Showing papers by "University of Houston published in 1999"

Yet another chaotic attractor

Abstract: This Letter reports the finding of a new chaotic at tractor in a simple three-dimensional autonomous system, which resembles some familiar features from both the Lorenz and Rossler at tractors.

Polymer-silicate nanocomposites : Model systems for confined polymers and polymer brushes

Abstract: The static and dynamic properties of polymer-layered silicate nanocomposites are discussed, in the context of polymers in confined spaces and polymer brushes. A wide range of experimental techniques as applied to these systems are reviewed, and the salient results from these are compared with a mean field thermodynamic model and non-equilibrium molecular dynamics simulations.

The arrangement of the three cone classes in the living human eye

Abstract: Human colour vision depends on three classes of receptor, the short- (S), medium- (M), and long- (L) wavelength-sensitive cones. These cone classes are interleaved in a single mosaic so that, at each point in the retina, only a single class of cone samples the retinal image. As a consequence, observers with normal trichromatic colour vision are necessarily colour blind on a local spatial scale1. The limits this places on vision depend on the relative numbers and arrangement of cones. Although the topography of human S cones is known2,3, the human L- and M-cone submosaics have resisted analysis. Adaptive optics, a technique used to overcome blur in ground-based telescopes4, can also overcome blur in the eye, allowing the sharpest images ever taken of the living retina5. Here we combine adaptive optics and retinal densitometry6 to obtain what are, to our knowledge, the first images of the arrangement of S, M and L cones in the living human eye. The proportion of L to M cones is strikingly different in two male subjects, each of whom has normal colour vision. The mosaics of both subjects have large patches in which either M or L cones are missing. This arrangement reduces the eye's ability to recover colour variations of high spatial frequency in the environment but may improve the recovery of luminance variations of high spatial frequency.

Prevention of secondary ischemic insults after severe head injury.

Abstract: Objective:The purpose of this study was to compare the effects of two acute-care management strategies on the frequency of jugular venous desaturation and refractory intracranial hypertension and on long-term neurologic outcome in patients with severe head injury.Design:Randomized clinical trial.Set

Persistence of dyslexia: the Connecticut Longitudinal Study at adolescence.

Abstract: Objective. The outcome in adolescence of children diagnosed as dyslexic during the early years of school was examined in children prospectively identified in childhood and continuously followed to young adulthood. This sample offers a unique opportunity to investigate a prospectively identified sample of adolescents for whom there is no question of the childhood diagnosis and in whom highly analytic measures of reading and language can be administered in adolescence. Design. Children were recruited from the Connecticut Longitudinal Study, a cohort of 445 children representative of those children entering public kindergarten in Connecticut in 1983. Two groups were selected when the children were in grade 9: children who met criteria for persistent reading disability in grades 2 through 6 (persistently poor readers [PPR]; n = 21) and a comparison group of nondisabled children, subdivided into average readers (n = 35) and superior readers (n = 39). In grade 9, each child received a comprehensive assessment of academic, language, and other cognitive skills. Results. Measures of phonological awareness (but not orthographic awareness) were most significant in differentiating the 3 reading groups, with smaller contributions from measures of word finding and digit-span. Academic measures that best separated good from poor readers were decoding and spelling, whereas measures of math and reading comprehension did not. Measures of phonological awareness, followed next by teacher rating of academic skills were the best predictors of decoding, reading rate, and reading accuracy. In contrast, the best predictor of reading comprehension was word finding, with digit span and socioeconomic status also contributing significantly. Using a growth curve model (quadratic model of growth to a plateau) all 3 groups demonstrated similar patterns of growth over time, with the superior group outperforming the average group, and the average group outperforming the PPR group. There was no evidence that the children in the PPR group catch up in their reading skills. Conclusions. Deficits in phonological coding continue to characterize dyslexic readers even in adolescence; performance on phonological processing measures contributes most to discriminating dyslexic and average readers, and average and superior readers as well. These data support and extend the findings of previous investigators indicating the continuing contribution of phonological processing to decoding words, reading rate, and accuracy and spelling. Children with dyslexia neither spontaneously remit nor do they demonstrate a lag mechanism for catching up in the development of reading skills. In adolescents, the rate of reading as well as facility with spelling may be most useful clinically in differentiating average from poor readers.

Uncertain Medical Expenses and Precautionary Saving Near the End of the Life Cycle

Abstract: This paper introduces a dynamic, structural model of household consumption decisions in which elderly families consider the effects of uncertain future medical expenses when deciding current levels of consumption. The model with uncertain medical expenses implies a potentially important role for precautionary saving incentives to explain slow rates of dissaving among elderly Americans during retirement. Rather than just simulating the stochastic dynamic model, preference parameters are estimated using panel data on health, wealth and expenditures for retired families. The health uncertainty model predicts consumption levels closer to observed expenditures than a life cycle model with uncertain longevity. However, elderly families typically dissave their financial assets more slowly than even the baseline health uncertainty model predicts is optimal.

Ganglion cell losses underlying visual field defects from experimental glaucoma

Abstract: PURPOSE. To investigate the relationship between ganglion cell losses and visual field defects caused by glaucoma. METHODS. Behavioral perimetry and histology data were obtained from 10 rhesus monkeys with unilateral experimental glaucoma that was induced by argon laser treatments to their trabecular meshwork. After significant visual field defects had developed, the retinas were collected for histologic analysis. The ganglion cells were counted by light microscopy in cresyl violet-stained retina sections, and the percentage of ganglion cell loss (treated to control eye counts) was compared with the depth of visual field defect (treated to control eye thresholds) at corresponding retinal and perimetry test locations. Sensitivity losses as a function of ganglion cell losses were analyzed for Goldmann III, white and Goldmann V, and short- and long-wavelength perimetry test stimuli. RESULTS. The relationship between the proportional losses of ganglion cells and visual sensitivity, measured with either white or colored stimuli, was nonlinear. With white stimuli, the visual sensitivity losses were relatively constant (approximately 6 dB) for ganglion cell losses of less than 30% to 50%, and then with greater amounts of cell loss the visual defects were more systematically related to ganglion cell loss (approximately 0.42 dB/percent cell loss). The forms of the neural-sensitivity relationships for visual defects measured with short- or long-wavelength perimetry stimuli were similar when the visual thresholds were normalized to compensate for differences in expected normal thresholds for white and colored perimetry stimuli. CONCLUSIONS. Current perimetry regimens with either white or monochromatic stimuli do not provide a useful estimate of ganglion cell loss until a substantial proportion have died. The variance in ganglion cell loss is large for mild defects that would be diagnostic of early glaucoma and for visual field locations near the fovea where sensitivity losses occur relatively late in the disease process. The neural-sensitivity relationships were essentially identical for both white and monochromatic test stimuli, and it therefore seems unlikely that the higher sensitivity for detecting glaucoma with monochromatic stimuli is based on the size-dependent susceptibility of ganglion cells to injury from glaucoma.

Eigenstructure-based coherence computations as an aid to 3-D structural and stratigraphic mapping

Abstract: Coherence measures applied to 3-D seismic data volumes have proven to be an effective method for imaging geological discontinuities such as faults and stratigraphic features. By removing the seismic wavelet from the data, seismic coherence offers interpreters a different perspective, often exposing subtle features not readily apparent in the seismic data. Several formulations exist for obtaining coherence estimates. The first three generations of coherence algorithms at Amoco are based, respectively, on cross correlation, semblance, and an eigendecomposition of the data covariance matrix. Application of these three generations to data from the Gulf of Mexico indicates that the implementation of the eigenstructure approach described in this paper produces the most robust results. This paper first introduces the basic eigenstructure approach for computing coherence followed by a comparison on data from the Gulf of Mexico. Next, Appendix A develops a theoretical connection between the well-known semblance and the less well-known eigenstructure measures of coherence in terms of the eigenvalues of the data covariance matrix. Appendix B further extends the analysis by comparing the semblance- and eigenstructure-based coherence measures in the presence of additive uncorrelated noise.

Interlocked feedback loops within the Drosophila circadian oscillator.

Abstract: Drosophila Clock ( dClk ) is rhythmically expressed, with peaks in mRNA and protein (dCLK) abundance early in the morning. dClk mRNA cycling is shown here to be regulated by PERIOD-TIMELESS (PER-TIM)–mediated release of dCLK- and CYCLE (CYC)–dependent repression. Lack of both PER-TIM derepression and dCLK-CYC repression results in high levels of dClk mRNA, which implies that a separate dClk activator is present. These results demonstrate that the Drosophila circadian feedback loop is composed of two interlocked negative feedback loops: a per-tim loop, which is activated by dCLK-CYC and repressed by PER-TIM, and a dClk loop, which is repressed by dCLK-CYC and derepressed by PER-TIM.

The photopic negative response of the macaque electroretinogram: reduction by experimental glaucoma.

Abstract: PURPOSE. TO investigate the photopic flash electroretinograms (ERGs) of macaque monkeys in which visual field defects developed as a consequence of experimental glaucoma. METHODS. Unilateral experimental glaucoma was induced in 10 monkeys by argon laser treatment of the trabecular meshwork. Visual field sensitivity was assessed behaviorally by static perimetry. Photopic ERGs were recorded to brief- (^5 msec) and long-duration (200 msec) red ganzfeld flashes on a rod-suppressing blue-adapting background. Electroretinograms were recorded in four other monkeys, after intravitreal injection of tetrodotoxin (TTX; 3-8-8 /xM) to suppress action potentials of retinal ganglion and amacrine cells, and in six normal adult human subjects. RESULTS. Experimental glaucoma removed a cornea-negative response, the photopic-negative response (PhNR), from the ERG. The PhNR in control eyes was maximal approximately 60 msec after a brief flash, 100 msec after onset, and 115 msec after offset of the long-duration stimulus. The PhNR in experimental eyes was greatly reduced when the mean deviation of the visual field sensitivity was as little as —6 dB. As visual sensitivity declined further, the PhNR was reduced only slightly more. The a- and b-waves were unchanged, even when sensitivity decreased by more than 16 dB. Tetrodotoxin also selectively reduced the PhNR. The PhNR was observed in normal human ERGs. CONCLUSIONS. The cornea-negative PhNR of the photopic ERG depends on spiking activity and is reduced in experimental glaucoma when visual sensitivity losses are still mild. The PhNR most likely arises from retinal ganglion cells and their axons, but its slow timing raises the possibility that it could be mediated by glia. Regardless of the mechanism of its generation, the PhNR holds promise as an indicator of retinal function in early glaucomatous optic neuropathy. (Invest Ophthalmol Vis Sci. 1999;40:1124 -1136)

Symmetry in locomotor central pattern generators and animal gaits.

Abstract: Animal locomotion is controlled, in part, by a central pattern generator (CPG), which is an intraspinal network of neurons capable of generating a rhythmic output1,2,3,4. The spatio-temporal symmetries of the quadrupedal gaits walk, trot and pace5,6,7,8 lead to plausible assumptions about the symmetries of locomotor CPGs9,10,11. These assumptions imply that the CPG of a quadruped should consist of eight nominally identical subcircuits, arranged in an essentially unique matter. Here we apply analogous arguments to myriapod CPGs. Analyses based on symmetry applied to these networks lead to testable predictions, including a distinction between primary and secondary gaits, the existence of a new primary gait called ‘jump’, and the occurrence of half-integer wave numbers in myriapod gaits. For bipeds, our analysis also predicts two gaits with the out-of-phase symmetry of the walk and two gaits with the in-phase symmetry of the hop. We present data that support each of these predictions. This work suggests that symmetry can be used to infer a plausible class of CPG network architectures from observed patterns of animal gaits.

Evaluating the Impact of Dss, Cognitive Effort, and Incentives on Strategy Selection

Abstract: Decision support system (DSS) researcher- have long debated whether or not the provision of a DSS would lead to greater decision-making effectiveness, efficiency, or both. The work described in thi- paper examine- how DSS designer- can guide user- toward- employing more normative decision strategies. Working from notion- of restrictiveness and decisional guid- ance (Silver 1990) supplemented by the cost-benefit framework of cognition, we explain how DSS capabilitie- influence decision behavior and performance through the manipulation of effort. The result- of thi- work should assist DSS developer- to devise directed or nondirected approache- to effect desired behaviors.

Incorporation of Titanium into Mesoporous Silica Molecular Sieve SBA-15

Abstract: Mesoporous silica molecular sieve SBA-15 has been synthesized and incorporated with variable amounts of titanium via incipient-wetness impregnation with titanium isopropoxide in ethanol followed by calcination. Characterization by powder X-ray diffraction, nitrogen adsorption, X-ray photoelectron, Raman, and diffuse reflectance ultraviolet spectroscopies has been carried out to understand the chemical nature of the titanium. The results suggest that titanium is present in two distinct chemical forms and that their relative amounts depend on the titanium loading. At low titanium loading of 1 atom % relative to silicon, the titanium ions are monatomically dispersed and the pore size of SBA-15 is not altered. This isolated titanium species reaches a maximum concentration around 6 atom % relative to silicon. At higher titanium loading, titanium dioxide (anatase) is also formed. The materials with significant titanium dioxide formation have reduced pore diameters, which suggests that the titanium dioxide exist...

Circadian rhythms in olfactory responses of Drosophila melanogaster

Abstract: The core mechanism of circadian timekeeping in arthropods and vertebrates consists of feedback loops involving several clock genes, including period (per) and timeless (tim)1,2. In the fruitfly Drosophila, circadian oscillations in per expression occur in chemosensory cells of the antennae, even when the antennae are excised and maintained in isolated organ culture3. Here we demonstrate a robust circadian rhythm in Drosophila in electrophysiological responses to two classes of olfactory stimuli. These rhythms are observed in wild-type flies during light–dark cycles and in constant darkness, but are abolished in per or tim null-mutant flies (per01 and tim01) which lack rhythms in adult emergence and locomotor behaviour. Olfactory rhythms are also abolished in the per 7.2:2 transgenic line in which per expression is restricted to the lateral neurons of the optic lobe4. Because per 7.2:2 flies do not express per in peripheral oscillators, our results provide evidence that peripheral circadian oscillators are necessary for circadian rhythms in olfactory responses. As olfaction is essential for food acquisition, social interactions and predator avoidance in many animals, circadian regulation of olfactory systems could have profound effects on the behaviour of organisms that rely on this sensory modality.

Functional vision and corneal changes after laser in situ keratomileusis determined by contrast sensitivity, glare testing, and corneal topography.

Abstract: Purpose To demonstrate the functional vision and corneal changes following laser in situ keratomileusis (LASIK) determined by contrast sensitivity, glare testing, and corneal topography. Setting University of Texas Medical School, Houston, Texas, USA. Methods Seven patients ranging in age from 20 to 61 years who had bilateral LASIK were evaluated preoperatively and 1 day, 1 week, and 1 and 6 months postoperatively. Visual acuity, using letters on the Baylor Visual Acuity Testor (BVAT) at 98% (standard acuity) and 13% contrast, and the contrast threshold were determined at 3 light levels (darkness, medium brightness acuity testor [BAT], high BAT). Pupil sizes were measured at each level, and corneal topography was performed at each visit. Results The greatest changes were found 1 day postoperatively: The contrast threshold worsened by a mean of 0.6 lines ± 1.0 (SD) (P = .05) in darkness, 0.4 ± 0.7 lines (P = .05) at medium BAT, and 0.8 ± 0.7 lines (P = .002) at high BAT. The 98% contrast acuity decreased a mean of 1.4 ± 1.6 lines (P = .01) in darkness, 1.0 ± 2.0 lines (P = .09) at medium BAT, and 0.8 ± 2.3 lines (P = .22) at high BAT. The 13% contrast acuity decreased a mean of 2.2 ± 2.6 lines (P = .01) in darkness, 1.3 ± 1.9 lines (P = .02) at medium BAT, and 1.4 ± 2.5 lines (P = .07) at high BAT. The predicted corneal acuity (PCA) obtained from corneal topography decreased by a mean of 3.3 ± 3.1 lines (P = .002), and the asphericity (Q-value) increased by an average of +0.35 ± 0.67 (P = .07). All values returned to the preoperative levels by 1 week except PCA, asphericity, visual acuity at 13%, and contrast threshold in darkness, which improved slightly but had not returned to baseline by 6 months. The 98% contrast acuity at medium BAT improved by 0.2 ± 1.0 lines (P = .34) and 0.3 ± 0.8 lines (P = .16) at high BAT at 1 month. The 98% contrast acuity values remained 0.3 lines over baseline through 6 months. Corneal topography showed that all corneas became oblate after LASIK to a mean Q-value of +0.47 ± 0.40 (P Conclusions Functional vision changes do occur after LASIK. The optical quality of the cornea is reduced and the asphericity becomes oblate. Changes in functional vision worsen as the target contrast diminishes and the pupil size increases. These findings indicate that the oblate shape of the cornea following LASIK is the predominant factor in the functional vision decrease.

A Model for Understanding How Virtual Reality Aids Complex Conceptual Learning

Abstract: Designers and evaluators of immersive virtual reality systems have many ideas concerning how virtual reality can facilitate learning. However, we have little information concerning which of virtual reality's features provide the most leverage for enhancing understanding or how to customize those affordances for different learning environments. In part, this reflects the truly complex nature of learning. Features of a learning environment do not act in isolation; other factors such as the concepts or skills to be learned, individual characteristics, the learning experience, and the interaction experience all play a role in shaping the learning process and its outcomes. Through Project Science Space, we have been trying to identify, use, and evaluate immersive virtual reality's affordances as a means to facilitate the mastery of complex, abstract concepts. In doing so, we are beginning to understand the interplay between virtual reality's features and other important factors in shaping the learning process and learning outcomes for this type of material. In this paper, we present a general model that describes how we think these factors work together and discuss some of the lessons we are learning about virtual reality's affordances in the context of this model for complex conceptual learning.

Chance‐constrained model predictive control

Abstract: This work focuses on robustness of model-predictive control with respect to satisfaction of process output constraints. A method of improving such robustness is presented. The method relies on formulating output constraints as chance constraints using the uncertainty description of the process model. The resulting on-line optimization problem is convex. The proposed approach is illustrated through a simulation case study on a high-purity distillation column. Suggestions for further improvements are made.

Reformulated radial basis neural networks trained by gradient descent

Abstract: This paper presents an axiomatic approach for constructing radial basis function (RBF) neural networks. This approach results in a broad variety of admissible RBF models, including those employing Gaussian RBFs. The form of the RBFs is determined by a generator function. New RBF models can be developed according to the proposed approach by selecting generator functions other than exponential ones, which lead to Gaussian RBFs. This paper also proposes a supervised learning algorithm based on gradient descent for training reformulated RBF neural networks constructed using the proposed approach. A sensitivity analysis of the proposed algorithm relates the properties of RBFs with the convergence of gradient descent learning. Experiments involving a variety of reformulated RBF networks generated by linear and exponential generator functions indicate that gradient descent learning is simple, easily implementable, and produces RBF networks that perform considerably better than conventional RBF models trained by existing algorithms.

The NA49 large acceptance hadron detector

Abstract: The NA49 detector is a wide acceptance spectrometer for the study of hadron production in p+p, p+A, and A+A collisions at the CERN SPS. The main components are 4 large-volume TPCs for tracking and particle identification via dE/dx. TOF scintillator arrays complement particle identification. Calorimeters for transverse energy determination and triggering, a detector for centrality selection in p+A collisions, and beam definition detectors complete the set-up. A description of all detector components is given with emphasis on new technical realizations. Performance and operational experience are discussed in particular with respect to the high track density environment of central Pb+Pb collisions.

The dimensionality of phonological awareness : An application of item response theory

Abstract: A battery of 7 tasks composed of 105 items thought to measure phonological awareness skills was administered to 945 children in kindergarten through 2nd grade. Results from confirmatory factor analysis at the task level and modified parallel analysis at the item level indicated that performance on these tasks was well represented by a single latent dimension. A 2-parameter logistic item response (IRT) model was also fit to the performance on the 105 items. Information obtained from the IRT model demonstrated that the tasks varied in the information they provided about a child's phonological awareness skills. These results showed that phonological awareness, as measured by these tasks, appears to be well represented as a unidimensional construct, but the tasks best suited to measure phonological awareness vary across development.

Biodegradation Rates for Fuel Hydrocarbons and Chlorinated Solvents in Groundwater

Abstract: Numerous studies presented in the general literature have shown that the key mechanism affecting the rate and extent of migration of a contaminant plume is biodegradation since it removes contaminant mass and reduces average plume concentrations. This paper attempts to address the importance of biodegradation for fuel and chlorinated solvent plumes and to present a comprehensive review of rates of biodegradation obtained from field and laboratory studies. Data from approximately 280 studies are statistically analyzed to determine ranges of biodegradation rates for various contaminants under different redox conditions. A review of 133 studies for fuel hydrocarbons has yielded first-order biodegradation coefficients up to 0.445 day−1 under aerobic conditions and up to 0.522−1 under anaerobic conditions in 90% of the cases. A median rate constant for benzene of 0.3% day−1 was estimated from all studies, while those for toluene, ethylbenzene, and xylenes were estimated to be 4, 0.3, and 0.4% day−1, respective...