Binocular rivalry allows for the unique examination of the neural processes associated with binocular vision by instigating a disruption of normal stereoscopic vision. Although binocular rivalry has been examined extensively in young adults, we know relatively little about its developmental trajectory across the human lifespan. This thesis provides a foundation for characterizing perceptual alternations during binocular rivalry in children and older adults, with a specific emphasis on expanding our understanding of binocular rivalry in older adults. From a theoretical perspective, my studies on aging and binocular rivalry have a specific significance, because unique changes that are known to occur with aging to certain neural mechanisms often associated with characteristics of perceptual alternations allows for the study of aging to serve as a test for many of the current models of binocular rivalry. Overall, my studies provide evidence for a significant transitional period in the binocular visual system at the age of 70 and older, and highlights the developmental trajectories of specific characteristics of binocular rivalry from childhood to senescence. / Thesis / Doctor of Philosophy (PhD)

Thesis (M.Sc.)--University of the Witwatersrand, 1961.

Thesis (Ph. D.)--University of Alabama at Birmingham, 2007. / Additional advisors: Timothy Gawne, Richard Gray, Michael Loop, Michael Sloane, Donald Twieg. Description based on contents viewed Mar. 3, 2008; title from title screen. Includes bibliographical references (p. 88-97).

Doctor of Philosophy / Binocular rivalry refers to the perceptual alternation that occurs while viewing incompatible images, in which one monocular image is dominant and the other is suppressed. Rivalry has been closely studied but the neural site at which it is initiated is still controversial. The central claim of this thesis is that primary visual cortex is responsible for its initiation. This claim is supported by evidence from four experimental studies. The first study (described in Chapter 4) introduces the methodology for measuring visual sensitivity during dominance and suppression and compares several methods to see which yields the greatest difference between these two sensitivities. Suppression depth was measured by comparing the discrimination thresholds to a brief test stimulus delivered during dominance and suppression phases. The deepest suppression was achieved after a learning period, with the test stimulus presented for 100 ms and with post-test masking. The second study (Chapter 5) compares two hypotheses for the mechanism of binocular rivalry. Under eye suppression, visibility decreases when the tested eye is being suppressed, regardless of the test stimulus’s features. Feature suppression, however, predicts that reduction of visibility is caused by suppression of a stimulus feature, no matter which eye is suppressed. Eye suppression claims that monocular channels in the visual system alternate between dominance and suppression, while Feature suppression assumes that the features of stimuli inhibit each other perceptually in the high-level cortex. The experiment used a test stimulus similar in features to one, but not the other, rivalry-inducing stimulus. Test sensitivity was found to be lowered when the test stimulus was presented to the eye whose rivalry-inducing stimulus was suppressed. Sensitivity was not lowered when the test stimulus was presented to the other eye, even when the test shared features with the suppressed stimulus. The conclusion is that feature suppression is weak or does not exist without eye suppression, and that rivalry therefore originates in the primary visual cortex. If binocular rivalry is initiated in the primary visual cortex, stimuli producing no coherent activity in that area should produce no rivalry. In the third study (Chapter 6) this idea was tested with rotating arrays of short-lifetime dots. The dots with the shortest lifetime produced an image with no rotation signal, and an infinite lifetime produced rigid rotation. Subjects could discriminate the rotation direction with high accuracy at all but the shortest lifetime. When the two eyes were presented with opposite directions of rotation, there was binocular rivalry only at the longest lifetimes. Stimuli with short lifetimes produce a coherent motion signal, since their direction can be discriminated, but do not produce rivalry. A simple interpretation of this observation is that binocular rivalry is initiated at a level in the visual hierarchy below that which supports the motion signal. The model supported by the results of previous chapters requires that binocular rivalry suppression be small in the primary visual cortex, and builds up as signals progress along the visual pathway. This model predicts that for judgements dependent on activity in high visual cortex: 1. Binocular rivalry suppression should be deep; 2. Responses should be contrast invariant. The fourth and last study (chapter 7) confirmed these predictions by measuring suppression depth in two ways. First, two similar forms were briefly presented to one eye: the difference in shapes required for their discrimination was substantially greater during suppression than during dominance. Second, the two forms were made sufficiently different in shape to allow easy discrimination at high contrast, and the contrast of these forms was lowered to find the discrimination threshold. The results in the second experiment showed that contrast sensitivity did not differ between the suppression and dominance states. This invariance in contrast sensitivity is interpreted in terms of steep contrast-response functions in cortex beyond the primary visual area. The work in this thesis supports the idea that binocular rivalry is a process distributed along the visual pathway. More importantly, the results provide several lines of evidence that binocular rivalry is initiated in primary visual cortex.

Thesis (Ph. D.)--School of Psychology, Faculty of Science, University of Sydney, 2004. / Bibliography: leaves 113-120. Also available in print form.

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 / Binocular rivalry refers to the perceptual alternation that occurs while viewing incompatible images, in which one monocular image is dominant and the other is suppressed. Rivalry has been closely studied but the neural site at which it is initiated is still controversial. The central claim of this thesis is that primary visual cortex is responsible for its initiation. This claim is supported by evidence from four experimental studies. The first study (described in Chapter 4) introduces the methodology for measuring visual sensitivity during dominance and suppression and compares several methods to see which yields the greatest difference between these two sensitivities. Suppression depth was measured by comparing the discrimination thresholds to a brief test stimulus delivered during dominance and suppression phases. The deepest suppression was achieved after a learning period, with the test stimulus presented for 100 ms and with post-test masking. The second study (Chapter 5) compares two hypotheses for the mechanism of binocular rivalry. Under eye suppression, visibility decreases when the tested eye is being suppressed, regardless of the test stimulus’s features. Feature suppression, however, predicts that reduction of visibility is caused by suppression of a stimulus feature, no matter which eye is suppressed. Eye suppression claims that monocular channels in the visual system alternate between dominance and suppression, while Feature suppression assumes that the features of stimuli inhibit each other perceptually in the high-level cortex. The experiment used a test stimulus similar in features to one, but not the other, rivalry-inducing stimulus. Test sensitivity was found to be lowered when the test stimulus was presented to the eye whose rivalry-inducing stimulus was suppressed. Sensitivity was not lowered when the test stimulus was presented to the other eye, even when the test shared features with the suppressed stimulus. The conclusion is that feature suppression is weak or does not exist without eye suppression, and that rivalry therefore originates in the primary visual cortex. If binocular rivalry is initiated in the primary visual cortex, stimuli producing no coherent activity in that area should produce no rivalry. In the third study (Chapter 6) this idea was tested with rotating arrays of short-lifetime dots. The dots with the shortest lifetime produced an image with no rotation signal, and an infinite lifetime produced rigid rotation. Subjects could discriminate the rotation direction with high accuracy at all but the shortest lifetime. When the two eyes were presented with opposite directions of rotation, there was binocular rivalry only at the longest lifetimes. Stimuli with short lifetimes produce a coherent motion signal, since their direction can be discriminated, but do not produce rivalry. A simple interpretation of this observation is that binocular rivalry is initiated at a level in the visual hierarchy below that which supports the motion signal. The model supported by the results of previous chapters requires that binocular rivalry suppression be small in the primary visual cortex, and builds up as signals progress along the visual pathway. This model predicts that for judgements dependent on activity in high visual cortex: 1. Binocular rivalry suppression should be deep; 2. Responses should be contrast invariant. The fourth and last study (chapter 7) confirmed these predictions by measuring suppression depth in two ways. First, two similar forms were briefly presented to one eye: the difference in shapes required for their discrimination was substantially greater during suppression than during dominance. Second, the two forms were made sufficiently different in shape to allow easy discrimination at high contrast, and the contrast of these forms was lowered to find the discrimination threshold. The results in the second experiment showed that contrast sensitivity did not differ between the suppression and dominance states. This invariance in contrast sensitivity is interpreted in terms of steep contrast-response functions in cortex beyond the primary visual area. The work in this thesis supports the idea that binocular rivalry is a process distributed along the visual pathway. More importantly, the results provide several lines of evidence that binocular rivalry is initiated in primary visual cortex.

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.) - University of Queensland, 2003. / Includes bibliography.

Thesis (Ph.D.) - University of Queensland, 2006. / Includes bibliography.