Are v1 simple cells optimized for visual occlusions? : A comparative study

Abstract: Simple cells in primary visual cortex were famously found to respond to low-level image components such as edges. Sparse coding and independent component analysis (ICA) emerged as the standard computational models for simple cell coding because they linked their receptive fields to the statistics of visual stimuli. However, a salient feature of image statistics, occlusions of image components, is not considered by these models. Here we ask if occlusions have an effect on the predicted shapes of simple cell receptive fields. We use a comparative approach to answer this question and investigate two models for simple cells: a standard linear model and an occlusive model. For both models we simultaneously estimate optimal receptive fields, sparsity and stimulus noise. The two models are identical except for their component superposition assumption. We find the image encoding and receptive fields predicted by the models to differ significantly. While both models predict many Gabor-like fields, the occlusive model predicts a much sparser encoding and high percentages of ‘globular’ receptive fields. This relatively new center-surround type of simple cell response is observed since reverse correlation is used in experimental studies. While high percentages of ‘globular’ fields can be obtained using specific choices of sparsity and overcompleteness in linear sparse coding, no or only low proportions are reported in the vast majority of studies on linear models (including all ICA models). Likewise, for the here investigated linear model and optimal sparsity, only low proportions of ‘globular’ fields are observed. In comparison, the occlusive model robustly infers high proportions and can match the experimentally observed high proportions of ‘globular’ fields well. Our computational study, therefore, suggests that ‘globular’ fields may be evidence for an optimal encoding of visual occlusions in primary visual cortex. Author Summary: The statistics of our visual world is dominated by occlusions. Almost every image processed by our brain consists of mutually occluding objects, animals and plants. Our visual cortex is optimized through evolution and throughout our lifespan for such stimuli. Yet, the standard computational models of primary visual processing do not consider occlusions. In this study, we ask what effects visual occlusions may have on predicted response properties of simple cells which are the first cortical processing units for images. Our results suggest that recently observed differences between experiments and predictions of the standard simple cell models can be attributed to occlusions. The most significant consequence of occlusions is the prediction of many cells sensitive to center-surround stimuli. Experimentally, large quantities of such cells are observed since new techniques (reverse correlation) are used. Without occlusions, they are only obtained for specific settings and none of the seminal studies (sparse coding, ICA) predicted such fields. In contrast, the new type of response naturally emerges as soon as occlusions are considered. In comparison with recent in vivo experiments we find that occlusive models are consistent with the high percentages of center-surround simple cells observed in macaque monkeys, ferrets and mice

Let Color Set the Mood

Few of us realize what a tangible effect color has on our lives. Did you ever stop to think just what you\u27re saying when you remark that He saw red. , I feel so blue. , I was green with envy. ? Psychologists have done a great deal of research on color. Not on the technical angle, but the human angle. How does color actually affect people? How can we use the psychologists\u27 findings to help us decorate our homes or business places

Consumer behavior in a multichannel context and its managerial implications

The dissertation project consists of four papers. Two papers are concerned with multichannel customer behavior: (a) drivers of competitive webrooming and (b) profitability effects of offline channel additions. The remaining two papers deal with consumer reactions towards different website cookie notifications

Pace-Setter Kitchens

New meal planning laboratories-being built in Home Economics Hall to replace last generation\u27s outdated kitchens-are aimed to provide the most practical kind of experience in many kinds of kitchens

Development of Charge Manipulation Nanoelectrospray Ion Mobility-Mass Spectrometry Techniques for Multiprotein Complex Analysis.

Macromoleclar protein complexes comprise a critical class of biomolecules unique in both their importance in biology and their relative impenetrability to detailed structural probes. Nanoelectrospray coupled to ion mobility-mass spectrometry (nESI-IM-MS) is an emerging tool for determining size and structure of protein complexes. However, its usefulness in such endeavors is largely dependent on the ability to accurately measure and correlate both intact assemblies and their protein building blocks in the gas phase to biologically-relevant structures in solution. Previous data have indicated that protein complex charge state has a demonstrated, yet currently unclear, influence on the dissociation pathways available to complexes upon collisional activation. Here, experiments designed to evaluate many different methods for ion charge state manipulation are described in the context of their potential applications in structural biology. In addition, the charge state-dependent mechanisms by which the building blocks of protein complexes are revealed via collisional activation in the gas phase are studied in detail, uncovering new intermediates and predicative correlations. Following a comprehensive introductory chapter, Chapter 2 describes a detailed set of experiments aimed at evaluating the relative merits of different charge manipulation protocols for protein complex structure analysis. Gas-phase methods, such as ion-neutral chemistry performed in the source region of the instrument, are found to provide a superior ability to reduce protein charge without leading to unwanted protein unfolding. In Chapter 3, the dissociation pathways of two protein complexes that take near-identical product ion formation pathways when high charge states are considered, are studied in detail revealing the role of previously-unknown compact states in their dissociation mechanisms upon charge state reduction. In Chapter 4, ion-ion chemistry is used to rapidly screen a relatively large number of charge-reduced protein complexes for charge states at which collision induced dissociation and unfolding energy thresholds converge, producing the first relationship capable of predicting the amount of charge reduction necessary to shift the dissociation mechanism of collisionally activated protein complexes generally toward the ejection of compact, native-like product ions. In Chapter 5, a final summary of this work is presented, along with a projected outlook of future endeavors in this area.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111426/1/rbornsc_1.pd

Heirs of the Round Table: French Arthurian Fiction from 1977 to the Present

While the English-speaking tradition has dominated the production of Arthurian-themed materials since the nineteenth-century Arthurian Revival, there is evidence that the publication of modern Arthurian fiction in French has enjoyed a major upswing over the past few decades. Notable contributions include Michel Rio\u27s Merlin-Morgane-Arthur trilogy, Jacques Roubaud and Florence Delay\u27s ten-volume cycle Graal théâtre, a half-dozen fantasy novels about the origins of the Arthurian world by Jean-Louis Fetjaine, and medievalist Michel Zink\u27s young adult novel Déodat, ou la transparence. Such texts are deeply anchored in the medieval tradition, invested in co-opting the flavor of medieval source texts at the level of narration as well as plot. Textual genealogies are frequently thematized in modern French Arthuriana by authors who credit a medieval parentage, whether through a narratorial intervention or paratexual references. As modern texts seek their own ground--whether as parodies, pastiches, entirely new adventures, or retellings of familiar stories from new perspectives--they continually draw upon the dozens of Arthurian works produced centuries before, presenting themselves as heirs to a literary tradition. With this implicit authorization, they continue its evolution. This paradigm replicates that which is already found in the medieval source material, whether in the Vulgate Cycle\u27s transformation of the Grail Quest from the romance conceived by Chrétien de Troyes into a Christian work exhorting scriptural exegesis, or in Wace\u27s appropriation of Geoffrey of Monmouth\u27s Historia Regum Brittaniae. Modern authors engage with the same process in ways that reflect a canny understanding of Arthurian literature, both its early iterations and its ongoing trajectory. Intertwined threads of genealogy, authority, legacy, and tradition in modern French Arthurian texts reveal an affinity between medieval and postmodern literary practice. As authors of the late twentieth- and early twenty-first centuries appropriate Arthurian material, they adopt techniques and textual strategies closely associated with medieval literature, recycling them to advance postmodern agendas