Modelling vergence, accommodation and their interaction.
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
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
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