Kinematically Redundant Octahedral Motion Platform for Virtual Reality Simulations

We propose a novel design of a parallel manipulator of Stewart Gough type for virtual reality application of single individuals; i.e. an omni-directional treadmill is mounted on the motion platform in order to improve VR immersion by giving feedback to the human body. For this purpose we modify the well-known octahedral manipulator in a way that it has one degree of kinematical redundancy; namely an equiform reconfigurability of the base. The instantaneous kinematics and singularities of this mechanism are studied, where especially "unavoidable singularities" are characterized. These are poses of the motion platform, which can only be realized by singular configurations of the mechanism despite its kinematic redundancy.Comment: 13 pages, 6 figure

The Dynamical Implications of Multiple Stellar Formation Events in Galactic Globular Clusters

Various galactic globular clusters display abundance anomalies that affect the morphology of their colour-magnitude diagrams. In this paper we consider the possibility of helium enhancement in the anomalous horizontal branch of NGC 2808. We examine the dynamics of a self-enrichment scenario in which an initial generation of stars with a top-heavy initial mass function enriches the interstellar medium with helium via the low-velocity ejecta of its asymptotic giant branch stars. This enriched medium then produces a second generation of stars which are themselves helium-enriched. We use a direct N-body approach to perform five simulations and conclude that such two-generation clusters are both possible and would not differ significantly from their single-generation counterparts on the basis of dynamics. We find, however, that the stellar populations of such clusters would differ from single-generation clusters with a standard initial mass function and in particular would be enhanced in white dwarf stars. We conclude, at least from the standpoint of dynamics, that two-generation globular clusters are feasible.Comment: 24 pages, 7 figures, 3 tables. Accepted for publication in Ap

Sospechas y evidencia: revisión de las causas de la extinción local y del declive de los mejillones de agua dulce

Sospechas y evidencia: revisión de las causas de la extinción local y del declive de los mejillones de agua dulce La conservación de la biodiversidad requiere pruebas fiables de las causas de extinción local. Utilizando los mejillones de agua dulce como ejemplo, llevamos a cabo esta valoración sistemática, la primera que se ha realizado, de la evidencia de peligro para cualquier grupo de organismos. Revisamos artículos que publicaban conclusiones sobre las causas de las extinciones locales, evaluando la calidad de las pruebas según una escala objetiva. Encontramos que únicamente el 48% de los estudios presentaban relaciones plausibles entre la extinción local y sus causas. Los análisis carecían de resolución, dado que más del 75% de los estudios considerados (n = 124) sugerían múltiples causas de extinción local. Los estudios llevados a cabo en grandes áreas, y los que presentaban menos pruebas, son los que abogaban por un mayor número de causas. A pesar de las evidencias, que frecuentemente eran débiles, existía un acuerdo sustancial sobre la identidad de las causas; la más frecuente era la destrucción o alteración del hábitat, pero se postulaban muchas más. A pesar de que la extinción local de los mejillones de agua dulce es indudablemente una realidad, las pruebas podrían ser más consistentes. En estos animales y en muchos otros, la evidencia de las causas de su extinción local a menudo ha sido circunstancial. Presentamos aquí un estudio sistemático que pueden utilizar los ecólogos, para fortalecer las evidencias concernientes a las causas de las extinciones locales. También hemos reflejado la relación entre la fortaleza de la evidencia y las prioridades económicas de las investigaciones. Palabras clave: Evidencia, Extinción, Extinción local, Agua dulce, Mejillones.Conservation of biodiversity requires reliable evidence of the causes of extirpation. Using freshwater mussels as an example, we performed the first–ever systematic assessment of the evidence for endangerment of any group of organisms. We surveyed articles publishing conclusions about the cause of local extirpation by assessing the quality of evidence on an objective scale. We found that only 48% of studies presented plausible links between extirpation and causes. Analyses lacked resolution since more than 75% of all studies considered (n = 124) suggested multiple causes of extirpation. Studies performed over large areas and those presenting less evidence postulated the most causes. Despite the frequently weak evidence, there was substantial agreement on the identity of causes; the most frequent was habitat destruction or alteration but many others were postulated. Although mussel extirpation is undoubtedly real, the evidence could be stronger. In these animals and others, evidence of the causes of extirpation has often been circumstantial. We present a systematic approach ecologists can use to strengthen the evidence concerning the causes of extirpation. We also reflect on the link between the strength of evidence and research funding priorities. Key words: Evidence, Extinction, Extirpation, Freshwater, Mussels.La conservación de la biodiversidad requiere pruebas fiables de las causas de extinción local. Utilizando los mejillones de agua dulce como ejemplo, llevamos a cabo esta valoración sistemática, la primera que se ha realizado, de la evidencia de peligro para cualquier grupo de organismos. Revisamos artículos que publicaban conclusiones sobre las causas de las extinciones locales, evaluando la calidad de las pruebas según una escala objetiva. Encontramos que únicamente el 48% de los estudios presentaban relaciones plausibles entre la extinción local y sus causas. Los análisis carecían de resolución, dado que más del 75% de los estudios considerados (n = 124) sugerían múltiples causas de extinción local. Los estudios llevados a cabo en grandes áreas, y los que presentaban menos pruebas, son los que abogaban por un mayor número de causas. A pesar de las evidencias, que frecuentemente eran débiles, existía un acuerdo sustancial sobre la identidad de las causas; la más frecuente era la destrucción o alteración del hábitat, pero se postulaban muchas más. A pesar de que la extinción local de los mejillones de agua dulce es indudablemente una realidad, las pruebas podrían ser más consistentes. En estos animales y en muchos otros, la evidencia de las causas de su extinción local a menudo ha sido circunstancial. Presentamos aquí un estudio sistemático que pueden utilizar los ecólogos, para fortalecer las evidencias concernientes a las causas de las extinciones locales. También hemos reflejado la relación entre la fortaleza de la evidencia y las prioridades económicas de las investigaciones. Palabras clave: Evidencia, Extinción, Extinción local, Agua dulce, Mejillones

Radiation-Resistant Solar Cells - A Panel Discussion

Radiation resistant silicon cells for solar energy conversio

Evidencing a place for the hippocampus within the core scene processing network

Functional neuroimaging studies have identified several “core” brain regions that are preferentially activated by scene stimuli, namely posterior parahippocampal gyrus (PHG), retrosplenial cortex (RSC), and transverse occipital sulcus (TOS). The hippocampus (HC), too, is thought to play a key role in scene processing, although no study has yet investigated scene-sensitivity in the HC relative to these other “core” regions. Here, we characterised the frequency and consistency of individual scene-preferential responses within these regions by analysing a large dataset (n = 51) in which participants performed a one-back working memory task for scenes, objects, and scrambled objects. An unbiased approach was adopted by applying independently-defined anatomical ROIs to individual-level functional data across different voxel-wise thresholds and spatial filters. It was found that the majority of subjects had preferential scene clusters in PHG (max = 100% of participants), RSC (max = 76%), and TOS (max = 94%). A comparable number of individuals also possessed significant scene-related clusters within their individually defined HC ROIs (max = 88%), evidencing a HC contribution to scene processing. While probabilistic overlap maps of individual clusters showed that overlap “peaks” were close to those identified in group-level analyses (particularly for TOS and HC), inter-individual consistency varied across regions and statistical thresholds. The inter-regional and inter-individual variability revealed by these analyses has implications for how scene-sensitive cortex is localised and interrogated in functional neuroimaging studies, particularly in medial temporal lobe regions, such as the H

Cell proliferation and differentiation kinetics during spermatogenesis in Hydra carnea

Spermatogenesis inHydra carnea was investigated. The cell proliferation and differentiation kinetics of intermediates in the spermatogenesis pathway were determined, using quantitative determinations of cell abundance, pulse and continuous labelling with3H-thymidine and nuclear DNA measurements. Testes develop in the ectoderm of male hydra as a result of interstitial cell proliferation. Gonial stem cells and proliferating spermatogonia have cell cycles of 28 h and 22 h, respectively. Stem cells undergo four, five or six cell divisions prior to meiosis which includes a premeiotic S+G2 phase of 20 h followed by a long meiotic prophase (22 h). Spermatid differentiation requires 12–29 h. When they first appear, testes contain only proliferating spermatogonia; meiotic and postmeiotic cells appear after 2 and 3 days, respectively and release of mature sperm begins after 4 days. Mature testes produce about 27,000 sperm per day over a period of 4–6 days: about 220 gonial stem cells per testis are required to support this level of sperm differentiation. Further results indicate that somatic (e.g. nematocyte) differentiation does not occur in testes although it continues normally in ectodermal tissue outside testes. Our results support the hypothesis that spermatogenesis is controlled locally in regions of the ectoderm where testes develop