published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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The vergence and accommodation systems, which are examples of physiological control systems, enable us to acquire and maintain clear single images of objects at a variety of distances in our visual world. Vergence and accommodation systems are interact with one each other and have both visual and non-visual components thus adding to their complexity. This thesis reviews the evolution of control theory models of vergence and accommodation from the 1960's to the present day and has outlined several properties of the systems which require further study. The thesis introduces the concept of Fuzzy Logic Control (FLC) to models of oculomotor control. FLC offers a new approach to modelling natural control systems and produces more realistic models than those obtained using conventional control theory techniques. Several characteristics of the vergence and accommodation systems were investigated with the aim of incorporating experimental data into control theory models using conventional techniques and FLC. The accommodation response to anisometropic stimuli was measured objectively. No evidence of a non-consensual response was found, from which it can be concluded that accommodation is consensual. A control theory model of binocular accommodation was simulated to illustrate the control strategies adopted by the accommodation during anisometropic stimulation. A Virtual Reality (VR) stimulus was used to investigate the possibility of adaptation of the crosslink components of vergence and accommodation by placing different demands on the vergence and accommodation systems. Crosslink behaviour was altered as a result of the VR stimulus which suggests that the links between vergence and accommodation (accommodative vergence and vergence accommodation) are amenable to adaptation. Control theory models were used to illustrate the effects of the VR stimulus on vergence and accommodation. The effect of proximity was investigated by measuring accommodation responses in the presence and absence of proximal cues. The effect of proximal cues under closed loop conditions was found to be minimal which suggests that proximal cues are only effective when visual cues are reduced. The results were extended to include the vergence system and a FLC model of proximal vergence and accommodation was implemented. Simulation of the model produced similar findings to a previous study which supports the use of FLC in models of oculomotor control. Voluntary vergence and accommodation were measured objectively under open loop conditions in a group of naive subjects. All subjects were able to produce voluntary responses corresponding to near and far. The ability of subjects to distinguish intermediate distances was more varied. The results show that voluntary responses can be produced without training and it is suggested that voluntary vergence and accommodation may be an important mode of response. The results were included in a control model of voluntary vergence and accommodation using FLC. The work presented provides support for the use of Fuzzy Logic in models of oculomotor control which can be used to improve models and complement existing models using conventional techniques.

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