90,122 research outputs found
Temporal structure in neuronal activity during working memory in Macaque parietal cortex
A number of cortical structures are reported to have elevated single unit
firing rates sustained throughout the memory period of a working memory task.
How the nervous system forms and maintains these memories is unknown but
reverberating neuronal network activity is thought to be important. We studied
the temporal structure of single unit (SU) activity and simultaneously recorded
local field potential (LFP) activity from area LIP in the inferior parietal
lobe of two awake macaques during a memory-saccade task. Using multitaper
techniques for spectral analysis, which play an important role in obtaining the
present results, we find elevations in spectral power in a 50--90 Hz (gamma)
frequency band during the memory period in both SU and LFP activity. The
activity is tuned to the direction of the saccade providing evidence for
temporal structure that codes for movement plans during working memory. We also
find SU and LFP activity are coherent during the memory period in the 50--90 Hz
gamma band and no consistent relation is present during simple fixation.
Finally, we find organized LFP activity in a 15--25 Hz frequency band that may
be related to movement execution and preparatory aspects of the task. Neuronal
activity could be used to control a neural prosthesis but SU activity can be
hard to isolate with cortical implants. As the LFP is easier to acquire than SU
activity, our finding of rich temporal structure in LFP activity related to
movement planning and execution may accelerate the development of this medical
application.Comment: Originally submitted to the neuro-sys archive which was never
publicly announced (was 0005002
Decorrelation of neural-network activity by inhibitory feedback
Correlations in spike-train ensembles can seriously impair the encoding of
information by their spatio-temporal structure. An inevitable source of
correlation in finite neural networks is common presynaptic input to pairs of
neurons. Recent theoretical and experimental studies demonstrate that spike
correlations in recurrent neural networks are considerably smaller than
expected based on the amount of shared presynaptic input. By means of a linear
network model and simulations of networks of leaky integrate-and-fire neurons,
we show that shared-input correlations are efficiently suppressed by inhibitory
feedback. To elucidate the effect of feedback, we compare the responses of the
intact recurrent network and systems where the statistics of the feedback
channel is perturbed. The suppression of spike-train correlations and
population-rate fluctuations by inhibitory feedback can be observed both in
purely inhibitory and in excitatory-inhibitory networks. The effect is fully
understood by a linear theory and becomes already apparent at the macroscopic
level of the population averaged activity. At the microscopic level,
shared-input correlations are suppressed by spike-train correlations: In purely
inhibitory networks, they are canceled by negative spike-train correlations. In
excitatory-inhibitory networks, spike-train correlations are typically
positive. Here, the suppression of input correlations is not a result of the
mere existence of correlations between excitatory (E) and inhibitory (I)
neurons, but a consequence of a particular structure of correlations among the
three possible pairings (EE, EI, II)
Jet formation and interference in a thin QCD medium
In heavy-ion collisions, an abundant production of high-energy QCD jets
allows to study how these multiparticle sprays are modified as they pass
through the quark-gluon plasma. In order to shed new light on this process, we
compute the inclusive two-gluon rate off a hard quark propagating through a
color deconfined medium at first order in medium opacity. We explicitly impose
an energy ordering of the two emitted gluons, such that the "hard" gluon can be
thought of as belonging to the jet substructure while the other is a "soft"
emission (which can be collinear or medium-induced). Our analysis focusses on
two specific limits that clarify the modification of the additional angle- and
formation time-ordering of splittings. In one limit, the formation time of the
"hard" gluon is short compared to the "soft" gluon formation time, leading to a
probabilistic formula for production of and subsequent radiation off a
quark-gluon antenna. In the other limit, the ordering of formation is reverted,
which automatically leads to the fact that the jet substructure is resolved by
the medium. We observe in this case a characteristic delay: the jet radiates as
one color current (quark) up to the formation of the "hard" gluon, at which
point we observe the onset of radiation of the new color current (gluon). Our
computation supports a picture in which the in-medium jet dynamics are
described as a collection of subsequent antennas which are resolved by the
medium according to their transverse extent.Comment: 33 page
A sparsity-driven approach for joint SAR imaging and phase error correction
Image formation algorithms in a variety of applications have explicit or implicit dependence on a mathematical model of the observation process. Inaccuracies in the observation model may cause various degradations and artifacts in the reconstructed images. The application of interest in this paper is synthetic aperture radar (SAR) imaging, which particularly suffers from motion-induced model errors. These types of errors result in phase errors in SAR data which cause defocusing of the reconstructed images. Particularly focusing on imaging of fields that admit a sparse representation, we propose a sparsity-driven method for joint SAR imaging and phase error correction. Phase error correction is performed during the image formation process. The problem is set up as an optimization problem in a nonquadratic regularization-based framework. The method involves an iterative algorithm each iteration of which
consists of consecutive steps of image formation and model error correction. Experimental results show the effectiveness of the approach for various types of phase errors, as well as the improvements it provides over existing techniques for model error compensation in SAR
The mechanisms of tinnitus: perspectives from human functional neuroimaging
In this review, we highlight the contribution of advances in human neuroimaging to the current understanding of central mechanisms underpinning tinnitus and explain how interpretations of neuroimaging data have been guided by animal models. The primary motivation for studying the neural substrates of tinnitus in humans has been to demonstrate objectively its representation in the central auditory system and to develop a better understanding of its diverse pathophysiology and of the functional interplay between sensory, cognitive and affective systems. The ultimate goal of neuroimaging is to identify subtypes of tinnitus in order to better inform treatment strategies. The three neural mechanisms considered in this review may provide a basis for TI classification. While human neuroimaging evidence strongly implicates the central auditory system and emotional centres in TI, evidence for the precise contribution from the three mechanisms is unclear because the data are somewhat inconsistent. We consider a number of methodological issues limiting the field of human neuroimaging and recommend approaches to overcome potential inconsistency in results arising from poorly matched participants, lack of appropriate controls and low statistical power
The Nonlinear Meissner Effect in Unconventional Superconductors
We examine the long-wavelength current response in anisotropic
superconductors and show how the field-dependence of the Meissner penetration
length can be used to detect the structure of the order parameter. Nodes in the
excitation gap lead to a nonlinear current-velocity constitutive equation at
low temperatures which is distinct for each symmetry class of the order
parameter. The effective Meissner penetration length is linear in and
exhibits a characteristic anisotropy for fields in the -plane that is
determined by the positions of the nodes in momentum space. The nonlinear
current-velocity relation also leads to an intrinsic magnetic torque for
in-plane fields that are not parallel to a nodal or antinodal direction. The
torque scales as for and has a characteristic angular
dependence. We analyze the effects of thermal excitations, impurity scattering
and geometry on the current response of a superconductor, and
discuss our results in light of recent measurements of the low-temperature
penetration length and in-plane magnetization of single-crystals of
and .Comment: 30 pages, RevTeX file with 16 postscript figures. Submitted to Phys.
Rev.
An introduction to the interim digital SAR processor and the characteristics of the associated Seasat SAR imagery
Basic engineering data regarding the Interim Digital SAR Processor (IDP) and the digitally correlated Seasat synthetic aperature radar (SAR) imagery are presented. The correlation function and IDP hardware/software configuration are described, and a preliminary performance assessment presented. The geometric and radiometric characteristics, with special emphasis on those peculiar to the IDP produced imagery, are described
Radiative energy loss of neighboring subjets
We compute the in-medium energy loss probability distribution of two
neighboring subjets at leading order, in the large- approximation. Our
result exhibits a gradual onset of color decoherence of the system and accounts
for two expected limiting cases. When the angular separation is smaller than
the characteristic angle for medium-induced radiation, the two-pronged
substructure lose energy coherently as a single color charge, namely that of
the parent parton. At large angular separation the two subjets lose energy
independently. Our result is a first step towards quantifying effects of energy
loss as a result of the fluctuation of the multi-parton jet substructure and
therefore goes beyond the standard approach to jet quenching based on single
parton energy loss. We briefly discuss applications to jet observables in
heavy-ion collisions.Comment: 34 pages, 15 figure
Retrieval of dispersive and convective transport phenomena in fluids using stationary and nonstationary time domain analysis
Simultaneously occuring dispersive and convective components of fluid kinematics are obtained by a time domain analysis of optically retrieved temporal histories of the transport phenomena. Utilizing triangulation of collimated optical fields of view from two radiometers to obtain the temporal histories of the intensity fluctuations associated with the transport phenomena has enabled investigators to retrieve the local convective transport by employing correlation statistics. The location of the peak in the covariance curve determines the transit time from which the convection velocity is calculated; whereas, the change in shape of the peak in the covariance curve determines the change in average frequency of the wave packet from which the dispersion velocity is calculated. Thus, two-component analysis requires the maximum possible enhancement of the delineation for the transport. The convection velocity is the result of a fixed reference frame calculation whereas, the dispersion velocity is the result of a moving reference frame calcuation
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