Visual Acuity and the Crowding Effect in 8- to 17-Year-Old Schoolchildren

Visual acuity (VA) in 292 schoolchildren aged 8–17 years was measured using Landolt Cs, E letters, and rectangular gratings. With the forced choice procedure, the VA measured with Landolt Cs was 1.4 times higher than with other targets, suggesting analysis of the whole image rather than its gaps by the visual system. In addition, the effect of surrounding images on VA estimates was studied with Landolt Cs, E letters, and gratings surrounded by bars, E letters, and gratings, respectively. A crowding effect, i.e., a worse perception of test objects surrounded by other images, was revealed. However, the perception of Landolt Cs, E letters, and gratings showed age-dependent differences. The separation between the stimulus and surrounding images at which the perception of gratings and other images worsened decreased with age increasing up to 16–17 and 12 years, respectively. The age-related differences were explained by the immaturity of selective-attention mechanisms.     

Measurement of visual acuity and crowding effect in 3–9-year-old children

Visual acuity was measured in 140 3–9-year-old children using Landolt’s rins with a gap in one of four orientations. The influence of surrounding additional contours on the detection of orientation of rings of a minimal visible size was studied in the same children, depending on the distance between the rings and the surrounding. The latter consisted of four bands, located symmetrically with reference to the ring. Visual acuity increased with age, reaching the adult level by approximately seven years. A deterioration of the orientation recognition was detected in all children when the surrounding was close to a ring. The distance that caused a perception deterioration decreased with age, reaching the adult level by nine years. The obtained data are discussed from the viewpoint of a possible influence of individual visual areas formed during ontogeny on task performance.     

Perception of visual image size by school students of different ages

In psychophysical experiments, 209 high school students with normal vision, 8 to 16 years old, were examined to study the perception of visual image sizes. Observers assessed the length of linear arrow-like figures (the Müller-Lyer illusion) or the length of single lines without arrow-like ends. Distortion of line size perception by 17–21% was obtained in the Müller-Lyer illusion independent of the age of observers. Distortion of size perception was absent in the case of estimation of the length of single line segments. The size differentiation thresholds gradually decreased in both cases with increasing age of observers and were correlated with the acuity of vision. For single lines, they were, on average, 1.3 times lower than the thresholds in the Müller-Lyer illusion. The probable mechanisms of the Müller-Lyer illusion are discussed. The experimental results demonstrate stability of illusion for observers aged 8–16 years, which may be connected with preferential contribution of the lower levels of the visual system to the appearance of the illusion.     

Visual acuity of the midland banded water snake estimated from evoked telencephalic potentials

The visual acuity of seven midland banded water snakes was measured by recording evoked responses from telencephalon to temporally modulated square wave grating patterns. Using conventional electrophysiological techniques and signal averaging, high contrast square wave gratings of different spatial frequencies were presented. Acuity was estimated by extrapolating relative response amplitude/log₁₀ spatial frequency functions which yielded an average acuity of 4.25 cycles/degree. Refractive state was also estimated by recording evoked potentials to intermediate spatial frequencies with different lenses in front of the eye. Polynomial fits indicated that under the experimental conditions the snakes were around 6.4 diopters hyperopic suggesting a corrected acuity of 4.89 cycles/degree. Reduction of grating luminance resulted in a reduction in evoked potential acuity measurements. These results indicate that the spatial resolution of midland banded water snakes is the equal of cat; about 20/120 in human clinical terms.     

The Visual Acuity of the Frog (Rana pipiens )

The frog's visual acuity for gratings was tested with a two-choice prey-dummy setup. The two dummies had a constant position in the visual field, but in one of them a striped pattern was constantly drifting, while the other was a potentially less attention arousing dummy with non-moving stripes. The acuity was tested in bright white light and with different brightness levels of green light (500 nm). The highest acuity 2.8 cycles per degree was achieved with the maximum level of green light. When the brightness was lowered by six orders of magnitude the acuity fell to 0.7 cycles per degree. These behavioural acuities are compared with a theoretically calculated resolving power of the retinal ganglion cell array.     

Visual acuity and contrast sensitivity of adult zebrafish

Background

The aim of this study was to evaluate the visual acuity of adult zebrafish by assessing the optokinetic reflex. Using a modified commercially available optomotor device (OptoMotry?), virtual three-dimensional gratings of variable spatial frequency or contrast were presented to adult zebrafish. In a first experiment, visual acuity was evaluated by changing the spatial frequency at different angular velocities. Thereafter, contrast sensitivity was evaluated by changing the contrast level at different spatial frequencies.

Results

At the different tested angular velocities (10, 15, 20, 25, and 30 d/s) and a contrast of 100%, visual acuity values ranged from 0.56 to 0.58 c/d. Contrast sensitivity measured at different spatial frequencies (0.011, 0.025, 0.5, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.55 c/d) with an angular velocity of 10 d/s and 25 d/s revealed an inverted U-shaped contrast sensitivity curve. The highest mean contrast sensitivity (±SD) values of 20.49?±?4.13 and 25.24?±?8.89 were found for a spatial frequency of 0.05 c/d (angular velocity 10 d/s) and 0.1 c/d (angular velocity 25 d/s), respectively.

Conclusions

Visual acuity and contrast sensitivity measurements in adult zebrafish with the OptoMotry? device are feasible and reveal a remarkably higher VA compared to larval zebrafish and mice.     

Screening for impaired visual acuity in middle age in general practice.

Screening for impaired distant visual acuity was one component of a controlled trial of multiphasic screening in middle age carried out in two general practices. The prevalence of impaired visual acuity (6/18 or worse in the better eye) at the initial screening in 1967 was 9.6% overall, ranging from 5.9% in people aged 40-49 years to 16.3% in those aged over 60. The question "Do you have difficulty seeing distant objects?" had a low sensitivity and high specificity, rendering it unsatisfactory for use in mass population screening for visual impairment. The prevalences of impaired visual acuity in the screening and control groups at the survey in 1972 showed no significant differences in any age group. Mass screening for defects of visual acuity in the course of a multiphasic examination is thus unlikely to reduce the prevalence of impaired distant visual acuity in the community.     

Wavelength dependence of visual acuity in goldfish

Visual acuity was measured in a two-choice training experiment with food reward. Four goldfish were trained to select a homogeneously illuminated testfield when a high-contrast grating (transparancy) was shown for comparison at the second testfield. Measurements were performed for white and monochromatic testfield illuminations in the light adapted state. Fourteen wavelengths between 404 nm and 683 nm were tested. For each wavelength (and white light) the testfield intensity was determined for which spatial resolution was highest. Between 446 nm and 683 nm maximal values of 2.0 cycles/deg (corresponding to a visual acuity of 15' of arc) were found. At 404 nm and in the ultraviolet resolution was lower (0.6 and ~0.25–0.35 cycles/deg, respectively). Cone and small ganglion cell densities may equally account for visual acuity. The action spectrum of maximal visual acuity is very similar to the spectral sensitivity function representing recognition of "colour". Measurements under reduced room illumination and after treatment with Ethambutol further indicate that the detection of high contrast gratings is processed by the same "channel" as colour vision. A similar separate and parallel processing of "colour" and "form" on the one hand, and "brightness" and "motion" on the other hand was found in humans.     

The tactile perception of stimulus orientation

Studies of the visual system suggest that, at an early stage of form processing, a stimulus is represented as a set of contours and that a critical feature of these local contours is their orientation. Here, we characterize the ability of human observers to identify or discriminate the orientation of bars and edges presented to the distal fingerpad. The experiments were performed using a 400-probe stimulator that allowed us to flexibly deliver stimuli across a wide range of conditions. Orientation thresholds, approximately 20 degrees on average, varied only slightly across modes of stimulus presentation (scanned or indented), stimulus amplitudes, scanning speeds, and different stimulus types (bars or edges). The tactile orientation acuity was found to be poorer than its visual counterpart for stimuli of similar aspect ratio, contrast, and size. This result stands in contrast to the equivalent spatial acuity of the two systems (at the limit set by peripheral innervation density) and to the results of studies of tactile and visual letter recognition, which show that the two modalities yield comparable performance when stimuli are scaled appropriately.     

Contrast transfer function of the visual system]

Visually evoked potentials were used to determine the spatial contrast response function of the visual system and the visual acuity of the pigeon. The spatial contrast response describes the relationship between the contrast in a pattern of vertical stripes, whose luminance is a function of position, and the amplitude of the visually evoked response at various spatial frequencies for a given temporal frequency (pattern reversal frequency); it indicates how particular spatial frequencies are attenuated in the visual system. The visually evoked responses were recorded using monopolar stainless steel electrodes inserted into the stratum griseum superficiale of the optic tectum; the depth of penetration was determined on the basis of a stereotactic atlas. The stimulus patterns were generated on a video monitor placed 75 cm in front of the animal's eye perpendicular to the optic axis. The spatial contrast response function measured at 10% contrast and 0.5 Hz reversal frequency shows a peak at a spatial frequency of 0.5 c/deg, corresponding to 1 degree of visual angle, and decreases progressively at higher spatial frequencies. The high-frequency limit (cut-off frequency) for resolution of sinusoidal gratings, estimated from the contrast response function, is 15.5 c/deg, corresponding to a visual acuity of 1.9 min of arc.     

THE RELATION BETWEEN VISUAL ACUITY AND ILLUMINATION

1. An apparatus for measuring the visual acuity of the eye at different illuminations is described. The test object is continuously variable in size and is presented at a fixed distance from the eye in the center of a 30° field. Observation of the field is through an artificial pupil. The maximum intensity obtainable is more than enough to cover the complete physiological range for the eye with white light though only 110 watts are consumed by the source. Means for varying the intensity over a range of 1:10¹⁰ in small steps are provided. 2. The relation of visual acuity and illumination for two trained observers was measured, using two different types of test object, a broken circle and a grating. The measurements with both test objects show a break at a visual acuity of 0.16, all values below that being mediated by the rods and those above by the cones. The grating gives higher visual acuities at intensities less than about 30 photons and lower visual acuities above that. The maximum visual acuity attainable with the grating under the same conditions is about 30 per cent lower than that with the C. It is shown that the limiting factor in the resolution of the eye for the grating is the diameter of the pupil when it is less than 2.3 mm. and the size of the central cones when the pupil is larger than that. The value of the diameter of the cone derived on that basis from the visual acuity data agrees with that derived from direct cone count in a unit of area. 3. The data for the cones made with both test objects are adequately described by one and the same form of the stationary state equation derived by Hecht for the photoreceptor system. This fact, together with certain considerations about the difference in the nature of the two test objects with regard to the resolvable area, leads to the conclusion that detail perception is a function of a distance rather than an area. All the data for the rods can likewise be described by another variety of the same equation, although the data are too fragmentary to make the choice of the form as certain as might be desired.     

Deterioration of visual acuity associated with growth hormone therapy in a child with extreme short stature and high hypermetropia

BACKGROUND: Growth hormone (GH) has long been implicated in the pathogenesis of diabetic retinopathy, although its precise role remains ill-defined. In 1998, an association between exogenous human GH and retinal pathology in non-diabetic subjects was described. CASE REPORT: A female child with extreme short stature of unknown aetiology (height -7.38 SD at 11.3 years) and severe hypermetropia developed retinopathy with visual deterioration during two separate empiric trials of GH therapy. On the first occasion, a relatively high dose of GH (10.5 mg/m2/week) administered from age 4.4 to age 4.7 years was associated with the development of central serous retinopathy, resulting in marked reduction in visual acuity. On cessation of GH, the macular oedema resolved, and visual acuity improved. At age 5.6 years, GH therapy was re-introduced at a lower dose (3.9 mg/m2/week) and her vision monitored closely. Bilateral retinal oedema recurred after 3 months, and GH therapy was stopped. Once again, the macular oedema regressed, and visual acuity improved following withdrawal of GH. These ophthalmic changes contra-indicated further GH therapy. CONCLUSION: We suggest that GH may be a risk factor in the development of retinopathy in certain non-diabetic patients, especially in the presence of a severe refractive error.     

用显示器测量办公亮度环境下的人眼对比度敏感视觉特性

对比度敏感是描述人眼视觉系统空间特性的主要指标之一,对比度敏感函数是反映不同条件下的对比度敏感与空间频率之间的关系。人眼对比度敏感数据的测量受到环境亮度较大的影响,为了研究常用办公环境条件下的人眼对比度敏感情况,对6位青年在环境亮度分别为153,312,470 cd/m2和暗室条件下,在距离为2米处观测11种空间频率的矩形光栅进行测量,光栅用显示器进行显示,其平均亮度分别为60和90 cd/m2。实验结果表明,对于相同频率的光栅,人眼对比度敏感程度随着环境亮度的增加而减小,而且人眼在暗室环境下比在办公环境条件下对亮度光栅更敏感;但是在观测平均亮度为60cd/m2的光栅时,人眼特殊地对在环境亮度为312 cd/m2的条件下更敏感。     

用显示器测量办公亮度环境下的人眼对比度敏感视觉特性

对比度敏感是描述人眼视觉系统空间特性的主要指标之一,对比度敏感函数是反映不同条件下的对比度敏感与空间频率之间的关系。人眼对比度敏感数据的测量受到环境亮度较大的影响,为了研究常用办公环境条件下的人眼对比度敏感情况,对6位青年在环境亮度分别为153,312,470 cd/m2和暗室条件下,在距离为2米处观测11种空间频率的矩形光栅进行测量,光栅用显示器进行显示,其平均亮度分别为60和90 cd/m2。实验结果表明,对于相同频率的光栅,人眼对比度敏感程度随着环境亮度的增加而减小,而且人眼在暗室环境下比在办公环境条件下对亮度光栅更敏感;但是在观测平均亮度为60cd/m2的光栅时,人眼特殊地对在环境亮度为312 cd/m2的条件下更敏感。     

Tobias Mayer--experiments on visual acuity (1755)

The pioneering experimental work of Tobias Mayer (1723-1762) on visual acuity, published in 1755 in Latin, is presented in English translation. Mayer distinguished between two kinds of visual acuity, the one (30 arc sec) for single objects seen against a uniform background, the other (1 arc min) for more complex objects such as gratings, grids, or checkerboards. Strong illumination did not improve visual acuity. For targets seen in the light of a candle, the visual angle needed for resolution was inversely proportional to the cube root of the distance of the candle and therefore to the sixth root of 'brightness'. The historical significance of Mayer's work on visual acuity is briefly outlined.     

Visual acuity and spatial contrast sensitivity in tree squirrels

The visual sensitivity of tree squirrels from three different species (two western gray squirrels, Sciurus griseus; three fox squirrels, Sciurus niger; and an eastern gray squirrel, Sciurus carolinensis) was measured for spatial patterns defined by luminance differences. Spatial contrast sensitivity functions were determined in behavioral discrimination experiments in which the stimuli were sinusoidally-modulated gratings. At an average luminance level of 3.4 cd/m² these squirrels were maximally sensitive to spatial frequencies of about 0.5 cycles/degree (c/d). These experiments, in conjunction with additional measurements involving discrimination of square-wave grating patterns, also indicate that the finest gratings these squirrels can discriminate average 2.2 c/d (SD = 0.42 c/d). There are no obvious differences in resolution acuity among the three species. Grating acuity also was measured at a much higher luminance level (340 cd/m²). The results of this experiment indicate that the spatial acuity of the tree squirrel increases with luminance level, reaching a maximum of 3.9 c/d at 340 cd/m².     

Visual acuity thresholds of juvenile loggerhead sea turtles (Caretta caretta): an electrophysiological approach

Visual evoked potentials measure dynamic properties of the visual system by recording transient electric responses of neural tissue identified to correspond to a specific visual stimulus, such as light or a striped grid. In this study, visual evoked potentials were used to test the visual acuity of juvenile loggerhead sea turtles (Caretta caretta) in water. Subject animals were fitted with a Plexiglas goggle filled with filtered seawater. Stimuli of black and white striped gratings were presented to the turtles using a slide projector directing an image onto a screen via a rotatable mirror that shifted the striped pattern laterally one-half cycle. Bioelectric activity was collected using a digital averaging computer and subdermal platinum electrodes, implanted under the head scutes directly above the optic nerve and the contralateral optic tectum. To isolate the response signal from the noise, signal averaging techniques were used when collecting visual evoked potentials. The resulting response waveforms included a robust positive-negative compound that was used to track the turtle's response to visual stimulation. Acuity thresholds for these sea turtles, which were derived from linear regressions analysis of the positive-negative compound amplitudes versus stripe size, ranged from 0.130 to 0.215. This acuity level is comparable to other inshore, shallow water marine species.     

Learning to identify near-acuity letters, either with or without flankers, results in improved letter size and spacing limits in adults with amblyopia

Amblyopia is a developmental abnormality that results in deficits for a wide range of visual tasks, most notably, the reduced ability to see fine details, the loss in contrast sensitivity especially for small objects and the difficulty in seeing objects in clutter (crowding). The primary goal of this study was to evaluate whether crowding can be ameliorated in adults with amblyopia through perceptual learning using a flanked letter identification task that was designed to reduce crowding, and if so, whether the improvements transfer to untrained visual functions: visual acuity, contrast sensitivity and the size of visual span (the amount of information obtained in one fixation). To evaluate whether the improvements following this training task were specific to training with flankers, we also trained another group of adult observers with amblyopia using a single letter identification task that was designed to improve letter contrast sensitivity, not crowding. Following 10,000 trials of training, both groups of observers showed improvements in the respective training task. The improvements generalized to improved visual acuity, letter contrast sensitivity, size of the visual span, and reduced crowding. The magnitude of the improvement for each of these measurements was similar in the two training groups. Perceptual learning regimens aimed at reducing crowding or improving letter contrast sensitivity are both effective in improving visual acuity, contrast sensitivity for near-acuity objects and reducing the crowding effect, and could be useful as a clinical treatment for amblyopia.     

Long-term result of autologous cultivated oral mucosal epithelial transplantation for severe ocular surface disease

The present study aimed to investigate the clinical outcomes of autologous cultivated oral mucosal epithelial transplantation (COMET) on human amniotic membrane (AM) for corneal limbal stem cell deficiency (LSCD). In this prospective, noncomparative case series, 20 eyes (18 patients) with bilateral severe ocular surface disease were chosen to undergo COMET on human AM. The primary outcome was clinical success, and the secondary outcomes were the best-corrected visual acuity difference, corneal opacification, symblepharon formation, and complications. The mean patient age was 48.2 ± 15.5 years. The mean follow-up time was 31.9 ± 12.1 months (range 8–50 months). All except one eye exhibited complete epithelialization within the first postoperative week. A successful clinical outcome, defined as a stable ocular surface without epithelial defects, a clear cornea without fibrovascular tissue invasion at the pupillary area, and no or mild ocular surface inflammation, was obtained in 15 of 20 eyes (75 %). The clinical success rate at 1 year was 79.3 %, and that at 4 years (end of follow-up) was 70.5 %. Fourteen of 20 (70 %) eyes exhibited improvement in visual acuity after COMET, and some required subsequent cataract surgery (2 eyes), penetrating keratoplasty (3 eyes), or keratoprosthesis implantation (1 eye). Preoperative symblepharon was eliminated in most eyes (8 of 13, 61.5 %) after COMET combined with eyelid reconstruction when needed. The only complication was corneal perforation (1 eye) induced by a severe eyelid abnormality; treatment with a tectonic corneal graft was successful. COMET can successfully restore ocular surface damage in most eyes with corneal LSCD.