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Husain, M. "On hemispheric specialisation and visual direction sensing." Thesis, University of Oxford, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.382681.
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Baker, Pamela Mary. "The contribution of cortical microcircuitry to stimulus masking effects in cat primary visual cortex /." Zürich : ETH, 2008. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=17615.
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Lin, Yan. "Investigating stimulus induced metabolic changes in human visual cortex using functional magnetic resonance spectroscopy at 7T." Thesis, University of Nottingham, 2011. http://eprints.nottingham.ac.uk/14589/.
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This thesis concerns the investigation of metabolic changes in 1H metabolite levels in the human visual cortex due to visual stimulation using proton magnetic resonance spectroscopy (1H-MRS) at 7T. The work described in this thesis has been undertaken by the author and collaborators at the Sir Peter Mansfield Magnetic Resonance Centre at the University of Nottingham. Detection of functional changes in 1H metabolites may enable a greater understanding of neurotransmitter activity and metabolic pathways used for energy synthesis during activation of brain tissue. Previous 1H MRS studies of the activated human brain mainly focused on observing lactate (Lac) changes. More recent studies by Mangia et al, taking advantage of the increased signal and spectral resolution at 7T, have investigated the change in the level of Lac, glutamate (Glu), Aspartate (Asp) and Glucose (Glc) during activation. However, Mangia, did not measure significant change in the level of gamma aminobutyric acid (GABA) and Glutamine (Gln), which might be expected to change due to increased neurotransmitter cycling rates during activation. Given that the metabolite changes observed due to visual stimulation were relatively small. We used a long, intense visual stimulus, designed to retain attention, to confirm and quantify the changes in the levels of Glu, GABA, and Gln, and to further investigate the Lac and Asp response to visual stimulation. Our present results using a moving stimulus of full-screen flickering contrast-defined wedges, have demonstrated many more metabolic changes throughout two different time scales of stimulation. Small (2~11%) but significant stimulation induced increases in Lac, Glu and glutathione (GSH) were observed along with decreases in Asp, GIn and glycine (Gly). In addition, decreases in (intracellular) Glc and increases in GABA were seen but did not reach significance. The opposite changes in Glu and Asp are indicative of increased activity of the malate-aspartate shuttle, which taken together with the opposite changes in Glc and Lac reflect the expected increase in brain energy metabolism. The increases in Glu and GABA coupled with the decrease in GIn can be interpreted in terms of increased activity of the Glu/Gln and Gln/Glu/GABA neurotransmitter cycles. An entirely new observation is the increase of GSH during prolonged visual stimulation. The similarity of its time course to that of Glu suggests that it may be a response to the increased release of Glu or to the increased production of reactive oxygen species. Gly is also a precursor of GSH and a decrease on activation is consistent with increased GSH synthesis. Together these observations constitute the most detailed analysis to date of functional changes in human brain metabolites. Interestingly, the Lac response was confined to the first visual stimulus. It is possible that processes triggered during the first period of visual stimulation, could continue for a while after stimulation has ended. If this is an important mechanism of the activity-stimulated brain Lac response, shortening the duration of the first stimulus might lead to an increase in Lac response during the second period of stimulation. With this in mind, we designed a repeated visual stimulation paradigm, varying the duration of the first stimulation (shorter than 9.9-min, based on our previous results), to see the effect on the Lac response during the second visual stimulation period. A gradual increase in Lac under the prolonged stimulation, following the first brief stimulation (1s, 16s and 48s, respectively), was observed and maintained until the end of these periods. Lac responses during the second stimulation period looked similar whether the first stimulation was 1s or 16s. With the increase of first visual stimulus duration (48s), the Lac response under the second stimulation period was slightly diminished. No significant Lac accumulation can be evident to the second stimulation, when the initial stimulation was 288s. The averaged Lac level was considerably below baseline after cessation of the first 288s stimulus. It is possible that the increased glycolytic flux, triggered during the initial longer stimulation, would still continue for a while during recovery, accounting for the decreased brain Lac level during resting periods from stimulation. Further experiments are ongoing, varying the duration of the second resting periods, to see the effect on the Lac response to the second stimulation.
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Pietravalle, Nadia. "How well does a linear model predict the responses of primary visual cortex neurons to a natural scene stimulus?" Thesis, University of British Columbia, 2010. http://hdl.handle.net/2429/30518.
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The goal was to test how well a linear model of the responses of neurons in area 18 of cat visual cortex, derived from recordings made in anaesthetized adult cats, predicts responses to natural scene stimuli.
Methods: Estimates of the spatio-temporal receptive field profile of the neurons were obtained by reverse correlation to an m-sequence stimulus (Reid et al., 1997). The receptive field estimate, together with a non-linear response function, was then used to give the expected probability, or rate, of spike firing (Chichilnisky, 2001; Ringach & Malone, 2007) during a time-varying natural scene stimulus. The ability of the model to describe the responses was assessed by computing the correlation coefficient between the rates predicted by the model and those observed during stimulation with a natural scene (Willmore & Smyth, David & Gallant, 2005). For each LN functional model identified for all real A18 neurons using m-sequence responses, a Poisson spike generator was added (Heeger, 2000) to simulate ‘LNP’ responses to m-sequence and natural scene stimuli, and was used to assess the statistical significance of the results.
Results: The LN model, with parameters derived from responses to m-sequence stimuli, was able to predict responses to m-sequence stimuli with fairly high reliability (correlation coefficients in the range 0.84 – 0.96). However the model was only able to weakly predict responses to natural scene stimuli. This result was confirmed by comparing the correlation coefficients between predicted and observed firing rates obtained for actual and for simulated responses to the natural scene stimulus; values ranged from 0.14 to 0.59, in marked contrast to the simulated ones ranging from 0.47 to 0.88. Reasons for the inability of the LNP model to predict responses to natural scene stimuli are discussed.
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Keemink, Sander Wessel. "Coding of multivariate stimuli and contextual interactions in the visual cortex." Thesis, University of Edinburgh, 2018. http://hdl.handle.net/1842/28969.
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The primary visual cortex (V1) has long been considered the main low level visual analysis area of the brain. The classical view is of a feedfoward system functioning as an edge detector, in which each cell has a receptive field (RF) and a preferred orientation. Whilst intuitive, this view is not the whole story. Although stimuli outside a neuron’s RF do not result in an increased response by themselves, they do modulate a neuron’s response to what’s inside its RF. We will refer to such extra-RF effects as contextual modulation. Contextual modulation is thought to underlie several perceptual phenomena, such as various orientation illusions and saliency of specific features (such as a contour or differing element). This gives a view of V1 as more than a collection of edge detectors, with neurons collectively extracting information beyond their RFs. However, many of the accounts linking psychophysics and physiology explain only a small subset of the illusions and saliency effects: we would like to find a common principle. So first, we assume the contextual modulations experienced by V1 neurons is determined by the elastica model, which describes the shape of the smoothest curve between two points. This single assumption gives rise to a wide range of known contextual modulation and psychophysical effects. Next, we consider the more general problem of encoding and decoding multi-variate stimuli (such as center surround gratings) in neurons, and how well the stimuli can be decoded under substantial noise levels with a maximum likelihood decoder. Although the maximum likelihood decoder is widely considered optimal and unbiased in the limit of no noise, under higher noise levels it is poorly understood. We show how higher noise levels lead to highly complex decoding distributions even for simple encoding models, which provides several psychophysical predictions. We next incorporate more updated experimental knowledge of contextual modulations. Perhaps the most common form of contextual modulations is center surround modulation. Here, the response to a center grating in the RF is modulated by the presence of a surrounding grating (the surround). Classically this modulation is considered strongest when the surround is aligned with the preferred orientation, but several studies have shown how many neurons instead experience strongest modulation whenever center and surround are aligned. We show how the latter type of modulation gives rise to stronger saliency effects and unbiased encoding of the center. Finally, we take an experimental perspective. Recently, both the presence and the underlying mechanisms of contextual modulations has been increasingly studied in mice using calcium imaging. However, cell signals extracted with calcium imaging are often highly contaminated by other sources. As contextual effects beyond center surround modulation can be subtle, a method is needed to remove the contamination. We present an analysis toolbox to de-contaminate calcium signals with blind source separation. This thesis thus expands our understanding of contextual modulation, predicts several new experimental results, and presents a toolbox to extract signals from calcium imaging data which should allow for more in depth studies of contextual modulation.
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Giacherio, Brenna. "Evaluation of Functional Near Infrared Spectroscopy (fNIRS) for Assessment of the Visual and Motor Cortices in Adults." Wright State University / OhioLINK, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=wright1401816241.
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Tigwell, D. A. "Directional and orientational tuning in the striate cortex of the cat for contrast and textured stimuli." Thesis, Keele University, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.237754.
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Shaw, Lynda Joan. "Emotional processing of natural visual images in brief exposures and compound stimuli : fMRI and behavioural studies." Thesis, Brunel University, 2009. http://bura.brunel.ac.uk/handle/2438/3203.
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Can the brain register the emotional valence of brief exposures of complex natural stimuli under conditions of forward and backward masking, and under conditions of attentional competition between foveal and peripheral stimuli? To address this question, three experiments were conducted. The first, a behavioural experiment, measured subjective valence of response (pleasant vs unpleasant) to test the perception of the valence of natural images in brief, masked exposures in a forward and backward masking paradigm. Images were chosen from the International Affective Picture System (IAPS) series. After correction for response bias, responses to the majority of target stimuli were concordant with the IAPS ratings at better than chance, even when the presence of the target was undetected. Using functional magnetic resonance imaging (fMRI), the effects of IAPS valence and stimulus category were objectively measured on nine regions of interest (ROIs) using the same strict temporal restrictions in a similar masking design. Evidence of affective processing close to or below conscious threshold was apparent in some of the ROIs. To further this line of enquiry, a second fMRI experiment mapping the same ROIs and using the same stimuli were presented in a foveal (‘attended’) peripheral (‘to-be-ignored’) paradigm (small image superimposed in the centre of a large image of the same category, but opposite valence) to investigate spatial parameters and limitations of attention. Results are interpreted as showing both valence and category specific effects of ‘to-be-ignored’ images in the periphery. These results are discussed in light of theories of the limitations of attentional capacity and the speed in which we process natural images, providing new evidence of the breadth of variety in the types of affective visual stimuli we are able to process close to the threshold of conscious perception.
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Kara, Prakash. "Processing of transient stimuli by the visual system of the rat." Master's thesis, University of Cape Town, 1993. http://hdl.handle.net/11427/26626.
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While three decades of intensive cortical electrophysiology using a variety of sustained visual stimuli has made a significant contribution to many aspects of visual function, it has not supported the existence of intracortical circuit operations in cortical processing. This study investigated cortical processing by a comparison of the response of primary visual cortical neurones to transient electrical and strobe-flash stimulation. Experiments were performed on 74 anaesthetised Long Evans rats. Standard stereotaxic and extracellular electrophysiological techniques were employed. Continuous (on-line) raster plots and peri-stimulus time histograms (PSTHs) of the extracellular spikes from 81 visual cortical and 55 lateral geniculate nucleus (LGN) neurones were compiled. The strobe-flash stimuli (0.05 ms) were applied to the contralateral eye while the monopolar or bipolar electrical stimuli (0.2 ms, 80-400 μA) were applied to the ipsilateral LGN. 60 of the 81 (74%) tested cortical units were found to be responsive to visual stimuli. A distinct and consistent difference in the cortical response to the two types of transient stimuli was found: (a) Electrical stimulation evoked a prolonged period (197 ± 61 ms) of inhibition in all cortical neurones tested (n=20). This was the case even in those cortical units that were completely unresponsive to visual stimulation. The protracted inhibition was usually followed by a 100-200 ms phase of rebound excitation. (b) Flash stimulation evoked a prominent excitatory discharge (5-30 ms duration) after a latency of 30-60 ms from the onset of the stimulus (n = 59). This was followed by either moderate inhibition or return to a firing rate similar to control activity, for a maximum of 40 ms. Thereafter, cortical neurones showed a sustained increased level of activity with superimposed secondary excitatory phases. The duration of this late re-excitatory phase was 200-300 ms. In 17 of 20 (85%) tested units, the temporal profile of the cortical response to flash stimulation was modulated by small changes in the level of background illumination. In 16 of the 17 units, this sensitivity was reflected primarily as an emergence of a brief secondary inhibitory phase at the lowest level of background illumination (0 lux). Only 1 of the 17 cortical units displayed a flash-evoked primary inhibitory phase at O lux. We explored the possibility that neurones in the lateral geniculate nucleus (LGN) of the thalamus were responsible for the late phase of cortical reexcitation. 49 of the 55 (89%) LGN neurones could be classified as either of the "ON type" i.e. excited by visual stimuli, or the "OFF type" i.e. inhibited by visual stimuli. The response of ON-like LGN neurones to strobe-flash stimulation of the contralateral eye was characterised by a primary excitatory or early discharge (ED) phase after a latency of 25-40 ms. Thereafter, a 200- 400 ms period of inhibition was observed. In 57% of the sample, a rebound excitatory or late discharge (LD) phase completed the response. OFF-like LGN neurones were inhibited by the strobe-flash stimuli after a latency of 30- 35 ms. This flash-evoked inhibition was maintained for 200-400 ms. The sensitivity of the flash-evoked LGN response to the level of background illumination was tested in 11 ON-like and 10 OFF-like neurones. No sustained secondary excitatory events, as observed in visual cortical neurones, were found in any of the ON- and OFF-like LGN neurones, irrespective of the level of background illumination. In conclusion, the data show that the late re-excitatory phase evoked in cortical neurones upon strobe-flash stimulation, is not due to sustained LGN (thalamic) input. Rather, it suggests that these re-excitatory phases are due to intracortical processing of the transient stimuli. These findings emphasize the independent role of the cortex in computing the response to visual stimuli, and cast doubt on traditional theories that have emphasised the role of the thalamus in shaping cortical responses. The difference in the flash and electrically evoked cortical response suggests that even though substantial inhibition is available to the cortex, only a small fraction of this inhibitory capacity is utilised during natural stimulation.
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Killian, Nathaniel J. "Bioelectrical dynamics of the entorhinal cortex." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/52148.
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The entorhinal cortex (EC) in the medial temporal lobe plays a critical role in memory formation and is implicated in several neurological diseases including temporal lobe epilepsy and Alzheimer’s disease. Despite the known importance of this brain region, little is known about the normal bioelectrical activity patterns of the EC in awake, behaving primates. In order to develop effective therapies for diseases affecting the EC, we must first understand its normal properties. To contribute to our understanding of the EC, I monitored the activity of individual neurons and populations of neurons in the EC of rhesus macaque monkeys during free-viewing of photographs using electrophysiological techniques. The results of these experiments help to explain how primates can form memories of, and navigate through, the visual world.
These experiments revealed neurons in the EC that represent visual space with triangular grid receptive fields and other neurons that prefer to fire near image borders. These properties are similar to those previously described in the rodent EC, but here the neuronal responses relate to viewing of remote space as opposed to representing the physical location of the animal. The representation of visual space may be aided by another EC neuron type that was discovered, free-viewing saccade direction cells, neurons that signaled the direction of upcoming saccades. Such a signal could be used by other cells to prepare to fire according to the future gaze location. Many of these spatially-responsive neurons also represented memory for images, suggesting that they may be useful for associating items with their locations.
I also examined the neuronal circuitry of recognition memory for visual stimuli in the EC, and I found that population synchronization within the gamma-band (30-140 Hz) in superficial layers of the EC was modulated by stimulus novelty, while the strength of memory formation modulated gamma-band synchronization in the deep layers and in layer III. Furthermore, the strength of connectivity in the gamma-band between different layers was correlated with the strength of memory formation, with deep to superficial power transfer being correlated with stronger memory formation and superficial to deep transfer correlated with weaker memory formation. These findings support several previous investigations of hippocampal-entorhinal connectivity in the rodent and advance our understanding of the functional circuitry of the medial temporal lobe memory system.
Finally, I explored the design of a device that could be used to investigate properties of brain tissue in vitro, potentially aiding in the development of treatments for disorders of the EC and other brain structures. We designed, fabricated, and validated a novel device for long-term maintenance of thick brain slices and 3-dimensional dissociated cell cultures on a perforated multi-electrode array. To date, most electrical recordings of thick tissue preparations have been performed by manually inserting electrode arrays. This work demonstrates a simple and effective solution to this problem by building a culture perfusion chamber around a planar perforated multi-electrode array. By making use of interstitial perfusion, the device maintained the thickness of tissue constructs and improved cellular survival as demonstrated by increased firing rates of perfused slices and 3-D cultures, compared to unperfused controls. To the best of our knowledge, this is the first thick tissue culture device to combine forced interstitial perfusion for long-term tissue maintenance and an integrated multi-electrode array for electrical recording and stimulation.
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Crook, J. M. "A neurophysiological investigation of the feline extrastriate visual cortex (area 18) using oriented and textured stimuli : A comparison with area 17." Thesis, Keele University, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.379422.
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Kremkow, Jens Oliver. "Correlating excitation and inhibition in visual cortical circuits : functional consequences and biological feasibility." Aix-Marseille 2, 2009. http://www.theses.fr/2009AIX20677.
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The primary visual cortex (V1) is one of the most studied cortical area in the brain. Together with the retina and the lateral geniculate nucleus (LGN) it forms the early visual system, which has become a common model for studying computational principles in sensory systems. Simple artificial stimuli (such as drifting gratings (DG)) have given insights into the neural basis of visual processing. However, recently more and more researchers have started to use more complex natural visual stimuli (NI), arguing that the low dimensional artificial stimuli are not sufficient for a complete understanding of the visual system. For example, whereas the responses of V1 neurons to DG are dense but with variable spike timings, the neurons respond with only few but precise spikes to NI. Furthermore, linear receptive field models provide a good fit to responses during simple stimuli, however, they often fail during NI. To investigate the mechanisms behind the stimulus dependent responses of cortical neurons we have built a biophysical, yet simple and comprehensible, model of the early visual system. We show how the spatial and temporal stimulus properties interact with the model architecture to give rise to the differential response behaviour. Our results show that during NI the LGN afferents show epochs of correlated activity. These temporal correlations induce transient excitatory synaptic inputs, resulting in precise spike timings in V1. Furthermore, the sparseness of the responses to NI can be explained by correlated and lagging inhibitory conductance, which is induced by the interactions of the thalamocortical circuit with the spatial-temporal correlations in the stimulus. We continue by investigating the origin of stimulus dependent nonlinear responses, by comparing models of different complexity. Our results suggest that adaptive processes shape the responses, depending on the temporal properties of the stimuli. The spatial properties can result in nonlinear inputs through the recurrent cortical network. We then study the functional consequences of correlated excitatory and inhibitory condutances in more details in generic models. These results show that: (1) spiking of individual neurons becomes sparse and precise, (2) the selectivity of signal propagation increases and the detailed delay allows to gate the propagation through feed-forward structures (3) and recurrent cortical networks are more stable and more likely to elicit in vivo type activity states. Lastly our work illustrates new advances in methods of constructing and exchanging models of neuronal systems by the means of a simulator independent description language (called PyNN). We use this new tool to investigate the feasibility of comparing software simulations with neuromorphic hardware emulations. The presented work gives new perspectives on the processing of the early visual system, in particular on the importance of correlated conductances. It thus opens the door for more elaborated models of the visual systemLe cortex visuel primaire (V1) est l’aire corticale la plus étudiée en neurosciences. En effet, ce système complété de la rétine et du corps genouillé latéeral forme le système visuel de bas niveau et constitue une référence pour l’étude de modèles de systèmes sensoriels. Des stimuli simples comme des réseaux sinusoïdaux en mouvement (DG) ont donné des informations fondamentales sur les bases neurales du traitement neural de l’information visuelle. Cependant, de nombreux chercheurs utilisent des signaux plus complexes basés sur des images naturelles (NI) car des signaux de faibles complexité ne sont pas pertinents pour une vision complète du système visuel. Par exemple, alors que les réponses des neurones de V1 sont denses et imprécises pour des réseaux (DG), elles sont parcimonieuses et de grande résolution temporelle pour des scènes naturelles (NI). De plus, le modèle d’un champ récepteur d’intégration linéaire décrit bien la réponse à ces premiers stimuli mais est en échec pour une réponses aux images naturelles. Pour comprendre ces mécanismes corticaux dépendants du stimulus, nous avons construit un modèle biophysique simple et réaliste du système visuel de bas niveau. Nous montrons de cette façon comment les propriétés spatio-temporelles du stimulus interagissent au niveau de la structure du modèle afin de donner ces réponses différenciées. Nos r´esultats montrent en particulier que, durant la stimulation NI, les afférents thalamiques montrent des phases d’activité corrélée. Ces corrélations temporelles sont nécessaires pour générer dans V1 une réponse synaptique excitatrice phasique qui cause une réponse temporelle précise. En particulier, la parcimonie de la réponse peut être expliquée par une phase inhibitrice corrélée et légèrement retardée, ou fenêtre temporelle de conductance, induite par un circuit thalamocortical spécialisé en interaction avec l’activité spatio-temporelle corrélée du stimulus entrant. Nous poursuivons en étudiant l’origine des réponses non-linéaires observées pour les images naturelles en comparant des modèles de complexités croissantes. Nos résultats suggèrent premièrement que des processus adaptatifs modèlent le stimulus en fonction des propriétés temporelles du stimulus. Le propriétés spatiales peuvent aussi générer des effets non-linéaires amplifiés par l’intermédiaire du réseau cortical récurrent que nous modélisons. Nous étudions alors les conséquences fonctionnelles de la phase corr´el´ee des conductances excitatrices et inhibitrices dans des modèles génériques. Nous montrons que : (1) des neurones individuels deviennent plus parcimonieux et précis, (2) la sélectivité de la propagation de l’information dans une structure de type ”en-avant” peut être contrôlée finement grâce au délai dans la fenêtre temporelle. (3) La r´eponse d’un modèle de réseau cortical récurrent est plus robuste et est compatible avec les états corticaux observés in vivo. En compl´ement, ce travail illustre des avancées méthodologiques pour construire et échanger des mod`eles neuraux grˆace `a un langage de description indépendant de l’architecture appelé PyNN. Nous utilisons cet outil pour d´evelopper ces modèles sur différentes solutions logicielles mais aussi sur des circuits intégrés neuromorphiques. En conclusion, ce travail ouvre des perspectives sur le rôle computationnel générique des conductances neurales et en particulier pour la mise en place de mod`ele plus ´elabor´es pour comprendre les m´ecanismes de la vision
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Khastkhodaei, Zeinab [Verfasser], and Laura [Akademischer Betreuer] Busse. "Processing of second-order, contrast-modulated stimuli in mouse visual cortex / Zeinab Khastkhodaei ; Betreuer: Laura Busse." Tübingen : Universitätsbibliothek Tübingen, 2017. http://d-nb.info/1167247108/34.
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Shulz, Daniel. "Un analogue cellulaire de la plasticité fonctionnelle dans le cortex visuel : étude des mécanismes neuronaux de l'épigénèse et de l'apprentissage." Paris 11, 1987. http://www.theses.fr/1987PA112420.
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Lewis, Lindsay Burke. "Cross-modal plasticity for tactile and auditory stimuli within the visual cortex of early blind human subjects." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2009. http://wwwlib.umi.com/cr/ucsd/fullcit?p3338839.
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Thesis (Ph. D.)--University of California, San Diego, 2009.Title from first page of PDF file (viewed January 13, 2009). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 194-211).
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Ocazionez, Sergio Andr?s Conde. "The influence of visual inter-hemispheric connections on spiking, assembly and LFP activities, and their phase relationship during figure-ground stimulation." Universidade Federal do Rio Grande do Norte, 2014. http://repositorio.ufrn.br:8080/jspui/handle/123456789/17032.
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Previous issue date: 2014-03-31Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior
Desde os descobrimentos pioneiros de Hubel e Wiesel acumulou-se uma vasta literatura descrevendo as respostas neuronais do c?rtex visual prim?rio (V1) a diferentes est?mulos visuais. Estes est?mulos consistem principalmente em barras em movimento, pontos ou grades, que s?o ?teis para explorar as respostas dentro do campo receptivo cl?ssico (CRF do ingl?s classical receptive field) a caracter?sticas b?sicas dos est?mulos visuais como a orienta??o, dire??o de movimento, contraste, entre outras. Entretanto, nas ?ltimas duas d?cadas, tornou-se cada vez mais evidente que a atividade de neur?nios em V1 pode ser modulada por est?mulos fora do CRF. Desta forma, ?reas visuais prim?rias poderiam estar envolvidas em fun??es visuais mais complexas como, por exemplo, a separa??o de um objeto ou figura do seu fundo (segrega??o figura-fundo) e assume-se que as conex?es intr?nsecas de longo alcance em V1, assim como as conex?es de ?reas visuais superiores, est?o ativamente envolvidas neste processo. Sua poss?vel fun??o foi inferida a partir da an?lise das varia??es das respostas induzidas por um est?mulo localizado fora do CRF de neur?nios individuais. Mesmo sendo muito prov?vel que estas conex?es tenham tamb?m um impacto tanto na atividade conjunta de neur?nios envolvidos no processamento da figura quanto no potencial de campo, estas quest?es permanecem pouco estudadas. Visando examinar a modula??o do contexto visual nessas atividades, coletamos potenciais de a??o e potenciais de campo em paralelo de at? 48 eletrodos implantados na ?rea visual prim?ria de gatos anestesiados. Estimulamos com grades compostas e cenas naturais, focando-nos na atividade de neur?nios cujo CRF estava situado na figura. Da mesma forma, visando examinar a influ?ncia das conex?es laterais, o sinal proveniente da ?rea visual isot?pica e contralateral foi removido atrav?s da desativa??o revers?vel por resfriamento. Fizemos isso devido a: i) as conex?es laterais intr?nsecas n?o podem ser facilmente manipuladas sem afetar diretamente os sinais que est?o sendo medidos, ii) as conex?es inter-hemisf?ricas compartilham as principais caracter?sticas anat?micas com a rede lateral intr?nseca e podem ser vistas como uma continua??o funcional das mesmas entre os dois hemisf?rios e iii) o resfriamento desativa as conex?es de forma causal e revers?vel, silenciando temporariamente seu sinal, permitindo conclus?es diretas a respeito da sua contribui??o. Nossos resultados demonstram que o mecanismo de segmenta??o figurafundo se reflete nas taxas de disparo de neur?nios individuais, assim como na pot?ncia do potencial de campo e na rela??o entre sua fase e os padr?es de disparo produzidos pela popula??o. Al?m disso, as conex?es laterais inter-hemisf?ricas modulam estas vari?veis dependendo da estimula??o feita fora do CRF. Observamos tamb?m uma influ?ncia deste circuito lateral na coer?ncia entre potenciais de campo entre eletrodos distantes. Em conclus?o, nossos resultados d?o suporte ? ideia de um mecanismo complexo de segmenta??o figura-fundo atuando desde as ?reas visuais prim?rias em diferentes escalas de frequ?ncia. Esse mecanismo parece envolver grupos de neur?nios ativos sincronicamente e dependentes da fase do potencial de campo. Nossos resultados tamb?m s?o compat?veis com a hip?tese que conex?es laterais de longo alcance tamb?m fazem parte deste mecanismo
Since Hubel and Wiesel s pioneer finding a vast body of literature has accumulated describing neuronal responses in the primary visual cortex (V1) to different visual stimuli. These stimuli mainly consisted of moving bars, dots or gratings which served to explore the responses to basic visual features such as orientation, direction of motion or contrast, among others, within a classical receptive field (CRF). However, in the last two decades it became increasingly evident that the activity of V1 neurons can be modulated by stimulation outside their CRF. Thus, early visual areas might be already involved in more complex visual tasks like, for example, the segmentation of an object or a figure from its (back)-ground. It is assumed that intrinsic long-range horizontal connections within V1 as well as feedback connections from higher visual areas are actively involved in the figure-ground segmentation process. Their possible role has been inferred from the analysis of the spike rate variations induced by stimuli placed outside the CRF of single neurons. Although it is very likely that those connections also have an impact on the joined activity of neurons involved in processing the figure and on their local field potentials (LFP), these issues remain understudied. In order to examine the context-dependent modulation of those activities, we recorded spikes and LFPs in parallel from up to 48 electrodes in the primary visual cortex of anesthetized cats. We stimulated with composite grating and natural scene stimuli focusing on populations of neurons whose CRFs were situated on the foreground figure. In addition, in order to examine the influence of horizontal connections we removed the inter-hemispheric input of the isotopic contralateral visual areas by means of reversible cooling deactivation. We did so because i) the intrinsic horizontal connections cannot be easily manipulated without directly affecting the measured signals, ii) because inter-hemispheric connections share the major anatomical features with the intrinsic lateral network and can be seen as a functional continuation of the latter across the two hemispheres and iii) because cooling causally and reversibly deactivates input connections by temporarily silencing the sending neurons and thus enables direct conclusions on their contribution. Our results demonstrate that the figure-ground segmentation mechanism is reflected in the spike rate of single neurons, as well as in their LFP power and its phase-relationship to the spike patterns produced by the population. In addition "lateral" inter-hemispheric connections modulate spike rates and LFP power depending on the stimulation of the neurons CRF surround. Further, we observe an influence of this lateral circuit on field- field coherences between remote recording sites. In conclusion, our findings support the idea of complex figure-ground segmentation mechanism acting already in early visual areas on different time scales. This mechanism seems to involve groups of neurons firing synchronously and dependent on the LFP s phase. Our results are also compatible with the hypothesis that long-range lateral connections contribute to that mechanism
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Thulin, Nilsson Linnea. "The Role of Primary Visual Cortex in Visual Awareness." Thesis, Högskolan i Skövde, Institutionen för biovetenskap, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:his:diva-11623.
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Despite its great complexity, a great deal is known about the organization and information-processing properties of the visual system. However, the neural correlates of visual awareness are not yet understood. By studying patients with blindsight, the primary visual cortex (V1) has attracted a lot of attention recently. Although this brain area appears to be important for visual awareness, its exact role is still a matter of debate. Interactive models propose a direct role for V1 in generating visual awareness through recurrent processing. Hierarchal models instead propose that awareness is generated in later visual areas and that the role of V1 is limited to transmitting the necessary information to these areas. Interactive and hierarchical models make different predictions and the aim of this thesis is to review the evidence from lesions, perceptual suppression, and transcranial magnetic stimulation (TMS), along with data from internally generated visual awareness in dreams, hallucinations and imagery, this in order to see whether current evidence favor one type of model over the other. A review of the evidence suggests that feedback projections to V1 appear to be important in most cases for visual awareness to arise but it can arise even when V1 is absent.
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Hardingham, Neil Robert. "Synaptic connections in rat visual cortex." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.325298.
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Fotheringhame, David K. "Temporal coding in primary visual cortex." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.339357.
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Nauhaus, Ian Michael. "Functional connectivity in primary visual cortex." Diss., Restricted to subscribing institutions, 2008. http://proquest.umi.com/pqdweb?did=1692099811&sid=1&Fmt=2&clientId=1564&RQT=309&VName=PQD.
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Peelen, Marius Vincent. "Body selectivity in human visual cortex." Thesis, Bangor University, 2006. https://research.bangor.ac.uk/portal/en/theses/body-selectivity-in-human-visual-cortex(4091f96c-dee2-42ec-9a32-c0a8cf17b288).html.
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Perceiving other people is a seemingly effortless process. Yet within a few hundred milliseconds we are aware of who we are looking at, what this person is doing, and even what this person feels. We derive this information from the form and motion of the face and body. Faces may be particularly important for some aspects of person perception (e. g., identity recognition), whereas bodies may be more important for others (e. g., action recognition). Furthermore, information from the body is important in cases where it is not possible to perceive the details of the face, for instance when the face is occluded, or when we see someone from a distance. In most cases, however, it is likely that information from both the face and the body are perceived in parallel and are integrated at an early stage. Previous research on person perception has mostly focused on the brain mechanisms underlying face perception. Much less research has focused on the brain mechanismsu nderlying body perception,w hich is the topic of this thesis. Using functional magnetic resonance imaging (fMRI) I provide evidence for a previously unknown body-selective visual area that overlaps a face-selective area. By employing novel analysis techniques that take into account patterns of activation across voxels I show that body- and face-selective areas can be functionally dissociated. Finally, I show that, in contrast to frontal and parietal action-recognition areas, visual body-selective areasd o not contain a dynamic representationo f observeda ctions. Together, thesef indings increaseo ur understandingo f the brain mechanismsu nderlying body, face and action perception, by showing both similarities and dissimilarities in the brain structures involved in these processes.
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Randell, Thomas David William. "Stimulus equivalence and naming." Thesis, University of Southampton, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.312896.
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Woodbury, Greg. "Modelling Emergent Properties of the Visual Cortex." University of Sydney. School of Mathematics and Statistics, 2003. http://hdl.handle.net/2123/695.
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Nicoll, A. J. "Excitatory synaptic connections in the visual cortex." Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.303635.
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Berman, Neil Jonathan. "Aspects of inhibition in the visual cortex." Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.303871.
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Ramachandran, Suchitra. "Visual Statistical Learning in Monkey Inferotemporal Cortex." Research Showcase @ CMU, 2014. http://repository.cmu.edu/dissertations/463.
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Despite living in noisy sensory environments, humans and non-human primates have the ability to learn regularities and patterns in the environment solely on the basis of passive exposure. This ability to learn what is statistically likely and predictable in the environment is called statistical learning. Visual statistical learning of image sequences has been demonstrated at the level of single neurons in the rhesus macaque (monkey) inferotemporal cortex (IT). Upon subjecting monkeys to extensive exposure to pairs of images presented sequentially such that the display of one image always predicted the subsequent display of another image, IT neurons showed suppressed responses to images that occurred in a predicted context, but not when the same effect, called prediction suppression, more thoroughly, we discovered that this effect depends on the conditional probability between the images presented sequentially. Further, the effect generalizes across time and space, it is domain specific, and it can be induced by training monkeys on longer sequences. These effects are long-lasting and robust: they persist at least for 20 months after initial training with no exposure to the stimuli in the interim. We have preliminary evidence for the existence of neurophysiological markers of statistical learning in areas upstream of IT in the ventral visual stream, suggesting that learning statistical regularities may be a fundamental function of sensory cortex. images occurred in an unpredicted context (Meyer & Olson, 2011). Upon investigating this effect, called prediction suppression, more thoroughly, we discovered that this effect depends on the conditional probability between the images presented sequentially. Further, the effect generalizes across time and space, it is domain specific, and it can be induced by training monkeys on longer sequences. These effects are long-lasting and robust: they persist at least for 20 months after initial training with no exposure to the stimuli in the interim. We have preliminary evidence for the existence of neurophysiological markers of statistical learning in areas upstream of IT in the ventral visual stream, suggesting that learning statistical regularities may be a fundamental function of sensory cortex.
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Lawrence, Samuel. "Global shape processing in human visual cortex." Thesis, University of York, 2016. http://etheses.whiterose.ac.uk/16448/.
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The research described in this thesis used a combination of neuroimaging, brain stimulation and psychophysical methods to explore the representation of global shape in human visual cortex. Global shape processing mechanisms integrate over local orientation information to form an abstracted representation of an object’s shape; an important processing step for accurate object recognition in the complex visual scenes of everyday life. This thesis presents evidence for a global shape processing stream in human lateral occipital cortex comprising brain areas LO1, LO2 and object-selective lateral occipital complex (LOC). First, brain responses to shape were shown to be more global and less dependent on task in LO2 compared to LO1. Second, a global shape aftereffect was used to demonstrate that global representations of object shape are formed by a series of integrative mechanisms, where each successive mechanism integrates over outputs from the preceding mechanisms to form increasingly global shape representations. Third, brain stimulation was used to determine whether area LO2 is causally important for global shape adaptation and shape discriminations, however brain stimulation had no effect on shape perceptions meaning conclusions about a causal role for LO2 in global shape processing could not be drawn. Finally, neural tuning to radial frequency, one possible dimension of a representational shape space, was modelled using Gaussian neural modelling. Tuning towards radial frequencies that are processed globally was localised to lateral occipital cortex, and shifted from local to global frequencies in LO2 and LOC. Overall, this thesis provides novel contributions to the current understanding of how the analysis of an object’s shape facilitates object recognition, and the nature and locus of global shape representations in the human brain.
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Mohamed, Abdelhack. "Top-down Modulation in Human Visual Cortex." Kyoto University, 2019. http://hdl.handle.net/2433/242434.
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Krug, Kristine. "Ordering geniculate input into primary visual cortex." Thesis, University of Oxford, 1997. https://ora.ox.ac.uk/objects/uuid:b342ffae-4a31-4171-94a6-83cb516e83fe.
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Precise point-to-point connectivity is the basis of ordered maps of the visual field in the brain. One point in the visual field is represented at one locus in the dLGN and one locus in primary visual cortex. A fundamental problem in the development of most sensory systems is the creation of the topographic projections which underlie these maps. Mechanisms ranging from ordered ingrowth of fibres, through chemical guidance of axons to sculpting of the map from an early exuberant input have been proposed. However, we know little about how ordered maps are created beyond the first relay. What we do know is that a topological mismatch requires the exchange of neighbours in the geniculo-cortical projection and that manipulating the input to the primary relay can affect the geniculo-cortical topography. Taking advantage of the immaturity of the newborn hamsterâs visual system, I studied the generation of an ordered map in primary visual cortex during the time of target innervation in normal and manipulated animals. I also investigated the patterning of neuronal activity prior to natural eye-opening. Paired injections of retrograde fluorescent tracers into visual cortex reveal that geniculate fibres are highly disordered at the time of invasion of the cortical plate. Topography in the geniculo-cortical projection emerges out of an unordered projection to area 17 in the first postnatal week. Furthermore, I show that manipulating the peripheral input can alter the topographic map which arises out of the early scatter. Removal of one eye at birth appears to slow the process of geniculo-cortical map formation ipsilateral to the remaining eye and at the end of the second postnatal week, a double projection between thalamus and cortex has formed. If retinal activity is blocked during this time, this double projection does not emerge. The results implicate retinal activity as the signal that induces the development of a different topographic order in the geniculo-cortical projection. It is generally believed that visual experience can influence development only after eye-opening. However, the final part of my thesis shows that neurons in the developing visual cortex of the ferret can not only be visually driven at least 10 days before natural eye-opening, but are also selective for differently oriented gratings presented through the closed eye-lid. Thus, visually-driven neuronal activity could influence development much earlier than previously assumed in many developmental studies.
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Rudiger, Philipp John Frederic. "Development and encoding of visual statistics in the primary visual cortex." Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/25469.
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How do circuits in the mammalian cerebral cortex encode properties of the sensory environment in a way that can drive adaptive behavior? This question is fundamental to neuroscience, but it has been very difficult to approach directly. Various computational and theoretical models can explain a wide range of phenomena observed in the primary visual cortex (V1), including the anatomical organization of its circuits, the development of functional properties like orientation tuning, and behavioral effects like surround modulation. However, so far no model has been able to bridge these levels of description to explain how the machinery that develops directly affects behavior. Bridging these levels is important, because phenomena at any one specific level can have many possible explanations, but there are far fewer possibilities to consider once all of the available evidence is taken into account. In this thesis we integrate the information gleaned about cortical development, circuit and cell-type specific interactions, and anatomical, behavioral and electrophysiological measurements, to develop a computational model of V1 that is constrained enough to make predictions across multiple levels of description. Through a series of models incorporating increasing levels of biophysical detail and becoming increasingly better constrained, we are able to make detailed predictions for the types of mechanistic interactions required for robust development of cortical maps that have a realistic anatomical organization, and thereby gain insight into the computations performed by the primary visual cortex. The initial models focus on how existing anatomical and electrophysiological knowledge can be integrated into previously abstract models to give a well-grounded and highly constrained account of the emergence of pattern-specific tuning in the primary visual cortex. More detailed models then address the interactions between specific excitatory and inhibitory cell classes in V1, and what role each cell type may play during development and function. Finally, we demonstrate how these cell classes come together to form a circuit that gives rise not only to robust development but also the development of realistic lateral connectivity patterns. Crucially, these patterns reflect the statistics of the visual environment to which the model was exposed during development. This property allows us to explore how the model is able to capture higher-order information about the environment and use that information to optimize neural coding and aid the processing of complex visual tasks. Using this model we can make a number of very specific predictions about the mechanistic workings of the brain. Specifically, the model predicts a crucial role of parvalbumin-expressing interneurons in robust development and divisive normalization, while it implicates somatostatin immunoreactive neurons in mediating longer range and feature-selective suppression. The model also makes predictions about the role of these cell classes in efficient neural coding and under what conditions the model fails to organize. In particular, we show that a tight coupling of activity between the principal excitatory population and the parvalbumin population is central to robust and stable responses and organization, which may have implications for a variety of diseases where parvalbumin interneuron function is impaired, such as schizophrenia and autism. Further the model explains the switch from facilitatory to suppressive surround modulation effects as a simple by-product of the facilitating response function of long-range excitatory connections targeting a specialized class of inhibitory interneurons. Finally, the model allows us to make predictions about the statistics that are encoded in the extensive network of long-range intra-areal connectivity in V1, suggesting that even V1 can capture high-level statistical dependencies in the visual environment. The final model represents a comprehensive and well constrained model of the primary visual cortex, which for the first time can relate the physiological properties of individual cell classes to their role in development, learning and function. While the model is specifically tuned for V1, all mechanisms introduced are completely general, and can be used as a general cortical model, useful for studying phenomena across the visual cortex and even the cortex as a whole. This work is also highly relevant for clinical neuroscience, as the cell types studied here have been implicated in neurological disorders as wide ranging as autism, schizophrenia and Parkinson’s disease.
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de, Haas B. "Contextual modulations of visual perception and visual cortex activity in humans." Thesis, University College London (University of London), 2014. http://discovery.ucl.ac.uk/1435556/.
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Visual perception and neural processing depend on more than retinal stimulation alone. They are modulated by contextual factors like cross-modal input, the current focus of attention or previous experience. In this thesis I investigate ways in which these factors affect vision. A first series of experiments investigates how co-occurring sounds modulate vision, with an emphasis on temporal aspects of visual processing. In three behavioral experiments I find that participants are unable to ignore the duration of co-occurring sounds when giving visual duration judgments. Furthermore, prolonged sound duration goes along with improved detection sensitivity for visual stimuli and thus extends beyond duration judgments per se. I go on to test a cross-modal illusion in which the perceived number of flashes in a rapid series is affected by the number of co-occurring beeps (the sound-Induced flash illusion). Combining data from structural magnetic resonance imaging (MRI) and a behavioral experiment I find that individual proneness to this illusion is linked with less grey matter volume in early visual cortex. Finally, I test how co-occurring sounds affect the cortical representation of more natural visual stimuli. A functional MRI (fMRI) experiment investigates patterns of activation evoked by short video clips in visual areas V1-3. The trial-by-trial reliability of such patterns is reduced for videos accompanied by mismatching sounds. Turning from cross-modal effects to more intrinsic sources of contextual modulation I test how attention affects visual representations in V1-3. Using fMRI and population receptive field (pRF) mapping I find that high perceptual load at fixation renders spatial tuning for the surrounding visual field coarser and goes along with pRFs being radially repelled. In a final behavioral and fMRI experiment I find that the perception of face features is modulated by retinal stimulus location. Eye and mouth stimuli are recognized better, and evoke more discriminable patterns of activation in face sensitive patches of cortex, when they are presented at canonical locations. Taken together, these experiments underscore the importance of contextual modulation for vision, reveal some previously unknown such factors and point to possible neural mechanisms underlying them. Finally, they argue for an understanding of vision as a process using all available cues to arrive at optimal estimates for the causes of sensory events.
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Ludwig, Casimir Johannes Hendrikus. "Stimulus-driven and goal-driven control over visual selection." Thesis, University of Bristol, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.269218.
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Herbst, Sophie. "How visual stimulus dynamics affect mechanisms of interval timing." Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät II, 2014. http://dx.doi.org/10.18452/17078.
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Gegenstand dieser Arbeit ist die Divergenz zwischen subjektiver und objektiver Zeitwahrnehmung. In drei empirischen Studien wird untersucht, wie Zeitwahrnehmung vom Inhalt eines Zeitintervalls beeinflusst wird. Dabei werden Paradigmen aus der Forschung zur visuellen Wahrnehmung mit Messungen der Zeitwahrnehmung kombiniert und durch Ableitungen neuronaler Aktivität (Elektroenzephalographie) ergänzt. Sensorische Modelle der Zeitwahrnehmung basieren auf der Annahme, dass die subjektive Dauer eines sensorischen Reizes in den selben neuronalen Netzwerken entsteht, die den Reiz selbst verarbeiten. Unter dieser Annahme müsste jede Veränderung des Reizes, welche neuronale Verarbeitung hervorruft, auch einen Effekt auf die wahrgenommene Dauer des Reizes haben, auch wenn diese Eigenschaft nicht bewusst wahrgenommen wird. Die Ergebnisse zeigen jedoch das nur Veränderungen die auch bewusst wahrgenommen werden einen Einflusss auf subjektive Dauer haben. Darüberhinaus zeigte sich, dass die subjektiv wahrgenommene Stärke der Veränderung die subjektive Dauer bestimmt. Die Befunde sprechen eher für das Modell einer zentrale inneren Uhr, das jedoch auch erklären muss, mittels welcher Mechanismen der Inhalt eines Zeitintervalls dessen subjektive Dauer beeinflusst. Anhand elektrophysiologischer Korrelate der Zeitwahrnehmung wird untersucht, ob die Dynamik visueller Reize die Zeitwahrnehmung schon während des Enkodierens der Zeit beeinflusst, oder erst später wenn eine Entscheidung über die Dauer des Zeitintervalls getroffen wird. Die Ergebnisse zeigen, dass die neuronalen Korrelate der Zeitwahrnehmung nicht die zeitliche Verzerrung widerspiegeln, die durch dynamische Reize hervorgerufen wird. Dies spricht dafür, dass diese Verzerrung auf einer späteren Prozessstufe eintritt. Insgesamt zeigen die Befunde der drei Studien, das Zeitwahrnehmung zwar stark vom sensorischen Inhalt beeinflusst wird, sich aber nicht direkt von der sensorischen Reizverarbeitung ableiten lässt.Often, perceived time differs from objective time. This work addresses how perceived time is influenced by the content of a time interval. Three empirical studies were conducted to assess how visual stimulus dynamics affect perceived duration. We combined paradigms from vision research with timing tasks and measures of neural processing using electroencephalogram (EEG). Sensory models of interval timing claim that duration of a time interval is encoded in the same neural networks that process its sensory content. Thus, even stimulus dynamics that are processed only on the sensory level but are not consciously perceived should affect perceived duration. In contrary, we showed that only consciously perceived stimulus dynamics affect perceived duration, with more perceived dynamics leading to longer perceived duration. Changes that were not perceived but evoked a neural response (measured in the EEG) did not affect perceived duration. These findings argue against the assumption of sensory timing models, but are consistent with models that assume a central internal clock. However, internal clock models do not sufficiently explain why stimulus dynamics affect perceived duration. We tested whether stimulus dynamics affect the stage of temporal encoding as postulated by internal clock models, by measuring neural correlates of temporal encoding in the EEG. We found that the neural correlates of temporal encoding reflected internal variations in perceived duration, but not the dilation induced by stimulus dynamics. We argue that visual stimulus dynamics affect perceived duration after temporal encoding. In sum, the findings show that duration perception is not grounded in early sensory processing, but is probably achieved by a specialized timing system that can be biased by the perception of dynamic stimuli. The discussion addresses theoretical implication of these findings for theories of time perception, and their implications for further research in the field.
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Patrick, Jonathan A. "Optimisation of peripheral visual function using stimulus-based manipulations." Thesis, University of Nottingham, 2017. http://eprints.nottingham.ac.uk/40607/.
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Ocular disorders that restrict visual capacity in the centre of the visual field, such as age-related macular degeneration (AMD) and Stargardt’s disease, force patients to perform important visual tasks in the periphery. It is well documented that visual performance is progressively limited as the peripheral eccentricity at which the task is performed increases. Since many of the disorders that cause central vision loss currently have no cure, adaptive techniques to optimise the remaining visual function are required. This thesis describes a series of psychophysical experiments that aim to optimise stimulus perception using manipulations to the stimulus input. Super-resolution (SR) is a form of image processing wherein multiple low-resolution images are merged over time to form a higher-resolution image. In many situations, the low- resolution sequence of images is produced by motion. Because of this, the effect of motion on peripheral acuity is first examined. The benefit of motion on acuity observed within 10° in the healthy periphery was very limited to specific combinations of target speed and retinal location. Thus, the investigation was extended to artificially undersampled stimuli. Spatial undersampling was achieved by presenting stimuli behind partially opaque masks. A significant benefit of motion was identified for the partially occluded stimuli, indicating a SR mechanism that operates when the visual input is sufficiently undersampled. In further experiments, it was established that smooth motion, originating from the target, is a key condition required for peripheral SR to be most effective. Since motion was shown to be insufficient to significantly improve resolution in the typical periphery, the effects of additional temporal modulations applied to static and moving stimuli were examined. Applying periodic temporal modulations to stimuli has the effect of creating temporal harmonics of the stimulus in the Fourier domain. The purpose of these experiments was thus to examine whether the visual system is capable of utilising these harmonics to better resolve the target. Temporally subsampling the stimulus, such that it appears with blank temporal intervals, was shown to drastically reduce the motion-related loss of acuity. However, at low target speeds, resolution thresholds were higher in the more subsampled conditions. It was shown that the loss at low speeds was driven by a reduction in the time-averaged contrast that accompanies temporal subsampling. Next, the effect of contrast polarity reversal was examined, whereby the target switches between black and white at periodic intervals, thus preserving the time-averaged contrast. Contrast polarity reversal diminished the motion-related loss, while also providing an overall reduction in resolution thresholds across speeds. Certain temporal modulations may therefore improve peripheral acuity for static and moving targets. To test whether the benefit of temporal modulations may be of use in a patient population, the effect of modulating the stimulus on resolution thresholds was examined in simulated conditions of ocular disease. A common comorbid symptom of central vision loss is exaggerated ocular jitter. The effects of subsampling and contrast polarity reversal were examined on resolution thresholds for targets jittering in accordance with ocular motion, multiplied by a variable gain factor. Temporal subsampling, as for smooth motion, was a hindrance to resolution. Contrast polarity reversal, however, was shown to improve performance at all levels of jitter. Contrast polarity reversal was also examined in simulated conditions of neuro-retinal matrix disorder (NRMD), whereby targets appear with spatial undersampling. There was no significant improvement in resolution for undersampled targets. Thus, while temporal modulations may be beneficial in some central vision loss disorders, the results do not support its use in NRMD patients. Additional temporal stimulus modulations therefore have diverse effects on resolution. To investigate the mechanisms driving these effects, a model was created to examine how the temporal modulations were influencing the perception of the stimulus. In the development of the model, the spatiotemporal characteristics of the stimulus were assessed. By calculating the extent to which the stimulus was compromised of frequencies to which the visual system is sensitive, an estimate of how visible the target should be in each condition was estimated. In assessment of the spatiotemporal characteristics of the stimuli, it was confirmed that contrast alone is not sufficient to explain the benefits of contrast polarity reversal. Further, the model indicated that the extended spectral range additional temporal modulations provide the stimulus is a reasonable explanation of the effects the modulations have on resolution, when combined with a description of the retinal response to temporally modulating stimuli. Finally, to confirm the use of contrast polarity reversal as a technique to optimise peripheral function in vision loss disorders, it was examined in a more salient task for patients: peripheral reading. Reading speed and accuracy were assessed for peripheral sentences with and without temporal modulation, in healthy observers and in patients with central vision loss. Both healthy observers and patients made significantly fewer errors in the contrast polarity reversal conditions than in the unmodulated conditions. However, only the healthy observers demonstrated a reduction in reading speed. While the results do not wholly support contrast polarity reversal, it was postulated that patients with more severe symptoms of AMD may reveal a stronger benefit. Thus, the experiments in this thesis have demonstrated that performance on several peripheral visual tasks can be improved by applying additional temporal modulations to the stimulus. Further, it has been indicated that this benefit stems from a combination of the contrast of the stimulus, and the effect of the modulation on the spatiotemporal characteristics of the target.
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Hodgskiss, Dean Leslie. "Towards improved visual stimulus discrimination in an SSVEP BCI." Master's thesis, University of Cape Town, 2010. http://hdl.handle.net/11427/11283.
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The dissertation investigated the influence of stimulus characteristics, electroencephalographic (EEG) electrode location and three signal processing methods on the spectral signal to noise ratio (SNR) of Steady State Visual Evoked Potentials (SSVEPs) with a view for use in Brain-Computer Interfaces (BCIs). It was hypothesised that the new spectral baseline processing method introduced here, termed the 'activity baseline', would result in an improved SNR.
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Duffy, Kevin R. "The maturation and experience-dependent plasticity of the developing visual cortex /." *McMaster only, 2001.
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Bartolucci, Marco. "Attentional modulations in the visual cortex in the absence of visual stimulation." Thesis, Royal Holloway, University of London, 2012. http://repository.royalholloway.ac.uk/items/5594dbe3-8d95-44e6-bda6-f0b0a37bfabe/1/.
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Every day we perceive visual scenes filled with different stimuli. Visual attention allows us to select the information that is most relevant to ongoing behaviour. The aim of this thesis is to explore how top-down modulations of activity in the human visual cortex affect perception and how attention interacts with visual processing in the brain. Specifically, we investigate the role of the modulation that occurs after a cue to attend but before onset of a visual stimulus, referred to in the literature as pre-stimulus attentional modulation, using fMRI methods alongside behavioural measurements. The main focus of the first three experiments is on the interactions between pre-stimulus attentional modulation and modulation by attention of the stimulus-evoked response. Results overall suggest that pre-stimulus activity is correlated with the effects of attention on the stimulus-evoked response and that the two attentional effects may therefore reflect a single process. The aim of the fourth experiment is to study the interaction of spatial and feature-based attention, and the results suggest that when both are engaged together, visual cortical areas do not benefit in an additive way, suggesting either that one dominates or that attentional resources saturate. The fifth experiment investigates the interaction of pre-stimulus activity with the speed and accuracy of saccade movements, and the results suggest no relation between those two processes. Finally the last two experiments focus on the role of pre-stimulus attentional modulation in perceptual learning, and the results strongly suggest that attentional modulation is involved in this process. Based on the results of these experiments, the role of pre- stimulus attentional modulation in visual processing is discussed.
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Jadauji, Jahan. "Modulation of olfactory processing by visual cortex stimulation." Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=106393.
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In order to view the world in a unified manner, information from the different senses must combine at some point in the sensory stream. This is known as perceptual binding and has led to the study of how various senses interact, termed multisensory integration. An intriguing example of a crossmodal interaction is the finding from various imaging studies that primary visual cortex is activated while subjects performed purely olfactory tasks. This is in line with the literature documenting a connection between vision and olfaction, but whether visual cortical activity and olfactory perception are connected causally is unknown. In this thesis I will investigate the connection between vision and olfaction using Transcranial Magnetic Stimulation (TMS). TMS is a non-invasive method of stimulating cortex via magnetic induction. The present research involved the application of TMS to early visual cortex, including V1, bilaterally in the context of experiments that test both visual and olfactory perception. In particular subjects were tested on a visual task (contrast detection) and two olfactory tasks that probe the ability to discriminate odor intensity and quality. Behavioral experiments were carried out before and after the application of either no TMS, real TMS to V1, sham TMS to V1 (replicates experimental conditions of real TMS but no current is produced in the cortex) and real TMS to primary auditory cortex. The results have revealed a significant improvement in performance on the visual task, replicating previous data. Interestingly, improvements in the ability of subjects to discriminate among odor qualities following TMS of V1 were also found, with a tendency for female subjects to show more improvement than their male counterparts. Similar improvements were not found in the ability to discriminate among odor intensities or on either olfactory task in any of the control conditions (no TMS or sham TMS or auditory cortex TMS). Implications and limitations of the present results are discussed.Afin de voir le monde d'une manière unifiée, les informations provenant des sens différents doivent combiner à un certain point dans le flux sensoriel. Ceci est connu comme la combinaison perceptuelle et a conduit à l'étude de la façon dont interagissent les différents sens, appelé intégration multisensorielle. Un exemple fascinant d'une interaction intermodale est la conclusion à partir des différents études d'imagerie que le cortex visuel primaire est activé alors que les sujets ont exécuté des tâches purement olfactif. Ceci est en accord avec la littérature documentant une connexion entre la vision et l'olfaction, mais si l'activité corticale visuelle et la perception olfactive sont connectés causalement est inconnue. Dans cette thèse, je vais étudier le lien entre la vision et l'olfaction en utilisant la stimulation magnétique transcrânienne (TMS). TMS est une méthode non invasive de stimulation du cortex via l'induction magnétique. La présente recherche concernait l'application de TMS au cortex visuel, y compris V1, bilatéralement dans le contexte des expériences qui testent à la fois la perception visuelle et olfactive. En particulier les sujets ont été testés sur une tâche visuelle (détection de contraste) et deux tâches olfactives qui sondent la capacité à discriminer l'intensité d'odeur et de qualité. Les expériences comportementales ont été réalisées avant et après l'application soit pas TMS, le TMS réelles à V1, faux TMS à V1 (réplique les conditions expérimentales de TMS réel, mais pas de courant est produit dans le cortex) et TMS réelles à cortex auditif primaire. Les résultats ont révélé une amélioration significant des performances sur la tâche visuelle, qui réplique des données précédentes. L'amélioration de la capacité des sujets à discriminer parmi les qualités d'odeurs suivantes TMS de V1 ont également été trouvés, avec une tendance pour les sujets féminins à montrer une amélioration plus que leurs homologues masculins. Des améliorations similaires n'ont pas été trouvées dans la capacité à discriminer entre les intensités d'odeur ou de chaque tâche olfactive dans l'une des conditions de contrôle (pas de TMS ou faux TMS ou le TMS du cortex auditif). Les implications et les limites des résultats actuels sont discutés.
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Gharat, Amol. "Motion-defined contour processing in early visual cortex." Thesis, McGill University, 2012. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=106530.
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From our daily experience it is very clear that relative motion cues can contribute to correctly identifying object boundaries and for perceiving depth. Motion-defined contours are not only generated by the motion of objects in a scene, but also by the movement of an observer's head and body. However the neural mechanism involved in detecting these contours is still unknown. To explore this mechanism, I extracellularly recorded responses of neurons in Area 18 of anesthetized and paralyzed cats while they were presented with visual stimuli. The goal of this study was to determine if neurons in Area 18 that have been previously shown to detect luminance, texture- and contrast-defined contours cue-invariantly could also detect motion-defined contours. Motion-defined contour stimuli were generated by modulating the velocity of high spatial frequency sinusoidal luminance gratings (carrier gratings) by a squarewave envelope. The carrier gratings used were outside the luminance passband of a neuron, such that presence of carrier alone within the receptive field of a neuron did not elicit a response. It was found that most of the neurons in Area 18 that responded to contrast-defined contours also respond to motion-defined contours. The orientation and direction selectivity of these neurons for motion-defined contours was similar to that of luminance gratings. A given neuron also exhibited similar selectivity for the spatial frequency of the carrier grating of contrast- and motion-defined contours. These results suggest that Area 18 is a common brain area where different second-order contours are detected in a form-cue invariant manner, through a common neural mechanism.Au quotidien, il est clair que les indices locaux de mouvement relatif contribuent à l'identification des bords des objets et à la perception de la profondeur. Les contours définis par le mouvement ne sont pas seulement générés par le mouvement des objets dans une scène, mais également par le mouvement de la tête et du corps de l'observateur. Cependant, les mécanismes neuronaux impliqués dans la détection de ces contours restent toujours inconnus. Pour étudier ces mécanismes, j'ai effectué des enregistrements électrophysologiques extracellulaires dans l'aire 18 de chats anesthésiés et paralysés pendant que des stimuli visuels leurs étaient présentés. Le but de cette étude était de déterminer si les neurones de l'aire 18 pour lesquels il a été montré qu'ils détectent indifféremment les contours définis par la luminance, la texture ou le contraste peuvent aussi détecter les contours définis par le mouvement. Les stimuli de contours définis par le mouvement étaient générés en modulant la vitesse de réseaux de luminances sinusoïdaux de haute fréquence spatiale (réseau porteur) par une enveloppe en créneau. Les réseaux porteurs utilisés étaient au-delà de la bande passante de luminance des neurones de façon à ce que la présentation du réseau porteur seul dans le champ récepteur d'un neurone ne génère pas de réponse. Il a alors été observé que la plupart des neurones de l'aire 18 qui répondent aux contours définis par le contraste répondent également aux contours définis par le mouvement. Les sélectivités à l'orientation et à la direction des contours définis par le mouvement de ces neurones étaient similaires à celles des réseaux de luminance. Un neurone donné présentait également la même sélectivité à la fréquence spatiale du réseau porteur pour les contours définis par le contraste et le mouvement. Ces résultats suggèrent que l'aire 18 est une aire d'intégration où différents contours de second-ordre sont détectés par un mécanisme commun, indépendamment de la nature des indices locaux.
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Jeong, Su Keun. "Flexible visual information representation in human parietal cortex." Thesis, Harvard University, 2014. http://nrs.harvard.edu/urn-3:HUL.InstRepos:13068539.
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In many everyday activities, we must visually process multiple objects embedded in complex real world scenes. Our visual system can flexibly extract behaviorally relevant visual information from such scenes, even though it has a severely limited processing capacity. This dissertation proposes that human superior intra-parietal sulcus (IPS) plays a central role in this flexible visual information processing. In Chapter 1, using functional magnetic resonance imaging (fMRI) with univariate analysis, I found that distractor processing in superior IPS was attenuated when target locations were known in advance. In Chapter 2, using multi-voxel pattern analysis (MVPA), I showed that superior IPS encoded object shapes, but only when such information was required by task. In Chapter 3, I showed that, given a set of perceptually distinct, but semantically grouped visual inputs, superior IPS could represent abstract object identity. The neural similarity of identities in superior IPS significantly correlated with perceived similarity between identities, confirming the representation in this region indeed reflected identity. Taken together, these results suggest that human superior IPS encodes a wide range of visual information, from simple features to abstract identities, in a task-dependent manner, enabling flexible goal-directed visual information processing in the human brain.Psychology
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Mineault, Patrick. "Parametric modelling of visual cortex at multiple scales." Thesis, McGill University, 2014. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=123020.
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The visual system is confronted with the daunting task of extracting behaviourally relevant visual information from noisy and ambiguous patterns of luminance falling on the retina. It solves this problem through a hierarchical architecture, in which the visual stimulus is iteratively re-encoded into ever more abstract representations which can drive behaviour. This thesis explores the question of how the computations performed by neurons in the visual hierarchy create behaviourally relevant representations. This question requires probing the visual system at multiple scales: computation is the role of single neurons and ensembles of neurons; representation is the function of multiple neurons within an area; hierarchical processing is an emergent process which involves multiple areas; and behaviour is defined at the full scale of the system, the psychophysical observer. To study visual processing at multiple scales, I propose to develop and apply parametric modelling methods in the context of systems identification. Systems identification seeks to establish the deterministic relationship between the input and the output of a system. Systems identification has proven particularly useful in the study of visual processing, where the input to the system can be easily controlled via sensory stimulation.Parametric modeling, built on the theory of Generalized Linear Models (GLMs), furnishes a common framework to analyze signals with different statistical properties which occur in the analysis of neural systems: spike trains, multi-unit activity, local field potentials and psychophysical decisions.In Chapter 2, I develop the parametric modeling framework which is used throughout this thesis in the context of psychophysical classification images. Results show that parametric modeling can infer a psychophysical observer's decision process with fewer trials than previously proposed methods. This allows the exploration of more complex, and potentially more informative, models of decision processes while retaining statistical tractability.In Chapter 3, I extend and apply this framework to the analysis of visual representations at the level of neuronal ensembles in area V4. The results show that it is possible to infer, from multi-unit activity and local field potential (LFP) signals, the representation of visual space at a fine-grained scale over several millimeters of cortex. Analysis of the estimated visual representations reveals that LFPs reflect both local sources of input and global biases in visual representation. These results resolve a persistent puzzle in the literature regarding the spatial reach of the local field potential.In Chapter 4, I extend and apply the same framework to the analysis of single-neuron responses in area MST of the dorsal visual stream. Results reveal that MST responses can be explained by the integration of their afferent input from area MT, provided that this integration is nonlinear. Estimated models reveal long suspected, but previously unconfirmed receptive field organization in MST neurons that allow them to respond to complex optic flow patterns. This receptive field organization and nonlinear integration allows more accurate estimation of the velocity of approaching objects from the population of MST neurons, thus revealing their possible functional role in vergence control and object motion estimation.Put together, these results demonstrate that with powerful statistical methods, it is possible to infer the nature of visual representations at multiple scales. In the discussion, I show how these results may be expanded to gain a better understanding of hierarchical visual processing at large.Le système visuel est confronté à la difficile tâche d'extraire de l'information utile au comportement à partir de motifs complexes et ambigus détectés par la rétine. Il résout ce problème grâce à une architecture hiérarchique, dans laquelle le stimulus visuel est itérativement ré-encodé dans une représentation abstraite. Ce mémoire explore la question suivante : comment les computations performées par des neurones de la hiérarchie visuelle créent-elles des représentations permettant des comportements complexes?Cette question nécessite l'étude du système visuel à plusieurs échelles : la computation est le rôle de neurones et d'ensembles de neurones; la représentation est une fonction des neurones dans une aire du cerveau; la hiérarchie émerge de la communication entre de multiples aires du cerveau; et le comportement est défini à l'échelle du système visuel complet, l'observateur psychophysique.Afin d'étudier le système visuel à de multiple échelles, je développe et applique des méthodes de modélisation paramétrique dans le cadre de l'identification de système. Celle-ci a pour but d'établir la relation déterministe entre l'entrée d'un système et sa sortie. L'identification de système est particulièrement utile dans l'étude de la vision, où l'entrée du système peut être facilement contrôlée par stimulation sensorielle.La modélisation paramétrique, bâtie sur la théorie des modèles linéaires généralisés, offre un paradigme commun pour analyser des signaux ayant des propriétés statistiques disparates, souvent rencontrés dans l'étude du système nerveux: les potentiels d'action, l'activité d'ensemble de neurones, et les décisions psychophysiques.Dans le 2ème chapitre, je développe le paradigme d'analyse par modélisation paramétrique qui sera utilisé tout au long de ce mémoire dans le contexte des images de classification psychophysiques. Je démontre qu'il est possible d'inférer, grâce à ces méthodes, le processus décisionnel d'un observateur psychophysique avec moins de données que ce qui était précédemment possible. Cette avancée permet l'exploration de modèles psychophysiques plus complexes, et potentiellement plus informatifs sur le processus décisionnel de l'observateur.Dans le 3ème chapitre, j'applique ce paradigme à l'analyse des représentations visuelles au niveau d'ensembles neuronaux dans l'aire V4 du système visuel. Les résultats démontrent qu'il est possible, à partir de l'activité des champs de potentiel locaux (CPL), d'inférer la représentation corticale de l'espace visuel sur une échelle de plusieurs millimètres. Je démontre ainsi que les CPL reflètent à la fois des sources synaptiques locales et des biais globaux dans la représentation visuelle. Ces résultats résolvent une controverse dans la littérature concernant l'intégration spatiale des CPL.Dans le 4ème chapitre, j'applique ce même paradigme dans l'analyse de neurones dans l'aire MST du système visuel dorsal. Je révèle que les réponses dans MST peuvent être expliquées par l'intégration de sources afférentes provenant de l'aire MT; cependant, cette intégration se révèle nonlinéaire. Cette analyse révèle des propriétés longtemps soupçonnées mais jusqu'ici non confirmées des champs réceptifs des neurones dans MST; celles-ci leur permettent de communiquer de l'information sur les motifs de flux optique complexes. Cette organisation des champs réceptifs et l'intégration nonlinéaire permet d'extraire plus facilement la vélocité d'objets s'approchant de l'observateur à partir des réponses de la population de neurones dans MST, révélant un rôle insoupçonné de ces neurones dans l'estimation de la vélocité des objets.Pris ensemble, ces résultats démontrent qu'à l'aide de méthodes statistiques puissantes, il est possible d'inférer la nature des représentations visuelles à de multiples échelles. Dans la discussion, je démontre comment généraliser ces résultats afin d'obtenir une meilleure compréhension des computations hiérarchiques dans le système visuel.
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Saenz, Melissa. "Global effects of attention in human visual cortex /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 2002. http://wwwlib.umi.com/cr/ucsd/fullcit?p3071014.
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Coggan, David. "The neural representation of objects in visual cortex." Thesis, University of York, 2019. http://etheses.whiterose.ac.uk/22899/.
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Neuroimaging studies have shown that different categories of object evoke different neural responses in the ventral visual pathway. This has been interpreted to suggest that these regions represent high-level conceptual or semantic properties of the stimulus, such as its category. However, images from different categories differ in low-level visual properties. Therefore, the extent to which category-specific neural responses indicate high-level or low-level representations is unclear. This thesis investigates the extent to which low-level properties of objects are important in the neural response of ventral visual pathway. The first study uses a data-driven approach to select clusters of objects based on the similarity of their low-level visual properties. These visually defined clusters did not correspond to typical object categories, but still evoked distinct patterns of response in the ventral stream. The second and third studies show category-specific patterns of response in the ventral stream to scrambled objects that are not recognizable, but nevertheless retain many of their low-level visual properties. The fourth study reveals that the bias toward natural object images found in the ventral stream begins to emerge in early visual areas. The final chapter shows that category-specific patterns of EEG response can be also explained by low-level image properties. Taken together, these results demonstrate the importance of low-level visual properties in the neural representation of objects. These findings suggest that the category-selectivity observed in high-level visual regions can be explained by a distributed organization based around more basic properties of the stimulus.
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Cadieu, Charles Fredrick. "Modeling shape representation in visual cortex area V4." Thesis, Massachusetts Institute of Technology, 2005. http://hdl.handle.net/1721.1/30367.
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Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Includes bibliographical references (p. 85-89).
Visual processing in biological systems is classically described as a hierarchy of increasingly sophisticated representations, originating in primary visual cortex (V1), progressing through intermediate area V4, and ascending to inferotemporal cortex. The computational mechanisms that produce representations in intermediate area V4 have remained a mystery. In this thesis I show that the standard model, a quantitative model which extends the classical description of visual processing, provides a computational mechanism capable of reproducing and predicting the responses of neurons in area V4 with a translation invariant combination of V1 responses. Using techniques I have developed, model neurons accurately predict the responses of 8 V4 neurons to within-class stimuli, such as closed contours and gratings, and achieve an average correlation coefficient of 0.77 between predicted responses and measured V4 responses. Furthermore, model neurons fit to a V4 neuron's grating stimulus response, can qualitatively predict the V4 neuron's 2-spot reverse correlation map. These results successfully demonstrate the first attempt to bridge V1 and V4 experimental data, by describing how representation in V4 could emerge from the nonlinear combination of V1 neural responses.
by Charles Fredrick Cadieu.
M.Eng.
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Freeborn, Robert Bruce. "A neural death model of the visual cortex." Thesis, University of Sussex, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.394270.
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Michel, Vincent. "Understanding the visual cortex by using classification techniques." Paris 11, 2010. http://www.theses.fr/2010PA112202.
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Dans cette thèse, nous présentons différentes méthodes d'apprentissage statistique pouvant être utilisées pour comprendre le code neuronal des fonctions cognitives, en se basant sur des données d'Imagerie par Résonance Magnétique fonctionnelle (lRMf). Nous nous intéressons principalement à l'étude de la localisation spatiale des entités impliquées dans le codage, et leur influence respective dans le processus cognitif, en nous focalisant sur l'étude du cortex visuel. Après avoir introduit les notions de codage neuronal et d'imagerie fonctionnelle, nous étudions les limites de l'approche classique d'analyse des données d'IRMf, et les différents avantages apportés par une méthode d'analyse récente, l'inférence inverse. Enfin, nous détaillons les méthodes d'apprentissage statistique utilisées dans le cadre de l'inférence inverse, et nous les évaluons sur un jeu de données réelles. Dans une seconde partie, nous décrivons les trois principales contributions de cette thèse. Tout d'abord, nous introduisons une approche Bayésienne pour la régularisation parcimonieuse, généralisant au sein d'un même modèle différentes régularisations Bayésiennes. Ensuite nous proposons un algorithme de coalescence supervisé qui tient compte de l'information spatiale contenue dans les images fonctionnelles, et qui semble être bien adaptée au cas de l'inférence entre sujets. Finalement, nous proposons d'inclure l'information spatiale au sein d'un modèle de régularisation, qui peut être utilisé dans un cadre de prédiction, et permet d'extraire des ensembles connexes de voxels prédictifs. Cette approche est bien adaptée à l'étude de la localisation spatiale du codage neuronalLn this thesis, we present different approaches for statistical learning that can be used for studying the neural code of cognitive functions, based on brain functional Magnetic Resonance Imaging (fMRI) data. Ln particular, we study the spatial organization of the neural code, i. E. The spatial localization and the respective weights of the different entities implied in the neural coding. Ln this thesis, we focus on the visual cortex. Ln the first part of this thesis, we introduce the notions of functional architecture, neural coding and functional imaging. Then, we study the limits of the classical approach for the characterization of the neural code fram fMRI images, and the advantages of a recent method of analysis, namely inverse inference. Finally, we detail the statistical learning approaches used for inverse inference, and we evaluate them on real data. Ln a second part, we describe the three main contributions of this thesis. First, we introduce a Bayesian framework for sparse regularization, that generalizes two reference approaches. Then, we propose a supervised clustering method, that takes into account the spatial structure of the images. The resulting weighted maps are easily interpretable, and this approach seems particularly interesting in the case of inter-subjects inference. The last contribution of this thesis aims at including the spatial information into the regularization framework. This regularization is th us used in both regression and classification settings, and extracts clusters of predictive voxels. This approach is well suited for the decoding problem addressed in this thesis
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Bedford, James L. "Neuro-electromagnetic imaging of the human visual cortex." Thesis, Aston University, 1995. http://publications.aston.ac.uk/14604/.
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Methods of solving the neuro-electromagnetic inverse problem are examined and developed, with specific reference to the human visual cortex. The anatomy, physiology and function of the human visual system are first reviewed. Mechanisms by which the visual cortex gives rise to external electric and magnetic fields are then discussed, and the forward problem is described mathematically for the case of an isotropic, piecewise homogeneous volume conductor, and then for an anisotropic, cocentric, spherical volume conductor. Methods of solving the inverse problem are reviewed, before a new technique is presented. This technique combines prior anatomical information gained from stereotaxic studies, with a probabilistic distributed-source algorithm to yield accurate, realistic inverse solutions. The solution accuracy is enhanced by using both visual evoked electric and magnetic responses simultaneously. The numerical algorithm is then modified to perform equivalent current dipole fitting and minimum norm estimation, and these three techniques are implemented on a transputer array for fast computation. Due to the linear nature of the techniques, they can be executed on up to 22 transputers with close to linear speedup. The latter part of the thesis describes the application of the inverse methods to the analysis of visual evoked electric and magnetic responses. The CIIm peak of the pattern onset evoked magnetic response is deduced to be a product of current flowing away from the surface areas 17, 18 and 19, while the pattern reversal P100m response originates in the same areas, but from oppositely directed current. Cortical retinotopy is examined using sectorial stimuli.
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Bernard, Clémence Francoise. "Otx2-glycosaminoglycan interaction to regulate visual cortex plasticity." Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066228/document.
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Pendant le développement postnatal du cortex cérébral visuel, l'homéoprotéine Otx2 est transférée préférentiellement dans les interneurones inhibiteurs à parvalbumine (cellules PV), induit leur maturation et régule la période critique de plasticité pour la dominance oculaire. Pendant cette période critique, les cellules PV sont progressivement entourées par une matrice extracellulaire riche en glycosaminoglycanes (GAGs), qui pourraient être impliqués dans la capture d'Otx2. Pour étudier comment l'interaction entre Otx2 et les GAGs à la surface des cellules PV régule la période critique, nous avons analysé une lignée de souris transgéniques Otx2-AA chez lesquelles cette interaction est perturbée. Ces souris présentent une spécificité réduite de l'Otx2 cortical pour les cellules PV et un retard dans l'ouverture et la fermeture de la période critique pour la dominance oculaire. Nous avons montré que la protéine Otx2 se lie aux chaines de chondroïtine sulfates à la surface des cellules PV et qu'elle a une forte affinité pour le chondroïtine sulfate CS-E. Chez l'adulte, le cortex est maintenu à l'état non plastique par un apport continuel d'Otx2. Afin de ré-ouvrir une fenêtre de plasticité chez l'adulte, nous avons développé deux modèles pour perturber le transfert d'Otx2 : un analogue synthétique de CS-E qui se lie à Otx2 et une souris knock-in inductible pour contrôler la sécrétion d'un anticorps simple chaine contre Otx2. Ces résultats confirment et précisent le rôle in vivo de l'interaction entre Otx2 et les GAGs, à la fois pour la mise en place des périodes critiques pendant le développement postnatal et pour le maintien de l'état non plastique du cortex chez l'adulteDuring postnatal development of the visual cerebral cortex, Otx2 homeoprotein is transferred preferentially into parvalbumin inhibitory interneurons (PV-cells), induces their maturation and regulates a critical period of plasticity for binocular vision. During the critical period, PV-cells are gradually enwrapped by perineuronal nets enriched in glycosaminoglycans (GAGs), which are likely involved in the capture of Otx2. To understand how Otx2 interacts with GAGs at the surface of PV-cells for critical period regulation, we have analyzed a transgenic Otx2-AA mouse line in which the interaction between Otx2 and GAGs is disrupted. These mice show a reduced specificity of cortical Otx2 for PV-cells with concomitant delayed onset and closure of critical period for ocular dominance. We have also identified that Otx2 protein binds chondroitin sulfate chains of the perineuronal nets and that it has a high affinity for the chondroitin sulfate CS-E. We have therefore developed a sugar-ase protection assay for identifying specific glycan sequences involved in homeoprotein recognition. Throughout adulthood, the cortex receives Otx2 to maintain a consolidated, non-plastic state. To interfere with Otx2 transfer in the adult and reopen a window of plasticity, we have developed two models: a synthetic hexasaccharide analogue of CS-E that binds to Otx2 and an inducible, knock-in mouse allowing spatio-temporal control of a secreted single chain antibody against Otx2. All these results confirm and clarify the in vivo role for Otx2-GAG interaction, both in the timing of critical periods during postnatal development and in the maintenance of the non-plastic state of the cortex in the adult
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Watson, David M. "The neural representation of scenes in visual cortex." Thesis, University of York, 2016. http://etheses.whiterose.ac.uk/12961/.
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Recent neuroimaging studies have identified a number of regions in the human brain that respond preferentially to visual scenes. These regions are thought to underpin our ability to perceive and interact with our local visual environment. However, the precise stimulus dimensions underlying the function of scene-selective regions remain controversial. Some accounts have proposed an organisation based on relatively high-level semantic or categorical properties of the stimulus. However, other accounts have suggested that lower-level visual features of the stimulus may offer a more parsimonious explanation. This thesis presents a series of fMRI experiments employing multivariate pattern analyses (MVPA) in order to test the role of low-level visual properties in the function of scene-selective regions. The first empirical chapter presents two experiments showing that patterns of neural response to different scene categories can be predicted by a model of the visual properties of scenes (GIST). The second empirical chapter demonstrates that direct manipulations of the spatial frequency content of the image significantly influence the patterns of response, with effects often being comparable to or greater than those of scene category. The third empirical chapter demonstrates that distinct patterns of response can be found to different scene categories even when images are Fourier phase scrambled such that low-level visual features are preserved, but perception of the categories is impaired. The fourth and final empirical chapter presents an experiment using a data-driven method to select clusters of scenes objectively based on their visual properties. These visually defined clusters did not correspond to typical scene categories, but nevertheless elicited distinct patterns of neural response. Taken together, these results support the importance of low-level visual features in the functional organisation of scene-selective regions. Scene-selective responses may arise from the combined sensitivity to multiple visual features that are themselves predictive of scene content.
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Pisauro, M. A. "Imaging haemodynamic activity in the mouse visual cortex." Thesis, University College London (University of London), 2014. http://discovery.ucl.ac.uk/1452981/.
Full textAbstract:
Neurovascular coupling, the relationship between neural and haemodynamic activity, is not a fixed property of the brain. Physiological, behavioural and cognitive factors can strongly modulate the degree to which haemodynamic responses to perceptual stimuli reflect co-localized neural responses. Moreover, a significant part of the haemodynamic activity is not directly coupled with the spiking activity. Here we show how anaesthesia can influence the relationship between haemodynamic and neural activity and how the former can be expressed as the sum of two components: one which is well coupled with neuronal responses and the other which seem to be independent of them and which correlates with alertness. We used wide-field optical imaging of intrinsic signals in mouse primary visual cortex (V1). Haemodynamic responses could be used to obtain clear maps of retinotopy in both anaesthetized and awake mice. However, when the mice were awake, responses were four times larger and twice as fast compared to when they were anaesthetized. By measuring neural responses we could establish that the effects of anaesthesia on haemodynamic responses were due to changes in neurovascular coupling. By activating V1 via optogenetics, we replicated the effects of anaesthesia in terms of delay of the response but not of amplitude. We then asked whether haemodynamic activity can all be explained in terms of local neural activity. By imposing a precise spatiotemporal pattern of neural responses in visual cortex we were able to distinguish two component of the haemodynamic activity: one reflects responses to visual stimuli, and is local to the retinotopic region activated by the stimuli. The second correlates strongly with pupil diameter, which reflects a measure of arousal, and is shared simultaneously by large regions of cortex.