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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Stochastic resonance in nanoscale systems

Saha, Aditya 06 1900 (has links)
This thesis considers the possibility of stochastic resonance (SR) in the following nanoscale systems: (i) hard-threshold devices; (ii) averaging structures of carbon nanotubes (CNTs); (iii) myoglobin atoms; and finally (iv) tubulin dimers. The description of SR is carried out using Kramers' rate theory in the adiabatic two-state approximation for continuous systems and using Shannon's information theoretic formalism for systems with static nonlinearities. The effective potentials are modelled by asymmetric or symmetric bistable wells in a single reaction co-ordinate. Quantum considerations have not been invoked. Hence, all results are implicitly valid in the high-temperature regime of relevance to industrial applications. It is established that information transmitted by arrays of identical CNTs is maximized by non-zero noise intensities and that the response of myoglobin and tubulin dimers to ambient molecular forces (as described by the signal-to-noise ratio or SNR) is enhanced by increasing temperature. Sample calculations are shown for solvent fluctuations, ligand interactions and dipole oscillations. These results can be used to explain: (i) the effects of temperature observed in fabrication processes for CNTs; (ii) the dynamical transition observed in myoglobin and (iii) the 8.085 MHz resonance observed in microtubules.
2

Stochastic resonance in nanoscale systems

Saha, Aditya 06 1900 (has links)
This thesis considers the possibility of stochastic resonance (SR) in the following nanoscale systems: (i) hard-threshold devices; (ii) averaging structures of carbon nanotubes (CNTs); (iii) myoglobin atoms; and finally (iv) tubulin dimers. The description of SR is carried out using Kramers' rate theory in the adiabatic two-state approximation for continuous systems and using Shannon's information theoretic formalism for systems with static nonlinearities. The effective potentials are modelled by asymmetric or symmetric bistable wells in a single reaction co-ordinate. Quantum considerations have not been invoked. Hence, all results are implicitly valid in the high-temperature regime of relevance to industrial applications. It is established that information transmitted by arrays of identical CNTs is maximized by non-zero noise intensities and that the response of myoglobin and tubulin dimers to ambient molecular forces (as described by the signal-to-noise ratio or SNR) is enhanced by increasing temperature. Sample calculations are shown for solvent fluctuations, ligand interactions and dipole oscillations. These results can be used to explain: (i) the effects of temperature observed in fabrication processes for CNTs; (ii) the dynamical transition observed in myoglobin and (iii) the 8.085 MHz resonance observed in microtubules.
3

Stochastic resonance in nanoscale systems

Saha, Aditya Unknown Date
No description available.
4

Stochastic resonance in a neuron model with application to the auditory pathway

Hohn, Nicolas Unknown Date (has links) (PDF)
In this thesis, the transmission of spike trains in a neuron model is studied in order to obtain a better understanding of the role played by stochastic activity, i.e. uncorrelated spikes, in the auditory pathway. Fluctuations of the neuron membrane potential are given by a first-order stochastic differential equation, using a leaky integrate-and-fire model. In contrast to most previous studies the model has a finite number of synapses, and the usual diffusion approximation does not hold. / The input signal is modeled by spike trains with spiking times described by inhomogenous Poisson processes. The membrane potential is a shot noise process for which statistical properties are derived with a Gaussian approximation. The statistics of the output spike train are obtained by using the property that a pool of a large number of output spike trains can be modeled by an inhomogeneous Poisson process. It is shown that, under certain conditions, the addition of uncorrelated input spikes, i.e. noise, can enhance the transmission of periodic temporal information. This phenomenon, called stochastic resonance, is demonstrated analytically and supported by computer simulations. / Results are compared with those obtained from the traditional leaky integrate-and- fire neuron receiving a continuous waveform input. The shot-noise property of the membrane potential, which implies that its variance is de facto modulated by the input stimulus, is shown to enhance the phenomenon of stochastic resonance. Indeed, for a given average noise level, a modulated noise gives a higher output signal-to-noise ratio than an unmodulated noise with the same average amplitude. / The derivation is then extended to certain polyperiodic stimuli mimicking vowel sounds. The fact that the addition of uncorrelated input spikes can enhance the transmission of information is discussed in the context of cochlear implants. The results provide supportive evidence to the postulate that a cochlear implant speech coding strategy that elicits stochastic firing neural activity might benefit the user.
5

Investigation of Stochastic Resonance in Directed Propagation of Cold Atoms

Jiang, Kefeng 26 July 2021 (has links)
No description available.
6

A Study of Stochastic Resonance in a Climate model

Olsson, Agnes, Jernmark Burrows, Ebba January 2022 (has links)
Historically, the earth’s fluctuation between interglacial and glacial climates has been observedto have a period of 105 years [1]. However, simulations of the global average temperature didn’tmanage to reproduce this cycle period until 1982, when Benzi et al. [2] introduced the combinationof long-term variations in incoming solar radiation and stochastic noise in an energy balancemodel. Using an energy balance model means that the change in global average temperature isset as proportional to the difference in ingoing and outgoing energy. The result of the simulationsdemonstrated so-called stochastic resonance, where small stochastic perturbations amplified thepattern of the variation in insolation, causing a pattern of large changes in the global averagetemperature, i.e. changes in the climate. The stochastic perturbations model unpredictable shorttime scale phenomena like the weather. Our study aimed to reproduce the result of Benzi et al.[2] and to investigate the model and its parameters. The presence of a 105-year climatic cycle insimulated data was found. The combination of both noise and varying incoming solar radiationwas necessary to observe the 105-year cycle. The characteristics of the climate cycle pattern did,however, vary greatly depending on the values of constants in the model, illustrating how themodel and constants were imprecise. Therefore, no conclusions can be drawn from this studyabout the earth’s current or future climate. However, the study still confirms that stochasticnoise is an important part of modeling the climate, and manages to simulate the earth’s observed105-year climate cycle.
7

Signal in the Noise? The Effect of Non-Invasive Brain Stimulation on Contrast Perception

Parrott, Danielle Elizabeth 13 July 2020 (has links)
A longstanding question in studies of cortical stimulation has been how does stimulation affect brain functioning and cognition, and what are its mechanisms of action. Brain stimulation has been traditionally seen either as a disrupting intervention or as a procedure to enhance cortical excitability and promote improvement in various modality from motor to visual performance. In vision, several hypotheses have been proposed and many experimental paradigms have been used to study how transcranial magnetic stimulation (TMS) and direct current stimulation, particularly transcranial random noise stimulation (tRNS) affect visual discrimination. Psychophysical paradigms are particularly useful to measure visual performance, whereby a stimulus is progressively changed from easy to difficult to perceive it, and accuracy threshold can be measured by titrating the stimulus discriminability. Stimuli that vary in contrast are typically used to study low-level visual functions and it is well known that neurons within the early visual areas in the brain, and primarily V1, are tuned to stimuli involved in contrast discrimination. Here we used an orientation discrimination task to study changes in contrast detection by varying stimulus contrast across different levels (Experiment 1, Chapter 2). We used neuro-navigated single-pulse TMS at different intensities to determine whether behavioral response changed linearly as a function of stimulus discriminability independently of TMSintensity, or whether TMS affected behavior depending on TMS intensity and contrast level. Moreover, we tested whether TMS had an effect selective for the field contralateral to stimulation or whether effects could be seen across the entire visual field. Single pulse TMS was delivered to left V1 while participants performed a 2-alternative forced choice orientation discrimination (OD) of one of two Gabor patches presented on either side of fixation at 5 contrast levels and 4 TMS intensities. Participants' performance on OD increased at all contrast levels in the right visual field (contralateral to stimulation) at 80% of phosphene thresholds (PT, individually measured at baseline). Furthermore, when TMS was delivered at 60% of PT, we found improved performance in the right visual field that was selective for the medium contrast, while performance increased at the highest contrast irrespective of TMS intensity, in the field ipsilateral to stimulation, thus both visual fields were affected by TMS, albeit differently. Since the improvement effects might be explained as the result of added noise to the system that paradoxically improves performance for justbelow threshold stimuli (middle contrasts), in Experiments 1 and 2 (Chapter 3) we used transcranial random noise stimulation, a neuromodulation procedure known to enhance cortical excitability when delivered at high frequencies, to further test the hypothesis that brain stimulation might work through a mechanism of stochastic resonance, whereby adding noise to a nonlinear system, the brain in our case, might paradoxically promote better performance by enhancing stimulus discriminability. This might happen only for selective stimulus intensities and stimulation strength. Based on previous successful work, we tested contrast discrimination changes as a function of four different tRNS low intensity levels of stimulation, and we found a decrease in performance selective for the condition with subthreshold stimuli and at .750 mA stimulation intensity. This result might indicate that low intensity stimulation is not enough to promote enhancement of stimuli under the stochastic mechanism effect, thereby suggesting that higher ranges of stimulation are necessary to create the optimal conditions for improvement.
8

The geometric stochastic resonance and rectification of active particles

Glavey, Russell January 2015 (has links)
This thesis describes the work of three research projects, the background research that motivated the work, and the resultant project findings. The three projects concerned: (i) Geometric stochastic resonance in a double cavity, (ii) Synchronisation of geometric stochastic resonance by a bi-harmonic drive, and (iii) Rectification of Brownian particles with oscillating radii in asymmetric corrugated channels. In the project 'Geometric stochastic resonance in a double cavity', we investigated synchronisation processes for the geometric stochastic resonance of particles diffusing across a porous membrane and subject to a periodic driving force. Non-interacting particle currents were driven through a symmetric membrane pore either parallel or perpendicular to the membrane. Then, harmonic mixing spectral current components were generated by the combined action of parallel and perpendicular drives. The role of the repulsive interaction of particles as a controlling factor with potential applications to the transport of colloids and biological molecules through narrow pores was also investigated. In 'Synchronisation of geometric stochastic resonance by a bi-harmonic drive', we simulated the stochastic dynamics of an elliptical particle using the Langevin equation. The particle was simultaneously driven by low and high frequency harmonic drives across a porous inter-cavity membrane. It was observed that the particle oscillated out of phase with the low frequency drive. This effect was due to the absolute negative mobility the particle would have exhibited if the low frequency drive had been replaced by a dc static force. It was also observed that the magnitude of this out-of-phase stochastic resonance depends on how the combined action of the driving forces and noise fluctuations affect the particle orientation, and as such was shown to be sensitive to the particle shape. This emphasises the importance of particle geometry, in addition to chamber geometry, to the realisation and optimisation of geometric stochastic resonance. In the project 'Rectification of Brownian particles with oscillating radii in asymmetric corrugated channels', we simulated the transport of a Brownian particle with an oscillating radius freely diffusing in an asymmetric corrugated channel over a range of driving forces for a series of temperatures and angular frequencies of radial oscillation. It was observed that there was a strong influence of self-oscillation frequency upon the average particle velocity. This effect can be used to control rectification of biologically active particles as well as for their separation according to their activity, for instance in the separation of living and dead cells. The background research is described in Chapter One and the research findings are described along with their related projects in Chapters Two and Three.
9

外力を受ける非線形振動子のエネルギー収集特性 / Energy Harvesting Characteristics of Nonlinear Oscillators under Excitation

窪田, まど華 23 March 2015 (has links)
Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第18991号 / 工博第4033号 / 新制||工||1621 / 31942 / 京都大学大学院工学研究科電気工学専攻 / (主査)教授 引原 隆士, 教授 土居 伸二, 教授 小林 哲生 / 学位規則第4条第1項該当
10

Encoding of Sensory Signals Through Balanced Ionotropic Receptor Dynamics and Voltage Dependent Membrane Noise

Marcoux, Curtis January 2016 (has links)
Encoding behaviorally relevant stimuli in a noisy background is critical for animals to survive in their natural environment. We identify core biophysical and synaptic mechanisms that permit the encoding of low frequency signals in pyramidal neurons of the weakly electric fish Apteronotus leptorhynchus, an animal that can accurately encode miniscule (0.1%) amplitude modulations of its self-generated electric field. We demonstrate that slow NMDA-R mediated EPSPs are able to summate over many interspike intervals of the primary electrosensory afferents (EAs), effectively eliminating the EA spike train serial correlations from the pyramidal cell input. This permits stimulus-evoked changes in EA spiking to be transmitted efficiently to downstream ELL pyramidal cells, where a dynamic balance of NMDA-R and GABA-A-R currents is critical for encoding low frequency signals. Interestingly, AMPA-R activity is depressed and plays a negligible role in the generation of action potentials; instead, cell intrinsic membrane noise implements voltage-dependent stochastic resonance to amplify weak sensory input and appears to drive a significant proportion of pyramidal cell spikes. Together, these mechanisms may be sufficient for the ELL to encode signals near the threshold of behavioral detection.

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