University of Houston

About: University of Houston is a education organization based out in Houston, Texas, United States. It is known for research contribution in the topics: Population & Poison control. The organization has 23074 authors who have published 53903 publications receiving 1641968 citations.

Are all aberrations equal

Abstract: PURPOSE: To determine for a fixed RMS error (25 μm, over a 6-mm pupil) how each mode of the normalized Zernike polynomial (second through the fourth radial order) affects high and low contrast logMAR visual acuity. METHODS: Three healthy volunteers served as subjects. CTView was used to generate optically aberrated logMAR charts. Accommodation was paralyzed and pupils dilated. The foveal achromatic axis of the eye was aligned to a 3-mm pupil and the eye was optimally refracted. Aberrated acuity charts were read until five letters were missed. Data were normalized for each subject to the acuity obtained by reading unaberrated charts and plotted as letters lost as a function of Zernike mode. RESULTS: Defocus (Z 0 2 ) decreased letter acuity more than astigmatism (Z 2 2 , Z - 2 2 ). Coma (Z 1 3 , Z - 1 3 ) decreased acuity more than trefoil (Z 3 3 , Z - 3 3 ). Spherical aberration (Z 0 4 ) and secondary astigmatism (Z 2 4 , Z - 2 4 ) decreased acuity much more than quadrafoil (Z 4 4 , Z - 4 4 ). CONCLUSIONS: 1. For an equal amount of RMS error not all coefficients of the Zernike polynomial induce equivalent losses in high and low contrast logMAR acuity. 2. Wavefront error concentrated near the center of the pyramid adversely affects visual acuity more than modes near the edge of the pyramid. 3. Large changes in chart appearance are not reflected in equally large decreases in visual performance (ie, subjects could correctly identify highly aberrated letters). 4. Interactions between modes complicate weighting each Zernike mode for visual impact.

Quantitative Diffusion Tensor Tractography of Association and Projection Fibers in Normally Developing Children and Adolescents

Abstract: Whole-brain diffusion tensor tractography (DTT) at high signal-to-noise ratio and angular and spatial resolutions were utilized to study the effects of age, sex differences, and lateral asymmetries of 6 white matter pathways (arcuate fasciculus [AF], inferior longitudinal fasciculus, inferior fronto-occipital fasciculus [IFOF], uncinate fasciculus [UF], corticospinal tract [CST], and somatosensory pathway [SS]) in 31 right-handed children (6-17 years). Fractional anisotropy (FA), a measure of the orientational variance in water molecular diffusivity, and the magnitude of water diffusivity (parallel, perpendicular, and mean diffusivity) along the pathways were quantified. Three major patterns of maturation were observed: 1) significant increase in FA with age, accompanied by significant decreases in all 3 diffusivities (e.g., left IFOF); 2) significant decreases in all three diffusivities with age without significant changes in FA (e.g., left CST); and 3) no significant age-related changes in FA or diffusivity (e.g., SS). Sex differences were minimal. Many pathways showed lateral asymmetries. In the right hemisphere, the frontotemporal (FT) segment of AF was not visualized in a substantial (29%) number of participants. FA was higher in the left hemisphere in the FT segment of AF, UF, and CST, whereas it was lower in the frontoparietal segment of AF. This study provides normative data essential for the interpretation of pediatric brain DTT measurements in both health and disease.

Observation of five-fold local symmetry in liquid lead.

Abstract: The local point symmetry of the short-range order in simple monatomic liquids remains a fundamental open question in condensed-matter science. For more than 40 years it has been conjectured that liquids with centrosymmetric interactions may be composed of icosahedral building blocks. But these proposed mobile, randomly orientated structures have remained experimentally inaccessible owing to the unavoidable averaging involved in scattering experiments, which can therefore determine only the isotropic radial distribution function. Here we overcome this limitation by capturing liquid fragments at a solid-liquid interface, and observing the scattering of totally internally reflected (evanescent) X-rays, which are sensitive only to the liquid structure at the interface. Using this method, we observe five-fold local symmetry in liquid lead adjacent to a silicon wall, and obtain an experimental portrait of the icosahedral fragments that are predicted to occur in all close-packed monatomic liquids. By shedding new light on local bond order in disordered structures such as liquids and glasses, these results should lead to a better microscopic understanding of melting, freezing and supercooling.