The modulation of emmetropization by visual inputs

Pao Yue-kong Library Electronic Theses Database

The modulation of emmetropization by visual inputs

 

Author: Tse, Yan-yin Dennis
Title: The modulation of emmetropization by visual inputs
Degree: Ph.D.
Year: 2008
Subject: Hong Kong Polytechnic University -- Dissertations.
Myopia.
Retina -- Physiology.
Department: School of Optometry
Pages: xi, 200 p. : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2239193
URI: http://theses.lib.polyu.edu.hk/handle/200/4868
Abstract: The epidemic of myopia in urbanized area of East Asia in recent years indicates that myopia may be determined by genetic make-up as well as other environmental factors. Emmetropization is a visually guided process of eye growth which is expected to produce eyes that are free from refractive error or emmetropic. Apparently, this process has somehow failed to prevent children from becoming myopic in a number of East Asian communities. In fact, the exact mechanism of emmetropization is unclear at present. The elucidation of the signal decoding mechanism of emmetropization may be the key to unravel the mystery of recent myopia epidemic. This thesis aimed to study the decoding process of optical signals in eye growth by three approaches. The research rationale was based on the fact that the natural environment usually comprises of objects at different positions of the view of field, imposing defocus of different signs and magnitudes. The decoding process was firstly investigated by introducing superimposed competing defocus (both myopic and hyperopic) on chick eye using dual-power lenses. The results showed that the eye growth could be modulated by both myopic and hyperopic defocus even when they were presented simultaneously. With different combinations of competing defocus, the refractive status and biometric data were shown to stabilize at intermediate end-points between the two focal planes of the lens. The result suggested that the visual system can discern the signs of defocus and integrate these signals quantitatively to guide its eye growth. It was also demonstrated that pre-existing experimental myopia can be reversed by wearing dual-power lenses of matching optical powers. This finding may carry practical implication of clinical myopia control. Secondly, the range of emmetropization response for myopic defocus was determined using a lens-cone device where the viewing distance was strictly controlled. It was found that the eye can decode and emmetropize quite accurately to myopic defocus of about 20D. The data also showed that the range of response was less accurate and narrower, but not abolished, when mid-high spatial frequency was removed from the visual target. This suggested that the mid-high spatial frequency is important in optimizing the range of emmetropization response. It also suggested that the emmetropization process is also robust and it may make use of available cues when others are absent. Thirdly, it was attempted to study the effect of controlled visual scene on ernmetropization. Competing myopic and hyperopic defocus was imposed in terms of various ratios of area subtended (solid angle) using a dual-plane lens-cone device. The results showed that the resultant emmetropization responses were quantitatively related to the amount of defocus as specified by the area (solid angle) subtended. Based on this finding, a spatial-temporal integration model for signal processing of emmetropization is proposed. The eye may be able to sample and integrate visual signals from the environment according to the sign, magnitude and spatial area of defocus so as to derive corresponding eye growth. Therefore, it is hypothesized that axial emmetropia, hyperopia and myopia may be resulted from different spatial ratio between myopic defocus and hyperopic defocus.

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