Refractive error and preferential looking visual acuity in infants 12-24 months of age: year 2 of a longitudinal study

This paper presents data from year 2 of a study assessing changes in visual acuity and refractive error in normal, healthy infants between birth and 24 months of age. Visual acuity and refractive error measurements were taken at 2-month intervals on 18 infants, 12-24 months of age. The acuity card preferential looking procedure and Mohindra's near retinoscopy were used for acuity and refractive error measurement. Response to the Stereo Fly was also observed. Mean acuity improved from 6.4 cycles per degree (cpd) (20/93) at 12 months to 20.5 cpd (20/29) at 24 months (SD = 0.5 octave). Refractive error remained in low hyperopia (X = 0.4D, SD = 0.5D). Amount and frequency of astigmatism showed little change. Response to the Stereo Fly improved from 0% at 12 months to 87% at 24 months. The acuity card procedure was easily accomplished, but more difficult with these children at 12-24 months of age than when previously tested between 0-12 months of age.     

Comparison between grating acuity measured by visual tracking and preferential looking in infants

Background The aim of this study was to compare the visual acuity measured by visual tracking and by preferential looking. Methods We examined 51 healthy full-term infants between three and 93 days of age. Visual acuity was tested with two different methods. One method was visual tracking (smooth and saccadic pursuit), recorded with an infrared photo-oculographic technique. For stimulation of visual tracking a square of 9.4 degrees of visual angle with vertical gratings (0.1, 0.2, 0.4, 0.8 and 1.6 cycles per degree) moved at a constant velocity of 7.5 degrees/sec. for 38sec. on a surface of equal luminance. The other method was preferential looking, determined by a second examiner, where we used plates with vertical gratings (0.1, 0.2, 0.4, 0.8, 1.6, 3.2 and 6.4 cycles per degree). Results The comparison of the two methods shows a significant correlation (p < 0.05). Visual acuity determined by visual tracking is lower than the acuity determined by preferential looking in the first 14 weeks of age. Conclusion Our study shows that visual tracking is a valid method; for visual acuity testing in infants. The need of sustained attention is a possible reason for the lower values in comparison with the acuity determined by preferential looking.     

Do preferential looking techniques underestimate infant visual acuity?

Held et al. (1979) Vision Res.19, 1377–1379, have recently provided evidence for the existence of non-monotonic psychometric functions in infant acuity testing. We argue that these non-monotonicities are statistically significant in the original report, but are not robust across small variations of stimuli and testing techniques. The existence of such non-monotonicities in some laboratories does not imply that infant acuity has been universally underestimated by all PL techniques, nor does it argue against the general sensitivity of PL techniques as measures of other visual capacities in infants.     

A new objective visual acuity test: an automated preferential looking

BACKGROUND: The Preferential Looking test using Teller acuity cards is widely used for preverbal children but has some weaknesses, in particular because it is examiner dependent and at best half-objective. We tested a new automated method in a pilot study. PATIENTS AND METHODS: In our preferential looking setting we project separate images into the left and right eye. This permits measuring each eye without having to cover the other. An infrared oculography system documents objectively and reproducibly whether or not the stimulus is seen. A square is shown which moves consecutively in a square formation on a background that has the same average brightness. Charting an XY-plot of the eye position results in a square with two diagonals, if the stimulus is seen. By using a specific baby examination unit, this test is already feasible in babies. In this study, the new setting was tested on two children. RESULTS: In two healthy children we found a good correlation between visual acuities determined with the new method and Teller visual acuity charts. Fogging one eye in each of the children resulted in fixation loss of the stimulus when the required visual acuity rose above the fogging value. CONCLUSIONS: This new technique permits one to perform an objective preferential looking test without the need of verbal answers. Larger studies have to validate these preliminary results.     

Comparison of preferential looking acuity and pattern reversal visual evoked response acuity in pediatric patients

? Background: We compared the visual acuities obtained with preferential looking (PL), the most widely used method of pediatric vision assessment, with those obtained with the spatial frequency sweep pattern-reversal visual evoked response (SPVER). ? Methods: Eighty patients (ages 1.5 months to 12 years) with various ocular pathologies pariticipated in this study. The PL acuity was determined using the up-and-down staircase procedure. The PVER was recorded with the spatial frequency sweep method using 10 spatial frequencies; the acuity was determined by placing the best-fit regression line on the descending slope of the PVER amplitude-spatial frequency function toward the higher spatial frequency to the baseline. ? Results: The PL acuities ranged from 20/25 to <20/1600 (mean 20/155). The correlation between the two methods was good (r=0.847). Fifty-six patients (70%) had an acuity agreement within 1.0 octave. When the PL acuity was >20/128, it was on average better than the PVER acuity. When the PL acuity was lower, the PVER acuity was usually better. This tendency was marked when the visual acuities were very poor (y=0.552x+0.362). ? Conclusion: The methods correlate well, although there is a dissociation of acuities in the presence of very low vision. PVER may be a useful addition to PL in assessment of vision in infants and young children.     

Measuring visual acuity in children using preferential looking and sine wave cards

Preferential looking using square waves is commonly used to measure visual acuity of infants. Since sine-wave gratings have the advantage of presenting only a single spatial frequency, we completed a study to develop and validate a set of acuity cards using sine waves. The subjects were 83 children (mean age = 41.5 months, range = 3 to 69 months). The sine-wave cards were compared with Teller cards. Identical visual acuity was determined in 83% of the cases. Wilcoxon non-parametric analysis provided no evidence to reject the null hypothesis of equal visual acuities between the two methods. We conclude that sine-wave cards may be used to measure the preferential looking acuity of children.     

Refractive error and visual acuity changes in orthokeratology patients.

PURPOSE: To evaluate the refractive error and visual acuity (VA) at various contrast levels in the two eyes of overnight orthokeratology (ortho-k) subjects, and to compare their postortho-k VA with the best corrected VA of spectacle-wearing control subjects matched for age, gender, and initial refractive error. METHODS: Distance postortho-k uncorrected and best corrected logMAR VA at four different contrast levels of 31 ortho-k (test) subjects (aged 7-35 years old) and the best corrected VA of 31 spectacle-wearing (control) subjects were measured and compared using the Waterloo Four-Contrast LogMAR VA Chart, which incorporated four sets of letters at different contrast levels: 90%, 48%, 21%, and 7%. Noncycloplegic manifest refractive error was measured in both eyes. RESULTS: The mean +/- SD percentage reductions in spherical equivalent achieved in the current study were 92% +/- 11% in the better eye and 84% +/- 14% in the worse eye of the test subjects. Postortho-k uncorrected VAs were significantly correlated with the residual overall blurring strength (length of the vector representing the residual refractive error) in both eyes at all contrast levels. The mean postortho-k uncorrected VA in the better eye were 0.00 +/- 0.11, 0.08 +/- 0.11, 0.21 +/- 0.12, and 0.46 +/- 0.13 with the 90%, 48%, 21%, and 7% contrast charts, respectively. These were comparable to the best corrected VA of the better eye of the control group with the 90% (-0.03 +/- 0.07) and 48% contrast charts (0.03 +/- 0.09), but worse than those of the control group with the 21% (0.13 +/- 0.10) and 7% (0.35 +/- 0.13) contrast charts. Postortho-k VA, with the four different contrast charts, improved by 0.07 to 0.12 log units in the better eye and 0.15 to 0.18 log units in the worse eye after correction of the residual refractive error; the improved VA was comparable to the best corrected VA of the control group. CONCLUSIONS: Postortho-k visual outcomes were compromised primarily due to the presence of residual refractive error. Although the uncorrected postortho-k VA was comparable to the best corrected VA of the spectacle wearers at high-contrast levels, it was worse at low-contrast levels and caused a significant between-eye difference at all contrast levels. Therefore, we suggested that monocular VA at high- and low-contrast levels should be evaluated for ortho-k patients.     

Clinical experience with preferential looking acuity tests in infants and young children

Early preferential looking methods for the assessment of infant visual acuity relied upon formal psychophysical procedures that were lengthy and only easily applicable to laboratory studies. Two clinically appropriate techniques evaluated in this study both maintain a forced-choice testing protocol and are administered by a single examiner. One method employs a specially constructed optical projection system to present gratings in a dark-room. The second method relies upon a commercially available acuity card test, used with good room lighting. Paediatric clinical patients and normal infants and young children were examined using both techniques. Findings demonstrate that suitably adapted preferential looking methods can provide rapid and valid estimates of visual acuity in infants and young children. The tests appear sufficiently robust to be appropriate for routine clinical use.     

Two years' clinical experience with the preferential looking technique for visual acuity determination in infants and young children

Two years' experience with a preferential looking technique for visual acuity determination was evaluated. Ninety-three patients were tested during this period, but selected groups (34 patients) were reported on earlier. In 59 patients aged 2 months to 13.75 years 131 testing procedures were performed. Diagnostic groups included uncomplicated strabismus (n = 26), strabismus combined with neurological disturbances (n = 16), neurological disturbances alone (n = 11) and other ocular diseases (n = 6). In addition, several of the children were mentally retarded. In 52 of the 59 patients in this study valuable results were obtained, while no useful information could be gathered from the remaining 7 patients. The findings influenced therapy strategies in 24 patients, and subsequent testing reflected therapeutic effectiveness. The test was successful in 7 of 8 patients referred from other clinics for estimation of visual function. Although established acuity thresholds should be regarded as conservative estimates, and not directly comparable to Snellen letter acuity notations, the preferential looking technique proved to be quite valuable in a paediatric ophthalmology unit. Main indications were amblyopia detection, control of therapeutic measures in strabismics, including pre-operative evaluation, and estimation of visual capacity in children with psychomotor retardation.     

Forced-choice preferential looking acuity in very young kittens: a model for human development

The development of visual acuity was studied longitudinally in young kittens, using a modification of the forced-choice preferential looking method (FPL). Acuity, defined as the spatial frequency which yields 70% correct responses by a naive observer, shows a 16-fold increase between 2 and 10 weeks of age. Acuity evaluated this way falls short of the acuity values obtained with the jumping stand or with electrophysiological methods. Acuity estimated with a criterion just above chance comes close to the resolution of the highest-resolving single cells in the striate cortex.     

The development of visual acuity in very young kittens. A study with forced-choice preferential looking

The development of visual acuity was studied longitudinally in young kittens, using a modification of the forced-choice preferential looking method (FPL) devised by Teller et al. [Vision Res. 14, 1433-1439 (1974)] for human infants. Acuity, defined as the spatial frequency which yields 70% correct responses by a naive observer, shows a 16-fold increase between 2 and 10 weeks of age. At comparable ages, acuity evaluated by this method falls short of the acuity values obtained with the mumping stand or with electrophysiological methods. FPL acuity estimated with a more lenient criterion (58%) comes close to the resolution of the highest-resolving single cells in the striate cortex. These results suggest that the preferential looking procedure provides a method that can be used in kittens over a wide age range, including ages at which it is impossible to use the jumping stand method.     

The effects of screen size and eccentricity on acuity estimates in infants using preferential looking

The study examined whether screen size (10° vs 19° dia.) and separation (3° vs 10° eccentricity of inner edges) affect the estimates of actuity obtained with 1–3 month infants tested by forced-choice preferential looking. One and 2-month infants (but not 3-month olds) showed higher acuity estimates with the larger screens. Screen separation did not significantly affect acuity estimates for any of the age groups. Possible factors underlying these results are discussed.     

Visual acuity development in infants and young children,as assessed by operant preferential looking

The operant preferential looking (OPL) procedure was used to obtain psychophysical estimates of visual acuity for square-wave gratings in 50 children between 5 months and 5 yr of age. Acuity developed systematically with age, from 6 min arc (5 c/deg) at 5 months to 0.75 min arc (40 c/deg) at 5 yr, a value close to adult acuity tested with the OPL stimuli. In addition, psychometric functions became steeper with age, suggesting that criterion for responding varies with age. OPL offers the possibility of using a single procedure to follow acuity development across a wide age range.     

Biofeedback training of visual acuity and myopia: a pilot study

Biofeedback training of accommodation was performed with nine subjects using the Accommotrac Vision Trainer to attempt to improve visual acuity (VA) and reduce myopia. A single-subject research design was used. Improvements in VA were seen with some subjects, but it is not clear whether the improvements were due to the biofeedback training alone, or to a learning effect observed during repeated measurements of VA. There was no change in refractive error. Implications for future research are discussed.     

Use of forced preferential looking for measurement of visual acuity in a population of neurologically impaired children

The forced preferential looking (FPL) technique has been used clinically, to assess visual acuity in infants for the past decade. It is generally accepted that the effectiveness of the procedure extends to the upper limit of 10 months of age. The authors discuss clinical observations on the viability of FPL as an effective technique in the measurement of visual acuity in an older population of neurologically impaired children.     

Effect of reinforcement on infants' performance in a preferential looking acuity task

Two experiments examined the effect of reinforcement on infants' performance in a preferential looking acuity task. In experiment 1, performance of 3- and 5-month-old infants was assessed under three conditions. In one condition, reinforcement was contingent upon performance. In another, no reinforcement was provided. In the last condition, reinforcement was provided, but it was not contingent upon performance. Similar performance was observed in all three conditions. In experiment 2, 7-month-olds were tested in the first and second conditions. No differences in performance were observed between these two groups. Thus, the reinforcement used in preferential looking measurements does not appear to improve performance significantly for 3- to 7-month-old infants. Several interpretations of these results are presented. The most plausible is that performance under nonreinforced conditions is already nearly optimal, so the addition of reinforcement has little impact. General implications for estimates of infant visual sensitivity in the laboratory and clinic are discussed.     

Refractive error in premature infants

PURPOSE: To determine the incidence and the degree of refractive error between the ages of 2 weeks and 6 months in premature infants without retinopathy of prematurity and to seek a correlation between refractive error and age at examination, birth weight, or gestational age. SUBJECTS AND METHODS: In this observational cross-sectional study, eye refraction in 390 premature infants, with no ocular pathology, was measured by cycloplegic retinoscopy at the age of 2 weeks to 6 months. A correlation was sought between refractive error and perinatal variables. RESULTS: Of the 390 infants reviewed, 347 (89%) had a refractive error and 43 (11%) were emmetropic in both eyes. Most of the infants were hyperopic (76.8%). Myopia was observed in only 11.9%. Astigmatism was found in 24.4% of the infants. The mean age at examination was 2.1 +/- 1 months; the mean birth weight was 1639 +/- 444 g, and the mean gestational age at birth was 32.2 +/- 2.4 weeks. The mean spherical equivalent of refraction was +1.56 +/- 1.82 diopters (D) in the right eye and +1.55 +/- 1.78 D in the left eye. Refractive error was positively correlated with age at examination ( R = 0.16, P = 0.001). The mean refractive error was +1.24 D in infants aged 1 month or less and reached +2.50 D at the age of 4 to 6 months. Refractive error was not correlated with birth weight or gestational age. CONCLUSIONS: The incidence of refractive error in premature infants without retinopathy of prematurity in the first 6 months of life may be as high as 89%. Most of these infants are hyperopic. Eye refraction is correlated with age at examination, but not with birth weight or gestational age.     

Normal and disordered visual development with preferential looking

Normal and disturbed development of grating acuity in preverbal infants has been studied using the Teller acuity card procedure. The results of 195 normal and 34 premature children are in good accordance with those reported in the literature. In more than 86% no interocular differences of grating acuity could be detected in these groups. On the other hand the incidence of interocular acuity differences reached 30% in 37 cases showing monolateral convergent squint. This is much higher than has been reported in current literature and may indicate that grating and Snellen acuity correlate better than expected in infants' squint amblyopia. As to the problem of monitoring early amblyopia therapy by PL, four longitudinal cases are presented.     

Measuring visual acuity in infants

This paper reviews the course of development of visual acuity in human infants and young children. Researchers have devised methods based on optokinetic nystagmus, visually evoked cortical potentials and preferential looking to assess visual acuity in infants and preverbal youngsters. During the first postnatal year and early childhood, acuity, measured by any of these methods, improves with increasing age. Each of these methods, has now been applied to the evaluation of clinical cases. Also presented are the results of a quick and simple test based on preferential looking, which has been incorporated into our clinical evaluation of infants. The data so far indicate that infants with normal eyes pass the test while infants with ocular problems which would interfere with vision fail. For evaluation of large numbers of infants this test appears to have the potential to assist nonspecialized personnel in the early identification of ophthalmic abnormalities.