Models of Oculomotor Control.
Baichuan Jiang · Optometry and Vision Science · 2002
Models of Oculomotor Control. George K. Hung. Singapore: World Scientific, 2001. Pages: 127. Price: $32.00. ISBN 981-02-4568-8. Models of Oculomotor Control, by George K. Hung, is based on the author’s research over the past 20 years. This book can serve as an introductory reference for biomedical engineers, vision researchers, and clinicians working on oculomotor systems. As described by the author, the purpose of this book is twofold. One purpose is to provide biomedical engineers with the knowledge of how various engineering control principles can be applied to the study of oculomotor systems. The second purpose is to provide vision researchers and clinicians with understanding of how control system models can quantitatively represent various physiological and clinical concepts of oculomotor systems. To cover the material in sufficient depth for the different kinds of readers mentioned above, the author includes an introduction on the basic anatomy and physiology of eye movements and the basic concepts of control systems. In addition, he defines basic measurement units, i.e., the use of diopters for accommodation, the use of prism diopters and meter angles for vergence, and the measurement methodology of both accommodation and vergence. I found the table in this chapter, which contains a glossary of terms used in describing oculomotor systems, to be particularly helpful because the table bridges the terminology gap between control model language and that used in vision research laboratories and clinics. I think that it would have been helpful for the reader with a nonengineering background if the author had included and explained more terms related to the control theory models, such as lamp stimulus, open-loop, closed-loop, feedback, Laplace transform, and different operators in the glossary. The book, then, is divided into two chapters based on the analysis techniques related to the treatments of static and dynamic responses of the oculomotor system. In the chapter on static analysis techniques, the author covers the static model of the accommodative system, the static interactive model of the accommodation and vergence systems, and the model that includes a proximal input. Using the nonlinear model, in which the dead-space operators DOF in the accommodative system and PFA in the vergence system are taken into account, the author was able to explain the discrepancies between the associated and disassociated phoria as well as the results when the AC/A ratio is measured by phoria and fixation disparity methods. Because most targets and measures used in the optometry clinic are static, I think that the static models may help the clinicians analyze their clinical results more easily. Although further studies are needed to apply the models to analyzing binocular vision disorders, the modeling work described in this book suggests that this could be possible. In the chapter on dynamic analysis techniques, the author covers several topics. The main sequence analysis provides information on the relationship between response amplitude and peak velocity. The root locus analysis is used for determining the dynamic stability of the accommodative system. The dual-mode models for the accommodation and vergence systems are the most important contribution of the author and his colleagues in this field. The underlying principle for the dual-mode models was based on the experimental recordings of the accommodation or vergence responses to slow and fast ramp stimuli. They suggested that there were two mutually-exclusive modes of responses: a fast, open-loop response that corrected most of the error between response and stimulus, followed by a slow, closed-loop response that reduced the residual error to a minimum. In the adaptation model of accommodation and vergence suggested by the author, an adaptive component is added into the modified static dual-interactive model of the accommodation and vergence systems, in which the controllers in the forward loops have been designed to have dual-mode responses to dynamic stimuli. Following this approach, the author further describes how the models for simulating nearwork-induced transient myopia, refractive error development, and saccade-vergence interactions were developed. Because the models described in this book are based on the author’s work, this may limit the reader’s views for understanding and comparing different models suggested by others. Disregarding this weakness, the progress in modeling the oculomotor control system illustrated in this book provides us with a vivid picture of how a biomedical scientist has combined his knowledge in control system theory with his experimental results in vision science and optometry to make a significant contribution to this field. FIGUREFigure