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Results for “Visual Acuity”

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At least 73 records · Page 4Linked to original sources

PubMed1985

Some statistical concepts in the analysis of vision and visual acuity.

As conventionally recorded, visual acuity data constitute an ordinal scale of measurement. An investigation of four different clinical samples shows that visual acuity is not normally distributed amongst these populations. Furthermore, acuity is often measured on charts which, by the inherent restriction of the stimulus size, have the effect of truncating the upper limit of the scale producing, in statistical terms, an incomplete distribution. The distribution of acuity for each sample is found to be adequately described in terms of the equation: F(x) = Ax alpha (T - x) beta, where F(x) is the cumulative distribution of the statistical population, T is the truncation level (known), x is the Snellen decimal acuity (known) and A, alpha and beta are parameters. Alternative analytical procedures based upon this model, which overcome these limitations, are discussed.

Humans↗

PubMed1990

A comparative study of grating and recognition visual acuity testing in children with anisometropic amblyopia without strabismus.

Bailey-Lovie-Ferris visual acuity charts and Teller visual acuity cards were used to compare recognition and grating visual acuity at near testing distances in 32 children with anisometropic amblyopia without strabismus. Appropriate optical corrections were worn. Test-retest intraobserver reliability was higher for letters (r =.95) than for gratings (r = .68). Using 20/30 visual acuity or better as the criterion for normal visual acuity, eight eyes with letter visual acuities ranging from 20/42 to 20/138 would have been inaccurately found to be normal by using the Teller visual acuity cards alone. Grating visual acuity measurements tended to be better than letter visual acuity; and, in general, they did not worsen proportionately with poorer letter visual acuity.

Adolescent↗

PubMed1987

Contact lens fitting relation and visual acuity in keratoconus.

The presence of visual decrement in keratoconus is widely recognized, but little is known about the effect of different contact lens fitting philosophies on visual acuity. We studied 10 eyes with keratoconus, each of which was tested with an automated visual acuity device while wearing rigid contact lenses of varying base curves and diameters. Lenses used were large, flat lenses and small, steep lenses, fitted from 0.4 mm flatter to 0.4 mm steeper than the average keratometric measurement in 0.1-mm increments, including parallel to the average corneal curvature. A small improvement in visual acuity was noted with the flat lenses, equalling approximately one-half line of visual acuity, which, although statistically significant, was not deemed to be clinically significant. In addition, a decrement in visual acuity of keratoconus patients with centrally placed cones was noted and is discussed.

Contact Lenses↗

PubMed1985

Maturation of the visual evoked response and its correlation with visual acuity in preterm infants.

Visual evoked responses (VERs) were elicited in 70 infants with postmenstrual ages between 30 and 39 weeks. On the basis of neurological and ultrasound examinations, 30 of the infants were classed as neurologically normal and 40 as abnormal: 26 of the latter had periventricular haemorrhage. Initially the VER consisted of a negative deflection only, and the appearance of a positive wave immediately preceding the negative deflection was taken to indicate maturation of the VER. Maturation was significantly delayed in the neurologically abnormal infants, and the delay was related to the degree of neurological insult. The visual acuity of 32 infants was estimated within seven days of the VER recording. There was a correlation of 79 per cent between the VER and the data for visual acuity.

Birth Weight↗

PubMedDate not supplied

Visual acuity measurements by swept spatial frequency visual-evoked-cortical potentials (VECPs): clinical application in children with various visual disorders.

Previous studies have indicated that visual acuities of normal infants can be estimated with good accuracy using swept spatial frequency visual-evoked-potentials (VECPs). In this report we describe acuity measurements obtained with this technique from 304 examinations performed on 135 children having various visual disorders. When possible, two or more different stimulation frequencies (8, 12, 15 or 24 contrast reversals/sec) were used in each patient, and three to eight sweep VECPs were obtained from each patient under each simulation and recording condition. High correlation coefficients (0.94 - 0.96) between the acuity estimated on each patient from either the single sweep giving the best visual acuity (BSS) or from vector averages (VeA) of the EEG data obtained from several sweeps confirmed previous findings in normal infants. We also found high correlation coefficients among BSS recorded at different temporal frequencies (0.79-0.97) and among comparisons of BSS or VeA acuity to optotype visual acuity (0.6-0.89). Children with clinically undetectable optokinetic responses showed lower visual acuity estimated by BSS than those who demonstrated optokinetic nystagmus. We conclude that the sweep VECP is a valid method, giving estimates of acuity which correlate well with optotype acuity and correspond well to other clinical findings, and that it can be useful in the clinical management of nonverbal patients.

Child↗

PubMed1986

Subfoveal choroidal neovascular membranes in age-related macular degeneration. Visual prognosis in eyes with relatively good initial visual acuity.

We determined the visual prognosis of 94 eyes in 92 patients having age-related macular degeneration, a discrete choroidal neovascular membrane directly under the center of the foveal avascular zone (subfoveal), and an initial visual acuity of 20/100 or better. Of the patients who were reexamined 24 months following their initial presentation, 77% had lost at least four lines of vision and 64% had lost at least six lines. Estimation of visual loss using a conservative assessment procedure showed four-line visual loss in 65% of the patients and six-line loss in 50%. In general, the better the visual acuity at the initial examination, the more likely the patient was to have a smaller choroidal neovascular membrane. These results suggest that it may be reasonable to consider a randomized clinical trial of laser photocoagulation for this group of patients with a relatively poor visual prognosis.

Age Factors↗

PubMed1992

[Visual acuity perimetry. Detection of glaucomatous visual field defects].

50 eyes of 50 patients with different types of glaucoma (27 eyes with POAG, 14 eyes with glaucoma due to pseudoexfoliation, 8 eyes with chronic narrow-angle glaucoma, 1 eye with normal-tension glaucoma) were examined with light-sense perimetry (Humphrey-Field-Analyzer, program 30-2) and resolution perimetry according to Frisén [2-7]. As the field area tested by Frisén's ring perimeter is smaller than that of the HFA, the comparison was restricted to the area of the ring perimeter, so that 50 out of 77 test locations of the HFA were included. In addition to a quantitative comparison with statistical criteria based on age-corrected normal values, a subjective qualitative assessment was established. The quantitative comparison of all hemifields showed good agreement in 61% of eyes, the ring perimeter indicating moderately and markedly increased numbers of defects in 13% and 4% of eyes and the HFA in 18% and 4% of eyes, respectively. Qualitative comparison of the entire fields revealed good agreement in 46% of eyes, with moderately and markedly more defects in 18% and 10% for the ring perimeter and 12% and 14% for the HFA, respectively. The average time needed for examination was 6.0 +/- 1.1 min for the ring perimeter and 17.2 +/- 2.7 min for the HFA. The Global Deviation of the ring perimeter correlates linearly with the Mean Deviation of the HFA (r = 0.4824, P < 0.001).

Adult↗

PubMed1988

The use of accurate visual acuity measurements in clinical anti-cataract formulation trials.

A standardized visual acuity technique is presented for use in anti-cataract drug trials. Ferris Logmar charts were used in repeatability studies of vision, visual acuity and pinhole visual acuity measurements for twenty normal subjects (mean (+/- SD) age 64 +/- 6.3 years). Visual acuity measurements were shown to be the most repeatable and thought to be the most suitable for monitoring cataract progression. Repeated visual acuity measurements were made on 29 cataractous eyes of 15 subjects (mean (+/- SD) age 67.8 +/- 7.2 years). A Logmar score change of 0.1 (one line) was shown to be a statistically significant change. This value can be used in statistical analyses of drug efficacy. The normal data gives a mean Logmar visual acuity of 1.15 (Snellen equivalent 6/5). This indicates the inadequacy of using 6/6 as a norm value for visual acuity, even for older patients. As the possibility of reversal of cataract theoretically exists in the early stages of cortical and capsular cataracts, patients with small amounts of these types of cataract are ideal patients for anti-cataract formulation trials. The normal visual acuity results indicate that the inclusion criteria for clinical trials can include patients with cataracts with visual acuity as good as 6/6.

Aged↗

PubMed1992

Contrast visual acuities in cataract patients. II. After IOL implantation.

Contrast visual acuities were measured in 100 eyes of 75 patients who attained a best-corrected visual acuity of greater than or equal to 0.8 (20/25) after intraocular lens (IOL) implantation. The variable contrast visual acuity chart (VCVAC), with three contrast levels of 90, 15, and 2.5% and reverse polarity of 90% contrast, was used to measure contrast visual acuities. The follow-up period ranged from 3 to 35 months (mean 7.41). The mean visual acuities measured with the 90, 15, and 2.5% charts were 0.92 (SD = 0.11), 0.59 (SD = 0.13), and 0.33 (SD = 0.14), respectively. The mean visual acuity measured with the 90% reverse polarity chart was 0.97 (SD = 0.11). The decreases in visual acuities compared with the 90% contrast were 0.64 and 1.48 octaves in the 15% and the 2.5% contrast charts, respectively. The pattern of the contrast acuity profile was comparable to normal subjects, but in 28 of 100 (28%) eyes, the visual acuities measured with the reverse polarity chart were slightly better than those measured with the standard 90% contrast chart, suggesting that the glare effect still exists after IOL implantation, though to a lesser degree than in cataractous eyes.

Adult↗

PubMed1991

The significance of testing preoperative visual function in cataract using laser interferometric visual acuity and ERG.

Tests of preoperative visual function and prediction of postoperative E chart visual acuity (ECVA) using laser interferometric visual acuity (LIVA) and electroretinogram (ERG) were performed in 16 cases (19 eyes) of cataract. The results showed that the coincident rate between preoperative LIVA and postoperative ECVA was 63.2%, and there was a parallel correlation between preoperative amplitude of photopic ERG b-wave and postoperative ECVA in 79.0% of the eyes. Comparing these two methods, the test of LIVA is very simple, fast, and is easily accepted by the patients with cataract. ERG is an objective method, which is slightly influenced by dense lens, but the test is more complex. Therefore, the combined use of these two methods will provide a more comprehensive and correct evaluation of preoperative visual function and a more reliable prediction for postoperative visual acuity in patients with cataract.

Adolescent↗

PubMed1979

Assessment of the visual acuity of human color mechanisms with the visually evoked cortical potential.

The amplitude of the human visually evoked cortical potential (VECP), which has been shown to vary as a function of the dimensions of checks in a checkerboard pattern, was used to objectively assess the visual acuity of the blue-sensitive compared with the red- and green-sensitive mechanisms in the eye. The results agree with psychophysical measures which have indicated poorer acuity of the blue system compared with the red and green systems.

Adult↗

PubMed1992

Simulation of a phosphene-based visual field: visual acuity in a pixelized vision system.

A visual prosthesis for the blind using electrical stimulation of the visual cortex will require the development of an array of electrodes. Passage of current through these electrodes is expected to create a visual image made up of a matrix of discrete phosphenes. The quality of the visual sense thus provided will be a function of many parameters, particularly the number of electrodes and their spacing. We are conducting a series of psychophysical experiments with a portable "phosphene" simulator to obtain estimates of suitable values for electrode number and spacing. The simulator consists of a small video camera and monitor worn by a normally sighted human subject. To simulate a discrete phosphene field, the monitor is masked by an opaque perforated film. The visual angle subtended by images from the masked monitor is 1.7 degrees or less, depending on the mask, and falls within the fovea of the subject. In the study presented here, we measured visual acuity as a function of the number of pixels and their spacing in the mask. Visual acuity was inversely proportional to pixel density, and trained subjects could achieve about 20/26 visual acuity with a 1024 pixel image. We conclude that 625 electrodes implanted in a 1 cm by 1 cm area near the foveal representation of the visual cortex should produce a phosphene image with a visual acuity of approximately 20/30. Such an acuity could provide useful restoration of functional vision for the profoundly blind.

Blindness↗

PubMed1989

[Objective and subjective determination of visual acuity].

The results of the present study show that the normal human visual acuity is not 1 (30 c/dg) but a remarkably higher value, about 1.73 (55 c/dg). Many authors, who studied the problem of objective determination of the visual acuity using visual evoked cortical potentials, corrected their results under the assumption that the objective method overestimates the real visual acuity. Now it is clear that the conventional determination of the visual acuity leads to serious underestimation of the real visual acuity. So, the way toward a more precise objective estimation of the human threshold visual acuity is now open.

Adolescent↗

PubMed1991

Visual acuity and contrast sensitivity in cataract: summation and inhibition of visual performance.

Patients exhibiting uniocular cataract often report an improvement in vision on closing their cataractous eye. Such qualitative evidence suggests the presence of binocular inhibition--the converse of binocular summation (that is the normal superiority of binocular over monocular vision). To quantify the extent of inhibition in cataract, binocular and monocular visual acuity and contrast sensitivity were measured in 28 patients. Twelve patients showed binocular inhibition for visual acuity whilst 11 showed inhibition for contrast sensitivity measured at four cycles per degree (c.deg-1). Contrast sensitivity for 8 c.deg-1 targets was further recorded in a subset of 14 patients in whom seven showed inhibition. In patients who demonstrated inhibition, the mean decrement in performance for visual acuity was 13%. For contrast sensitivity at 4 and 8 c.deg-1 the mean decrement was 25% and 32% respectively. The clinical significance of these findings is discussed with respect to the assessment of visual function and management of the cataract patient.

Adult↗

PubMed1990

Binocular summation in visually evoked responses and visual acuity.

Monocular and binocular transient visually evoked responses (VER) were recorded on 50 adult subjects using a pattern reversal stimulus (check size 5.5 min arc). The peak-to-trough amplitude of the VER wave was measured and compared with the subjective visual acuity (Landolt C). The binocular amplitude enhancement over the monocular amplitude was around 26% across the acuity range studied, whereas the mean binocular subjective visual acuity improvement was around 11.3%, but this varied from 1% at the high acuity end to 17% at the low acuity end of the range.

Adolescent↗

PubMedDate not supplied

Assessment of visual acuity in children with severe neurological impairments.

We used the Teller Acuity Cards to assess visual acuity in children with severe neurological handicaps (N = 12), all of whom had been previously untestable. With every child we were able to obtain at least one estimate of visual acuity, even with those described as likely having little or no visual function. However, in all cases, performance was poorer than that of unimpaired children of the same age. Our results confirm that the test is useful for assessing visual acuity in populations who, because of significant cognitive, linguistic, and motor deficits, cannot be assessed by more traditional techniques.

Adolescent↗

PubMed1989

[Statistical evaluation of visual acuity].

The statistical evaluation of the visual acuity has to be based on the non-parametric test as the decimal scale of the visual acuity forms a serial scale and not the intervallic one. From the theoretic point of view, the most advantageous approach uses the paremetric statistical tests. Visual acuity is measured on optotypic tables with log gradiation of the optotype size. At present time optotypes of this kind are not available in Czechoslovakia.

Humans↗

PubMed1992

Set shot shooting performance and visual acuity in basketball.

Common sense suggests that decreasing visual acuity will have a negative effect on basketball shooting performance. To test the hypothesis that basketball shooting performance monotonically decreases with decreasing acuity, 19 subjects attempted 25 set shots from a fixed location at each of 5 different acuity levels: 6/6 or better and vision blurred (by optical defocus) to visual acuities of 6/12, 6/24, 6/48, and 6/75. Our results revealed a small but statistically nonsignificant decrease in shooting performance between the 6/6+ and 6/12 conditions. For visual acuities between 6/12 and 6/75, the number of baskets made remained constant. We conclude that decreases in visual acuity over the range of 6/6+ to 6/75 resulting from defocus do not significantly reduce set shot shooting performance.

Adolescent↗