15,586 research outputs found
Potential mechanisms for imperfect synchronization in parkinsonian basal ganglia
Neural activity in the brain of parkinsonian patients is characterized by the
intermittently synchronized oscillatory dynamics. This imperfect
synchronization, observed in the beta frequency band, is believed to be related
to the hypokinetic motor symptoms of the disorder. Our study explores potential
mechanisms behind this intermittent synchrony. We study the response of a
bursting pallidal neuron to different patterns of synaptic input from
subthalamic nucleus (STN) neuron. We show how external globus pallidus (GPe)
neuron is sensitive to the phase of the input from the STN cell and can exhibit
intermittent phase-locking with the input in the beta band. The temporal
properties of this intermittent phase-locking show similarities to the
intermittent synchronization observed in experiments. We also study the
synchronization of GPe cells to synaptic input from the STN cell with
dependence on the dopamine-modulated parameters. Dopamine also affects the
cellular properties of neurons. We show how the changes in firing patterns of
STN neuron due to the lack of dopamine may lead to transition from a lower to a
higher coherent state, roughly matching the synchrony levels observed in basal
ganglia in normal and parkinsonian states. The intermittent nature of the
neural beta band synchrony in Parkinson's disease is achieved in the model due
to the interplay of the timing of STN input to pallidum and pallidal neuronal
dynamics, resulting in sensitivity of pallidal output to the phase of the
arriving STN input. Thus the mechanism considered here (the change in firing
pattern of subthalamic neurons through the dopamine-induced change of membrane
properties) may be one of the potential mechanisms responsible for the
generation of the intermittent synchronization observed in Parkinson's disease.Comment: 27 pages, 9 figure
How Noise and Coupling Induce Bursting Action Potentials in Pancreatic beta-cells
Unlike isolated beta-cells, which usually produce continuous spikes or fast
and irregular bursts, electrically coupled beta-cells are apt to exhibit robust
bursting action potentials. We consider the noise induced by thermal
fluctuations as well as that by channel gating stochasticity and examine its
effects on the action potential behavior of the beta-cell model. It is observed
numerically that such noise in general helps single cells to produce a variety
of electrical activities. In addition, we also probe coupling via gap junctions
between neighboring cells,with heterogeneity induced by noise, to find that it
enhances regular bursts.Comment: 40 pages, 10 figure
Optimal first-passage time in gene regulatory networks
The inherent probabilistic nature of the biochemical reactions, and low copy
number of species can lead to stochasticity in gene expression across identical
cells. As a result, after induction of gene expression, the time at which a
specific protein count is reached is stochastic as well. Therefore events
taking place at a critical protein level will see stochasticity in their
timing. First-passage time (FPT), the time at which a stochastic process hits a
critical threshold, provides a framework to model such events. Here, we
investigate stochasticity in FPT. Particularly, we consider events for which
controlling stochasticity is advantageous. As a possible regulatory mechanism,
we also investigate effect of auto-regulation, where the transcription rate of
gene depends on protein count, on stochasticity of FPT. Specifically, we
investigate for an optimal auto-regulation which minimizes stochasticity in
FPT, given fixed mean FPT and threshold.
For this purpose, we model the gene expression at a single cell level. We
find analytic formulas for statistical moments of the FPT in terms of model
parameters. Moreover, we examine the gene expression model with
auto-regulation. Interestingly, our results show that the stochasticity in FPT,
for a fixed mean, is minimized when the transcription rate is independent of
protein count. Further, we discuss the results in context of lysis time of an
\textit{E. coli} cell infected by a phage virus. An optimal lysis
time provides evolutionary advantage to the phage, suggesting a
possible regulation to minimize its stochasticity. Our results indicate that
there is no auto-regulation of the protein responsible for lysis. Moreover,
congruent to experimental evidences, our analysis predicts that the expression
of the lysis protein should have a small burst size.Comment: 8 pages, 3 figures, Submitted to Conference on Decision and Control
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An alternative to the plasma emission model: Particle-In-Cell, self-consistent electromagnetic wave emission simulations of solar type III radio bursts
1.5D PIC, relativistic, fully electromagnetic (EM) simulations are used to
model EM wave emission generation in the context of solar type III radio
bursts. The model studies generation of EM waves by a super-thermal, hot beam
of electrons injected into a plasma thread that contains uniform longitudinal
magnetic field and a parabolic density gradient. In effect, a single magnetic
line connecting Sun to earth is considered, for which several cases are
studied. (i) We find that the physical system without a beam is stable and only
low amplitude level EM drift waves (noise) are excited. (ii) The beam injection
direction is controlled by setting either longitudinal or oblique electron
initial drift speed, i.e. by setting the beam pitch angle. In the case of zero
pitch angle, the beam excites only electrostatic, standing waves, oscillating
at plasma frequency, in the beam injection spatial location, and only low level
EM drift wave noise is also generated. (iii) In the case of oblique beam pitch
angles, again electrostatic waves with same properties are excited. However,
now the beam also generates EM waves with the properties commensurate to type
III radio bursts. The latter is evidenced by the wavelet analysis of transverse
electric field component, which shows that as the beam moves to the regions of
lower density, frequency of the EM waves drops accordingly. (iv) When the
density gradient is removed, electron beam with an oblique pitch angle still
generates the EM radiation. However, in the latter case no frequency decrease
is seen. Within the limitations of the model, the study presents the first
attempt to produce simulated dynamical spectrum of type III radio bursts in
fully kinetic plasma model. The latter is based on 1.5D non-zero pitch angle
(non-gyrotropic) electron beam, that is an alternative to the plasma emission
classical mechanism.Comment: Physics of Plasmas, in press, May 2011 issue (final accepted version
Stochasticity from function -- why the Bayesian brain may need no noise
An increasing body of evidence suggests that the trial-to-trial variability
of spiking activity in the brain is not mere noise, but rather the reflection
of a sampling-based encoding scheme for probabilistic computing. Since the
precise statistical properties of neural activity are important in this
context, many models assume an ad-hoc source of well-behaved, explicit noise,
either on the input or on the output side of single neuron dynamics, most often
assuming an independent Poisson process in either case. However, these
assumptions are somewhat problematic: neighboring neurons tend to share
receptive fields, rendering both their input and their output correlated; at
the same time, neurons are known to behave largely deterministically, as a
function of their membrane potential and conductance. We suggest that spiking
neural networks may, in fact, have no need for noise to perform sampling-based
Bayesian inference. We study analytically the effect of auto- and
cross-correlations in functionally Bayesian spiking networks and demonstrate
how their effect translates to synaptic interaction strengths, rendering them
controllable through synaptic plasticity. This allows even small ensembles of
interconnected deterministic spiking networks to simultaneously and
co-dependently shape their output activity through learning, enabling them to
perform complex Bayesian computation without any need for noise, which we
demonstrate in silico, both in classical simulation and in neuromorphic
emulation. These results close a gap between the abstract models and the
biology of functionally Bayesian spiking networks, effectively reducing the
architectural constraints imposed on physical neural substrates required to
perform probabilistic computing, be they biological or artificial
Neurosystems: brain rhythms and cognitive processing
Neuronal rhythms are ubiquitous features of brain dynamics, and are highly correlated with cognitive processing. However, the relationship between the physiological mechanisms producing these rhythms and the functions associated with the rhythms remains mysterious. This article investigates the contributions of rhythms to basic cognitive computations (such as filtering signals by coherence and/or frequency) and to major cognitive functions (such as attention and multi-modal coordination). We offer support to the premise that the physiology underlying brain rhythms plays an essential role in how these rhythms facilitate some cognitive operations.098352 - Wellcome Trust; 5R01NS067199 - NINDS NIH HH
Electrophysiological analysis of transcranial direct current stimulation and its effect on cortical spreading depression
Transcranial direct current stimulation (TDCS) allows for the noninvasive modulation of cortical activity. In this study, the effects of cathodal and anodal TDCS treatment on baseline activity in the motor cortex of rats were investigated via translaminar electroencephalogram (EEG) recording and power spectral density analysis. Treatment with low intensity anodal TDCS for five minutes was found to increase delta and theta frequency cortical activity during and for up to five minutes following treatment.
This study also assessed the interaction of TDCS with the phenomenon of cortical spreading depression (CoSD), which has been implicated in numerous disease states, including migraine and stroke. TDCS treatment was given concurrently with induction of CoSD via administration of potassium chloride to the surface of the dura. The presence of the spreading depression event, a characteristic low frequency wave observed to travel outwards from the point of CoSD induction and downwards through the cortex, was used as a proxy measure for the occurrence of CoSD. It was observed that animals treated with cathodal TDCS exhibited fewer spreading depression events relative to those treated with anodal TDCS or those receiving sham treatment.
In this study, animals were segregated into groups that exhibited stimulus artifact during TDCS treatment and those that did not. Stimulus artifact was defined as a characteristic alpha and/or beta frequency activity spike lasting throughout and not longer than the period of stimulation. Those animals receiving TDCS without exhibiting stimulus artifact were considered for the purposes of this study to not have received proper TDCS treatment, and acted as a sham treatment group. Because salient differences emerged between the stimulus artifact positive and stimulus artifact negative groups, this study suggests that the presence of stimulus artifact could be used as a proxy measure for successful TDCS dosage
The design of a turboshaft speed governor using modern control techniques
The objectives of this program were: to verify the model of off schedule compressor variable geometry in the T700 turboshaft engine nonlinear model; to evaluate the use of the pseudo-random binary noise (PRBN) technique for obtaining engine frequency response data; and to design a high performance power turbine speed governor using modern control methods. Reduction of T700 engine test data generated at NASA-Lewis indicated that the off schedule variable geometry effects were accurate as modeled. Analysis also showed that the PRBN technique combined with the maximum likelihood model identification method produced a Bode frequency response that was as accurate as the response obtained from standard sinewave testing methods. The frequency response verified the accuracy of linear models consisting of engine partial derivatives and used for design. A power turbine governor was designed using the Linear Quadratic Regulator (LQR) method of full state feedback control. A Kalman filter observer was used to estimate helicopter main rotor blade velocity. Compared to the baseline T700 power turbine speed governor, the LQR governor reduced droop up to 25 percent for a 490 shaft horsepower transient in 0.1 sec simulating a wind gust, and up to 85 percent for a 700 shaft horsepower transient in 0.5 sec simulating a large collective pitch angle transient
Shaping bursting by electrical coupling and noise
Gap-junctional coupling is an important way of communication between neurons
and other excitable cells. Strong electrical coupling synchronizes activity
across cell ensembles. Surprisingly, in the presence of noise synchronous
oscillations generated by an electrically coupled network may differ
qualitatively from the oscillations produced by uncoupled individual cells
forming the network. A prominent example of such behavior is the synchronized
bursting in islets of Langerhans formed by pancreatic \beta-cells, which in
isolation are known to exhibit irregular spiking. At the heart of this
intriguing phenomenon lies denoising, a remarkable ability of electrical
coupling to diminish the effects of noise acting on individual cells.
In this paper, we derive quantitative estimates characterizing denoising in
electrically coupled networks of conductance-based models of square wave
bursting cells. Our analysis reveals the interplay of the intrinsic properties
of the individual cells and network topology and their respective contributions
to this important effect. In particular, we show that networks on graphs with
large algebraic connectivity or small total effective resistance are better
equipped for implementing denoising. As a by-product of the analysis of
denoising, we analytically estimate the rate with which trajectories converge
to the synchronization subspace and the stability of the latter to random
perturbations. These estimates reveal the role of the network topology in
synchronization. The analysis is complemented by numerical simulations of
electrically coupled conductance-based networks. Taken together, these results
explain the mechanisms underlying synchronization and denoising in an important
class of biological models
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