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Purpose: Simultaneous comparisons of the circumferential morphological tissue profiles and final refractions from form-deprivation myopia (FDM), defocus-induced myopia (DIM), and defocus-induced hyperopia (DIH) models of ametropia have been made to test the hypothesis that changes in the thickness profiles of the three coats of the eye, and particularly that of the choroid, can be predicted from the degree of induced refractive error., Methods: Hatchling chickens (n = 23) were raised for 2 weeks wearing either a monocular translucent diffuser (FDM, n = 8), monocular -10.00 D lens goggle (DIM, n = 7), monocular +10.00 D lens goggle (DIH, n = 7), or nothing (Norm, n = 1). All animals were refracted using retinoscopy and were then sacrificed, and whole eyes were processed for scanning electron microscopy. Retinal, choroidal, and cartilaginous sclera (CS) thickness measurements were made from photographic collages of the entire circumference of the globe. Of the 23 chickens, complete morphological profile data were available for both eyes of 10 animals (nine treated and one normal). The contralateral fellow eyes (FEyes) of all nine experimental chickens were used as experimental controls as paired comparisons for statistical analyses., Results: Morphological profiles of control and experimental eyes revealed significant systematic regional variations in tissue thickness. This variation was related to nasal or temporal eccentricity with the nasal side generally thinner than the temporal. Retinal, choroidal, and CS tissue from FDM and DIM eyes showed very similar anatomical responses despite significantly different degrees of refractive change. DIH eyes showed significant increases in choroidal thickness but none in retinal or CS thickness. Analysis of fellow control eyes indicated that in both myopia models (FDM and DIM), significant changes in all tissues of the untreated fellow eyes occur whereas only the choroid of the fellow eye was affected in the hyperopic (DIH) model., Conclusions: The morphological similarity observed in the circumferential profiles of the retina, choroid, and cartilaginous sclera of the FDM and DIM eyes despite approximately 20 D difference in final refraction suggests that choroidal thickness is not a good predictor of final refractive error across models. Similarly, the final refractive difference of approximately 20 D between the DIM and the DIH eyes did not receive a major contribution from the final difference in choroidal thickness (with its implied effect on vitreous chamber length).

Background: The choroid of the chick swells markedly during recovery from experimentally induced myopia. It has been demonstrated previously that the lymphatic sinusoids of the choroid contribute most to the expansion. This raises important questions about the particular ultrastructural changes occurring in choroidal lymphatics as a means of understanding the role these vessels might play in emmetropization., Methods: Thirteen hatchling chicks were monocularly occluded for 2 weeks to induce myopia and then allowed normal visual experiences during recovery for periods of 0 to 72 h before sacrifice., Results: Electron microscopic analysis detailed the temporal progression of vascular changes and provides qualitative evidence for edema in the extravascular space. Quantitative analysis showed that the frequency of open junctions between lymphatic endothelial cells (an indicator of passive fluid transfer) increased over the 3 days of recovery. Lymphatic fenestrations (an indicator of active fluid transfer) were rare in both nondeprived eyes and in form-deprived eyes at the time of occluder removal, but increased in density significantly over the first 24 h of recovery before returning to control levels by 72 h. The number of lymphatic endothelial caveolae did not change significantly during recovery, nor did the number of fenestrations along the walls of choriocapillaris vessels., Conclusions: The walls of the lymphatics of the chick choroid open to allow greater fluid transfer during re-emmetropization than normal; the lymphatics may play an important role in the maintenance of chorioretinal fluid balance and homeostasis.

Purpose: The effect of form deprivation myopia (FDM) on cone photoreceptor function was investigated., Methods: Photopic electroretinogram (ERG) a-waves were recorded from control and form-deprived eyes of chickens. Cone-generated P3 responses were derived from the leading edge of ERG a-waves using an analytical expression derived from a quantitative model of phototransduction. The parameters obtained were the maximum cone P3 response (RmaxP3), sensitivity (S), and delay (t(d))., Results: P3 response sensitivity is significantly higher in form-deprived eyes, at lower retinal irradiances. At higher flash intensities, in form-deprived eyes, P3 response sensitivity declines at a significantly greater rate, per unit increase in retinal irradiance, than in control eyes. Visual deprivation does not significantly affect RmaxP3 or t(d)., Conclusions: Hypotheses to explain the altered cone photoreceptor sensitivity in form-deprived eyes are proposed. Changes in the biochemistry of phototransduction, or intrinsic geometric and physical attributes of photoreceptors or their waveguide modal properties, could account for the findings.

Background: Subjects with velo-cardio-facial syndrome (VCFS) were assessed to determine whether the learning disabilities (LDs) characterizing the syndrome are compounded by visual function deficits, which would be predicted on the basis of the proposed embryonic derivation of the genetic anomaly., Methods: Twelve subjects diagnosed with VCFS underwent a vision screening with emphasis on binocular function., Results: Subjects exhibited deficits in accommodation (N = 5), convergence (N = 7), fusional reserves (N = 9), and stereopsis (N = 10). Three patients were strabismic at near. No patient was in the clinically normal range for all tests of binocular function., Conclusions: Individuals with VCFS have a chromosome 22 deficit thought to alter the migration of neural crest (NC) cells, which are the embryonic precursors of the autonomic innervation of the smooth muscle components of the eye and of cranial nerves III, IV, and VI. It was predicted that VCFS patients would demonstrate reduced accommodation and convergence flexibility, which would reduce visual comfort for near work. The pathological etiology of the visual anomalies suggests that if visual training is not successful, these patients should be provided with a suitable optical reading correction from an early age.

Background: Abnormal functioning of the transient visual pathway (the M-pathway) has been implicated in specific reading disability (SRD). The aim of this study is to examine the contrast thresholds for flicker-defined form discrimination in primary school children, and to compare its development with reading and mentation development as a means of identifying children at risk of SRD., Methods: One hundred eighty-seven children (aged 4 to 13 years) and 22 adults (aged 18 to 45 years) were assessed for contrast sensitivity to an illusory, flicker contrast-defined form (the letter 'E')--a task which was designed to rely to a large extent on magnocellular pathway function. Reading age (Neale Analysis of Reading) and mental age (Raven's Coloured Progressive Matrices) were assessed in the children, who had been previously screened for clinically normal binocular vision and refractive anomalies., Results: The ability of primary school-age children to discriminate the orientation of the low contrast flickered letters (from a choice of 4) showed a significant improvement from kindergarten (ages 4 to 6 years) to grade 3 (ages 8 to 10 years) and older age groups. No significant difference was found between good and disabled readers (at least 1-year lag in reading readiness for the kindergarten group and 2-year lag in reading performance for 8 to 10 and 10 to 12-year-olds)., Conclusions: It appears that there is a developmental improvement in perceptual capacity for tasks attributed to magnocellular function, which plateaus at the age of about 8 to 10 years. However, despite the reported reduction of magnocellular function in specific reading disabled children, no significant difference in contrast threshold for flicker-defined letter discrimination was found between good and poor readers.