Visual and retinal control of eye growth and refraction

In contrast to previous investigations of myopia development in humans that have primarily examined the effectiveness of therapies, the present thesis examines the visual factors underlying the emmetropization process and the formation of form-deprivation myopia. The chick has\ud been used as a model in this research. The experiments have been designed on the premise that emmetropization is a vision-dependent phenomenon, with information provided by the visual image guiding the growth of the eye.\ud Initially the effects of physiological variants were studied. The results showed that normal ocular growth rates are both breed- and sexdependent in the chick, and also that different breeds of chicks differ in both their susceptibility to form-deprivation myopia and in the speed of their recovery from form-deprivaiton myopia. This result has important implications with respect to interpretations and comparisons of studies using differenct breeds and/or sexes of chicks. Interrupting deprivation with even a very short period of normal vision had a marked effect on deprivation-induced myopia. Neutralization of a "myopic defocus signal" was found to occur in less\ud than 20 minutes. Periods of normal vision from 20 min to 60 min were equally effective at reducing the high myopia and suggest that defocus is not sampled continually; most of the recovery appeared to be due to choroidal expansion. The effectiveness of normal vision in guiding emmetropization was found to be independent of the timing of the experience. Periods of normal vision given in the morning, i.e. at the start of the light cycle, were as effective as those given in the afternoon, i.e. at the end of the light cycle, in preventing occlusion-induced myopia.\ud Low powered spectacle lenses, i.e. +1 D and -1 D were used to determine the sensitivity of the emmetropization process to defocus. Compensation for the lenses occurred even though the depth-of-focus of the chick eye was estimated to be between ±0.75 D and ±2 D and the defocus may have been below the detection threshold of the retina.Similarly compensation for larger induced refractive errors, i.e. +10 D and -10 D occurred. Hyperopia in response to +10 D lens wear occurred very rapidly and some hyperopia was seen even when lens wear was interrupted by long periods of normal vision. In contrast, the response to -10 D lenses was much slower and myopia was only observed if the lens was applied continuously. As a possible explanation for the differential\ud response to different types of lenses it was suggested that accommodation would differentially affect the degree of "image blur", with large degrees of defocus for positive lenses and effectively none for the negative lenses.\ud The experiment was thus repeated following dliary nerve section to eliminate accommodation as a factor. The results found with ciliary nerve section were similar to those obtained without and thus accommodation does not explain the nonlinearity of effect. An alternative model involving the speed of physiological growth responses\ud was suggested. The similarity of the effect of periods of normal vision on form-deprivation myopia and lens-induced myopia suggests that similar emmetropizing growth signals are experienced during the period of normal vision regardless of the method of myopia production.\ud To investigate further the role of accommodation in\ud emmetropization, compensation to negative spectacle and hard contact lenses were compared. Hard contact lenses were presumed to decrease the gain of the accommodation system by eliminating the refractive effects of corneal accommodation; thus for any given accommodative "effort" the\ud refractive effect would be lower. Emmetropization occurred in response to both types of lenses, again suggesting that accommodation has a limited role in the emmetropization system. Other likely defocus signals were thus investigated.\ud The chick eye was found to possess 3.7 D of longitudinal chromatic aberration, measured by chromoretinoscopy. Longitudinal chromatic aberration can potentially provide information about defocus and hence its role in emmetropization was studied by rearing chicks under\ud monochromatic light. It was found that adequate information was provided by monochromatic light to guide emmetropization in the chick,whether the monochromatic light was from the extremes (red and blue\ud light), or the centre (yellow light), of the visible spectrum. However, it was also found that emmetropization to the refractive difference present in different coloured lights does not occur. The results of this study were\ud supported by an experiment using an interrupted-occlusion paradigm. It was assummed that during the period of "normal vision", if the information provided was adequate, that defocus cues would be detected and the effect of occlusion reduced. Chicks were monocularly occluded and the occlusion interrupted by a brief exposure to "normal vision" per\ud day under monochromatic light. Prevention of myopia, i.e.\ud emmetropizing growth, was observed. The results of both of these studies suggest the presence of non-chromatic cue or cues to defocus.The interrupted-occlusion paradigm was likewise used to investigate possible non-chromatic cues to defocus. During the period of occlude removal stimuli of either restricted contrast or restricted spatial frequency\ud information were presented. Restricted contrast environments were found to be as effective as normal vision at reducing the magnitude of occlusion-induced myopia. The data indicated that a varied contrast environment is not required for emmetropization but suggest the\ud presence of a contrast threshold, less than 4% for the chick, below which emmetropization becomes inaccurate. Emmetropization was spatialfrequency-dependent; the ability of restricted spatial frequency environments to reduce occlusion-induced myopia varied with the spatial frequency presented. The data further indicated that mid-spatial\ud frequencies are required for emmetropization. The results suggest that there are additional as yet unknown non-chromatic cue for defocus.\ud The main findings of this thesis show that in the chick:\ud i) myopic defocus is detected within 20 min,\ud ii) emmetropization to refractive defocus of opposite signs is highly nonlinear\ud and differentially affected by normal vision,\ud iii) emmetropization occurs under monochromatic light from the centre and extremes of the visible spectrum,\ud iv) there is at least one non-chromatic visual cue involved in emmetropization,\ud v) a varied contrast environment is not required for emmetropization,\ud vi) emmetropization is spatial frequency dependent,\ud vii) intermediate spatial frequencies appear to be required for emmetropization.\ud A preliminary model is proposed for the refractive error detector of the visual system