Eye movements in hemianopia and the rehabilitation of hemianopic dyslexia

This thesis is a study of the nature and rehabilitation of the functional impairments in unilateral homonymous hemianopia (HH), with a major focus on hemianopic dyslexia. The reading, visual exploration and line bisection impairments associated with homonymous visual field loss are frequent and well-established clinical phenomena. Yet, it is still unknown whether the reading and visual exploration impairments are caused by the visual field defect or by additional extrastriate injury preventing efficient spontaneous oculomotor adaptation. It is also unclear whether the line bisection impairment directly arises from the visual field defect or its adaptive oculomotor consequences, or whether it indicates an associated visual-spatial deficit that is caused by injury to regions involved in visual-spatial perception (Introduction). Based on a critical review of research into hemianopic dyslexia since its original description in 1881, it is suggested that the visual field defect is a major component of hemianopic dyslexia but possibly not its sole cause (Chapter 1). This assumption was confirmed in six experiments whose purpose was to establish the extent to which the reading, visual exploration and line bisection impairments associated with HH are purely visually elicited. To study the behavioural changes associated with the visual field defect that are not caused by brain injury, a gaze-contingent display paradigm was used to simulate HH in healthy participants. Simulated HH induced the reading and visual exploration impairments of hemianopic patients. However, all participants showed efficient spontaneous oculomotor adaptation to simulated HH which was associated with highly specific and task-dependent improvements in reading and visual exploration (Chapters 2 and 3). Moreover, simulated HH did not induce the main feature of the hemianopic line bisection impairment, i.e., the contralateral line bisection error, albeit it nevertheless impaired line bisection performance (Chapter 4). The final study investigated the basis and specificity of the therapeutic effect of an efficient compensatory oculomotor treatment method for hemianopic dyslexia in patients with unilateral homonymous visual field loss. The results demonstrate that using text-material and, thus, lexical-semantic processes, is not critical to the treatment effect, which was also found to be specific to reading (Chapter 5). The concluding chapter reviews the main findings and suggests that the functional impairments associated with visual field loss may not simply be failures of vision. Although the hemianopic visual field defect is a major component of hemianopic dyslexia and possibly contributes to the visual exploration and line bisection impairments, additional injury to specific extrastriate regions seems to be the critical causative factor. The implications for understanding, assessing and rehabilitating functional impairments in homonymous visual field disorders are discussed. The important future research directions arising from this thesis are also identified and presented (Conclusion)

Eye movements, search and perception of visual field defects in glaucoma

Glaucoma is a progressive disease of the optic nerve that can result in irreversible loss of visual function and impairment in everyday visual tasks. The experimental studies described in this thesis primarily aim to investigate the performance of people with glaucoma on search and other visual tasks whilst simultaneously monitoring eye movements, making comparison with age-related visually healthy people. In an experiment focussing on visual search, a patient group (n=30) took significantly longer on average to find a target in images of everyday scenes than controls (n=30). Furthermore, comparison of eye movements made by the participants during this task revealed there was a statistically significant reduction (6%) in saccade rate in the patients compared to the controls, and that saccade rate correlated with performance. Similar differences in eye movements were observed when the same groups passively viewed a selection of images in a slideshow. A bivariate contour ellipse (BCE) analysis revealed that, on average, patients viewed smaller regions of the images compared to the controls. Eye movement differences between patients and controls were also examined in a different cohort of people with glaucoma (n=14) and visually healthy controls (n=22) whilst they watched a selection of Hazard Perception Test driving films. Saccade rate of the patients was found to increase by 9%, though results from the BCE analysis suggested the average size of viewing area was similar in both groups. Finally, a novel interview-based study of 50 people with glaucoma provides evidence that patients do not perceive their visual field defect as a black ‘tunnel’ effect, or as ‘black patches’, but more like blurred regions: this finding may, for example, impact on how glaucomatous visual field loss is depicted in patient information about the condition. In conclusion, the results from this thesis show how visual loss from glaucoma influences how patients perceive and react to their visual environment. The principal findings from the studies described in this thesis also show, for the first time, that eye movement analysis could provide a window into the functional deficits associated with glaucoma

Motion-induced position shifts in visual perception tasks and eye movement

Movement has an effect upon the perceived spatial position of moving objects, such that they are not perceived at their instantaneous spatial position. Vision scientists named this phenomenon motion-induced position shift (MIPS). The reason, neural loci, and the mechanisms causing the positional illusion have challenged scientists over the last century. Nowadays, many vehicles, such as cars, planes and submarines are equipped with onboard computers containing touchscreens. Active controls of those on-board computers require visuomotor-actions, which could be affected by perceptual illusions, but also require time, and attention. Hence, it is becoming more crucial to fully understand how the visual system generates visuomotor-guided actions, and how it copes with visual illusions. Human-machine interactions could be designed such that perceptual illusions would be 1) avoided, or 2) predicted, and considered in human actions, or such that 3) the user interacted with visuomotor actions that resisted visual illusions. One alternative to finger points towards on-board computers is saccadic eye movements. The saccadic system is very fast, and therefore, would not require as much time and attention as a finger point task towards the touch screen. Saccades are constantly facing the challenge of localising objects, which makes it interesting to study how they cope with visual illusions like the motion-induced position shift. The purpose of this thesis was to establish if the saccadic system was affected by the motion-induced position shift in the same manner as the perceptual system was affected. I confirmed that movement had an effect upon the perceived spatial position of moving objects in perception-tasks and in volitional saccades. A previous study showed that reflexive saccades resisted the illusion, indicating that they were more accurate than other visually guided actions. I replicated these results, but claimed that the results are not representative. As a consequence, there is no evidence that reflexive saccades do escape the visual illusion while volitional saccades do not

Adaptations of the human eye to reduce the impact of chromatic aberrations on vision

Sharpness and contrast of the retinal image are affected by two types of optical aberrations, monochromatic and polychromatic. Monochromatic aberrations result from imperfections in the refracting surfaces, while polychromatic aberrations result from the dispersion of light in the ocular media. Wavelength-dependent differences in focal lengths are referred to as longitudinal chromatic aberration (LCA) while differences in image position and image magnification result from transverse chromatic aberration (TCA). The primary objective of my PhD work is to further clarify the relationship between two chromatic aberrations, longitudinal chromatic aberration (LCA) and transverse chromatic aberration (TCA), and visual perception. I have studied the morphological and optical adaptations of the visual system that were developed in the course of evolution to cope with chromatic aberrations. Generally, transverse chromatic aberration (TCA) has been less studied even though it causes more loss in retinal image contrast. While LCA is similar in different eyes, TCA shows large inter-individual variability. It is not known which ocular variables determine this variability. Therefore, in project 1 I have measured chromatic differences in perceived image magnification (determined by TCA) in different subjects with a newly established psychophysical procedure and found that a major part of the inter-individual variance in CDM (64%) was explained by lens thickness. Since lens thickness increase with age, also TCA will increase. This study was published in the Journal of the Optical Society of America A, 2014. Due to longitudinal chromatic aberration (LCA), the focus of the image on the retina cannot be equally good at all wavelengths. Human eyes are about 2 D more myopic in blue light (450 nm) than in red (650 nm). For this reason, the retinal image in the fovea is typically in best focus for the mid- and long-wavelengths but severely out of focus for the blue (>1D). Probably for this reason, the short wavelength sensitive cones (the S-cones) are lacking from the foveal center, causing a “foveal blue scotoma”. I found that the foveal blue scotoma is highly variable among subjects but it is not known, why. Therefore, in project 2, I have studied the variables that might influence the appearance of the foveal blue scotoma: shape of the foveal pit and distribution of macular pigment. I found that the shape of the foveal pit is a strong predictor of the foveal blue scotoma - the steeper the foveal slopes, the larger the blue scotoma. Macular pigment distribution, on the other, gave rise to the percept of Maxwell’s spot, but was not correlated to the size of the blue scotoma. This study was published in Vision Research, 2015. My third project deals with new technology to measure LCA. In our laboratory we use routinely eccentric infrared photorefraction to measure refractive states in human and animal eye. The use 6 of infrared light has the advantage that the subject is not aware that is being measured and that pupils remain large which increases the signal-to-noise ratio. However eccenetric photorefraction could also provide information on LCA if it is used in white light. The differences in refractions measured in the R, G and B channel of the video camera should provide LCA but this technique was never established even though it would be a great advance to obtain LCA from single pictures of the eye. Therefore, in project 3, I studied the potential of polychromatic eccentric photorefraction in measuring LCA. I found that the calibration of photorefraction in white light is much more variable in different subjects, than in infrared light. The major reason was the large individual variability in fundal reflectance in visible light and less variability in the near infrared. Fundal reflectance has a major effect on the brightness of the pupil during the measurements. Because the technique uses a brightness slope in the pupil, and determines the gradient of pixel values, the slope of pupil brightness depends on the absolute pixel brightness. This finding explains a lot of the variability of photorefraction and will be of interest to researchers using this technique. The work was submitted to BOE in June 2015