Developing visual systems and exposure to virtual reality and stereo displays: some concerns and speculations about the demands on accommodation and vergence

Little is known about the developmental plasticity of the vergence and accommodative systems, an important issue since abnormalities can lead to visual problems, e.g. strabismus. One way of artificially altering the links between accommodation and vergence is to vary the stimulus to vergence while fixing the accommodative stimulus, as is found in virtual reality displays. While it would be of interest to study developmental plasticity in this situation, since many children are exposed to games machines which use this arrangement, no studies to date have tackled this issue. There is, however, some indication that long-term VR viewing in adults can lead to visual problems. It seems important to determine the safety of these systems for the developing human visual system before they come into common use. In this paper, adaptation of the accommodation and vergence systems and the effect of VR viewing in adults is discussed. The sparse literature on adaptation in children is then reviewed, and suggestions made for approaches that would enhance our knowledge of plasticity of accommodation and vergence in children.     

Polypyrrole Based Microwave Absorbers

Reflection of microwave radiations from single layer and two-layer materials is calculated. Microwave absorbing materials are formulated by mixing a commercially available paint or rubber with the conducting polypyrrole (PPy) powder. The reflection loss strongly depends on thickness and complex permittivity of the material. For a single layer material, optimum values of the real part, , and imaginary part, , of the complex permittivity are found by calculations which lead to a minimum reflectivity at a given sample thickness. The ability to readily tailor the conductivity of the PPy powder enables the design of microwave absorbers according to theoretical desired values of and . A paint panel containing 2 wt% of PPy powder with a thickness of 2.5 mm exhibits a reflectivity < – 10 dB (i.e. at least 90% absorption of the incident radiation) over 12 to 18 GHz. Blending and milling during the manufacturing process can destroy the original fibrous shape of PPy aggregates leading to low radiation absorption. In an attempt to achieve a broadband absorber, a two-layer system consisting of a first layer containing PPy powder and a second layer containing carbonyl iron has been fabricated.     

The approximate inverse and conjugate gradient: non-symmetrical algorithms for fast attenuation correction in SPECT

Hybrid methods have been known for a long time as very efficient algorithms for attenuation correction in single-photon emission computed tomography, but only recently have efforts been made to formulate them with more rigorous mathematics. This has allowed us to explain their efficiency in terms of approximate inversion, and to establish a convergence condition. The present study focuses on the convergence problem and emphasizes the question of symmetry. Hybrid method operators are not symmetrical; therefore the convergence condition is not easily verified. New schemes based on a modified conjugate gradient method are presented. Convergence is proved and performances are shown to be at least as good as the standard hybrid schemes on perfect and noisy simulated data.     

Carbon-13 Variable Temperature NMR Studies of Some Dimethyl-1-oxa-3,4-diazacyclohexanes1

Conformational equilibria are elucidated and rationalised for 3,4-dimethyl-1-oxa-3,4-diazacyclohexane, and a trans-bicyclic analogue. The major conformer has 3ax-NMe, 4eq-NMe: two minor conformations are also significantly populated. Kinetic and thermodynamic parameters are correlated with those for analogous heterocycles.     

Topographical organization of group II afferent input in the rat spinal cord

Recent data suggest that patients with Alzheimer's disease (AD) are able to show perceptual priming and, to some extent, conceptual priming for material which has preexisting representations in memory, and that normal elderly subjects are able to automatically activate pre-existing representations in both perceptual and conceptual priming tasks. An important question concerns the capacity of showing priming for materials without pre-existing representations in memory in normal and pathological aging. In order to address this issue, 20 patients with mild AD, 20 elderly controls and 20 young controls subjects were assessed with a paradigm of priming for new verbal associations. Neither the patients nor the normal elderly subjects demonstrated priming effects for new associations, while young subjects showed significant priming effects. These results suggest that the absence of priming for new verbal associations is attributable more to an effect of aging than to a specific effect of Alzheimer's disease.     

Worldwide bioassay data resources for plutonium/americium internal dosimetry studies

Biokinetic models are the scientific underpinning of internal dosimetry and depend, ultimately, for their scientific validation on comparisons with human bioassay data. Three significant plutonium/americium bioassay databases, known to the authors, are described: (1) Sellafield, (2) Los Alamos and (3) the United States Transuranium Registry. A case is made for a uniform standard for database format, and the XML standard is discussed.     

Comparison of dose estimation from occupational exposure to 239Pu using different modelling approaches

Several approaches are available for bioassay interpretation when assigning Pu doses to Mayak workers. First, a conventional approach is to apply ICRP models per se. An alternative method involves individualised fitting of bioassay data using Bayesian statistical methods. A third approach is to develop an independent dosimetry system for Mayak workers by adapting ICRP models using a dataset of available bioassay measurements for this population. Thus, a dataset of 42 former Mayak workers, who died of non-radiation effects, with both urine bioassay and post-mortem tissue data was used to test these three approaches. All three approaches proved to be adequate for bioassay and tissue interpretation, and thus for Pu dose reconstruction purposes. However, large discrepancies are observed in the resulting quantitative dose estimates. These discrepancies can, in large part, be explained by differences in the interpretation of Pu behaviour in the lungs in the context of ICRP lung model. Thus, a careful validation of Pu lung dosimetry model is needed in Mayak worker dosimetry systems.     

Fatigue mechanisms of thermoplastics

Failure in fatigue by the mechanism of crack initiation and propagation originationg at flaws and stress concentrations has been fairly well established. It explains the fatigue phenomena in highly elastic materials, namely metals and certain thermosetting plastics such as reinforced polyester and epoxy resins. We have found, however, that a different mechanism dominates the fatigue behavior of thermoplastics. Specifically, the mechanism involves the generationa of heat within the material due to viscous damping or hysteresis. This paper presents fatigue, damping data and temperature measurements during cyclic stress to support this conclusion on three themoplastic resins of widely different mechanical properties, polyetrafluouoethylene (PTFE), Nylon 6, and polymethyl-methacrylate (PMMA). In addition to the usual S-N fatigue curves, we wil show how surface temperature changes with fatigue life and how this change is affected by stress, Frequency, crystallinity, specimen geometry and other parameters. Alos, the loss compliance of the materials will be presented as afunction of temperature to show the relationship of fatigue to damping properties.