Large deployable antenna program. Phase 1: Technology assessment and mission architecture

The program was initiated to investigate the availability of critical large deployable antenna technologies which would enable microwave remote sensing missions from geostationary orbits as required for Mission to Planet Earth. Program goals for the large antenna were: 40-meter diameter, offset-fed paraboloid, and surface precision of 0.1 mm rms. Phase 1 goals were: to review the state-of-the-art for large, precise, wide-scanning radiometers up to 60 GHz; to assess critical technologies necessary for selected concepts; to develop mission architecture for these concepts; and to evaluate generic technologies to support the large deployable reflectors necessary for these missions. Selected results of the study show that deployable reflectors using furlable segments are limited by surface precision goals to 12 meters in diameter, current launch vehicles can place in geostationary only a 20-meter class antenna, and conceptual designs using stiff reflectors are possible with areal densities of 2.4 deg/sq m

Individual accessibility and travel possibilities: A literature review on time geography

In the late 1960s, Torsten Hägerstrand introduced the conceptual framework of time geography which can be deemed an elegant tool for analysing individual movement in space and time. About a decade later, the auspicious time-geographic research has gradually lost favour, mainly due to the unavailability of robust geocomputational tools and the lack of georeferenced individual-level travel data. It was only from the early 1990s that new GISbased research gave evidence of resurgence in popularity of the field. From that time on, several researchers have steadily been publishing work at the intersection of time geography, disaggregate travel modeling, and GI-science. This paper reviews the most important timegeographic contributions. From this exercise, some prevailing research gaps are deduced and a way to deal with these gaps is presented. In particular, we focus on space-time accessibility measures, geovisualisation of activity patterns, human extensibility and fuzzy space-time prisms in relation to CAD

pypet: A Python Toolkit for Data Management of Parameter Explorations

pypet (Python parameter exploration toolkit) is a new multi-platform Python toolkit for managing numerical simulations. Sampling the space of model parameters is a key aspect of simulations and numerical experiments. pypet is designed to allow easy and arbitrary sampling of trajectories through a parameter space beyond simple grid searches. pypet collects and stores both simulation parameters and results in a single HDF5 file. This collective storage allows fast and convenient loading of data for further analyses. pypet provides various additional features such as multiprocessing and parallelization of simulations, dynamic loading of data, integration of git version control, and supervision of experiments via the electronic lab notebook Sumatra. pypet supports a rich set of data formats, including native Python types, Numpy and Scipy data, Pandas DataFrames, and BRIAN(2) quantities. Besides these formats, users can easily extend the toolkit to allow customized data types. pypet is a flexible tool suited for both short Python scripts and large scale projects. pypet's various features, especially the tight link between parameters and results, promote reproducible research in computational neuroscience and simulation-based disciplines.DFG, 103586207, GRK 1589: Verarbeitung sensorischer Informationen in neuronalen Systeme

The computational neurology of active vision

In this thesis, we appeal to recent developments in theoretical neurobiology – namely, active inference – to understand the active visual system and its disorders. Chapter 1 reviews the neurobiology of active vision. This introduces some of the key conceptual themes around attention and inference that recur through subsequent chapters. Chapter 2 provides a technical overview of active inference, and its interpretation in terms of message passing between populations of neurons. Chapter 3 applies the material in Chapter 2 to provide a computational characterisation of the oculomotor system. This deals with two key challenges in active vision: deciding where to look, and working out how to look there. The homology between this message passing and the brain networks solving these inference problems provide a basis for in silico lesion experiments, and an account of the aberrant neural computations that give rise to clinical oculomotor signs (including internuclear ophthalmoplegia). Chapter 4 picks up on the role of uncertainty resolution in deciding where to look, and examines the role of beliefs about the quality (or precision) of data in perceptual inference. We illustrate how abnormal prior beliefs influence inferences about uncertainty and give rise to neuromodulatory changes and visual hallucinatory phenomena (of the sort associated with synucleinopathies). We then demonstrate how synthetic pharmacological perturbations that alter these neuromodulatory systems give rise to the oculomotor changes associated with drugs acting upon these systems. Chapter 5 develops a model of visual neglect, using an oculomotor version of a line cancellation task. We then test a prediction of this model using magnetoencephalography and dynamic causal modelling. Chapter 6 concludes by situating the work in this thesis in the context of computational neurology. This illustrates how the variational principles used here to characterise the active visual system may be generalised to other sensorimotor systems and their disorders

Mechanisms of memory consolidation : Analyzing the coordinated activity of concept neurons in the human medial temporal lobe during waking and sleep

The aim of this thesis is to investigate the role of human concept neurons in memory consolidation during sleep. Memory consolidation is a process by which memories initially dependent on the hippocampus are transferred to cortical areas, thereby gradually becoming independent of the hippocampus. Theories of memory consolidation posit that memory traces encoding autobiographic episodes are rapidly formed in the hippocampus during waking, and reactivated during subsequent slow-wave sleep to be transformed into a long-lasting form. Concept neurons in the human medial temporal lobe are neurons tuned to semantic concepts in a selective, sparse, and invariant manner. These neurons respond to pictures or written and spoken words representing their preferred concept (for example, a person, an animal, an object), regardless of physical stimulus properties. Concept neurons have been speculated to be building blocks for episodic memory. We used whole-night recordings from concept neurons in the medial temporal lobe of epilepsy patients implanted with depth electrodes for presurgical monitoring to test the hypothesis that the coordinated activity of concept neurons during sleep is a neurophysiological correlate of memory consolidation in humans. To conduct this study, we developed software methods for artifact removal and spike sorting of long-term recordings from single neurons. In an evaluation on both simulated model data and visual stimulus presentation experiments, our software outperformed previous methods. Starting from the conceptual analogy between rodent place cells and human concept neurons, we developed an episodic memory task in which participants learned a story eliciting sequential activity in concept neurons. We found that concept neurons preserved their semantic tuning across whole-night recordings. Hippocampal concept neurons had, on average, lower firing rates during rapid-eye-movement (REM) sleep than during waking. During slow-wave sleep, firing rates did not significantly differ from waking. The activity of concept neurons increased during ripples in the local field potential. Furthermore, concept neurons whose preferred stimuli participated in the memorized story were conjointly reactivated after learning, most pronouncedly during slow-wave sleep. Cross-correlations of concept neurons were most asymmetric during slow-wave sleep. Cross-correlation peak times were often in the range believed to be relevant for spike-timing-dependent plasticity. However, time lags of peak cross-correlations did not correlate with the positional order of stimuli in the memorized story. Our findings support the hypothesis that concept neurons rapidly encode a memory trace during learning, and that the reactivation of the same neurons during subsequent slow-wave sleep and ripples contributes to the consolidation of the memory episode. However, the consolidation of the temporal order of events in humans appears to differ from what rodent research suggests.Mechanismen der Gedächtniskonsolidierung : Analyse der Aktivität von Konzeptzellen im menschlichen Schläfenlappen während Wachheit und Schlaf In dieser Arbeit wird die Rolle von Konzeptzellen ("concept neurons") im Gehirn des Menschen bei der Gedächtniskonsolidierung im Schlaf untersucht. Gedächtniskonsolidierung ist ein Prozess, durch den Gedächtnisinhalte, die zunächst vom Hippokampus abhängen, in die Großhirnrinde übertragen werden. Dadurch reduziert sich im Laufe der Zeit die Abhängigkeit der Gedächtnisinhalte vom Hippokampus. In der Theorie der Gedächtniskonsolidierung wird angenommen, dass während wachem Erleben sehr schnell Gedächtnisspuren im Hippokampus entstehen, welche im darauffolgenden Tiefschlaf reaktiviert werden, um so eine langfristig stabile Gedächtnisspur zu erzeugen. Konzeptzellen im Schläfenlappen des Menschen sind Nervenzellen, die auf den semantischen Inhalt eines Stimulus selektiv und semantisch invariant reagieren. Konzeptzellen antworten auf Abbildungen ihres präferierten Konzepts (zum Beispiel einer Person, eines Tieres oder eines Objekts) oder auf geschriebene und gesprochene Wörter, die das gleiche Konzept darstellen, unabhängig von den speziellen Eigenschaften des Stimulus, wie zum Beispiel Bildgröße oder -farbe. Auf jedes Konzept reagiert dabei nur ein sehr kleiner Teil dieser Zellen. Man vermutet, dass Konzeptzellen Bausteine des episodischen Gedächtnisses sind. Die vorliegende Studie nutzt Aufzeichnungen der Aktivität einzelner Konzeptzellen während ganzer Nächte, um zu untersuchen, inwiefern die koordinierte Aktivität von Konzeptzellen im Schlaf ein neurophysiologisches Korrelat der Gedächtniskonsolidierung darstellt. Die Teilnehmer der Studie waren Epilepsiepatienten, in deren mediale Schläfenlappen aus klinischen Gründen Tiefenelektroden zur Anfallsaufzeichnung implantiert worden waren. Zur Analyse der Daten wurde zunächst eine Software entwickelt, die eine Artefaktbereinigung und das Spike-Sorting von neuronalen Langzeitaufzeichnungen leistet. Diese Software zeigte deutliche Vorteile gegenüber vorhandenen Methoden, und zwar sowohl in Tests mit simulierten Modelldatensätzen als auch im Falle tatsächlicher Aufzeichnungen (hier Experimente, in denen visuelle Stimuli auf einem Laptop dargestellt wurden). Ausgehend von einer Analogie zwischen Ortszellen ("place cells") bei Nagetieren und Konzeptzellen bei Menschen wurde ein Experiment entwickelt, das episodisches Gedächtnis operationalisierte. Darin lernten die Teilnehmer eine kurze Geschichte auswendig, was sequentielle Aktivität von Konzeptzellen auslöste. Konzeptzellen zeigten ein stabiles Antwortverhalten: am Abend und nächsten Morgen antworteten sie auf die gleichen Stimuli. Konzeptzellen im Hippokampus hatten im Mittel im Rapid-Eye-Movement-Schlaf (REM-Schlaf) niedrigere Feuerraten als während Wachheit. Im Tiefschlaf unterschieden sich die Feuerraten nicht signifikant von Wachheit. Die Aktivität der Konzeptzellen war während "ripples" im lokalen Feldpotential erhöht, und Konzeptzellen, deren präferierte Stimuli in der erinnerten Geschichte auftauchten, feuerten im darauffolgenden Schlaf gemeinsam, ein Effekt, der im Tiefschlaf besonders ausgeprägt war. Die Kreuzkorrelationen von Konzeptzellen waren im Tiefschlaf asymmetrischer als während Wachheit und REM-Schlaf, und die typischen Zeitabstände des Feuerns von Konzeptzellen lagen in einem Bereich, der als relevant für "spike-timing-dependent plasticity" gilt. Die Zeitabstände waren jedoch nicht mit dem Abstand der präferierten Stimuli in der erinnerten Geschichte korreliert. Diese Befunde stützen die Theorie, dass die Aktivität von Konzeptzellen während des Lernens instantan eine Gedächtnisspur erzeugt, und dass die Reaktivierung der gleichen Nervenzellen im Tiefschlaf nach dem Lernen zur Konsolidierung der Gedächtnisinhalte beiträgt. Die zeitliche Reihenfolge von Ereignissen wird offenbar im menschlichen Gehirn nicht auf die Weise konsolidiert, die sich aus der Forschung an Nagetieren nahelegte

Individual accessibility and segregation on activity spaces: an agent-based modelling approach

One of the main challenges of cities is the increasing social inequality imposed by the way population groups, jobs, amenities and services, as well as the transportation infrastructure, are distributed across urban space. In this thesis, the concepts of accessibility and segregation are used to study these inequalities. They can be defined as the interaction of individuals with urban opportunities and with individuals from other population groups, respectively. Interactions are made possible by people’s activities and movement within a city, which characterise accessibility and segregation as inherently dynamic and individual-based concepts. Nevertheless, they are largely studied from a static and place-based perspective. This thesis proposes an analytical and exploratory framework for studying individual-based accessibility and segregation in cities using individuals’ travel trajectories in space and time. An agent-based simulation model was developed to generate individual trajectories dynamically, employing standard datasets such as census and OD matrices and allowing for multiple perspectives of analysis by grouping individuals based on their attributes. The model’s ability to simulate people’s trajectories realistically was validated through systematic sensitivity tests and statistical comparison with real-world trajectories from Rio de Janeiro, Brazil, and travel times from London, UK. The approach was applied to two exploratory studies: São Paulo, Brazil, and London, UK. The first revealed inequalities in accessibility by income, education and gender and also unveiled within-group differences beyond place-based patterns. The latter explored ethnic segregation, unveiling patterns of potential interaction among ethnic groups in the urban space beyond their residential and workplace locations. Those studies demonstrated how inequality in accessibility and segregation can be studied both at large metropolitan scales and at fine level of detail, using standard datasets, with modest computational requirements and ease of operationalisation. The proposed approach opens up avenues for the study of complex dynamics of interaction of urban populations in a variety of urban contexts

Non-photorealistic rendering: a critical examination and proposed system.

In the first part of the program the emergent field of Non-Photorealistic Rendering is explored from a cultural perspective. This is to establish a clear understanding of what Non-Photorealistic Rendering (NPR) ought to be in its mature form in order to provide goals and an overall infrastructure for future development. This thesis claims that unless we understand and clarify NPR's relationship with other media (photography, photorealistic computer graphics and traditional media) we will continue to manufacture "new solutions" to computer based imaging which are confused and naive in their goals. Such solutions will be rejected by the art and design community, generally condemned as novelties of little cultural worth ( i.e. they will not sell). This is achieved by critically reviewing published systems that are naively described as Non-photorealistic or "painterly" systems. Current practices and techniques are criticised in terms of their low ability to articulate meaning in images; solutions to this problem are given. A further argument claims that NPR, while being similar to traditional "natural media" techniques in certain aspects, is fundamentally different in other ways. This similarity has lead NPR to be sometimes proposed as "painting simulation" — something it can never be. Methods for avoiding this position are proposed. The similarities and differences to painting and drawing are presented and NPR's relationship to its other counterpart, Photorealistic Rendering (PR), is then delineated. It is shown that NPR is paradigmatically different to other forms of representation — i.e. it is not an "effect", but rather something basically different. The benefits of NPR in its mature form are discussed in the context of Architectural Representation and Design in general. This is done in conjunction with consultations with designers and architects. From this consultation a "wish-list" of capabilities is compiled by way of a requirements capture for a proposed system. A series of computer-based experiments resulting in the systems "Expressive Marks" and 'Magic Painter" are carried out; these practical experiments add further understanding to the problems of NPR. The exploration concludes with a prototype system "Piranesi" which is submitted as a good overall solution to the problem of NPR. In support of this written thesis are : - • The Expressive Marks system • Magic Painter system • The Piranesi system (which includes the EPixel and Sketcher systems) • A large portfolio of images generated throughout the exploration