The results of an investigation into the nature of the visual information obtained from pages of text and used in the visual processing of text during reading are reported. An initial investigation into the visual processing of text by applying a computational model of early vision (MIRAGE: Watt & Morgan, 1985; Watt, 1988) to pages of text (Computational Analysis 1) is shown to extract a range of features from a text image in the representation it delivers, which are organised across a range of spatial scales similar to those spanning human vision. The features the model extracts are capable of supporting a structured set of text processing tasks of the type required in reading. From the findings of this analysis, a series of psychophysical and computational studies are reported which exan-dne whether the type of information used in the human visual processing of text can be described by this modelled representation of information in text images. Using a novel technique to measure the 'visibility' of the information in text images, a second stage of investigation (Experiments 1-3) shows that information used to perform different text processing tasks of the type performed in reading is contained at different spatial scales of visual analysis. A second computational analysis of the information in text demonstrates how the spatial scale dependency of these text processing tasks can be accounted for by the model of early vision. In a third stage, two further experiments (Experiments 4-5) show how the pattern of text processing performance is determined by typographical parameters, and a third computational analysis of text demonstrates how changes in the pattern of text processing performance can be modelled by changes in the pattern of information represented by the model of vision. A fourth stage (Experiments 6-7 and Computational Analysis 4) examines the time-course of the visual processing of text. The experiments show how the duration required to reach a level of visual text processing performance varies as a function of typographical parameters, and comparison of these data with the model shows that this is consistent with a time-course of visual analysis based on a coarse-to-fine spatial scale of visual processing. A final experiment (Experiment 8) examines how reading performance varies with typographical parameters. It is shown how the pattern of reading performance and the pattern of visual text processing performance are related, and how the model of early vision might describe the visual processing of text in reading. The implications of these findings for theories of reading and theories of vision are finally discussed.

Discrimination thresholds were obtained during a delayed spatial frequency discrimination task. In Experiment 1, we found that presentation of a mask 3 s before onset of a reference Gabor patch caused selective interference in a subsequent discrimination task. However, a 10 s interval abolished this masking effect. In Experiment 2, the mask was associated with a second spatial frequency discrimination task so that a representation of the mask had to be coded into short-term perceptual memory. The presence of this second discrimination task now caused similar interference effects on the primary discrimination task at both the 3 s and 10 s ISI conditions. The different results from these two experiments are best explained by a two-step perceptual memory mechanism. The results also provide further insight into the conditions under which stimulus representations are shared between the perceptual and memory domains.

The present thesis reports psychophysical and modeling studies on the spatial properties of visual mechanisms mediating the perception of mirror symmetry in human vision. In a first set of experiments, patterns were filtered for power spectra that decayed with spatial frequency according to variable slopes. Results revealed that symmetry detection is optimal if contrast energy is roughly equated across log-frequency bands (i.e. 1/f2) and that, under such conditions, spatial scales contribute equally and independently to symmetry perception. In a second study, random-noise patterns were filtered for various orientation bands. Results showed that symmetry perception is possible at all orientations, is mediated by oriented mechanisms, and is computed independently in different orientation channels. Data also revealed that the dimensions of the spatial integration region (IR) for symmetry vary with orientation in a way that approximately matches the spatial distribution of information in the stimulus. Finally, symmetry detection was measured for bandpass textures of variable spatial density and variable contrast polarity. For such patterns, it was found that symmetry is computed at a spatial scale proportional to stimulus density and that mechanisms insensitive to contrast polarity (i.e. second-order) are involved in the scale-selection process. / Overall, results from empirical and modeling work revealed an intimate link between symmetry perception and the properties of spatial filters. In particular, I argue that the size of the IR tends to vary such that a fixed amount of information is integrated irrespective of the spatial properties of the stimulus. Implications for the functional architecture of symmetry perception are discussed, and a paradigm for future research in symmetry perception is proposed in which spatial filtering is extended to higher orders of spatial complexity.

Doctor of Philosophy / The human, along with other primates, has forward placed eyes, and an area of acute vision (the fovea) on each retina. The overlap of the visual fields and the hemi-decussation of the visual pathways at the optic chiasm provide the basis for binocular vision, in particular stereopsis, the accurate perception of the position of objects in three dimensional space and an improved ability to perceive the form of solid objects. An intricate system of eye movements is needed to achieve and maintain stable foveal fixation on each eye in an environment where visual targets vary in direction and depth, where the visual environment may be moving, the eyes or the rest of the body is moving. The purpose of this study is to evaluate the accuracy of binocular alignment for far and near fixations, under relatively natural conditions. To achieve binocular fixation, accurate vergence eye movements are required to align the eyes, and to maintain this alignment when a person changes fixation to objects situated at different distances from the eyes. ‘Pure’ vergence eye movements occur when these objects are situated along the mid sagittal plane, however, in natural conditions other eye movement systems are also involved. To understand the contribution of different eye movement systems to binocular fixation at different distances, the accuracy of binocular alignment in subjects with normal binocular single vision was evaluated in subjects with normal binocular vision under the following conditions • Fixation on targets along the mid sagittal plane (vergence eye movements only) • Fixation on targets displaced to either side of the mid sagittal plane (combined vergence eye movements and saccades • Fixation on earth fixed targets situated straight ahead in space, but with the head tilted to either side (combined vergence eye movements, saccades and torsional eye movements). The protocol for all experiments was approved by the Human Ethics Committee of the University of Sydney and followed the tenets of the Declaration of Helsinki. Throughout this thesis the term ‘binocular alignment’ will be used to describe the position of each eye during or following a change in vergence. The term ‘vergence error’ will refer to situations where the angle of vergence alignment is different from that required, so that the image of the fixation target does not fall on the fovea of one or both eyes.

Doctor of Philosophy / The human, along with other primates, has forward placed eyes, and an area of acute vision (the fovea) on each retina. The overlap of the visual fields and the hemi-decussation of the visual pathways at the optic chiasm provide the basis for binocular vision, in particular stereopsis, the accurate perception of the position of objects in three dimensional space and an improved ability to perceive the form of solid objects. An intricate system of eye movements is needed to achieve and maintain stable foveal fixation on each eye in an environment where visual targets vary in direction and depth, where the visual environment may be moving, the eyes or the rest of the body is moving. The purpose of this study is to evaluate the accuracy of binocular alignment for far and near fixations, under relatively natural conditions. To achieve binocular fixation, accurate vergence eye movements are required to align the eyes, and to maintain this alignment when a person changes fixation to objects situated at different distances from the eyes. ‘Pure’ vergence eye movements occur when these objects are situated along the mid sagittal plane, however, in natural conditions other eye movement systems are also involved. To understand the contribution of different eye movement systems to binocular fixation at different distances, the accuracy of binocular alignment in subjects with normal binocular single vision was evaluated in subjects with normal binocular vision under the following conditions • Fixation on targets along the mid sagittal plane (vergence eye movements only) • Fixation on targets displaced to either side of the mid sagittal plane (combined vergence eye movements and saccades • Fixation on earth fixed targets situated straight ahead in space, but with the head tilted to either side (combined vergence eye movements, saccades and torsional eye movements). The protocol for all experiments was approved by the Human Ethics Committee of the University of Sydney and followed the tenets of the Declaration of Helsinki. Throughout this thesis the term ‘binocular alignment’ will be used to describe the position of each eye during or following a change in vergence. The term ‘vergence error’ will refer to situations where the angle of vergence alignment is different from that required, so that the image of the fixation target does not fall on the fovea of one or both eyes.

Thesis (Ph.D.)--Tufts University, 2003. / Adviser: Phillip J. Holcomb. Submitted to the Dept. of Psychology. Includes bibliographical references (leaves 123-135). Access restricted to members of the Tufts University community. Also available via the World Wide Web;

Thesis (M.S.) in Computer Engineering--University of Maine, 2007. / Includes vita. Includes bibliographical references (leaf 104).

Thesis (M.S.)--University of British Columbia, 2006. / Includes bibliographical references (leaves 45-48).

Thesis (Ph.D.) -- McMaster University, 1998. / Includes bibliographical references (leaves 191-212). Also available via World Wide Web.

Thesis (M.A.)--University of Toronto, 2005.