27,457 research outputs found
Flexor Dysfunction Following Unilateral Transient Ischemic Brain Injury Is Associated with Impaired Locomotor Rhythmicity
Functional motor deficits in hemiplegia after stroke are predominately associated with flexor muscle impairments in animal models of ischemic brain injury, as well as in clinical findings. Rehabilitative interventions often employ various means of retraining a maladapted central pattern generator for locomotion. Yet, holistic modeling of the central pattern generator, as well as applications of such studies, are currently scarce. Most modeling studies rely on cellular neural models of the intrinsic spinal connectivity governing ipsilateral flexor-extensor, as well as contralateral coupling inherent in the spinal cord. Models that attempt to capture the general behavior of motor neuronal populations, as well as the different modes of driving their oscillatory function in vivo is lacking in contemporary literature. This study aims at generating a holistic model of flexor and extensor function as a whole, and seeks to evaluate the parametric coupling of ipsilateral and contralateral half-center coupling through the means of generating an ordinary differential equation representative of asymmetric central pattern generator models of varying coupling architectures. The results of this study suggest that the mathematical predictions of the locomotor centers which drive the dorsiflexion phase of locomotion are correlated with the denervation-type atrophy response of hemiparetic dorsiflexor muscles, as well as their spatiotemporal activity dysfunction during in vivo locomotion on a novel precise foot placement task. Moreover, the hemiplegic solutions were found to lie in proximity to an alternative task-space solution, by which a hemiplegic strategy could be readapted in order to produce healthy output. The results revealed that there are multiple strategies of retraining hemiplegic solutions of the CPG. This solution may modify the hemiparetic locomotor pattern into a healthy output by manipulating inter-integrator couplings which are not damaged by damage to the descending drives. Ultimately, some modeling experiments will demonstrate that the increased reliance on intrinsic connectivity increases the stability of the output, rendering it resistant to perturbations originating from extrinsic inputs to the pattern generating center
Polarization entangled photon-pair source based on quantum nonlinear photonics and interferometry
We present a versatile, high-brightness, guided-wave source of polarization
entangled photons, emitted at a telecom wavelength. Photon-pairs are generated
using an integrated type-0 nonlinear waveguide, and subsequently prepared in a
polarization entangled state via a stabilized fiber interferometer. We show
that the single photon emission wavelength can be tuned over more than 50 nm,
whereas the single photon spectral bandwidth can be chosen at will over more
than five orders of magnitude (from 25 MHz to 4 THz). Moreover, by performing
entanglement analysis, we demonstrate a high degree of control of the quantum
state via the violation of the Bell inequalities by more than 40 standard
deviations. This makes this scheme suitable for a wide range of quantum optics
experiments, ranging from fundamental research to quantum information
applications. We report on details of the setup, as well as on the
characterization of all included components, previously outlined in F. Kaiser
et al. (2013 Laser Phys. Lett. 10, 045202).Comment: 16 pages, 7 figure
A compact, multi-pixel parametric light source
The features of a compact, single pass, multi-pixel optical parametric
generator are discussed. Several hundreds of independent high spatial-quality
tunable ultrashort pulses were produced by pumping a bulk lithium triborate
crystal with an array of tightly focussed intense beams. The array of beams was
produced by shining a microlenses array with a large pump beam. Overall
conversion efficiency to signal and idler up to 30% of the pump beam has been
reported. Shot-to-shot energy fluctuation down to 3% was achieved for the
generated radiation.Comment: 11 pages, 6 figures, submitted to "Optics Communications
Parametric generation of second sound in superfluid helium: linear stability and nonlinear dynamics
We report the experimental studies of a parametric excitation of a second
sound (SS) by a first sound (FS) in a superfluid helium in a resonance cavity.
The results on several topics in this system are presented: (i) The linear
properties of the instability, namely, the threshold, its temperature and
geometrical dependencies, and the spectra of SS just above the onset were
measured. They were found to be in a good quantitative agreement with the
theory. (ii) It was shown that the mechanism of SS amplitude saturation is due
to the nonlinear attenuation of SS via three wave interactions between the SS
waves. Strong low frequency amplitude fluctuations of SS above the threshold
were observed. The spectra of these fluctuations had a universal shape with
exponentially decaying tails. Furthermore, the spectral width grew continuously
with the FS amplitude. The role of three and four wave interactions are
discussed with respect to the nonlinear SS behavior. The first evidence of
Gaussian statistics of the wave amplitudes for the parametrically generated
wave ensemble was obtained. (iii) The experiments on simultaneous pumping of
the FS and independent SS waves revealed new effects. Below the instability
threshold, the SS phase conjugation as a result of three-wave interactions
between the FS and SS waves was observed. Above the threshold two new effects
were found: a giant amplification of the SS wave intensity and strong resonance
oscillations of the SS wave amplitude as a function of the FS amplitude.
Qualitative explanations of these effects are suggested.Comment: 73 pages, 23 figures. to appear in Phys. Rev. B, July 1 st (2001
Experimental study of parametric subharmonic instability for internal waves
Internal waves are believed to be of primary importance as they affect ocean
mixing and energy transport. Several processes can lead to the breaking of
internal waves and they usually involve non linear interactions between waves.
In this work, we study experimentally the parametric subharmonic instability
(PSI), which provides an efficient mechanism to transfer energy from large to
smaller scales. It corresponds to the destabilization of a primary plane wave
and the spontaneous emission of two secondary waves, of lower frequencies and
different wave vectors. Using a time-frequency analysis, we observe the time
evolution of the secondary waves, thus measuring the growth rate of the
instability. In addition, a Hilbert transform method allows the measurement of
the different wave vectors. We compare these measurements with theoretical
predictions, and study the dependence of the instability with primary wave
frequency and amplitude, revealing a possible effect of the confinement due to
the finite size of the beam, on the selection of the unstable mode
Development of monitoring techniques by acoustical means for mechanical checkouts Final report, 15 May - 30 Sep. 1965
Automated pattern recognition devices using sonic signature data for detecting S3D and F-1 engine valve malfunction
Coupling the neural and physical dynamics in rhythmic movements
A pair of coupled oscillators simulating a central pattern generator (CPG) interacting with a pendular limb were numerically integrated. The CPG was represented as a van der Pol oscillator and the pendular limb was modeled as a linearized, hybrid spring-pendulum system. The CPG oscillator drove the pendular limb while the pendular limb modulated the frequency of the CPG. Three results were observed. First, sensory feedback influenced the oscillation frequency of the coupled system. The oscillation frequency was lower in the absence of sensory feedback. Moreover, if the muscle gain was decreased, thereby decreasing the oscillation amplitude of the pendular limb and indirectly lowering the effect of sensory feedback, the oscillation frequency decreased monotonically. This is consistent with experimental data (Williamson and Roberts 1986). Second, the CPG output usually led the angular displacement of the pendular limb by a phase of 90° regardless of the length of the limb. Third, the frequency of the coupled system tuned itself to the resonant frequency of the pendular limb. Also, the frequency of the coupled system was highly resistant to changes in the endogenous frequency of the CPG. The results of these simulations support the view that motor behavior emerges from the interaction of the neural dynamics of the nervous system and the physical dynamics of the periphery
New Concept of Solvability in Quantum Mechanics
In a pre-selected Hilbert space of quantum states the unitarity of the
evolution is usually guaranteed via a pre-selection of the generator (i.e., of
the Hamiltonian operator) in self-adjoint form. In fact, the simultaneous use
of both of these pre-selections is overrestrictive. One should be allowed to
make a given Hamiltonian self-adjoint only after an {\em ad hoc} generalization
of Hermitian conjugation. We argue that in the generalized, hidden-Hermiticity
scenario with nontrivial metric, the current concept of solvability (meaning,
most often, the feasibility of a non-numerical diagonalization of Hamiltonian)
requires a generalization allowing for a non-numerical form of metric. A few
illustrative solvable quantum models of this type are presented.Comment: 24 pages, 8 figure
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