Differences in orientation and mobility of people with different types of visual impairment have received little attention in the literature. Typically, studies have assessed the mobility performance of either a heterogenous group of subjects with low vision (ie. mixed ocular diseases) or a homogenous group of subjects with a particular eye disease (E§ Retinitis Pigmentosa: RP). There have been few investigations of both mobility performance and vision determinants of mobility in subjects with central visual field (VF) loss. Similarly, mobility performance and its relationship with vision of subjects with contrasting types of VF loss have not been systematically investigated. The work described in this thesis aims to provide a better understanding in these areas. The first study compared the vision and mobility performance of subjects with central VF loss, modelled through Age-related Macular Degeneration (ARMD), and subjects of similar age with normal vision. Elderly subjects with ARMD, when considered as a homogenous group, had no more difficulty with mobility than the normal vision subjects. This was true for both the efficiency (speed) and safety (errors) aspects of mobility performance. The height and luminance of objects did not affect the number of errors made by central VF loss subjects. The sub-group of central VF loss subjects with an absolute binocular scotoma had significantly more difficulty with mobility than the normally sighted subjects. The significance of a binocular scotoma was further demonstrated through the results of the vision determinants of mobility performance. VF measures, relating directly or indirectly to the presence or size of a binocular scotoma, emerged as the best predictors of mobility performance. The second study compared the vision and mobility performance of subjects with peripheral VF loss, modelled through RP, and subjects of similar age with normal vision. Peripheral VF loss subjects had significantly impaired mobility compared to normal vision subjects. This was evident in both the efficiency (speed) and safety (errors) aspects of mobility performance. In particular, the peripheral VF loss subjects found low luminance and inferiorly- and superiorly-placed objects hazardous while navigating through the mobility course. Residual VF measures related to the central 10° radius VF zone emerged as significant predictors of mobility performance. The third study compared the vision and mobility performance of subjects with central and peripheral VF loss. The results of this study showed that, unlike central VF loss, peripheral VF loss was associated with decreased mobility performance. Peripheral VF loss subjects, despite being younger, walked significantly slower and made mo~e errors than the central VF loss subjects. Other differences in mobility performance between the contrasting VF loss groups included the impact of object luminance and position. Similarities in performance included the benefits of familiarity with the course and the degradation of performance under conditions of glare and reduced illumination. Again the importance of the central 10° radius VF for mobility was shown, these VF measures being the best predictors of mobility performance for both VF loss groups. Another aim of this work was to develop and assess the effectiveness of VF scoring based on cortical magnification. Despite the fact that these methods of VF scoring emphasise the central VF, which was shown to be a significant predictor of mobility performance, these methods did not emerge as better determinants of mobility performance than conventional VF scoring methods. This may be a consequence of the available equations and measurement errors associated with kinetic VF measurement. The work described in this thesis provides an improved understanding of the mobility performance and its relationship with vision in low vision subjects with contrasting types of VF loss. The results of these experiments lead to recommendations relating to the vision assessment and referral criteria of low vision patients. Since many determinants of mobility performance related to the central 10° radius VF zone, future work should aim to improve VF measurement and scoring methods including scoring techniques based on cortical magnification, where central VF areas are given prominence. Other future studies should evaluate the vision and mobility performance of low vision subjects with a greater number of forms of VF loss to extend on the work reported in this thesis
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