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Results for “Visual Acuity”

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PubMed1977

Relation of visual acuity to illumination, contrast, and distance in the partially sighted.

Visual acuity (minimum target size for threshold visibility) was measured as a function of luminance, contrast, and distance in experiments using 16 partially sighted persons. The results indicate large individual differences in dependence of visual acuity on both luminance and contrast. Visual acuity often failed to change systematically with variations in the testing distance; the effects of contrast and luminence on visual acuity also frequently failed to show any systematic dependence on viewing distance. The relation of acuity in the partially sighted to medical diagnosis is discussed along with some practical implications.

Albinism↗

PubMed1978

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.

Adult↗

PubMed1977

Visual acuity and contrast sensitivity in multiple sclerosis--hidden visual loss: an auxiliary diagnostic test.

In 48 patients with multiple sclerosis sine-wave gratings were used to test visual sensitivity for coarse, medium, and fine detail rather than measuring visual acuity for fine detail only, as in conventional clinical tests. In 20/48 patients the test revealed a visual defect of neural origin, qualitatively different from that caused by refractive error. In 11 of these 20 patients, visual sensitivity to detail of medium coarseness was markedly degraded, even though sensitivity to both coarse and fine detail was unimpaired. In 3 of these 20 patients visual sensitivity to coarse detail was selectively degraded. These visual defects could not be detected by the Snellen test, yet the patient might experience visual problems in everyday life and also experience distorted visual perception. Possible neural bases for these visual impairments are discussed. Since 8 of the 14 patients with selective loss showed no clinical evidence of visual involvement, the test can aid the earlier diagnosis of multiple sclerosis.

Adolescent↗

PubMed1978

Fading and feedback in the modification of visual acuity.

Two experiments assessing the modification of visual acuity by fading plus feedback procedures are presented. The training procedure involved a gradual increase of the distance myopic subjects could accurately discriminate letters requiring a minimum of 20/20 vision at approximately 23 feet. In Experiment I experimental and matched control groups of mildly to moderately myopic subjects were compared on repeated Ortho-rater examinations of visual acuity. The experimental group showed a significant improvement relative to the control group who received no training. Multiple-baseline across-stimuli designs were used in Experiment II to evaluate training effects for another group of subjects with moderate to severe myopia. Two measures of acuity were obtained. Accuracy of discrimination of letters at various preselected distances was determined each session while Ortho-rater examinations were given periodically throughout training. Changes in discrimination accuracy over sessions appeared to be related to fading for three of the four subjects, and Ortho-rater acuity changes were observed for each subject. Some short-term maintenance of improved vision was evident on both acuity measures.

Accommodation, Ocular↗

PubMed1978

Visual acuity in hard and soft contact lens wearers: a comparison.

Visual acuity measurements with spectacles, with hard contact lenses and with hard contact lenses plus overrefraction were made on 75 eyes. Similar data was taken on 75 eyes wearing soft contact lenses. In each case, some eyes showed a decrease in visual acuity with contact lenses but the soft lenses wearers showed a greater percentage of eyes with visual acuity decrease.

Contact Lenses↗

PubMed1976

Subjective and objective visual acuity testing techniques.

The Catford apparatus for determining the objective visual acuity was elevated with 20 normal (20 eyes) and 40 abnormal (75 diseased eyes) patients. The vision of the normal individuals was fogged with neutral-density filters and convex lenses. Eyes with normal or near normal vision showed good correlation between optokinetic response and visual acuity, but no correlation was observed in eyes with poor vision. These findings, which vary from those of Catford, indicate that objective methods of visual acuity testing using a nystagmoid response do not appear useful for general clinical purposes.

Evaluation Studies as Topic↗

PubMed1978

[The importance of the interference fringe visual acuity in the indication of a cataract-extraction (author's transl)].

100 patients have been examined before and after cataract-extraction using the Retinal Visual Acuity Tester, Takata, Japan. Laser produced interference fringes are formed on the retina, as a result the visual acuity can be determined easily before the operation. -- With immature cataracts we obtained a good correlation between the preoperative interference fringe visual acuity and the postoperative visual acuity tested by Landoltrings. Therefore, it can be differentiated whether worse visual acuity is caused by the cataract or by a retinal disease.

Adult↗

PubMed1979

Residual astigmatism and visual acuity with hydrogel contact lenses: a comparative study.

Residual astigmatism, visual acuity, and transferred corneal toricity were measured in a double masked fashion on 24 randomly selected patients fitted with Buasch and Lomb Soflens contact lenses, the Hydrocurve II contact lenses, and the AOSoftTM hydrophillic contact lenses. None of the subjects had any contraindications to hydrogel lens wear. All three lens types produced a small but similar reduction in refractive astigmatism. However, because of residual astigmatism, fewer eyes achieved 20/20 or better visual acuity with each of the hydrogel lens types than with the best spectacle correction. Corneal toricity was found to be transferred to the front surface of all three lens types. Our findings indicate that generally these three types of hydrogel lenses result in similar residual astigmatism, visual acuity and transferred corneal toricity. However, individual differences may occur. Therefore, the choice of lens type should be based on the individual patient's response to various lens types.

Adult↗

PubMedDate not supplied

Physiological basis of visual acuity and its development in kittens.

To answer the questions, (1) Which cells in the visual system are responsible for high visual acuity and (2) Does the function of the cells which provide high visual acuity develop postnatally; single cell studies have been made in the retina, lateral geniculate nucleus (LGN) and visual cortex of cats of different ages. Sustained-X retinal ganglion cells in the area centralis (the equivalent retinal position to the human fovea) set the upper limit of visual acuity. The cellular acuity develops postnatally until it reaches the adult level at 3--4 months-of-age. The improvement of acuity is associated with an increase in the strength of the inhibitory surround mechanism of the receptive field of sustained cells in the area centralis. The maturation of cellular acuity coincides with maturation of retinal and LGN synaptic organisation and of optic nerve myelination.

Age Factors↗

PubMed1979

The effect of pupil size on visual acuity in uncorrected and corrected myopia.

The effect of pupil size on the relation between Snellen visual acuity and corrected and uncorrected myopia was examined for 22 young subjects with degrees of myopia ranging from 0.75 D to 7.5 D. Effective pupil size was varied by inducing mydriasis and then placing artificial pupils of between 1.0 and 8.0 mm diameter before the eye. Both a constant chart luminance of 120 cd/m2 and a constant retinal illuminance of 2150 trolands were used. There was little difference in results for the two lighting conditions. For the corrected myopes considered as a group, maximum visual acuity occurred for 2--3 mm diameter pupils, but larger pupils reduced acuity only marginally. For the uncorrected myopes, variation in pupil size produced a large variation in visual acuity, and for refractive errors greater than about 1.5 D, the optimum pupil diameter was less than 1 mm. For uncorrected myopes of 3.0 D or less, visual acuity was nearly as good with a 1-mm pupil as for corrected myopes. The presented data are a useful guide to the clinician.

Adult↗

PubMed1976

New design principles for visual acuity letter charts.

This paper intoduces new principles for the design and use of letter charts for the measurement of visual acuity. It is advocated that the test task should be essentially the same at each size level on the chart. Such standardization of the test task requires the use of letters of equal legibility, the same number of letters on each row, and uniform between-letter and between-row spacing. It is also advocated that, combined with the test task standardization, there should be a logarithmic progression of letter size. Charts incorporating these design features have been made. These charts facilitate the use of nonstandard testing distances which might be used when there is low visual acuity, when examination room layout prevents testing at the standard distance, or when it is necessary to validate visual acuity scores or detect malingering. Adjusting the visual acuity score according to the chosen testing distance is simplified by the use of logarithmic scaling.

Humans↗

PubMed1976

Readability of approach charts as a function of visual acuity, luminance, and printing format.

This study determined the ability of 12 presbyopic subjects to read numerals from aeronautical approach procedure charts. The readability of chart numerals was determined for 15 size and contrast combinations as a function of near visual acuity (equivalent 20/20, 20/40, and 20/60) and chart brightness (100 and 1.0 ft L). Test subjects were evaluated at the normal near-visual-acuity level (20/20) and the minal near-visual-acuity levels (20/40) for Classes I and II and 20/60 for Class III) as specified in the FAA's Guide for Aviation Medical Examiners. The results indicated that subjects with 20/20 near visual acuity could read all chart numerals under bright and dim luminance conditions. Subjects with 20/40 and 20/60 levels of near visual acuity experienced reading difficulty under bright conditions and increased difficulty under dim luminance.

Aerospace Medicine↗

PubMed1979

[The determination of the maximal angular visual acuity and of the Vernier acuity (author's transl)].

In clinical diagnosis it is important to determine the maximal angular visual acuity by using for example the TNO Landolt C ring chart of Vos. The determination of the Vernier- or Nonius acuity is hardly of any clinical diagnostic significance because in several pathological conditions and in normal eyes the visual acuity is always three times higher than the angular visual acuity. Therefore this does not contribute to further differentiation.

Amblyopia↗

PubMed1979

Reflectance of visual acuity screens.

The luminance of the background and of the target (visual acuity letter) was measured for five commonly used visual acuity screen materials. Most materials were found to be highly nonlambertian. A greater than threefold difference in reflectance normal to the surface was found for the respective screens tested. The percent polarization maintained by each screen material, using American Optical Vectograph systems, was measured. The relative illuminations generated by eight projector systems were compared. A discussion of the factors and recommendations for improvements related to projected chart visibility are made.

Field Dependence-Independence↗

PubMed1979

A new test of visual acuity using a holographic phase grating and a laser.

A new visual acuity testing device is able to determine a specific visual acuity of the retina even in the presence of opacities of the ocular media, such as a cataract. This is achieved by forming two spatially coherent, quasimonochromatic point sources near the nodal point of the eye. The two spherical waves that emanate from the point sources interact to produce a pattern of stripes on the retina. The intensity of the pattern is easily varied. More importantly, the pattern can be projected over different-sized retinal areas corresponding to visual fields of 0.5 to 15 degrees in diameter. The device may be useful for vision research and preoperative assessments of visual potential.

Holography↗

PubMed1979

Late diagnosis of choroidal malignant melanomas in eyes with clear media and low visual acuity.

Five patients seen in a one-year period with advanced malignant melanoma of the choroid in eyes with low visual acuity and clear media are reported. The low visual acuity appeared clinically to be antecedent to the malignant tumor; although in certain cases it may have been related to the occult melanoma, this was not appreciated clinically. Because of the low visual acuity, these patients did not have vision symptoms referable to the melanoma, and the malignancy was diagnosed late in its course. All five cases had extrascleral extension of the tumor at the time of surgical treatment; three are dead of metastatic disease, and two have undergone orbital exenteration with only brief follow-up periods. This experience suggests that eyes with clear media and low vision owing to presumably unrelated causes should undergo periodic examination to rule out the presence of a growing malignant melanoma and to prevent its late diagnosis.

Abdominal Neoplasms↗

PubMed1976

Visual acuity at hyperbaric air pressure.

The visual acuity of five divers in a dry pressure chamber was measured with a Landolt C test. Only one of them showed a significant reduction at 7.0 atm. These results contrast sharply with open sea experiments, which show a considerable reduction at much lower pressures (Baddeley, 1968). It is suggested that besides stresses, luminance differences, also turbidity, as a function of depth may account for the discrepancy.

Air Pressure↗

PubMed1977

Effects of luminance and contrast on visual acuity, ages 16 to 90 years.

Visual acuity of persons aged 16 to 90 years was measured with Snellen letters of varied contrast at 10, 1, 0.1, and 0.01 fl chart luminances. Percentage losses of seeing with age were computed. At 10 fl luminance, perception of high- and medium-contrast letters has changed little at age 40, but about twice as much light is needed to see low-contrast letters as at age 20. By age 70, no 2-min-subtense (20/40) letters were seen at 0.01 fl luminance. The need of older people for increased lighting during indoor tasks and night driving is discussed.

Adolescent↗