Abstract
Ocular accommodation is the process whereby the power of the crystalline lens changes in order to achieve clear retinal images of fixated targets. The accommodation system is said to respond to two categories of stimuli: retinotopic and spatiotopic. The influence of retinotopic cues on the accommodation response and its dynamics has been heavily investigated. Previous studies have also demonstrated that spatiotopic cues, such as size and proximity, can influence the accommodation response. However, less is understood about their influence on the dynamics of the accommodation response including latency, velocity and maximum velocity. This thesis has investigated the influence of size and proximity on the accommodation response. Dynamic accommodation responses were investigated when subjects were presented with targets of different size and in the presence and absence of blur feedback. Changes in response amplitude could be demonstrated with familiar and unfamiliar targets.When targets (familiar or unfamiliar) are presented with a size and blur change, response amplitudes are similar. This holds for both coherent and conflicting blur and size cues. However, presenting conflicting blur and size changes results in a surprising finding for some subjects. These subjects appear to respond initially to the size change, before the accommodation response is corrected according to the blur change. The magnitude of the initial, wrong, response to the size change appears to be limited by the subject's depth of focus. No differences in response latency are observed in these subjects. This confirms the initial fast phase of the accommodation response is made by spatiotopic cues prior to the overriding blur cue independent of whether the cues correlate.
Subjectively interpreting accommodative demand between paired targets that have no size change reveals a perceived blur threshold of 1.92 ± 0.62D. Such a high threshold demonstrates the difficult nature of the task. Although subjects may perceive a difference in blur, they usually fail to identify the correct direction of the target vergence change. Perceived blur thresholds can be lowered by providing congruent target size information, in addition to blur information; while providing disruptive target size information was shown to increase perceived blur thresholds even further.
Having established that changing target size can influence the accommodation response, irrespective of whether the target was familiar or not, the minimum change in target size to produce an accommodation response was investigated. By employing different types of targets at varying initial target vergences, it was observed that size had a greater influence with increasing proximity, and when the subject had no prior knowledge of the target dimensions. Considering the conditions examined in this thesis, a target must change in size by a minimum of 66% to induce an accommodation response in the absence of any further target information.
This thesis has elucidated accommodation response dynamics under unnatural viewing conditions where size and proximity on one hand and blur on the other provide conflicting information to the accommodation controller. The results may contribute to the ever growing field of representing objects in three dimensional, virtual environments, or fly glass displays where size and proximity cues are commonly in conflict with optical blur, and thereby improving the observers' visual experience by reducing the frequently reported visual discomfort and eye strain.
| Date of Award | 2014 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Dirk Seidel (Supervisor), Niall Strang (Supervisor) & Lyle Gray (Supervisor) |