2,150 research outputs found
GABA-enhanced collective behavior in neuronal axons underlies persistent gamma-frequency oscillations
Gamma (30–80 Hz) oscillations occur in mammalian electroencephalogram in a manner that indicates cognitive relevance. In vitro models of gamma oscillations demonstrate two forms of oscillation: one occurring transiently and driven by discrete afferent input and the second occurring persistently in response to activation of excitatory metabotropic receptors. The mechanism underlying persistent gamma oscillations has been suggested to involve gap-junctional communication between axons of principal neurons, but the precise relationship between this neuronal activity and the gamma oscillation has remained elusive. Here we demonstrate that gamma oscillations coexist with high-frequency oscillations (>90 Hz). High-frequency oscillations can be generated in the axonal plexus even when it is physically isolated from pyramidal cell bodies. They were enhanced in networks by nonsomatic -aminobutyric acid type A (GABAA) receptor activation, were modulated by perisomatic GABAA receptor-mediated synaptic input to principal cells, and provided the phasic input to interneurons required to generate persistent gamma-frequency oscillations. The data suggest that high-frequency oscillations occurred as a consequence of random activity within the axonal plexus. Interneurons provide a mechanism by which this random activity is both amplified and organized into a coherent network rhythm
Conditional sampling for barrier option pricing under the LT method
We develop a conditional sampling scheme for pricing knock-out barrier
options under the Linear Transformations (LT) algorithm from Imai and Tan
(2006). We compare our new method to an existing conditional Monte Carlo scheme
from Glasserman and Staum (2001), and show that a substantial variance
reduction is achieved. We extend the method to allow pricing knock-in barrier
options and introduce a root-finding method to obtain a further variance
reduction. The effectiveness of the new method is supported by numerical
results
A role for fast rhythmic bursting neurons in cortical gamma oscillations in vitro
Basic cellular and network mechanisms underlying gamma frequency oscillations (30–80 Hz) have been well characterized in the hippocampus and associated structures. In these regions, gamma rhythms are seen as an emergent property of networks of principal cells and fast-spiking interneurons. In contrast, in the neocortex a number of elegant studies have shown that specific types of principal neuron exist that are capable of generating powerful gamma frequency outputs on the basis of their intrinsic conductances alone. These fast rhythmic bursting (FRB) neurons (sometimes referred to as "chattering" cells) are activated by sensory stimuli and generate multiple action potentials per gamma period. Here, we demonstrate that FRB neurons may function by providing a large-scale input to an axon plexus consisting of gap-junctionally connected axons from both FRB neurons and their anatomically similar counterparts regular spiking neurons. The resulting network gamma oscillation shares all of the properties of gamma oscillations generated in the hippocampus but with the additional critical dependence on multiple spiking in FRB cells
Stability Analysis of Asynchronous States in Neuronal Networks with Conductance-Based Inhibition
Oscillations in networks of inhibitory interneurons have been reported at various sites of the brain and are thought to play a fundamental role in neuronal processing. This Letter provides a self-contained analytical framework that allows numerically efficient calculations of the population activity of a network of conductance-based integrate-and-fire neurons that are coupled through inhibitory synapses. Based on a normalization equation this Letter introduces a novel stability criterion for a network state of asynchronous activity and discusses its perturbations. The analysis shows that, although often neglected, the reversal potential of synaptic inhibition has a strong influence on the stability as well as the frequency of network oscillations
The Vector Vortex Coronagraph: Laboratory Results and First Light at Palomar Observatory
High-contrast coronagraphy will be needed to image and characterize faint
extra-solar planetary systems. Coronagraphy is a rapidly evolving field, and
many enhanced alternatives to the classical Lyot coronagraph have been proposed
in the past ten years. Here, we discuss the operation of the vector vortex
coronagraph, which is one of the most efficient possible coronagraphs. We first
present recent laboratory results, and then first light observations at the
Palomar observatory. Our near-infrared H-band (centered at ~ 1.65 microns) and
K-band (centered at ~ 2.2 microns) vector vortex devices demonstrated excellent
contrast results in the lab, down to ~ 1e-6 at an angular separation of 3 lb/d.
On sky, we detected a brown dwarf companion 3000 times fainter than its host
star (HR 7672) in the Ks band (centered at ~2.15 microns), at an angular
separation of ~ 2.5 lb/d. Current and next-generation high-contrast instruments
can directly benefit from the demonstrated capabilities of such a vector
vortex: simplicity, small inner working angle, high optical throughput (>90%),
and maximal off-axis discovery space
Genetically altered AMPA-type glutamate receptor kinetics in interneurons disrupt long-range synchrony of gamma oscillation
Gamma oscillations synchronized between distant neuronal populations may be critical for binding together brain regions devoted to common processing tasks. Network modeling predicts that such synchrony depends in part on the fast time course of excitatory postsynaptic potentials (EPSPs) in interneurons, and that even moderate slowing of this time course will disrupt synchrony. We generated mice with slowed interneuron EPSPs by gene targeting, in which the gene encoding the 67-kDa form of glutamic acid decarboxylase (GAD67) was altered to drive expression of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor subunit GluR-B. GluR-B is a determinant of the relatively slow EPSPs in excitatory neurons and is normally expressed at low levels in γ-aminobutyric acid (GABA)ergic interneurons, but at high levels in the GAD-GluR-B mice. In both wild-type and GAD-GluR-B mice, tetanic stimuli evoked gamma oscillations that were indistinguishable in local field potential recordings. Remarkably, however, oscillation synchrony between spatially separated sites was severely disrupted in the mutant, in association with changes in interneuron firing patterns. The congruence between mouse and model suggests that the rapid time course of AMPA receptor-mediated EPSPs in interneurons might serve to allow gamma oscillations to synchronize over distance
Imaging the asymmetric dust shell around CI Cam with long baseline optical interferometry
We present the first high angular resolution observation of the B[e]
star/X-ray transient object CI Cam, performed with the two-telescope Infrared
Optical Telescope Array (IOTA), its upgraded three-telescope version (IOTA3T)
and the Palomar Testbed Interferometer (PTI). Visibilities and closure phases
were obtained using the IONIC-3 integrated optics beam combiner. CI Cam was
observed in the near-infrared H and K spectral bands, wavelengths well suited
to measure the size and study the geometry of the hot dust surrounding CI Cam.
The analysis of the visibility data over an 8 year period from soon after the
1998 outburst to 2006 shows that the dust visibility has not changed over the
years. The visibility data shows that CI Cam is elongated which confirms the
disc-shape of the circumstellar environment and totally rules out the
hypothesis of a spherical dust shell. Closure phase measurements show direct
evidence of asymmetries in the circumstellar environment of CI Cam and we
conclude that the dust surrounding CI Cam lies in an inhomogeneous disc seen at
an angle. The near-infrared dust emission appears as an elliptical skewed
Gaussian ring with a major axis a = 7.58 +/- 0.24 mas, an axis ratio r = 0.39
+/- 0.03 and a position angle theta = 35 +/- 2 deg.Comment: 9 pages, 5 figures, accepted MNRA
Low-resolution spectrograph for the IOTA interferometer
The design and scientific objectives of a near infrared channeled spectrometer planned at the IOTA interferometer are discussed. The spectrometer has the flexibility to reconfigure easily for conventional broadband operations in addition to multi-channel mode. This instrument makes use of the existing PICNIC camera at the IOTA in order to be cost efficient. The spectrometer has been designed specifically for studying Mira stars. However, it will find its application in other areas of astrophysical interests such as studies of circumstellar disks around young stars and binary stars
New dynamics in cerebellar Purkinje cells: torus canards
We describe a transition from bursting to rapid spiking in a reduced
mathematical model of a cerebellar Purkinje cell. We perform a slow-fast
analysis of the system and find that -- after a saddle node bifurcation of
limit cycles -- the full model dynamics follow temporarily a repelling branch
of limit cycles. We propose that the system exhibits a dynamical phenomenon new
to realistic, biophysical applications: torus canards.Comment: 4 pages; 4 figures (low resolution); updated following peer-review:
language and definitions updated, Figures 1 and 4 updated, typos corrected,
references added and remove
Imaging the spotty surface of Betelgeuse in the H band
This paper reports on H-band interferometric observations of Betelgeuse made
at the three-telescope interferometer IOTA. We image Betelgeuse and its
asymmetries to understand the spatial variation of the photosphere, including
its diameter, limb darkening, effective temperature, surrounding brightness,
and bright (or dark) star spots. We used different theoretical simulations of
the photosphere and dusty environment to model the visibility data. We made
images with parametric modeling and two image reconstruction algorithms: MIRA
and WISARD. We measure an average limb-darkened diameter of 44.28 +/- 0.15 mas
with linear and quadratic models and a Rosseland diameter of 45.03 +/- 0.12 mas
with a MARCS model. These measurements lead us to derive an updated effective
temperature of 3600 +/- 66 K. We detect a fully-resolved environment to which
the silicate dust shell is likely to contribute. By using two imaging
reconstruction algorithms, we unveiled two bright spots on the surface of
Betelgeuse. One spot has a diameter of about 11 mas and accounts for about 8.5%
of the total flux. The second one is unresolved (diameter < 9 mas) with 4.5% of
the total flux. Resolved images of Betelgeuse in the H band are asymmetric at
the level of a few percent. The MOLsphere is not detected in this wavelength
range. The amount of measured limb-darkening is in good agreement with model
predictions. The two spots imaged at the surface of the star are potential
signatures of convective cells.Comment: 10 pages, 10 figures, accepted for publication in A&A, references
adde
- …