19,963 research outputs found
Gating Input to Visual Cortex by Feedback to LGN
Anatomical studies have documented massive back-projections from higher to lower visual cortices and to the lateral geniculate nucleus (LGN). The large number of synapses from these sources suggest that they should have a profound influence on the information carried by feed-forward inputs to these cells. However, the functional role of these connections is unclear. In order to explore the role of the feedback connections, we have recorded spike trains from electrodes placed in LGN in the macaque monkey under sufenta anesthesia, and have compared LGN cells' activity with and without suppression by cooling of feedback from primary visual cortex (V1). Normally, magno and parvo LGN cells show a wide range over which their responses are proportional to stimulus contrast. Inactivation of V1 feedback causes LGN cells to become more nonlinear and less sensitive to high contrast than during normal conditions. Responses during V1 inactivation have a similar shape to those of retinal ganglion cells. We have also tested the properties of the so-called extended surround as they relate to cortical activity and to influences on responses to LGN stimulation. A model of this data suggests an interpretation in terms of two fnuctional components of feedback: a contrast-dependent component which dominates at high input contrast, and a constant baseline level of inhibitory feedback. We also show that the influence of the extended surround on the classical center mechanism is more complicated than a simple integration model.National Institutes of Health (EY-05156); Office of Naval Research (N00014-95-1-409
Helium Recombination Lines as a Probe of Abundance and Temperature Problems
The paper presents a simplified formula to determine an electron temperature,
Te(He I), for planetary nebulae (PNe) using the He I 7281/6678 line flux ratio.
In our previous studies of Te(He I) (Zhang et al. 2005), we used the He I line
emission coefficients given by Benjamin et al. (1999). Here we examine the
results of using more recent atomic data presented by Porter et al. (2005). A
good agreement is shown, suggesting that the effect of uncertainties of atomic
data on the resultant Te(He I) is negligible. We also present an analytical
formula to derive electron temperature using the He I discontinuity at 3421 A.
Our analysis shows that Te(He I) values are significantly lower than electron
temperatures deduced from the Balmer jump of H I recombination spectra, Te(H
I), and that inferred from the collisionally excited [O III] nebular-to-auroral
forbidden line flux ratio, Te([O III]). In addition, Te(H I) covers a wider
range of values than either Te(He I) or Te([O III]). This supports the
two-abundance nebular model with hydrogen-deficient material embedded in
diffuse gas of a ``normal'' chemical composition (i.e. ~solar).Comment: 5 pages, 3 figures. To appear in the RevMexAA proceedings of "The
Ninth Texas-Mexico Conference on Astrophysics
Buffered Reset Leads to Improved Compression in Fuzzy ARTMAP Classification of Radar Range Profiles
Fuzzy ARTMAP has to date been applied to a variety of automatic target recognition tasks, including radar range profile classification. In simulations of this task, it has demonstrated significant compression compared to k-nearest-neighbor classifiers. During supervised learning, match tracking search allocates memory based on the degree of similarity between newly encountered and previously encountered inputs, regardless of their prior predictive success. Here we invesetigate techniques that buffer reset based on a category's previous predictive success and thereby substantially improve the compression achieved with minimal loss of accuracy.Office of Naval Research (N00014-95-1-0657, N00014-95-1-0409, N00014-96-1-0659
Dual time scales in simulated annealing of a two-dimensional Ising spin glass
We apply a generalized Kibble-Zurek out-of-equilibrium scaling ansatz to
simulated annealing when approaching the spin-glass transition at temperature
of the two-dimensional Ising model with random couplings.
Analyzing the spin-glass order parameter and the excess energy as functions of
the system size and the annealing velocity in Monte Carlo simulations with
Metropolis dynamics, we find scaling where the energy relaxes slower than the
spin-glass order parameter, i.e., there are two different dynamic exponents.
The values of the exponents relating the relaxation time scales to the system
length, , are for the relaxation of the order
parameter and for the energy relaxation. We argue that the
behavior with dual time scales arises as a consequence of the entropy-driven
ordering mechanism within droplet theory. We point out that the dynamic
exponents found here for simulated annealing are different from the
temperature-dependent equilibrium dynamic exponent , for which
previous studies have found a divergent behavior; . Thus, our study shows that, within Metropolis dynamics, it is easier
to relax the system to one of its degenerate ground states than to migrate at
low temperatures between regions of the configuration space surrounding
different ground states. In a more general context of optimization, our study
provides an example of robust dense-region solutions for which the excess
energy (the conventional cost function) may not be the best measure of success.Comment: 13 pages, 16 figure
Calculation of the Two-body T-matrix in Configuration Space
A spectral integral method (IEM) for solving the two-body Schroedinger
equation in configuration space is generalized to the calculation of the
corresponding T-matrix. It is found that the desirable features of the IEM,
such as the economy of mesh-points for a given required accuracy, are carried
over also to the solution of the T-matrix. However the algorithm is
considerably more complex, because the T-matrix is a function of two variables
r and r', rather than only one variable r, and has a slope discontinuity at
r=r'. For a simple exponential potential an accuracy of 7 significant figures
is achieved, with the number N of Chebyshev support points in each partition
equal to 17. For a potential with a large repulsive core, such as the potential
between two He atoms, the accuracy decreases to 4 significant figures, but is
restored to 7 if N is increased to 65.Comment: 22 pages, 1 table 8 figure
Threshold Determination for ARTMAP-FD Familiarity Discrimination
The ARTMAP-FD neural network performs both identification (placing test patterns in classes encountered during training) and familiarity discrimination (judging whether a test pattern belongs to any of the classes encountered during training). ARTMAP-FD quantifies the familiarity of a test pattern by computing a measure of the degree to which the pattern's components lie within the ranges of values of training patterns grouped in the same cluster. This familiarity measure is compared to a threshold which can be varied to generate a receiver operating characteristic (ROC) curve. Methods for selecting optimal values for the threshold are evaluated. The performance of validation-set methods is compared with that of methods which track the development of the network's discrimination capability during training. The techniques are applied to databases of simulated radar range profiles.Advanced Research Projects Agency; Office of Naval Research (N00011-95-1-0657, N00011-95-0109, NOOOB-96-0659); National Science Foundation (IRI-94-01659
Related Services for Vermont\u27s Students with Disabilities
The purpose of Related Services for Vermont’s Students with Disabilities is to offer information regarding related services that is consistent with IDEA and with Vermont Law and regulations. It also describes promising or exemplary practices in education, special education, and related services. The manual’s content applies to all related services disciplines which serve students with disabilities, ages 3 through 21, who have an Individualized Education Program (IEP)
How well do we know the neutron structure function?
We present a detailed analysis of the uncertainty in the neutron F2n
structure function extracted from inclusive deuteron and proton deep-inelastic
scattering data. The analysis includes experimental uncertainties as well as
uncertainties associated with the deuteron wave function, nuclear smearing, and
nucleon off-shell corrections. Consistently accounting for the Q^2 dependence
of the data and calculations, and restricting the nuclear corrections to
microscopic models of the deuteron, we find significantly smaller uncertainty
in the extracted F2n/F2p ratio than in previous analyses. In addition to
yielding an improved extraction of the neutron structure function, this
analysis also provides an important baseline that will allow future,
model-independent extractions of neutron structure to be used to examine
nuclear medium effects in the the deuteron.Comment: 5 pages, 6 figure
Spitzer reveals what's behind Orion's Bar
We present Spitzer Space Telescope observations of 11 regions SE of the
Bright Bar in the Orion Nebula, along a radial from the exciting star
theta1OriC, extending from 2.6 to 12.1'. Our Cycle 5 programme obtained deep
spectra with matching IRS short-high (SH) and long-high (LH) aperture grid
patterns. Most previous IR missions observed only the inner few arcmin. Orion
is the benchmark for studies of the ISM particularly for elemental abundances.
Spitzer observations provide a unique perspective on the Ne and S abundances by
virtue of observing the dominant ionization states of Ne (Ne+, Ne++) and S
(S++, S3+) in Orion and H II regions in general. The Ne/H abundance ratio is
especially well determined, with a value of (1.01+/-0.08)E-4. We obtained
corresponding new ground-based spectra at CTIO. These optical data are used to
estimate the electron temperature, electron density, optical extinction, and
the S+/S++ ratio at each of our Spitzer positions. That permits an adjustment
for the total gas-phase S abundance because no S+ line is observed by Spitzer.
The gas-phase S/H abundance ratio is (7.68+/-0.30)E-6. The Ne/S abundance ratio
may be determined even when the weaker hydrogen line, H(7-6) here, is not
measured. The mean value, adjusted for the optical S+/S++ ratio, is Ne/S =
13.0+/-0.6. We derive the electron density versus distance from theta1OriC for
[S III] and [S II]. Both distributions are for the most part decreasing with
increasing distance. A dramatic find is the presence of high-ionization Ne++
all the way to the outer optical boundary ~12' from theta1OriC. This IR result
is robust, whereas the optical evidence from observations of high-ionization
species (e.g. O++) at the outer optical boundary suffers uncertainty because of
scattering of emission from the much brighter inner Huygens Region.Comment: 60 pages, 16 figures, 10 tables. MNRAS accepte
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