35 research outputs found
Pilot-trial : high frequency, Poisson distributed cortical stimulation in a screening model for epileptic seizures
Functional MRI during hippocampal deep brain stimulation in the healthy rat brain
Deep Brain Stimulation (DBS) is a promising treatment for neurological and psychiatric disorders. The mechanism of action and the effects of electrical fields administered to the brain by means of an electrode remain to be elucidated. The effects of DBS have been investigated primarily by electrophysiological and neurochemical studies, which lack the ability to investigate DBS-related responses on a whole-brain scale. Visualization of whole-brain effects of DBS requires functional imaging techniques such as functional Magnetic Resonance Imaging (fMRI), which reflects changes in blood oxygen level dependent (BOLD) responses throughout the entire brain volume. In order to visualize BOLD responses induced by DBS, we have developed an MRI-compatible electrode and an acquisition protocol to perform DBS during BOLD fMRI. In this study, we investigate whether DBS during fMRI is valuable to study local and whole-brain effects of hippocampal DBS and to investigate the changes induced by different stimulation intensities. Seven rats were stereotactically implanted with a custom-made MRI-compatible DBS-electrode in the right hippocampus. High frequency Poisson distributed stimulation was applied using a block-design paradigm. Data were processed by means of Independent Component Analysis. Clusters were considered significant when p-values were <0.05 after correction for multiple comparisons. Our data indicate that real-time hippocampal DBS evokes a bilateral BOLD response in hippocampal and other mesolimbic structures, depending on the applied stimulation intensity. We conclude that simultaneous DBS and fMRI can be used to detect local and whole-brain responses to circuit activation with different stimulation intensities, making this technique potentially powerful for exploration of cerebral changes in response to DBS for both preclinical and clinical DBS
Observing FRB 121102 with VERITAS; Searching for Associated TeV Emission
Fast radio bursts are bright, unresolved and short flashes of radio emission
originating from outside the Milky Way. The origin of these mysterious
outbursts is unknown, but their high luminosity and short duration has prompted
much speculation. The discovery that FRB 121102 repeats has enabled
multiwavelength follow up, which has identified the host galaxy. VERITAS has
observed the location of FRB 121102, including coincident observations with
Arecibo. We present the results of a search for steady very high energy
gamma-ray emission and the methodology for searching for short timescale,
transient optical and very high energy gamma-ray emission.Comment: 8 pages, 1 figure, 1 table. In Proceedings of the 35th International
Cosmic Ray Conference (ICRC 2017), Busan (South Korea
A NICER Discovery of a Low-Frequency Quasi-Periodic Oscillation in the Soft-Intermediate State of MAXI J1535-571
We present the discovery of a low-frequency Hz quasi-periodic
oscillation (QPO) feature in observations of the black hole X-ray binary MAXI
J1535-571 in its soft-intermediate state, obtained in September-October 2017 by
the Neutron Star Interior Composition Explorer (NICER). The feature is
relatively broad (compared to other low-frequency QPOs; quality factor
) and weak (1.9% rms in 3-10 keV), and is accompanied by a weak
harmonic and low-amplitude broadband noise. These characteristics identify it
as a weak Type A/B QPO, similar to ones previously identified in the
soft-intermediate state of the transient black hole X-ray binary XTE J1550-564.
The lag-energy spectrum of the QPO shows increasing soft lags towards lower
energies, approaching 50 ms at 1 keV (with respect to a 3-10 keV continuum).
This large phase shift has similar amplitude but opposite sign to that seen in
Rossi X-ray Timing Explorer data for a Type B QPO from the transient black hole
X-ray binary GX 339-4. Previous phase-resolved spectroscopy analysis of the
Type B QPO in GX 339-4 pointed towards a precessing jet-like corona
illuminating the accretion disk as the origin of the QPO signal. We suggest
that this QPO in MAXI J1535-571 may have the same origin, with the different
lag sign depending on the scale height of the emitting region and the observer
inclination angle.Comment: Accepted for publication in ApJ Letter
Discovery of kHz Fluctuations in Centaurus X-3: Evidence for Photon Bubble Oscillations (PBO) and Turbulence in a High Mass X-ray Binary Pulsar
We report the discovery of kHz fluctuations, including quasi-periodic
oscillations (QPO) at ~330 Hz and ~760 Hz and a broadband kHz continuum in the
power density spectrum of the high mass X-ray binary pulsar Centaurus X-3.
These observations of Cen X-3 were carried out with the Rossi X-ray Timing
Explorer (RXTE). The fluctuation spectrum is flat from mHz to a few Hz, then
steepens to behavior between a few Hz and ~100 Hz. Above a hundred Hz,
the spectrum shows the QPO features, plus a flat continuum extending to ~1200
Hz and then falling out to ~1800 Hz. These results, which required the
co-adding three days of observations of Cen X-3, are at least as fast as the
fastest known variations in X-ray emission from an accreting compact object
(kHz QPO in LMXB sources) and probably faster since extension to ~1800 Hz is
indicated by the most likely parameterization of the data.
Multi-dimensional radiation hydrodynamics simulations of optically thick
plasma flow onto the magnetic poles of an accreting neutron star show that the
fluctuations at frequencies above 100 Hz are consistent with photon bubble
turbulence and oscillations (PBO) previously predicted to be observable in this
source. For a polar cap opening angle of 0.25 radians, we show that the
spectral form above 100 Hz is reproduced by the simulations, including the
frequencies of the QPO and the relative power in the QPO and the kHz continuum.
This has resulted in the first model-dependent measurement of the polar cap
size of an X-ray pulsar.Comment: received ApJ: April 1, 1999 accepted ApJ: September 1, 199
Detection of Very Low-Frequency Quasi-Periodic Oscillations in the 2015 Outburst of V404 Cygni
In June 2015, the black hole X-ray binary (BHXRB) V404 Cygni went into
outburst for the first time since 1989. Here, we present a comprehensive search
for quasi-periodic oscillations (QPOs) of V404 Cygni during its recent
outburst, utilizing data from six instruments on board five different X-ray
missions: Swift/XRT, Fermi/GBM, Chandra/ACIS, INTEGRAL's IBIS/ISGRI and JEM-X,
and NuSTAR. We report the detection of a QPO at 18 mHz simultaneously with both
Fermi/GBM and Swift/XRT, another example of a rare but slowly growing new class
of mHz-QPOs in BHXRBs linked to sources with a high orbital inclination.
Additionally, we find a duo of QPOs in a Chandra/ACIS observation at 73 mHz and
1.03 Hz, as well as a QPO at 136 mHz in a single Swift/XRT observation that can
be interpreted as standard Type-C QPOs. Aside from the detected QPOs, there is
significant structure in the broadband power, with a strong feature observable
in the Chandra observations between 0.1 and 1 Hz. We discuss our results in the
context of current models for QPO formation.Comment: 17 pages, 9 figures, published in Ap
The effects of periodic and non-periodic inputs on the dynamics of a medial entorhinal cortex layer II stellate cell model
Various neuron types exhibit sub-threshold and firing frequency resonance in which the sub-threshold membrane potential or firing frequency responses to periodic inputs peak at a preferred frequency (or frequencies). Previous experimental work has shown that medial entorhinal cortex layer II stellate cells (SCs) exhibit sub-threshold and firing frequency resonance in the theta frequency band (4 - 10 Hz). In this thesis we seek to understand the biophysical and dynamic mechanism underlying these phenomena and how they are related. We studied the effects of sinusoidal current and synaptic conductance inputs at various frequencies, with and without noise, on the supra-threshold dynamics of a SC model. For current inputs, our results show that while the SC model exhibits a single frequency preference peak (in the theta frequency band) for low sinusoidal input levels, it exhibits three preferred frequency peaks for larger input levels. These additional peaks occur at frequencies that are roughly a multiple of the theta one. For synaptic conductance inputs, we observe an additional peak in the signal gain which occurs at a much higher frequency (in the high gamma frequency band). These findings depart from the linear prediction. The corresponding linearized model does not exhibit three preferred frequency peaks for current inputs and a much higher frequency for conductance inputs under the same conditions (such as parameters, noise, amplitude of inputs and maximal synaptic conductance) in the nonlinear model.
Previous experimental work has shown high-frequency Poisson-distributed trains of combined excitatory and inhibitory conductance- and current-based synaptic inputs reduce amplitude of subthreshold oscillations of SCs. The second goal of this thesis is to investigate the mechanism underlying these phenomena in the context of the model. More specially, we studied the effects of both conductance- and current-based synaptic inputs at various maximal conductance values on a SC model. Our numerical simulations show that conductance-based synaptic inputs reduce the amplitude of SC\u27s subthreshold oscillations for low enough value of the maximal synaptic conductance value but amplify these oscillations at a higher range. These results are in contrast to the experimental results