Finite sample properties of estimators of spatial autoregressive models with autoregressive disturbances

The article investigates the finite sample properties of estimators for spatial autoregressive models where the disturbance terms may follow a spatial autoregressive process. In particular we investigate the finite sample behavior of the feasible generalized spatial two-stage least squares (FGS2SLS) estimator introduced by Kelejian and Prucha (1998), the maximum likelihood (ML) estimator, as well as that of several other estimators. We find that the FGS2SLS estimator is virtually as efficient as the ML estimator. This is important because the ML estimator is computationally burdensome, and may even be forbidding in large samples, while the FGS2SLS estimator remains computationally feasible in large samples. Received: 20 January 2001 / Accepted: 31 August 2001     

Acute Weston Hurst necrotizing hemorrhagic leukoencephalitis

The clinical and pathological findings of a 43-year-old woman, diagnosed as having acute hemorrhagic leukoencephalitis at postmortem examination, are presented. The acute hemorrhagic leukoencephalitis affects mainly young adults and is the most fulminant from of demyelinating disease. It is frequently preceded by a respiratory infection. Diagnosis is facilitated by CT scanning and MRI, which reveal the massive lesion in the cerebral white matter. Many cases terminate fatally in 2 or 4 days, but in others survival is longer. The pathological findings are distinctive.     

Adaptive training interfaces for less-experienced,elderly users of electronic devices

A great number of complex electronic devices are now part of our everyday lives. While many of us learn to handle these products by trial and error; others, especially older users with little experience in using electronic devices, need support. In order to allow the user maximum flexibility in terms of learning time and location, a training programme is presented which is implemented as part of the software embedded in the product itself. Particular focus is placed on the effect of adaptive training on learning. In this study, the training versions differed in their ability to adjust their complexity to the user's experience (adaptive user interface complexity) and their capability to support the learner by prompting them during the learning process (adaptive training advice). The results show that the adjustment of complexity had a positive effect on users’ experience: elderly users who trained with an adaptive interface were more successful in learning to use a mobile phone. Adaptive training advice, however, was found to have no significant effects on learners’ success and reduced their self-efficacy. This work offers guidelines on how to design integrated training applications for electronic devices that successfully help elderly users with little prior experience.     

Natural perspective projections for head-mounted displays

The display units integrated in today's head-mounted displays (HMDs) provide only a limited field of view (FOV) to the virtual world. In order to present an undistorted view to the virtual environment (VE), the perspective projection used to render the VE has to be adjusted to the limitations caused by the HMD characteristics. In particular, the geometric field of view (GFOV), which defines the virtual aperture angle used for rendering of the 3D scene, is set up according to the display field of view (DFOV). A discrepancy between these two fields of view distorts the geometry of the VE in a way that either minifies or magnifies the imagery displayed to the user. It has been shown that this distortion has the potential to affect a user's perception of the virtual space, sense of presence, and performance on visual search tasks. In this paper, we analyze the user's perception of a VE displayed in a HMD, which is rendered with different GFOVs. We introduce a psychophysical calibration method to determine the HMD's actual field of view, which may vary from the nominal values specified by the manufacturer. Furthermore, we conducted two experiments to identify perspective projections for HMDs, which are identified as natural by subjects--even if these perspectives deviate from the perspectives that are inherently defined by the DFOV. In the first experiment, subjects had to adjust the GFOV for a rendered virtual laboratory such that their perception of the virtual replica matched the perception of the real laboratory, which they saw before the virtual one. In the second experiment, we displayed the same virtual laboratory, but restricted the viewing condition in the real world to simulate the limited viewing condition in a HMD environment. We found that subjects evaluate a GFOV as natural when it is larger than the actual DFOV of the HMD--in some cases up to 50 percent--even when subjects viewed the real space with a limited field of view.     

Estimation of Detection Thresholds for Redirected Walking Techniques

In immersive virtual environments (IVEs), users can control their virtual viewpoint by moving their tracked head and walking through the real world. Usually, movements in the real world are mapped one-to-one to virtual camera motions. With redirection techniques, the virtual camera is manipulated by applying gains to user motion so that the virtual world moves differently than the real world. Thus, users can walk through large-scale IVEs while physically remaining in a reasonably small workspace. In psychophysical experiments with a two-alternative forced-choice task, we have quantified how much humans can unknowingly be redirected on physical paths that are different from the visually perceived paths. We tested 12 subjects in three different experiments: (E1) discrimination between virtual and physical rotations, (E2) discrimination between virtual and physical straightforward movements, and (E3) discrimination of path curvature. In experiment E1, subjects performed rotations with different gains, and then had to choose whether the visually perceived rotation was smaller or greater than the physical rotation. In experiment E2, subjects chose whether the physical walk was shorter or longer than the visually perceived scaled travel distance. In experiment E3, subjects estimate the path curvature when walking a curved path in the real world while the visual display shows a straight path in the virtual world. Our results show that users can be turned physically about 49 percent more or 20 percent less than the perceived virtual rotation, distances can be downscaled by 14 percent and upscaled by 26 percent, and users can be redirected on a circular arc with a radius greater than 22 m while they believe that they are walking straight.     

Zyklisches Werkstoffverhalten sprühkompaktierter Aluminiumwerkstoffe. Kennwerte für die Bauteilbemessung: Von der Probe zum Bauteil

The fatigue behaviour of spray compacted aluminium materials. Material properties for component design data: From specimen to component. Spray compacted, high performance aluminium alloys (DISPAL = DISpersion hardened Aluminium) are characterised by their high strength, high Young’s modulus, good wear resistance and low coefficient of thermal expansion. These properties vindicate both the application of these materials and the increasing interest of the automobile manufacturers. Within the framework of a bilateral project, tests were carried out by varying different parameters under both strain‐ and load‐controlled conditions in order to describe the fatigue behaviour of these materials.     

Visibility of the Aharonov–Bohm Effect in a Ring Coupled to a Fluctuating Magnetic Flux

We consider the visibility of the Aharonov–Bohm effect for cotunneling transport through a clean one-channel ring coupled to a fluctuating magnetic flux. We concentrate on the modification of the destructive interference at ₀ /2 by the fluctuating flux, since changes in the magnitude of the current away from this point can also be caused by renormalization effects and do not necessarily indicate dephasing. For fluctuations arising from the Nyquist noise in an external coil at T=0, the suppression of the destructive interference shows up only in a contribution proportional to V ³ , and therefore does not affect the linear conductance. In this sense, the Nyquist bath does not lead to dephasing in the linear transport regime at zero temperature in our model.     

High-speed path-length scanning with a multiple-pass cavity delay line

Techniques for high-speed delay scanning are important for low-coherence interferometry, optical coherence tomography, pump probe measurements, and other applications. We demonstrate a novel scanning delay line using a multiple-pass cavity. Differential delays are accumulated with each pass so that millimeter delays can be generated with tens of micrometer mirror displacements. With special design criteria, misalignment sensitivity can be dramatically reduced. The system is demonstrated to scan 6 m/s at 2-kHz repetition rates. Real-time optical coherence tomography imaging with 500 pixel images at four frames/s is performed. Using a Cr:forsterite laser source, we obtained axial image resolutions of 6 microm with 92-dB sensitivity.     

Benecol®—一种有效、安全降低血清胆固醇水平的独特食品配料

在世界范围内,心血管疾病——冠心病(CHD)、中风、周围动脉疾病的患病数量正在增加.据世界卫生组织(WHO)统计估计,每年大约有1600万人死于各种形式的心血管疾病.高血清胆固醇是导致各种心血管疾病的主要危险因素之一.近来,通过饮食方法如功能食品来降低胆固醇水平已经引起了极大关注.植物甾醇和植物甾烷醇是动物胆固醇的植物等价物,因为它们在植物细胞中的作用与胆固醇在动物细胞中的作用是一样的.如果在日常的饮食当中含有足量的植物甾醇或植物甾烷醇,则它们可以有效地降低胃肠道中对胆固醇的吸收量,从而降低血清胆固醇浓度.植物甾烷醇酯是世界上第一个基于植物甾醇的降低胆固醇的商业食品配料.1995年芬兰首次生产出添加植物甾烷醇酯的降胆固醇食品.如今,在全世界的24个国家销售Benecol品牌的各种植物甾烷醇酯强化食品,包括植物黄油、酸奶、牛奶、橙汁、通心粉等.Benecol食品含有植物甾烷醇酯,专门针对想通过饮食方法降低血清胆固醇水平的人群.植物甾烷醇酯的安全性已经得到了严格的安全性测试,并得到了40多项人体临床研究的证实.国际专业团体已经充分肯定了植物甾烷醇酯食品的降低胆固醇的功效.