects that are detected at very-high energies (VHE) are
of fundamental importance to study multiple astrophysical processes, including
the physics of jets, the properties of the extragalactic background light and
the strength of the intergalactic magnetic field. Unfortunately, since most
blazars have featureless optical spectra that preclude a redshift
determination, a substantial fraction of these VHE extragalactic sources cannot
be used for cosmological studies. To assess whether molecular lines are a
viable way to establish distances, we have undertaken a pilot program at the
IRAM 30m telescope to search for CO lines in three BL Lac objects with known
redshifts. We report a positive detection of M_H2 ~ 3x10^8 Msun toward 1ES
1959+650, but due to the poor quality of the baseline, this value is affected
by a large systematic uncertainty. For the remaining two sources, W Comae and
RGB J0710+591, we derive 3sigma upper limits at, respectively, M_H2 < 8.0x10^8
Msun and M_H2 < 1.6x10^9 Msun, assuming a line width of 150 km/s and a standard
conversion factor alpha=4 M_sun/(K km/s pc^2). If these low molecular gas
masses are typical for blazars, blind redshift searches in molecular lines are
currently unfeasible. However, deep observations are still a promising way to
obtain precise redshifts for sources whose approximate distances are known via
indirect methods. Our observations further reveal a deficiency of molecular gas
in BL Lac objects compared to quasars, suggesting that the host galaxies of
these two types of active galactic nuclei (AGN) are not drawn from the same
parent population. Future observations are needed to assess whether this
discrepancy is statistically significant, but our pilot program shows how
studies of the interstellar medium in AGN can provide key information to
explore the connection between the active nuclei and the host galaxies.Comment: 8 pages, 3 figures. MNRAS accepte
The architecture of Abell 1386 and its relationship to the Sloan Great Wall
We present new radial velocities from AAOmega on the Anglo-Australian
Telescope for 307 galaxies (b_J < 19.5) in the region of the rich cluster Abell
1386. Consistent with other studies of galaxy clusters that constitute
sub-units of superstructures, we find that the velocity distribution of A1386
is very broad (21,000--42,000 kms^-1, or z=0.08--0.14) and complex. The mean
redshift of the cluster that Abell designated as number 1386 is found to be
~0.104. However, we find that it consists of various superpositions of
line-of-sight components. We investigate the reality of each component by
testing for substructure and searching for giant elliptical galaxies in each
and show that A1386 is made up of at least four significant clusters or groups
along the line of sight whose global parameters we detail. Peculiar velocities
of brightest galaxies for each of the groups are computed and found to be
different from previous works, largely due to the complexity of the sky area
and the depth of analysis performed in the present work. We also analyse A1386
in the context of its parent superclusters: Leo A, and especially the Sloan
Great Wall. Although the new clusters may be moving toward mass concentrations
in the Sloan Great Wall or beyond, many are most likely not yet physically
bound to it.Comment: 21 pages, 9 figures, includes the full appendix table. Accepted for
publication in MNRA
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