Thesis (M. Med. Sc.)--University of Hong Kong, 2002. / Includes bibliographical references (46-49).

Thesis (Ph. D.)--University of Texas at Austin, 2001. / Vita. Includes bibliographical references (leaves 237-244). Available also in a digital version from Dissertation Abstracts.

Purpose: WHO reports 46% of world vision impairment from refractive error (RE) among children occurs in China. We estimated RE prevalence and associated vision impairment (VI) among Chinese children and adults. Methods: Data from population-based studies were stratified by gender in age intervals of 3 years (ages 3-17 y) or 10 years (ages >= 30 y): counts of persons with myopia (worse eye spherical equivalent <= -1.0D, <= -2.0D, <= -6.0D) and prevalence of low vision (< 6/12 in the better-seeing eye for children and < 6/18 for adults) and blindness (<=6/60) attributable to RE. Figures for VI included persons with habitual vision below the cutoff improving to above the cutoff with refraction, and those with myopic retinopathy. Estimates for ages 18-29 y were obtained from regression models derived from the pooled estimates. Prevalence of myopia and VI attributable to RE in each age/gender category was calculated by applying modeled rates to 2000 China census figures and projections for 2020. Association with VI attributable to RE was tested for: gender, urban versus rural residence, and residence in provinces with per capita GDP in the upper versus lower 50% for China. Results: Data were obtained from 5 cohorts for children and 14 for adults. There were 291 million and 21.4 million persons with myopia <= -1.0D and <= -6.0D respectively in 2000, expected to rise to 306 million and 36.9 million by 2020. Of these, 18.4 million were blind and 116 million had low vision in 2000, with figures of 25.3 million and 123 million in 2020. Children accounted for the following proportion of RE disease burden in China in 2000: myopia <= -1.0D: 19.0%; RE-associated low vision: 56.1%; blindness: 14.1%. Refractive error was responsible for 82.3% of blindness and 90.5% of low vision among children, and 11.6% and 64.4% of blindness and low vision among adults. Urban residence (OR 1.85, P = 0.004) and higher GDP (OR 10.6, P < 0.001) were associated with refractive blindness among children. For adults, lower GDP was associated with refractive blindness (OR 1.47, P = 0.01). Gender was un-associated with refractive blindness among children or adults. Conclusions: Both children and adults suffer a heavy burden of VI associated with RE in China. Income may affect risk for such VI differently among children and adults. / published_or_final_version / Public Health / Master / Master of Public Health

It has been shown in experiments that a method of teaching where students are encouraged to create their own solution methods to mathematical problems (creative mathematically founded reasoning, CMR) results in better learning and proficiency than one where students are provided with solution methods for them to practice by repetition (algorithmic reasoning, AR). The present study investigated whether students in an AR practice condition pay less attention to information relevant for mathematical problem solving than students in a CMR condition. To test this, attentional behavior during practice was measured using eye-tracking equipment. These measurements were then associated with task proficiency in a follow-up test one week after the practice session. The findings support the theory and confirm previous studies in that CMR leads to better task performance in the follow-up test. The findings also suggest that students within the CMR condition whom focus less on extraneous information perform better. / Experiment har visat att en undervisningsmetod i vilken elever uppmuntras att själva komma på lösningsmetoder till matematiska problem (creative mathematically founded reasoning, CMR) resulterar i bättre inlärning och färdighet än en metod i vilken eleverna ges en färdig en lösningsmetod att öva på genom repetition (algorithmic reasoning, AR). Denna studie undersöker om elever under en AR-träningsbetingelse ägnar mindre uppmärksamhet åt information som är relevant för matematisk problemlösning än vad elever under en CMR-träningsbetingelse gör. För att testa detta mättes elevernas uppmärksamhetsbeteende under träning med hjälp av ögonrörelsekamera. Måtten ställdes sedan i relation till uppgiftsfärdighet i ett uppföljningstest en vecka efter träningssessionen. Resultaten stödjer teorin och bekräftar tidigare studier som visat att CMR leder till bättre prestation i uppföljningstestet. Resultaten tyder även på att de elever under CMR-betingelsen som fokuserar minst på ovidkommande information presterar bättre.

Visual search can simply be defined as the task of looking for an object of interest in a visual environment. Due to its foveated nature, the human visual system succeeds at such task by making many discrete fixations linked by rapid eye movements called saccades. However, very little is known about how saccadic targets (fixation loci) are selected by the brain in such naturalistic tasks. Discoveries to be made are not only invaluable to the field of vision science but are very important in designing automated vision systems, which to this day lag in performance vis-à-vis human observers. What I have sought to accomplish in this dissertation has been to reveal previously unknown saccadic targeting and target selection strategies used by human observers in naturalistic visual search tasks. My driving goal has been to understand how the brain selects fixation loci and target candidates upon fixation, with the objective of using these findings for automated fixation selection algorithms employed for visual search. I have proposed a novel and efficient technique akin to psychophysical reverse correlation to study human observer strategies in locating low-contrast targets under a variety of experimental conditions. My technique has successfully been used to study saccadic programming and target selection in various experimental conditions, including visual searches for targets with known characteristics, targets whose orientation attributes are not known a priori, and targets containing multiple orientations. I have found visual guidance in saccadic targeting and target selection under all experimental conditions, revealed by observers' selectivity for spatial frequencies and/or orientations of stimuli close to that of the target. I have shown that under uncertainty, observers rely on known target characteristics to direct their saccades and to select target candidates upon foveal scrutiny. Moreover, I have demonstrated that multiple orientation characteristics of targets are represented in observer search strategies, modulated by their sensitivity / selectivity for each orientation. Some of my findings have been applied towards applications for automated visual search algorithms.

In this dissertation, we conducted a stereoscopic eye tracking experiment using naturalistic stereo images. We analyzed low level 2D and 3D scene features at binocular fixations and randomly selected places. The results reveal that humans tend to fixate on regions with higher luminance variations, but lower disparity variations. Because of the often observed co-occurrence of luminance and depth changes in natural environments, the dichotomy between luminance features and disparity features inspired us to study the accurate statistics of 2D and 3D scene properties. Using a range map database, we studied the distribution of disparity in natural scenes. The natural disparity distribution has a high peak at zero, and heavier tails that are similar to a Laplace distribution. The relevance of natural disparity distribution to other studies in neurobiology and visual psychophysics are discussed in detail. We also studied luminance, range and disparity statistics in natural scenes using a co-registered luminance-range database. The distributions of bandpass 2D and 3D scene features can be well modeled by generalized Gaussian models. There are positive correlations between bandpass luminance and depth, which can be captured by varying shape parameters in the probability density functions of the generalized Gaussians. In another study on suprathreshold luminance and depth discontinuities, we show that observing a significant luminance edge at a significant depth edge is much more likely than at homogeneous depth surfaces. It is also true that a significant depth edge happens at a significant luminance edge with a greater probability than at homogeneous luminance regions. Again, the dependency between luminance and depth discontinuities can be modeled successfully by generalized Gaussians. We applied our statistical models in 3D natural scenes to stereo correspondence. A Bayesian framework is proposed to incorporate the bandpass disparity prior, and the luminance-disparity dependency in the likelihood function. We compared our algorithm with a classical simulated annealing method based on heuristically defined energy functions. The computed disparity maps show great improvements both perceptually and objectively. / text

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published_or_final_version / Medical Sciences / Master / Master of Medical Sciences

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