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Spatiotemporal receptive fields of simple cells in early visual cortex
No full textIn order to claim that the behaviours of early visual cortex neurons are fully understood, it must be possible to create models of them that can predict responses to any arbitrary type of stimuli. Thus the underlying theme of this thesis is to explore stimuli and implement system identification approaches appropriate for the estimation of receptive field (RF) models, as well as to quantitatively examine their response properties. Specifically, I aim to address the following questions: 1) what types of stimuli provide the most robust and generalizable RF models? 2) are cortical simple cell RF models classified into distinct categories? and 3) what are the spatiotemporal response properties of the RF models?To answer these questions, I use artificial and natural stimuli in conjunction with regularized regression system identification, to estimate RF models of visual cortical neurons. The use of natural stimuli is more appropriate as they drive more ecologically relevant responses. Regularized regression system identification techniques, which have antecedents in the field of machine learning, have a number of advantages: 1) they are based on regression and therefore appropriate for a wide range of stimuli including natural images; 2) they incorporate regularization to prevent overfitting; and 3) they allow for the estimation of full 3D (i.e., space-space-time) spatiotemporal RFs in a computationally tractable manner. I investigate simple cells of a somewhat higher cortical processing area (cat area 18), thereby providing a more challenging medium for our experiments. Results from Chapter 2 showed that white noise stimuli usually elicited weak responses, yielding poor RF models, while short bar and natural image stimuli drove strong responses yielding RF models with high predictive ability. However, RF models derived from natural images better predicted responses to other types of novel stimuli, when compared to models derived from broadband artificial stimuli (i.e., white noise, short bars). Furthermore, natural image-derived RF models performed well when used to predict responses to sinewave gratings.Chapter 3 revealed that early cortical simple cells could be delineated into three distinct categories, based on spatial RF structure and output nonlinearity. These three classes were: 1) non-oriented cells with expansive nonlinearities; 2) oriented cells with expansive nonlinearities; and 3) oriented cells with compressive nonlinearities. Our sample was split between these three types, suggesting that large numbers of non-oriented cells could be readily found in brain areas beyond the lateral geniculate nucleus and layer 4 striate cortex. Additionally, our results suggested that compressive output nonlinearities were much more common than previously believed. In Chapter 4 I identified a number of spatiotemporal response properties that varied amongst the three types of simple cells. These included optimal spatial frequency, latency, duration, responsivity, and direction selectivity. Of these, optimal spatial frequency was the most interesting as it showed segregation of our sample into distinct clusters, suggesting that optimal spatial frequency may be another dimension along which categorically distinct classes of cortical neurons could be established. Results also revealed that the three cell types effectively covered a large range of temporal latencies and durations. Finally, the two classes of oriented cells differed in their levels of responsivity, suggesting that expansive versus compressive nonlinearities may reflect the degree to which a nonlinear gain control mechanism is engaged. Collectively, the three studies in this thesis demonstrate the powerful potential of natural image stimuli and regularized regression system identification techniques, making them ideal candidates for routine use in visual neuroscience studies. RF models derived from natural images have revealed that physiological response...Afin de prétendre que les comportements des neurones du cortex visuel sont pleinement compris, il doit être possible de créer des modèles de ceux-ci qui peuvent entièrement prédire leurs réponses à tout type arbitraire de stimuli. Le thème sous-jacent de cette thèse est donc d'explorer les stimuli et les approches d'identification de système appropriés pour l'estimation de tels modèles de champ récepteur (CR), ainsi que d'examiner de façon quantitative les propriétés de leur réponse. Plus précisément, je cherche à répondre aux questions suivantes: 1) quels types de stimuli donnent des modèles de CR les plus robustes et généralisables?; 2) est-ce que les modèles de CR des cellules corticales simples sont classés en catégories distinctes?; et 3) quelles sont les propriétés des réponses spatiotemporelles des modèles de CR?Pour répondre à ces questions, j'utilise des stimuli artificiels et naturels en conjonction avec l'identification de système par régression régularisée, pour estimer les modèles de CR des neurones du cortex visuel. L'utilisation de stimuli naturels est plus appropriée, car elle entraîne des réponses plus écologiquement pertinentes. Les techniques d'identification de système par régression régularisée, qui ont des antécédents dans le domaine de l'apprentissage automatique, comportent un certain nombre d'avantages: 1) elles sont basées sur la régression et donc appropriées pour un large éventail de stimuli, y compris les images naturelles; 2) elles incorporent la régularisation afin d'éviter le sur-ajustement; et 3) elles permettent l'estimation de CR spatiotemporels 3D complets (c.-à-d., espace-espace-temps) de manière calculable par ordinateur. J'étudie les cellules simples d'une région corticale de niveau de traitement légèrement plus haut (chat, aire 18), offrant ainsi un milieu plus stimulant pour nos expériences. Les résultats du Chapitre 2 ont démontré que l'utilisation de bruit blanc comme stimuli suscitait généralement de faibles réponses, produisant de pauvres modèles de CR, tandis que l'utilisation de barres courtes et d'images naturelles comme stimuli provoquait de fortes réponses, ce qui donne des modèles de CR ayant une grande capacité prédictive. Toutefois, les modèles de CR dérivés d'images naturelles ont mieux prédit les réponses à d'autres types de nouveaux stimuli, par rapport aux modèles dérivés de stimuli artificiels à large bande (c.-à-d., bruit blanc, barres courtes). De plus, les modèles de CR dérivés d'images naturelles ont bien performé lorsqu'utilisés pour prédire les réponses aux grilles sinusoïdales. Le Chapitre 3 a révélé que les cellules corticales simples pouvaient être délimitées en trois catégories distinctes, basées sur la structure spatiale des CR et sur la non-linéarité de sortie. Ces trois catégories étaient: 1) les cellules non-orientées avec des non-linéarités à expansion; 2) les cellules orientées avec des non-linéarités à expansion; et 3) les cellules orientées avec des non-linéarités à compression. Notre échantillon a été réparti entre ces trois types, ce qui suggère qu'un grand nombre de cellules non-orientées peuvent être facilement trouvées dans les zones du cerveau au-delà du corps genouillé latéral et de la couche 4 du cortex strié. De plus, nos résultats suggèrent que les non-linéarités à compression de sortie sont beaucoup plus fréquentes qu'on ne le croyait auparavant. Dans le Chapitre 4, j'ai identifié un certain nombre de propriétés des réponses spatiotemporelles qui variaient entre les trois types de cellules simples. Il s'agissait notamment de la fréquence spatiale optimale, de la latence, de la durée, de la réactivité et de la sélectivité directionnelle. Parmi celles-ci, la fréquence spatiale optimale était la plus intéressante, car elle a démontré la ségrégation de notre échantillon en grappes distinctes. Ceci suggère que la fréquence spatiale optimale peut être une autre dimension au long de laquelle des classes de neurones corticaux..
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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