635,760 research outputs found
Neuronal oscillations enhance stimulus discrimination by ensuring action potential precision
No full textAlthough oscillations in membrane potential are a prominent feature of sensory, motor, and cognitive function, their precise role in signal processing remains elusive. Here we show, using a combination of in vivo, in vitro, and theoretical approaches, that both synaptically and intrinsically generated membrane potential oscillations dramatically improve action potential (AP) precision by removing the membrane potential variance associated with jitter-accumulating trains of APs. This increased AP precision occurred irrespective of cell type and—at oscillation frequencies ranging from 3 to 65 Hz—permitted accurate discernment of up to 1,000 different stimuli. At low oscillation frequencies, stimulus discrimination showed a clear phase dependence whereby inputs arriving during the trough and the early rising phase of an oscillation cycle were most robustly discriminated. Thus, by ensuring AP precision, membrane potential oscillations dramatically enhance the discriminatory capabilities of individual neurons and networks of cells and provide one attractive explanation for their abundance in neurophysiological systems
Mixed-mode oscillations with multiple time scales
No full textMixed-mode oscillations (MMOs) are trajectories of a dynamical system in which there is an alternation between oscillations of distinct large and small amplitudes. MMOs have been observed and studied for over thirty years in chemical, physical and biological systems. Few attempts have been made thus far to classify different patterns of MMOs, in contrast to the classification of the related phenomena of bursting oscillations. This paper gives a survey of different types of MMOs, concentrating its analysis on MMOs whose small-amplitude oscillations are produced by a local, multiple-time-scale "mechanism." Recent work gives substantially improved insight into the mathematical properties of these mechanisms. In this survey, we unify diverse observations about MMOs and establish a systematic framework for studying their properties. Numerical methods for computing different types of invariant manifolds and their intersections are an important aspect of the analysis described in this paper
Disrupted dopamine transmission and the emergence of exaggerated beta oscillations in subthalamic nucleus and cerebral cortex
No full textIn the subthalamic nucleus (STN) of Parkinson’s disease (PD) patients, a pronounced synchronization of oscillatory activity at beta frequencies (15–30 Hz) accompanies movement difficulties. Abnormal beta oscillations and motor symptoms are concomitantly and acutely suppressed by dopaminergic therapies, suggesting that these inappropriate rhythms might also emerge acutely from disrupted
dopamine transmission. The neural basis of these abnormal beta oscillations is unclear, and how they might compromise information processing, or how they arise, is unknown. Using a 6-hydroxydopamine-lesioned rodent model of PD, we demonstrate that beta oscillations are inappropriately exaggerated, compared with controls, in a brain-state-dependent manner after chronic dopamine loss. Exaggerated
beta oscillations are expressed at the levels of single neurons and small neuronal ensembles, and are focally present and spatially distributed within STN. They are also expressed in synchronous population activities, as evinced by oscillatory local field potentials, in STN and cortex. Excessively synchronized beta oscillations reduce the information coding capacity of STN neuronal ensembles, which
may contribute to parkinsonian motor impairment. Acute disruption of dopamine transmission in control animals with antagonists of D1/D2 receptors did not exaggerate STN or cortical beta oscillations. Moreover, beta oscillations were not exaggerated until several days after 6-hydroxydopamine injections. Thus, contrary to predictions, abnormally amplified beta oscillations in cortico-STN circuits do not result simply from an acute absence of dopamine receptor stimulation, but are instead delayed sequelae of chronic dopamine depletion.
Targeting the plastic processes underlying the delayed emergence of pathological beta oscillations after continuing dopaminergic dysfunction may offer considerable therapeutic promise
Neutrino oscillations at high densities: cosmological and astrophysical aspects
No full textThis doctoral thesis treats the effects of neutrino oscillations in the early universe and supernovae. The main probe for the context of the early universe is big bang nucleosynthesis (BBN). I explain the general link between neutrinos and BBN with an emphasis on the degeneracy between neutrino asymmetry and extra degrees of freedom. I show how the degenerate effects of both oscillations and collisions lead to observables detectable in the near future that brake this degeneracy and draw bounds on neutrino asymmetry and their contribution to the energy density of the universe in the current epoch. Considering astrophysical aspects, I analyze the significance of a proper treatment of neutrino evolution to understand the neutrino signal from the next galactic supernova (SN). I show the type of effects collective neutrino conversions can produce and the numerical difficulties confronted. Finally, I show how that stability analysis can complement a numerical treatment and provide definitive answers in
some example cases
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Feedback control of oscillations in combustion and cavity flows
No full textThis thesis considers the control of combustion oscillations, motivated by the susceptibility of lean premixed combustion to such oscillations, and the long and expensive development and commissioning times that this is giving rise to. The controller used is both closed-loop, employing an actuator to modify some system parameter in response to a measured signal, and adaptive, meaning that it is able to maintain control over a wide range of operating conditions. The controller is applied to combustion systems with annular geometries, where instabilities can occur both longitudinally and azimuthally, and which require multiple sensors and multiple actuators for control.
One of the requirements of Lyapunov-based adaptive control which is particularly troublesome for combustion systems is then addressed: that the sign of the high-frequency gain of the open-loop system is known. We address it by using an adaptive controller which employs a Nussbaum gain, and successfully apply it experimentally to combustion oscillations in a Rijke tube.
Another type of fluid-acoustic resonance is then considered: the compressible flow past a shallow cavity. We start by finding a linear model of the cavity flow's dynamics, or its `transfer function', which we identify from direct numerical simulations. We compare this measured transfer function to that given by a conceptual model which is based on the Rossiter mechanism, and which models each component of the flow physics separately.
We then look at using closed-loop control to eliminate these cavity oscillations. We start by designing a robust controller based on a balanced reduced order model of the system, the model being provided by the Eigensystem Realization Algorithm (ERA). The robust controller provides closed-loop stability over a much wider Mach number range than seen in previous studies. Finally, we look at the suitability of the adaptive controller, earlier developed for combustion oscillations, for the cavity. Based on some general properties of the cavity flow, and by using collocated control, the oscillations are eliminated at all Mach numbers tested in the range
Predicting psychosis in at-risk patients using abnormal neural oscillations and synchrony in conjunctions with machine learning algorithms
No full textIn the last 20 years, there has been a marked increase in interest in the early detection and treatment of psychosis. Despite the various potential “prodromes” that have been identified and have helped to increase the accuracy in the detection of persons at-risk of developing psychoses, it is still not possible to predict the transition to psychosis with sufficient accuracy. Although some electroencephalography (EEG) studies, based on basic power-spectral and event-related potential analyses, have been conducted in the field of early detection, neural oscillations and their phase-synchronization across brain areas have been ignored.
The present dissertation covers three different studies which, together, demonstrate that neural oscillations are disturbed in emerging psychosis. The first paper shows that at-risk patients with later transition to psychosis are characterized by abnormal localized brain activity and that inter-cortical areas of the brain are poorly synchronized. The second study shows that machine learning algorithms can detect patterns of abnormal brain activity predictive of later transitions to psychosis with promising accuracy. The third study reveals, in a cross-sectional manner, that patients who already had a first episode of psychosis at inclusion, already demonstrated the same abnormal patterns of brain activity revealed in at-risk patients with later transition to psychosis
Effect of structuring on coronal loop oscillations
No full textIn this Thesis the theoretical understanding of oscillations in coronal structures is developed. In particular, coronal loops are modelled as magnetic slabs of plasma. The effect of introducing inhomogeneities on the frequency of oscillation is studied. Current observations indicate the existence of magnetohydrodynamic (MHD) modes in the corona, so there is room for improved modelling of these modes to understand the physical processes more completely. One application of the oscillations, on which this Thesis concentrates, is coronal seismology. Here, the improved theoretical models are applied to observed instances of coronal MHD waves with the aim of determining information regarding the medium in which these waves propagate.
In Chapter two, the effect of gravity on the frequency of the longitudinal slow MHD mode is considered. A thin, vertical coronal slab of magnetised plasma, with gravity acting along the longitudinal axis of the slab is studied, and the effect on the frequency of oscillation for the uniform, stratified and structured cases is addressed. In particular, an isothermal plasma, a two-layer plasma and a plasma with a linear temperature profile are studied. Here, a thin coronal loop, with its footpoints embedded in the chromosphere-photosphere is modelled, and the effects introduced by both gravity and the structuring of density at the footpoint layers are studied. In this case, gravity increases the frequency of oscillation and causes amplification of the eigenfunctions by stratification. Furthermore, density enhancements at the footpoints cause a decrease in the oscillating frequency, and can inhibit wave propagation, depending on the parameter regime.
In Chapter three, the effects introduced to the transverse fast MHD mode when gravity acts across a thin coronal slab of magnetised plasma are considered. This study concentrates on the modification of the frequency due to the dynamical effect of gravity in the equation of motion, neglecting the effect of stratification. Here, gravity causes a reduction of the oscillating frequency of the fundamental fast mode, and increases the lower cutoff frequency. In effect, for this configuration, gravity allows the transition between body and surface modes, in a slab geometry.
It is found, in these two studies, that each harmonic is affected in a unique manner due to structuring or stratification of density. With this knowledge, in Chapter four, a new parameter is derived; P1/2P2, the ratio of the period of the fundamental harmonic of oscillation to twice the period of its first harmonic. This parameter is shown to be a measure of the longitudinal structuring of density along a coronal loop, and the departure of this ratio from unity can yield information regarding the lengthscales of the structure. This process is highlighted using the known observations, indicating that P1/2P2 may prove to be a useful diagnostic tool for coronal seismology.
Finally, in Chapter five, outwardly propagating coronal slow MHD modes are observed and are used to infer coronal parameters. The possibility of using these oscillations to infer near-resolution lengthscales in coronal loops -- fine-scale strands -- is also discussed. TRACE observations are used to determine the average period, phase speed, detection length, amplitude and energy flux for the propagating slow MHD mode. The indication is that the source of these oscillations appears very localised in space, and the driver only acts for a few periods, suggesting the perturbations are driven by leaky p-modes (solar surface modes)
Daily to intraseasonal oscillations at Antarctic research station Neumayer
No full textHigh temporal resolution (three hours) records of temperature, wind speed and sea level pressure
recorded at Antarctic research station Neumayer (708S, 88W) during 1982–2011 are analysed to identify
oscillations from daily to intraseasonal timescales. The diurnal cycle dominates the three-hourly time series
of temperature during the Antarctic summer and is almost absent during winter. In contrast, the three-hourly
time series of wind speed and sea level pressure show a weak diurnal cycle. The dominant pattern of the
intraseasonal variability of these quantities, which captures the out-of-phase variation of temperature and
wind speed with sea level pressure, shows enhanced variability at timescales of , 40 days and , 80 days,
respectively. Correlation and composite analysis reveal that these oscillations may be related to tropical
intraseasonal oscillations via large-scale eastward propagating atmospheric circulation wave-trains. The
second pattern of intraseasonal variability, which captures in-phase variations of temperature, wind and sea
level pressure, shows enhanced variability at timescales of , 35, , 60 and , 120 days. These oscillations
are attributed to the Southern Annular Mode/Antarctic Oscillation (SAM/AAO) which shows enhanced
variability at these timescales. We argue that intraseasonal oscillations of tropical climate and SAM/AAO are
related to distinct patterns of climate variables measured at Neumayer
Performance- and stimulus-dependent oscillations in monkey prefrontal cortex during short-term memory
No full textShort-term memory requires the coordination of sub-processes like encoding, retention, retrieval and comparison of stored material to subsequent input. Neuronal oscillations have an inherent time structure, can effectively coordinate synaptic integration of large neuron populations and could therefore organize and integrate distributed sub-processes in time and space. We observed field potential oscillations (14–95 Hz) in ventral prefrontal cortex of monkeys performing a visual memory task. Stimulus-selective and performance-dependent oscillations occurred simultaneously at 65–95 Hz and 14–50 Hz, the latter being phase-locked throughout memory maintenance. We propose that prefrontal oscillatory activity may be instrumental for the dynamical integration of local and global neuronal processes underlying short-term memory
Linear oscillations of axisymmetric viscous liquid bridges
No full textSmall amplitude free oscillations of axisymmetric capillary bridges are considered for varying values of the capillary Reynolds number C-1 and the slenderness of the bridge Λ . A semi-analytical method is presented that provides cheap and accurate results for arbitrary values of C-1 and Λ ; several asymptotic limits (namely, C>> 1, C>>1, Λ >> 1 \ {and} \ |π -Λ |>> 1 ) are considered in some detail, and the associated approximate results are checked. A fairly complete picture of the (fairly complex) spectrum of the linear problem is obtained for varying values of C and Λ . Two kinds of normal modes, called capillary and hydrodynamic respectively, are almost always clearly identified, the former being associated with free surface deformation and the latter, only with the internal flow field; when C is small the damping rate associated with both kind of modes is comparable, and the hydrodynamic ones explain the appearance of secondary (steady or slowly-varying) streaming flow
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