1,184 research outputs found

    Constrained Monte Carlo Method and Calculation of the Temperature Dependence of Magnetic Anisotropy

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    We introduce a constrained Monte Carlo method which allows us to traverse the phase space of a classical spin system while fixing the magnetization direction. Subsequently we show the method's capability to model the temperature dependence of magnetic anisotropy, and for bulk uniaxial and cubic anisotropies we recover the low-temperature Callen-Callen power laws in M. We also calculate the temperature scaling of the 2-ion anisotropy in L10 FePt, and recover the experimentally observed M^2.1 scaling. The method is newly applied to evaluate the temperature dependent effective anisotropy in the presence of the N'eel surface anisotropy in thin films with different easy axis configurations. In systems having different surface and bulk easy axes, we show the capability to model the temperature-induced reorientation transition. The intrinsic surface anisotropy is found to follow a linear temperature behavior in a large range of temperatures

    A guide to finance for Social Enterprises in South Africa

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    The global economic crisis and its aftermath deepen the challenge of decent employment creation. The Global Jobs Pact developed in response to the crisis sets out a framework that ensures linkages between social progress and economic development. In this context, there is increasing interest in the social economy as a way to combine social and economic goals. The Ouagadougou Symposium on the Global Jobs Pact as it relates to Africa included a recommendation to increase support to the social economy. A regional conference in October 2009 on the social economy as a response to the economic crisis in Africa defined the social economy as "a concept designating enterprises and organizations, in particular cooperatives, mutual benefit societies, associations, foundations and social enterprises, which have the specific feature of producing goods, services and knowledge while pursuing both economic and social aims and fostering solidarity.

    Supplementary Motor Area Encodes Reward Expectancy in Eye-Movement Tasks

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    Neural activity signifying the expectation of reward has been found recently in many parts of the brain, including midbrain and cortical structures. These signals can facilitate goal-directed behavior or the learning of new skills based on reinforcements. Here we show that neurons in the supplementary motor area (SMA), an area concerned with movements of the body and limbs, also carry a reward expectancy signal in the postsaccadic period of oculomotor tasks. While the monkeys performed blocks of memory-guided and object-based saccades, the neurons discharged a burst after a ∼200-ms delay following the target-acquiring saccade in the memory task but often fired concurrently with the target-acquiring saccade in the object task. The hypothesis that this postsaccadic bursting activity reflects the expectation of a reward was tested with a series of manipulations to the memory-guided saccade task. It was found that although the timing of the bursting activity corresponds to a visual feedback stimulus, the visual feedback is not required for the neurons to discharge a burst. Second, blocks of no-reward trials reveal an extinction of the bursting activity as the monkeys come to understand that they would not be rewarded for properly generated saccades. Finally, the delivery of unexpected rewards confirmed that in many of the neurons, the activity is not related to a motor plan to acquire the reward (e.g., licking). Thus we conclude that reward expectancy is represented by the activity of SMA neurons, even in the context of an oculomotor task. These results suggest that the reward expectancy signal is broadcast over a large extent of motor cortex, and may facilitate the learning of new, coordinated behavior between different body parts

    Representation of Object-Centered Space by Neurons of the Supplementary Eye Field

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    The supplementary eye field (SEF) is a region of cortex located on the dorsomedial shoulder of the frontal lobe, considered to be involved in the control of eye movements. SEF neurons show spatially selective activity during visually- and memory-guided saccades. The selectivity exhibited by SEF neurons has been described as being related to an eye- or head-centered reference frame. We have previously shown that SEF neurons exhibit selectivity in an object-centered reference frame: neurons will fire selectively when saccades are directed to one end of a bar or another, irrespective of the absolute location of the bar in space.It is not well known how SEF neurons display selectivity for object-centered locations. In order to better understand the mechanism of this phenomenon, we performed three studies. In the first study, we asked how SEF neurons encode locations in both egocentric and object-centered reference frames. We recorded from single SEF neurons while monkeys performed tasks requiring spatial representation in either eye-centered or object-centered reference frames. Different SEF neurons encoded locations in eye-centered coordinates only, object-centered coordinates only, or in complex combinations of the two.In the second study, we tested whether object-centered selectivity is an innate property of SEF neurons or whether it is acquired through learning. We recorded the activity of SEF neurons before and after training monkeys to perform an object-centered task. Some SEF neurons exhibited object-centered selectivity before training. Following training, this number was increased, as was the intensity of object-centered spatial selectivity.In the third study, we investigated whether the object-centered selectivity seen in SEF neurons during performance of an object-centered task is reduced during performance of a non-object-centered task. We recorded from SEF neurons while monkeys performed either an object-centered task or a color matching task with an object as a target. An equivalent number of neurons showed object-centered selectivity in both tasks, but the strength of selectivity was slightly higher during performance of the object-centered task. We conclude from the results of these studies that neurons in the SEF are critically involved in the dynamic representation of locations using multiple spatial reference frames

    Microstructural quantification of collagen fiber orientations and its integration in constitutive modeling of the porcine carotid artery

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    Background Mechanical characteristics of vascular tissue may play a role in different arterial pathologies, which, amongst others, requires robust constitutive descriptions to capture the vessel wall’s anisotropic and non-linear properties.Specifically, the complex 3D network of collagen and its interaction with other structural elements has a dominating effect of arterial properties at higher stress levels.The aim of this study is to collect quantitative collagen organization as well as mechanical properties to facilitate structural constitutive models for the porcine carotid artery.This helps the understanding of the mechanics of swine carotid arteries, being a standard in clinical hypothesis testing, in endovascular preclinical trials for example. Method Porcine common carotid arteries (n = 10) were harvested and used to (i) characterize the collagen fiber organization with polarized light microscopy, and (ii) the biaxial mechanical properties by inflation testing.The collagen organization was quantified by the Bingham orientation density function (ODF), which in turn was integrated in a structural constitutive model of the vessel wall.A one-layered and thick-walled model was used to estimate mechanical constitutive parameters by least-square fitting the recorded in vitro inflation test results.Finally, uniaxial data published elsewhere were used to validate the mean collagen organization described by the Bingham ODF. Results Thick collagen fibers, i.e.the most mechanically relevant structure, in the common carotid artery are dispersed around the circumferential direction.In addition, almost all samples showed two distinct families of collagen fibers at different elevation, but not azimuthal, angles.Collagen fiber organization could be accurately represented by the Bingham ODF (¿1,2,3=[13.5,0.0,25.2] and ¿1,2,3=[14.7,0.0,26.6]; average error of about 5%), and their integration into a structural constitutive model captured the inflation characteristics of individual carotid artery samples.Specifically, only four mechanical parameters were required to reasonably (average error from 14% to 38%) cover the experimental data over a wide range of axial and circumferential stretches.However, it was critical to account for fibrilar links between thick collagen fibers.Finally, the mean Bingham ODF provide also good approximation to uniaxial experimental data. Conclusions The applied structural constitutive model, based on individually measured collagen orientation densities, was able to capture the biaxial properties of the common carotid artery. Since the model required coupling amongst thick collagen fibers, the collagen fiber orientations measured from polarized light microscopy, alone, seem to be insufficient structural information. Alternatively, a larger dispersion of collagen fiber orientations, that is likely to arise from analyzing larger wall sections, could have had a similar effect, i.e. could have avoided coupling amongst thick collagen fibers.Peer ReviewedPostprint (author's final draft

    Journal of South Carolina Water Resources Volume 6, Issue 1

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    The Journal of South Carolina Water Resources (JSCWR) is an annual peer-reviewed journal dedicated to scientific research and policy on all aspects of water management to prepare for and meet the growing challenge of providing water resources for the sustainable growth of South Carolina’s economy, while preserving its natural resources

    Contextual control of orienting eye-head gaze shifts in the monkey

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    Vision is one of the principal methods used by primates to acquire information about the surrounding environment. As a result, both humans and monkeys have a highly evolved oculomotor system that functions to rapidly relocate the line of sight to areas of interest. These orienting movements are called gaze shifts. Gaze shifts commonly include the coordinated movement of the eyes-in-head and the head-in-space. This thesis examines the muscular and neural control of orienting head movements. The contextual control of behavior is important as it allows one to act appropriately in response to different situations. A common task used to examine the contextual control of behavior is the pro- and anti-saccade task. Pro-saccades simply require a subject to look towards a stimulus. Anti-saccades require a subject to inhibit a movement towards a stimulus in favor of a volitional movement to the diametrically opposite position. This task is can reveal capabilities of the oculomotor system and its response to varying behavioral states. To understand the neuromuscular control of orienting head movements during various tasks, we recorded the electromyographic (EMG) activity in ten turner and extensor neck muscles. Recording neck EMGs provides an objective and precise measurement of the neural signals received at the neck muscles, circumventing some of the structural and biomechanical complexities of head motion. Chapter two examines neck muscle activity in a pro- and anti-saccade task. Many neural areas and certain neck muscles become active in response to the presentation of a visual stimulus. This visual response on the neck muscles can result in a head turning synergy that orients the head towards the stimulus. By dissociating the typical stimulus-response paradigm, we can analyze if and how the bottom-up visual activity changes in relation to different contexts. A number of cortical and subcortical areas are involved in the generation of correct anti-saccades. By combining EMG recordings while subjects perform this task, we can examine whether top-down task-related activity is present in the neck muscles. This experiment could reveal flexibility in the eye-head gaze shift system that has previously gone unreported. Chapter three will elucidate the supplementary eye fields (SEF) role in the control of orienting eye-head gaze shifts. Neck EMG activity was recorded while providing electrical microstimulation to the SEF in a pro-saccade task The combination of EMGs and SEF stimulation is the first to systematically study the cephalomotor command during head-restrained and head-unrestrained orienting eye-head gaze shifts. The evoked activity of EMGs could reveal functional properties of the neural circuitry between the SEF and the motor related neurons responsible for eye and head movements. The timing and metrics of evoked EMG activity and eye-head gaze shifts are consistent with other frontal areas suggesting a functional role of the frontal cortex in influencing eye-head gaze shifts. Chapter four will combine EMG recordings with SEF stimulation during a pro- and anti-saccade task. The SEF is thought to serve as an interface between high-level cognitive control of gaze shifts and low-level activity associated with the production of saccades. As will be described later in the thesis, neck muscles demonstrate top-down task related activity during anti-saccades. The SEF is a likely candidate for the generation of task-dependent signals observed during anti-saccades. By combining SEF stimulation and neck EMGs in an anti-saccade task, we can reveal if neck muscle activity is consistent with a role for the SEF in the contextual control of eye-head gaze shifts. In summary, this thesis identifies three central point’s concerning orienting eye-head gaze shifts. First, chapter two emphasizes the complex interaction of sensori-motor processes in orienting head movements. Second, chapter three attests to the consistent nature of certain areas in frontal cortex and their impact on eye-head gaze shifts. Finally, chapter four demonstrates a potential candidate for influencing the contextual control of cephalomotor commands. Combined, these results highlight the complex interactions of sensori-motor transformations in the motor periphery and emphasize the parallel nature of information processing during the contextual control of eye-head gaze shifts
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