No / A PDMS stamp is used to transfer a synthetic peptide in a given pattern to any suitable surface. Using this method two-dimensional neuronal model networks could be formed on glass substrates as well as on electronic devices and adjusted to the given microelectronic structure. The present work focuses on the mechanism of neurite guidance under simplified in vitro conditions, using in vitro guidance cues and outline the incorporation of these interfacial methods into microelectronic sensor devices.

This thesis solves estimation and control problems in computational neuroscience, mathematically dealing with the first-passage times of diffusion stochastic processes. We first derive estimation algorithms for model parameters from first-passage time observations, and then we derive algorithms for the control of first-passage times. Finally, we solve an optimal design problem which combines elements of the first two: we ask how to elicit first-passage times such as to facilitate model estimation based on said first-passage observations. The main mathematical tools used are the Fokker-Planck partial differential equation for evolution of probability densities, the Hamilton-Jacobi-Bellman equation of optimal control and the adjoint optimization principle from optimal control theory. The focus is on developing computational schemes for the solution of the problems. The schemes are implemented and are tested for a wide range of parameters.

No / Motion is known to distort visual space, producing illusory mislocalizations for flashed objects. Previously, it has been shown that when a stationary bar is flashed in the proximity of a moving stimulus, the position of the flashed bar appears to be shifted in the direction of nearby motion. A model consisting of predictive projections from the sub-system that processes motion information onto the sub-system that processes position information can explain this illusory position shift of a stationary flashed bar in the direction of motion. Based on this model of motion¿position interactions, we predict that the perceived position of a flashed stimulus should also be attracted towards a nearby moving stimulus. In the first experiment, observers judged the perceived vertical position of a flash with respect to two horizontally moving dots of unequal contrast. The results of this experiment were in agreement with our prediction of attraction towards the high contrast dot. We obtained similar findings when the moving dots were replaced by drifting gratings of unequal contrast. In control experiments, we found that neither attention nor eye movements can account for this illusion. We propose that the visual system uses predictive influences from the motion processing sub-system on the position processing sub-system to overcome the temporal limitations of the position processing system.

Cochlear implants are electronic devices intended for restoring hearing to the profoundly deaf. Unfortunately the degree of restored hearing varies greatly between subjects. To investigate some of the mechanisms that determine this variability, mathematical models of the auditory system are used. The level of detail that these models incorporate varies greatly. The present study describes the development of a method to create high detail, subject specific cochlea models. μ-CT scans and photomicrographs were used to obtain the morphology and histology of a specific guinea pig cochlea. A 3D model was constructed from this data and the finite element method was used to determine the potential distribution inside the cochlea. The potential distribution was calculated for different stimulus protocols applied to different modelled electrodes. A neuron model was then used to obtain neural excitation profiles. The modelled excitation profiles were compared to data from literature and it was found that this model is valid and can be used as a tool in electric hearing research. The model output was also compared to brainstem response data from the specific subject to assess the degree to which this model can predict brain stem data from a specific subject. Possible improvements to the model were also discussed. / Dissertation (MEng)--University of Pretoria, 2009. / Electrical, Electronic and Computer Engineering / unrestricted

To advance the understanding of electric hearing, from both a theoretical and practical perspective, the present study employs an engineering approach to examine whether a fundamental stochastic link exists between neural stimulation and perception. Through the use of custom-developed psychophysics software, temporal gap-detection experiments were carried out and compared with simulation results of a theoretical model. The results are informative, and the suggested modeling principles may be a step forward to a clearer understanding of how the hearing system perceives temporal stimuli. To enable the implementation of psycho-electric experiments involving cochlear implants, a software framework was developed for Matlab version 6.5, called the Psychoacoustics Toolbox, which can present stimuli either acoustically or (for interfacing with cochlear implants) using Cochlear Ltd. hardware. This toolbox facilitates easy setup of experiments based on extensible markup language (XML) templates, and allows for both adaptivestaircase procedures and presentation of a fixed set of stimuli to a participant. Multi-track interleaving of stimuli is also supported, as put forward by Jesteadt (1980), to allow for capturing of subjective responses (such as loudness perception). As part of this research, experiments were performed with three subjects, with a total of four cochlear implants. For the temporal gap-detection experiments, the rate of electrical stimulation varied over a range from 100 to 2700 pulses per second; both periodic stimulus sequences and stimuli reflecting a dead-time-modified Poisson process were used. Also, three spatially distinct stimulation sites were used with each implant to allow comparison among basal, central and apical cochlear responses. A biologically plausible psychophysical model (in contrast with a phenomenological one) was developed for predicting temporal gap-detection thresholds in electric hearing. The model was applied to both periodic and Poisson stimuli, but can easily be used with other kinds of stimuli. For comparison with experimental results, model predictions were made over the same range of stimulus rates. As a starting point, the model takes the neural stimuli, runs them through a neural filter, and then draws statistical interspike-interval (ISI) distribution data from the generated spikes. From the ISI statistics, psychometric curves can be calculated using the principles of Green and Swets (1966), from which predictions can be made for threshold measurements based on the percentage-correct mark for the specific experimental setup. With a model in place, simulations were executed to compare the model results with experimental measurements. In addition to the simulations, mathematical equations for the periodic types of stimuli were derived, given that numerical calculations could be made with higher computational e ciency for this kind of stimulus. These equations allowed for an investigation into the implications of varying the values of different neuron-model parameters. Clear similarities were found between the shapes of gap-threshold curves for experimental and modeled data, and qualitative links have been identified between model parameters and features recognized in threshold curves. For periodic stimuli, quantitative predictions of gap thresholds are close to experimental ones, although measured values cover a larger range. The results of experimental measurements using Poisson stimuli are generally somewhat larger than model predictions, although the shapes of the curves show resemblance. A possible explanation is that participants may find decision tasks involving Poisson stimuli, as opposed to periodic stimuli, confusing. Overall, model predictions and experimental results show close correspondence, suggesting Department of Electrical, Electronic and Computer Engineering. University of Pretoria. ii that the principles underlying the model are fundamentally correct. Copyright 2007, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: Geldenhuys, TA 2007, Temporal gap detection in electric hearing : modelling and experiments, MEng dissertation, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-02232012-131459 / > E1091/gm / Dissertation (MEng)--University of Pretoria, 2012. / Electrical, Electronic and Computer Engineering / Unrestricted

The present study intended to examine the neural basis of audiovisual integration, hypothetically achieved by synchronized gamma-band oscillations (30--80 Hz) that have been suggested to integrate stimulus features and top--down information. To that end, we studied the impact of visual symbolic information on early auditory sensory processing of upcoming sounds. In particular, we used a symbol-to-sound--matching paradigm in which simple score-like patterns predict corresponding sound patterns. Occasionally, a single sound is incongruent with the corresponding element of the visual pattern. In response to expected sounds congruent with the corresponding visual symbol, a power increase of phase-locked (evoked) activity in the 40-Hz band was observed peaking 42-ms poststimulus onset. Thus, for the first time, we demonstrated that the comparison process between a neural model, the expectation, and the current sensory input is implemented at very early levels of auditory processing. Subsequently, expected congruent sounds elicited a broadband power increase of non--phase-locked (induced)activity peaking 152-ms poststimulus onset, which might reflect the formation of a unitary event representation including both visual and auditory aspects of the stimulation. Gamma-band responses were not present for unexpected incongruent sounds. A model explaining the anticipatory activation of cortical auditory representations and the match of experience against expectation is presented

Perceptual rivalry refers to visual phenomena that are characterised by alternations between different percepts, despite an unchanging sensory input. Two common types of perceptual rivalry are (i) reversible figures — two-dimensional stable images that when viewed, are perceived to switch between different interpretations, and (ii) binocular rivalry — the alternations in image dominance resulting from the presentation of conflicting stimuli, one to each eye. Several investigators have suggested that these rivalling phenomena are mediated by similar neural mechanisms. Such a view, however, has not only been inadequately substantiated, but has also yet to be assessed in the context of a directly testable neurophysiological model. Miller and Pettigrew have proposed a novel, high-level interhemispheric switch (IHS) explanatory model of binocular rivalry. This model conceptualises the perceptual alternations as being mediated by alternations between one hemisphere’s selected image and the other hemisphere’s selected (rival) image. To assess their hypothesis, caloric vestibular stimulation (CVS) was used. CVS is a simple, inexpensive, and non-invasive brain stimulation technique that unilaterally activates high-level attentional areas. In accordance with the IHS model, CVS was found to significantly affect predominance (the duration that one image is perceived relative to the other, within a given viewing period) during conventional binocular rivalry with horizontal/vertical gratings and with orthogonal oblique gratings. The present thesis therefore aims, through the use of CVS, to extend this IHS model of binocular rivalry to reversible-figure alternations and propose a general IHS model of perceptual rivalry. Chapter 1 provides a detailed literature review of the field within the context of comparing both perceptual rivalries. In Chapter 2, investigations are presented on two different reversible figures — the perspective-reversing Necker cube and the figure–ground reversing Rubin’s vase–faces illusion. In these experiments, CVS was found to significantly change observers’ predominance compared to their baseline predominance. These results demonstrate that interhemispheric switching also mediates the alternations of these visual phenomena, in addition to binocular rivalry, thereby extending the IHS model to one of perceptual rivalry in general. Moreover, the findings are interpreted in a cognitive neuroscience context, including a novel proposal of a forebrain framework for the IHS model. Chapter 3 presents CVS experiments that address the issue of percept–to–hemisphere selection and the reproducibility of CVS effects, following Miller’s initial work on two types of conventional binocular rivalry. In planned analyses, significant predominance changes were not found in horizontal/vertical rivalry, oblique rivalry and Necker-cube rivalry. In post-hoc analyses that accounted for study-design differences between Miller’s original experiments and the present experiment, CVS was again not shown to induce significant predominance changes in any of the rivalry types. Assessment of directional predominance changes following CVS appeared to suggest an arbitrary selection of percept–to–hemisphere in all rivalry types, although no firm conclusions could be drawn from the obtained data on this issue. Nevertheless, the experiments further extend upon Miller’s earlier work by examining the inter- and intra-individual reproducibility of CVS-induced effects on predominance. Such reproducibility was found to be low and potential reasons for this are discussed. The experiments in Chapter 4 examine a type of binocular rivalry in which dichoptic presentation of Díaz-Caneja stimuli yields rivalry among four different stable images: half-field rivalry between the images presented to the eyes, and coherence rivalry in which aspects of each eye’s presented image are perceptually regrouped into rivalling coherent images. Each of these rivalries was found to occur for about half the given viewing time. Furthermore, CVS significantly shifted the predominance of perceived coherent images (coherence rivalry) but not half-field images (eye rivalry). This finding suggests that coherence rivalry (like conventional rivalry according to previous experiments) is mediated by interhemispheric switching at a high level, while eye rivalry is mediated by intrahemispheric mechanisms, most likely at a low level. In addition, it is proposed that Díaz-Caneja stimuli induce ‘meta-rivalry’ whereby these discrete high- and low-level competitive processes themselves rival for visual consciousness. The current thesis thus presents a novel meta-rivalry model of multistable binocular rivalry. It also presents the first direct evidence that interhemispheric switching mediates reversible-figure alternations, thereby supporting a generalised IHS model of perceptual rivalry. It is argued that both models provide a parsimonious exploratory framework within which specific predictions can be made and readily tested. Finally, the findings of all experiments in the current thesis are summarised.

Perceptual rivalry refers to visual phenomena that are characterised by alternations between different percepts, despite an unchanging sensory input. Two common types of perceptual rivalry are (i) reversible figures — two-dimensional stable images that when viewed, are perceived to switch between different interpretations, and (ii) binocular rivalry — the alternations in image dominance resulting from the presentation of conflicting stimuli, one to each eye. Several investigators have suggested that these rivalling phenomena are mediated by similar neural mechanisms. Such a view, however, has not only been inadequately substantiated, but has also yet to be assessed in the context of a directly testable neurophysiological model. Miller and Pettigrew have proposed a novel, high-level interhemispheric switch (IHS) explanatory model of binocular rivalry. This model conceptualises the perceptual alternations as being mediated by alternations between one hemisphere’s selected image and the other hemisphere’s selected (rival) image. To assess their hypothesis, caloric vestibular stimulation (CVS) was used. CVS is a simple, inexpensive, and non-invasive brain stimulation technique that unilaterally activates high-level attentional areas. In accordance with the IHS model, CVS was found to significantly affect predominance (the duration that one image is perceived relative to the other, within a given viewing period) during conventional binocular rivalry with horizontal/vertical gratings and with orthogonal oblique gratings. The present thesis therefore aims, through the use of CVS, to extend this IHS model of binocular rivalry to reversible-figure alternations and propose a general IHS model of perceptual rivalry. Chapter 1 provides a detailed literature review of the field within the context of comparing both perceptual rivalries. In Chapter 2, investigations are presented on two different reversible figures — the perspective-reversing Necker cube and the figure–ground reversing Rubin’s vase–faces illusion. In these experiments, CVS was found to significantly change observers’ predominance compared to their baseline predominance. These results demonstrate that interhemispheric switching also mediates the alternations of these visual phenomena, in addition to binocular rivalry, thereby extending the IHS model to one of perceptual rivalry in general. Moreover, the findings are interpreted in a cognitive neuroscience context, including a novel proposal of a forebrain framework for the IHS model. Chapter 3 presents CVS experiments that address the issue of percept–to–hemisphere selection and the reproducibility of CVS effects, following Miller’s initial work on two types of conventional binocular rivalry. In planned analyses, significant predominance changes were not found in horizontal/vertical rivalry, oblique rivalry and Necker-cube rivalry. In post-hoc analyses that accounted for study-design differences between Miller’s original experiments and the present experiment, CVS was again not shown to induce significant predominance changes in any of the rivalry types. Assessment of directional predominance changes following CVS appeared to suggest an arbitrary selection of percept–to–hemisphere in all rivalry types, although no firm conclusions could be drawn from the obtained data on this issue. Nevertheless, the experiments further extend upon Miller’s earlier work by examining the inter- and intra-individual reproducibility of CVS-induced effects on predominance. Such reproducibility was found to be low and potential reasons for this are discussed. The experiments in Chapter 4 examine a type of binocular rivalry in which dichoptic presentation of Díaz-Caneja stimuli yields rivalry among four different stable images: half-field rivalry between the images presented to the eyes, and coherence rivalry in which aspects of each eye’s presented image are perceptually regrouped into rivalling coherent images. Each of these rivalries was found to occur for about half the given viewing time. Furthermore, CVS significantly shifted the predominance of perceived coherent images (coherence rivalry) but not half-field images (eye rivalry). This finding suggests that coherence rivalry (like conventional rivalry according to previous experiments) is mediated by interhemispheric switching at a high level, while eye rivalry is mediated by intrahemispheric mechanisms, most likely at a low level. In addition, it is proposed that Díaz-Caneja stimuli induce ‘meta-rivalry’ whereby these discrete high- and low-level competitive processes themselves rival for visual consciousness. The current thesis thus presents a novel meta-rivalry model of multistable binocular rivalry. It also presents the first direct evidence that interhemispheric switching mediates reversible-figure alternations, thereby supporting a generalised IHS model of perceptual rivalry. It is argued that both models provide a parsimonious exploratory framework within which specific predictions can be made and readily tested. Finally, the findings of all experiments in the current thesis are summarised.

Submitted by Patrícia Cerveira (pcerveira1@gmail.com) on 2017-05-29T17:39:15Z No. of bitstreams: 1 Daniel Souto Dissertação.pdf: 2907570 bytes, checksum: 0379fd854c367debb9d8b6237a881084 (MD5) / Rejected by Biblioteca da Escola de Engenharia (bee@ndc.uff.br), reason: Bom dia, Patrícia! Rejeitei este item, pois fiquei com dúvida se é realmente acesso restrito. Conforme orientações da Jane, o acesso é aberto. Você está colocando acesso restrito. O acesso somente é restrito quando a tese ou dissertação serão publicadas e se exige ineditismo, ou quando o produto do trabalho tem segredo empresarial ou virará patente. Nos demais casos, é aberto. Aguardo retorno. Atenciosamente, Catarina Ribeiro Bibliotecária BEE - Ramal 5992 on 2017-06-29T13:54:55Z (GMT) / Submitted by Patrícia Cerveira (pcerveira1@gmail.com) on 2017-06-29T18:56:23Z No. of bitstreams: 1 Daniel Souto Dissertação.pdf: 2907570 bytes, checksum: 0379fd854c367debb9d8b6237a881084 (MD5) / Approved for entry into archive by Biblioteca da Escola de Engenharia (bee@ndc.uff.br) on 2017-07-03T12:39:19Z (GMT) No. of bitstreams: 1 Daniel Souto Dissertação.pdf: 2907570 bytes, checksum: 0379fd854c367debb9d8b6237a881084 (MD5) / Made available in DSpace on 2017-07-03T12:39:19Z (GMT). No. of bitstreams: 1 Daniel Souto Dissertação.pdf: 2907570 bytes, checksum: 0379fd854c367debb9d8b6237a881084 (MD5) / O problema de diagnóstico de faltas em linhas de transmissão constitui um dos principais desafios para gestão técnica de instalações de transmissão. A assertividade nesta atividade é fundamental para suporte à tomada de decisão, reduzindo as taxas de indisponibilidade e promovendo o restabelecimento célere da função transmissão, contribuindo para melhoria da qualidade do serviço e reduzindo os impactos financeiros advindos de reduções na parcela variável. Este documento apresenta uma proposta de sistema inteligente para classificação e localização de faltas em linhas de transmissão. Os algoritmos utilizados são baseados nos chamados modelos neurais autônomos, que incluem técnicas analíticas para seleção de entradas e especificação automática da estrutura sem a necessidade do uso de um conjunto independente de dados para validação. Ao utilizar a inferência bayesiana para especificação e treinamento de perceptrons de múltiplas camadas (MLPs), o sistema inteligente fornece respostas probabilísticas para classificação do tipo de defeito e também para a distância da falta em relação à subestação monitorada. Para desenvolvimento dos modelos são utilizados dados técnicos de uma linha de transmissão integrante do Sistema Interligado Nacional (SIN), a qual é modelada em um “software” de simulação de transitórios eletromagnéticos, ATP, visando estabelecer os diversos cenários de falta. Foram analisados dois tipos de rede equivalente, uma detalhada e outra simples, de forma a precisar qual o melhor modelo e se há diferenças significativas nos resultados em termos de representação das faltas. As bases de dados com as oscilografias de tensão e corrente obtidas para cada tipo de defeito são utilizadas para treinamento e teste do sistema inteligente, sendo demonstrando o potencial dos algoritmos utilizados. / The problem of fault diagnosis in transmission lines is one of the main challenges for the technical management of transmission facilities. The assertiveness on this activity is crucial to support decision making, reducing unavailability rates and promoting rapid reinstatement of the transmission function, contributing to the improvement of service quality and reducing the financial impacts arising from reductions in the variable portion. This document presents a proposal of intelligent system for classification and location of faults in transmission lines. The algorithms used are based on the so-called autonomous neural models which include analytical techniques for input selection and automatic structure specification without the need for an independent set of data for validation. Using Bayesian inference for specification and training of multilayer perceptrons (MLPs), the intelligent system provides probabilistic responses for classification of the type of fault and also for the distance of the fault from the monitored substation. Thus for the development of the models, technical data are used of a transmission line that is part of the National Interconnected System (SIN) which is modeled in an electromagnetic transient simulation software, ATP, aiming to establish the various fault scenarios. Furthermore, two types of equivalent network were analyzed, one detailed and one simple, in order to specify the best model and if there were significant differences in results in terms of fault representation. The databases with voltage and current oscillographs obtained for each type of fault are used for training and testing of the intelligent system, demonstrating the potential of the algorithms used.

Dans cette thèse, le comportement d'un modèle mathématique permettant de transcrire la dynamique neuronale est étudié : le système de FitzHugh-Nagumo. En particulier, nous nous intéressons au caractère aléatoire d'ouverture et de fermeture des canaux ioniques d'un neurone qui reçoit ou non un stimulus. Ce caractère aléatoire de la dynamique neuronale est considéré, dans notre modèle, comme un bruit. Dans un premier temps, le comportement du modèle de FitzHugh-Nagumo a été caractérisé au voisinage de la bifurcation d'Andronov-Hopf qui traduit la transition entre l'état d'activation et l'état de repos du neurone. Classiquement, un neurone positionné à l'état de repos ne produit aucun potentiel d'action. Cependant, il a été montré un phénomène pour lequel une quantité appropriée de bruit permet la production de potentiels d'action des plus réguliers : la résonance cohérente. Le deuxième effet observé lors de simulations numériques permet au neurone d'améliorer la détection et l'encodage d'un signal subliminal : il s'agit de la résonance stochastique. De plus, cette thèse s'inscrit dans un contexte électronique puisqu'en plus de simuler numériquement le système de FitzHugh-Nagumo, les résultats de simulations ont également été confirmés en réalisant un circuit électronique. En effet, nous avons reproduit la dynamique non linéaire du système de FitzHugh-Nagumo à l'aide de ce circuit électronique. Cela a permis de mettre en évidence expérimentalement les deux phénomènes de résonance cohérente et de résonance stochastique pour lesquelles le bruit peut avoir une influence constructive sur le comportement de notre circuit électronique. / We study the nonlinear FitzHugh-Nagumo model witch describes the dynamics of excitable neural element. It is well known that this system exhibits three different possible responses. Indeed, the system can be mono-stable, oscillatory or bistable. In the oscillatory regime, the system periodically responds by generating action potential. By contrast, in the mono-stable state the system response remains constant after a transient. Under certain conditions, the system can undergo a bifurcation between the stable and the oscillatory regime via the so called Andronov-Hopf bifurcation. In this Phd thesis, we consider the FitzHugh-Nagumo model in the stable state, that is set near the Andronov-Hopf bifurcation. Moreover, we take into account the contribution of noise witch can induces two phenomena coherence resonance and stochastic resonance. First, without external driving, we show the effect of coherence resonance since a critical noise level enhances the regularity of the system response. Another numerical investigation reports how noise can allow to detect a subthreshold deterministic signal applied to the system. In this case, an appropriate amount of noise maximizes the signal to noise ratio reveling the stochastic resonance signature. Besides this numerical studies, we have also built a non linear circuit simulating the FitzHugh-Nagumo model under the presence of noise. This circuit has allowed to confirm experimentally the numerical observation of stochastic resonance and coherence resonance. Therefor, this electronic circuit contributes a framework for further experimental investigation in the field of neural sciences to better understand the role of noise in neural encoding.