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PubMed2000

Corneal shape of Hong Kong-Chinese.

Corneal asphericity (the p-value, p) and apical corneal radius (r0) were calculated for 63 Hong Kong-Chinese (39 male and 24 female), using the Topographic Modeling System (TMS-1). The values of p and r0 in, and between, the two principal meridians were compared, and the effects of refractive error and gender were also investigated. The mean +/- SD r0 and p along the flattest meridian of the right eye were 7.82 +/- 0.26 mm and 0.78 +/- 0.12 respectively. The mean +/- SD r0 and p along the steepest meridian of the right eye were 7.64 +/- 0.26 mm and 0.83 +/- 0.15 respectively. Meridional variations were found in r0 and in p. There was no correlation between p and r0 along the two principal meridians. p was not significantly correlated to the refractive error but r0 was. Our results agree with previous findings that while males have longer r0 than females, their p are not significantly different.

China↗
PubMed1992

Refractive plasticity of the developing chick eye.

We have developed a lightweight plastic goggle with rigid contact lens inserts that can be applied to the eyes of newly hatched chicks to explore the range and accuracy of the developmental mechanism that responds to retinal defocus. Convex and concave lenses of 5, 10, 15, 20 and +30 D were applied to one eye on the day of hatching. The chick eye responds accurately to defocus between -10 and +15 D, although hyperopia develops more rapidly than myopia. Beyond this range there is first a levelling off of the response and then a decrease. The resulting refractive errors are caused mainly by increases and decreases in axial length, although high levels of hyperopia are associated with corneal flattening. If +/- 10 D defocusing lenses are applied nine days after hatching the resulting myopia and hyperopia are equal to about 80% of the inducing power. After one week of inducing myopia and hyperopia with +/- 10 D lenses, the inducing lenses were reversed. In this case, the refractive error did not reach the power of the second lens after another week of wear. Instead, astigmatism in varying amounts (0-12 D) was produced, being greater when reversal was from plus to minus. Finally, astigmatism can also be produced by applying 9 D toric inducing lenses on the day of hatching. The astigmatism produced varies from 2 to 6 D, and the most myopic meridian coincides with the power meridian of the inducing lens. This astigmatism appears to be primarily due to corneal toricity.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed2004

Productivity associated with visual status of computer users.

BACKGROUND: The aim of this project is to examine the potential connection between the astigmatic refractive corrections of subjects using computers and their productivity and comfort. We hypothesize that improving the visual status of subjects using computers results in greater productivity, as well as improved visual comfort. METHODS: Inclusion criteria required subjects 19 to 30 years of age with complete vision examinations before being enrolled. Using a double-masked, placebo-controlled, randomized design, subjects completed three experimental tasks calculated to assess the effects of refractive error on productivity (time to completion and the number of errors) at a computer. The tasks resembled those commonly undertaken by computer users and involved visual search tasks of: (1) counties and populations; (2) nonsense word search; and (3) a modified text-editing task. RESULTS: Estimates of productivity for time to completion varied from a minimum of 2.5% upwards to 28.7% with 2 D cylinder miscorrection. Assuming a conservative estimate of an overall 2.5% increase in productivity with appropriate astigmatic refractive correction, our data suggest a favorable cost-benefit ratio of at least 2.3 for the visual correction of an employee (total cost 268 dollars) with a salary of 25,000 dollars per year. CONCLUSIONS: We conclude that astigmatic refractive error affected both productivity and visual comfort under the conditions of this experiment. These data also suggest a favorable cost-benefit ratio for employers who provide computer-specific eyewear to their employees.

Adult↗
PubMed2003

Prevalence and causes of blindness and visual impairment in Bangladeshi adults: results of the National Blindness and Low Vision Survey of Bangladesh.

AIM: To determine the age, sex, and cause specific prevalences of blindness and visual impairment in adults 30 years of age and older in Bangladesh. METHODS: A nationally representative sample of 12 782 adults 30 years of age and older was selected based on multistage, cluster random sampling with probability proportional to size procedures. The breakdown of the cluster sites was proportional to the rural/urban distribution of the national population. The examination protocol consisted of an interview, visual acuity (VA) testing, autorefraction, and optic disc examination on all subjects. Corrected VA retesting, cataract grading, and a dilated fundal examination were performed on all visually impaired subjects. The definitions of blindness (<3/60) and low vision (<6/12 to >or=3/60) were based on the presenting visual acuity in the better eye. The World Health Organization/Prevention of Blindness proforma and its classification system for identifying the main cause of low vision and blindness for each examined subject was used. RESULTS: In total, 11 624 eligible subjects were examined (90.9% response rate) across the 154 cluster sites. A total of 162 people were bilaterally blind (1.53% age standardised prevalence) while a further 1608 subjects (13.8%) had low vision (<6/12 VA) binocularly. Visual acuity was >6/12 in the "better eye" in the remaining 9854 subjects (84.8%); however, 748 of these people had low vision in the fellow eye. The main causes of low vision were cataract (74.2%), refractive error (18.7%), and macular degeneration (1.9%). Cataract was the predominant cause (79.6%) of bilateral blindness followed by uncorrected aphakia (6.2%) and macular degeneration (3.1%). CONCLUSIONS: There are an estimated 650 000 blind adults (95% CI 552 175 to 740 736) aged 30 and over in Bangladesh, the large majority of whom are suffering from operable cataract. This survey indicates the need for the development and implementation of a national plan for the delivery of effective eye care services, aimed principally at resolving the large cataract backlog and the inordinate burden of refractive error.

Adult↗
PubMed1995

Ophthalmoscopic measurement of the optic disc.

PURPOSE: The authors evaluated whether the optic disc dimensions can be measured directly by ophthalmoscopy without the use of a fundus camera or special ophthalmoscope. MATERIALS AND METHODS: The horizontal and vertical disc diameters were measured ophthalmoscopically in 158 eyes of 95 subjects using a standard Goldmann three-mirror contact lens and a commercial slit lamp with adjustable length of the beam. The refractive error ranged from -7.25 to 3.5 diopters. Based on these direct measurements, the authors calculated the disc area by applying a modified formula for an ellipse, where area = pi/4 x the horizontal diameter x the vertical diameter. Additionally, the authors determined by planimetry the diameters and area of the disc on color photographs after correcting the ocular and photographic magnification. RESULTS: The values of the disc diameters as determined by photography were x1.27 larger than those measured with the Goldmann contact lens. Taking into account this constant correction factor of 1.27 for all eyes examined, the disc diameters, as measured by the Goldmann lens, varied by 0.11 +/- 0.06 mm or 5.8% +/- 3.5% from the values measured on the photographs. This error decreased slightly (P = 0.04) with increasing disc size. It was independent of the refractive error. The calculated disc area based on the ophthalmoscopically determined disc diameters differed by 9.4% +/- 6.6% from the disc area as measured by planimetry on the photographs. CONCLUSIONS: For clinical purposes, the optic disc and other structures of the posterior fundus can be measured by ophthalmoscopy using a Goldmann contact lens and a slit lamp with adjustable beam length. The optic disc area can be calculated by the formula: horizontal disc diameter x vertical disc diameter x pi/4.

Adult↗
PubMedDate not supplied

Pattern of childhood blindness at a referral center in Saudi Arabia.

BACKGROUND: An understanding of the causes of blindness and the magnitude of the problem is crucial in designing effective intervention and prevention programs. We undertook this retrospective review to determine the causes of childhood blindness at an eye referral center. PATIENTS AND METHODS: We reviewed charts of children who presented between August 1997 and August 2003. All children had a complete ophthalmologic examination. Blindness was defined as a visual acuity < 20/400, visual impairment as visual acuity between 20/400 and 20/60, and visual loss as a visual acuity < 20/60. RESULTS: A total of 5217 children included 220 (59%) males and 152 (41%) females (age range 2 to 18 years, mean age, 10 years). One hundred twelve (2%) were blind and 260 (5%) had visual impairment. The most common causes of bilateral blindness included optic nerve diseases, retinal disorders, and cataract. The most common causes of unilateral blindness included trauma, retinal diseases, refractive errors, and optic nerve diseases. The most common causes of bilateral visual impairment included refractive errors, corneal diseases, retinal disorders, cataract, and congenital nystagmus. Genetically determined disorders were observed in 37 (70%) of 53 patients with bilateral blindness, and in 52 (56%) of 93 patients with bilateral visual impairment. The incidence of consanguinity among parents of children with acquired causes was 2 (3%) of 59 patients compared to 34 (38%) of 89 among genetically determined causes (P < 0.001). CONCLUSION: Genetically determined disorders continue to play an important role in the causation of childhood blindness among patients attending our referral center in Saudi Arabia. Genetic counseling, early eye screening of children and public education may help in the prevention of visual disorders in children.

Adolescent↗
PubMed2006

Corneal astigmatic effects of conventional recession vs suspension recession ("hang-back") strabismus surgery: a pilot study.

INTRODUCTION: Changes in refractive error after strabismus surgery occur secondary to changes in corneal curvature. The change in refractive error is usually transient but may lead to a temporary decrease in vision. We compared the change in corneal curvature following strabismus surgery in eyes utilizing a suspension (so-called "hang-back") recession technique to eyes undergoing conventional recession surgery. METHODS: Sixteen patients undergoing bilateral medial or lateral rectus recessions were prospectively entered into the study. One eye was randomly selected to undergo conventional strabismus surgery and the other eye underwent strabismus surgery using the suspension technique. Keratometry measurements were taken in the operating room immediately prior to and immediately after surgery. The change in corneal curvature was calculated and analyzed for each technique. RESULTS: The mean change in corneal curvature was -0.06 diopters (D) for the suspension group and +0.18 D for the conventional group, confirming prior reports. However, all eyes undergoing suspension surgery had a change within 1 D whereas only 75% of those eyes undergoing conventional surgery fell within the same range. Overall there was twice the variation in the degree of astigmatism in the conventional group when compared with the suspension group. CONCLUSIONS: No eye undergoing suspension surgery experienced a change in corneal curvature greater than 1 D while 25% of those undergoing conventional surgery did. Therefore, possible benefits to the suspension recession technique might include less overall variability of astigmatism change and decreased amounts of induced astigmatism. Patients undergoing strabismus surgery using a suspension technique may be less likely to notice a change in their vision in the immediate postoperative period.

Astigmatism↗
PubMedDate not supplied

Blindness and vision impairment in the elderly of Papua New Guinea.

BACKGROUND: To estimate the magnitude and causes of blindness and vision impairment in Papua New Guinea for service delivery planning and ophthalmic education development. METHODS: Using the World Health Organization standardized Rapid Assessment of Cataract Surgical Services protocol, a population-based cross-sectional survey was conducted in 2005. By systematic, two-stage cluster random sampling, 39 clusters each of 30 people aged 50 years and over were selected from urban and rural locations. A cause of vision loss was determined for each eye with a presenting visual acuity worse than 6/18. RESULTS: Of the 1191 people enumerated, 1174 were examined (98.6%). The 50 years and older age-gender adjusted prevalence of vision impairment (presenting visual acuity less than 6/18 in the better eye) was 29.2% (95% Confidence Interval [CI]: 27.6, 35.1, Design Effect [deff] = 2.3). That of functional blindness (presenting visual acuity less than 6/60 in the better eye) was 8.9% (95% CI: 8.4, 12.0, deff = 1.2), and of World Health Organization blindness (but presenting, rather than best corrected, visual acuity of less than 3/60 in the better eye) was 3.9% (95% CI: 3.4, 6.1, deff = 1.0). Uncorrected refractive error (13.1%, 95% CI: 11.3, 15.1, deff = 1.2) and cataract (7.4%, 95% CI: 6.4, 10.2, deff = 1.3) were leading causes of vision impairment, age-gender adjusted. Cataract was the most common (age-gender adjusted 6.4%, 95% CI: 5.1, 7.3, deff = 1.1) cause of functional blindness. On bivariate analysis, increasing age (P < 0.001), illiteracy (P < 0.001) and unemployment (P < 0.001) were associated with functional blindness. Gender was not. CONCLUSIONS: The identification and treatment of refractive error and cataract need to be priorities for eye health services in Papua New Guinea if the burden of vision impairment and blindness is to be diminished. The education of community and hospital eye care providers, whether medical, nursing or other cadres, must emphasize these. Eye care services must be structured and provided to allow and encourage accessibility and uptake, with satisfactory treatment outcomes for these conditions.

Aged↗
PubMed1989

Effect of defocus on visual acuity as measured by source and observer methods.

The relation between refractive error and visual acuity has been measured by two very different methods. In one called "source methods," emmetropes or corrected ametropes view defocused stimuli presented on projection screens or photographs. In the type called "observer methods," focused stimuli are presented to the observers who are either uncorrected ametropes or emmetropes defocused by lenses placed (usually), in the spectacle plane. The study reported in this paper demonstrates for the first time that these two methods of defocusing retinal images and their effects on visual acuity can be correlated. Results show that the source method of producing defocus could be used interchangeably with the observer method in investigating the rates of change of visual acuity with defocus for young normal observers. The angular diameter of the defocused image of a point, the blur disc diameter in object space, allows the two methods to be compared. Although the results show that the two methods are highly correlated, they show that the source method gives a statistically but not clinically significant lower acuity. The results of both methods are used to derive an equation linking refractive error, visual acuity, and pupil diameter.

Accommodation, Ocular↗
PubMed2000

Prevalence of eye diseases in primary school children in a rural area of Tanzania.

AIMS: The study measured the prevalence of eye diseases in primary school children between 7 and 19 years of age in a rural area of Tanzania, and investigated whether teachers could successfully provide the first component of a school eye screening service. METHODS: Teachers from each of three primary schools in Mwanza Region tested visual acuity using a Snellen's E chart in 1438 pupils. 1386 of these pupils were then interviewed and underwent a full eye examination by an eye team. RESULTS: 10 pupils (0.7%) had bilateral poor eyesight (visual acuity worse than 6/12), and an additional 14 pupils (1.0%) had unilateral poor eyesight. Significant refractive errors causing visual acuity less than 6/12 (1.0%), strabismus (0. 5%), and amblyopia (0.2%) were uncommon. Overall, 76 pupils (5.5%) had active trachoma, though the prevalence was 15.5% in the poorest school. 73 pupils (5.3%) reported night blindness, eight (0.6%) had Bitot's spots, and 11 (0.8%) had corneal scars. Simple screening by teachers correctly identified 80% of the pupils who were found to have bilateral poor eyesight by the eye team, with 91% specificity. CONCLUSION: The prevalence of significant refractive errors was not high enough to justify a school eye screening programme solely for this purpose. However, a programme may be justified in areas where trachoma is common. Further research is needed to validate the frequent reports of night blindness and to establish the public health importance of vitamin A deficiency in this age group.

Adolescent↗
PubMed2005

Ophthalmologic abnormalities in high school students with mental retardation in Taiwan.

BACKGROUND AND PURPOSE: Visual and ocular disorders are common in mental retardation (MR), and can influence sensory-motor development and learning. This study investigated visual and ocular abnormalities in a population of high school students with MR. METHODS: All students with MR in their first year at a special education high school who underwent ophthalmic examinations in December 2001 were included. Data were collected by retrospective review of hospital records and student health records. RESULTS: A total of 68 students with MR, including 45 males and 23 females, completed ophthalmologic examinations. Abnormal ophthalmologic findings in these 68 students (15-23 years old) included astigmatism (74.4%), myopia (53.7%), amblyopia (29.3%), exodeviation (23.5%), anisometropia (22.0%), blepharoconjunctivitis (20.6%), hyperopia (18.2%), cataract (13.2%) and suspected glaucoma (11.8%). Amblyopia was mainly attributed by refractive errors and strabismus. Myopia was less prevalent than in the general population of the same age in Taiwan, but astigmatism and anisometropia were more prevalent. More than one-third of the exodeviations were large-angle exotropia. The presence and type of cataract were highly associated with Down's syndrome. CONCLUSION: The prevalence of visual and ocular disorders in teenage and young-adult students with MR is high. Because these disorders can be detrimental to learning, early and regular ophthalmic examinations of children with MR are mandatory, with particular attention to screening for refractive error, amblyopia, strabismus and cataract and treatment of these conditions when identified.

Adolescent↗
PubMed1999

Amblyopia and visual acuity in children with Down's syndrome.

BACKGROUND/AIMS: Amblyopia in people with Down's syndrome has not been well investigated. This study was designed to determine the prevalence and associated conditions of amblyopia in a group of home reared children with Down's syndrome. METHODS: All children in the study group underwent an evaluation of visual acuity. In addition, previous ophthalmological records were reviewed, and a subgroup of children was examined. For the purposes of this study, amblyopia was defined quantitatively as a difference of two Snellen acuity lines between eyes or if unilateral central steady maintained (CSM) vision and a clear fixation preference was observed. A high refractive error was defined as a spherical equivalent more than 3 dioptres and astigmatism more than 1.75 dioptres. Anisometropia was defined as a difference of at least 1.5 dioptres of sphere and/or 1.0 dioptre of cylinder between eyes. 68 children with Down's syndrome between the ages of 5 and 19 years were enrolled in the final study group. RESULTS: Amblyopia was observed in 15 (22%) of 68 patients. An additional 16 (24%) patients had bilateral vision less than 20/50. Strabismus, high refractive errors, and anisometropia were the conditions most commonly associated with decreased vision and amblyopia CONCLUSION: This study suggests that the prevalence of amblyopia is higher than previously reported. Fully 46% of these children with Down's syndrome had evidence of substantial visual deficits. These patients may be at higher risk for visual impairment and should be carefully examined for ophthalmological problems.

Adolescent↗
PubMed1999

Development of refraction and strabismus.

In the past year, as in recent years, most of the research on the development of refraction has focused on the following: 1) mechanisms whereby the eye can maintain coordinated growth to achieve emmetropia and 2) disruptions of emmetropization resulting in myopia or hyperopia. Preterm children and those with Down syndrome have higher refractive errors than other children, suggesting a failure of emmetropization. One of the most intriguing studies of the past year and one certain to lead to follow-up studies reported that ambient room illumination at night in a child's first two years is associated with a higher prevalence of myopia than sleeping in darkness. Reports on the development of hyperopia showed that it is axial in nature, similar to myopia. The effects of spectacle interventions to correct refractive errors are still being debated, with recent evidence from children suggesting that lenses do not exacerbate myopia. Analyses of risk factors and numerous new screening procedures detect patients with strabismus for referral at a variety of sensitivity and specificity levels. Hyperopia and high AC/A ratios are most clearly associated as causal agents for esotropia and intermittent exotropia. However, the action of even these simple mechanisms is confounded by abnormal binocular fusion mechanisms and the inability of optical correction to align the eyes of many patients. Asymmetric optokinetic nystagmus, latent nystagmus, and dissociated vertical deviation appear to be linked to infantile esotropia from before its onset. But the way the mechanisms underlying these oculomotor anomalies are causally related to the onset of infantile esotropia remains a mystery.

Humans↗
PubMed2005

Optometric and ophthalmic contact in elderly hip fracture patients with visual impairment.

AIM: To describe previous contact with optometry and ophthalmic services in a group of elderly patients with and without visual impairment (VI) who had fallen and sustained a fractured neck of femur. METHOD: A cross sectional study of 537 patients aged 65 and over who had undergone hip fracture surgery in four Scottish centres (Glasgow, Ayr, Dundee and Fife). All patients had an in-depth optometric history, ophthalmic history and examination. RESULTS: Three hundred and ninety-three (79%) patients reported optometric contact in the 3 years preceding surgery and 107 (21%) patients had not seen an optometrist for more than 3 years. In the latter group, 64 had VI, which was due to uncorrected refractive error in 17 (27%) and untreated cataract in 20 (31%). VI (best binocular visual acuity of 6/18 or less) was found in 239 (46%) patients. A past ophthalmic history was present in 257 (50%) patients. Only 39 (16%) patients with VI were under ophthalmic care at the time of the study. CONCLUSIONS: There was significantly poor optometric and ophthalmic contact in patients who had VI and had fallen and sustained hip fracture. A proportion of the VI (66%) was due to uncorrected refractive error and untreated cataract. Public health providers should be made aware of the fact that current optometric and ophthalmic care pathways are not accessed by this group of elderly patients with VI and at risk of falling.

Accidental Falls↗
PubMed2004

Causes of visual impairment in people aged 75 years and older in Britain: an add-on study to the MRC Trial of Assessment and Management of Older People in the Community.

BACKGROUND: Visual impairment and blindness are common in older people in Britain. It is important to know the causes of visual impairment to develop health service and research priorities. The authors aimed to identify the causes of visual impairment in people aged 75 years and older in Britain. METHODS: In the MRC Trial of the Assessment and Management of Older People in the Community, trial nurses tested visual acuity in everyone aged 75 years and older in 53 general practices. For all visually impaired patients in 49 of the 53 medical practices, data regarding the cause of vision loss were extracted from the general practice medical notes. Additional follow up questionnaires were also sent to the hospital ophthalmologist to confirm the cause of vision loss. Visual impairment was defined as a binocular acuity of less than 6/18. RESULTS: There were 1742 (12.5%) people visually impaired in the 49 participating practices. Of these, 450 (26%) achieved a pinhole visual acuity in either eye of 6/18 or better. In these people, the principal reason for visual loss was considered to be refractive error. The cause of visual loss was available for 976 (76%) of the remaining 1292 visually impaired people identified. The main cause of visual loss was age related macular degeneration (AMD); 52.9% (95% confidence interval 49.2 to 56.5) of people had AMD as a main or contributory cause. This was followed by cataract (35.9%), glaucoma (11.6%), myopic degeneration (4.2%), and diabetic eye disease (3.4%). CONCLUSIONS: A substantial proportion of visual impairment in our sample of older people in Britain can be attributed to remediable causes-refractive error and cataract. There is considerable potential for visual rehabilitation in this age group. For the large proportion with macular degeneration, low vision services will be important.

Aged↗
PubMed1997

Visual impairment in school children in southern India.

This study was done to determine the prevalence of visual impairment due to refractive errors and ocular diseases in lower middle class school children of Hyderabad, India. A total of 4,029 children, which included 2,348 males and 1,681 females, in the age range of 3 to 18 years from 9 schools were screened with a detailed ocular examination protocol. Among 3,669 children in whom visual acuity could be recorded, on presentation 115 (3.1%) had visual acuity < 6/18 in the better eye (equivalent to visual impairment), while 41 (1.1%) had visual acuity < or = 6/60 [corrected] in the better eye (equivalent to legal blindness) out of which 18 (0.5%) had visual acuity < 6/60 in the better eye (equivalent to economic blindness). Of 115 children who presented with initial visual acuity < 6/18, vision improved to > or = 6/18 with refraction in 109 (94.8%). No child was legally or economically blind after refractive correction. Prevalence of hyperopia was 22.6%, myopia 8.6% and astigmatism 10.3%. The prevalence of myopia was significantly higher among children > or = 10 years of age (P < 0.001). The maximum, mean and median values for myopia were 10.00, 1.35 and 0.75 D in the better eye. For hyperopia these values were 8.50, 0.65 and 0.50 D. The major causes for best corrected visual acuity < 6/9 in the worse eye for 51 (1.4%) children included amblyopia in 40 (1.1%), corneal diseases in 5 (0.1%), cataract in 2 (0.05%) and others in 4 (0.1%). Out of the total, 30 (0.7%) children had strabismus. These data support the assumption that vision screening of school children in developing countries could be useful in detecting correctable causes of decreased vision, especially refractive errors, and in minimising long term permanent visual disability.

Adolescent↗
PubMed1995

[Sensitivity of refraction measurement with the infrared videorefractometer VRB 200].

AIM OF THE STUDY: This study investigates the sensitivity of a computerised videorefractor (VRB 200) for refractive errors, especially amblyopiogenic, in babies and infants. PATIENTS AND METHODS: Videopictures from 312 eyes of 156 babies and infants were taken, evaluated and compared with the current measurements (skiaskopy or refractometry) of the university eye hospital Erlangen-Nürnberg. RESULTS: 255 (81.7%) of 312 eyes were evaluable. The VRB 200 achieves a sensitivity of 96.1% in hyperopic eyes (219 of 228 eyes) and 94.7% in myopic eyes respectively (18 of 19 eyes). Of 8 emmetropic eyes 6 were recognized correctly negative. Astigmatism could be estimated coarsely quantitatively in power and qualitativ in the axis with a sensitivity of 80% (92 of 115 eyes) and a specificity of 86% (121 of 140). Anisometropia over 2 dpt was recognized in 5 of 6 patients. A general statement concerning the specificity is not possible with this study, because it is not a mass screening procedure. CONCLUSION: This videorefractor is able to recognize semi-quantitatively ambylopiogenic refractive errors with high sensitivity.

Amblyopia↗
PubMed2000

Development of refraction and strabismus.

Research on the etiology and causes of refractive errors has become a very active field of study during the past few years. Most of this research has focused on myopia. But hyperopia and astigmatism are also being examined both in comparison to myopia and in their own right. Animal models have also been developed for the study of experimentally induced myopia and hyperopia. These studies demonstrate the chain of neural and molecular events that occurs in induced myopia and hyperopia with increasing precision. In the future, these results may elucidate the mechanisms that underlie the refractive errors seen in human populations. Research into the development of strabismus has not progressed with the same vigor. The links among hyperopia, accommodative convergence, and strabismus are well established. Numerous neural, oculomotor, and subjective correlates of strabismus are now well established, but there has been a failure to develop the experimental paradigms needed to demonstrate the causal relations among these different factors.

Adaptation, Ocular↗