Eye movements in hemianopia and the rehabilitation of hemianopic dyslexia

Abstract

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).