摘要:

In a previous study, it was found that oblique gaze and the prismatic effect inherent in dynamic aniseikonia combine to affect the ability to fixate centrally. In the current study, retinal image differences were introduced, whereas prismatic effects were eliminated. This simulates static aniseikonia as opposed to dynamic aniseikonia. At the same time, oblique gaze was simulated by presenting the stimuli in directions other than straight ahead. The directions of presentation contained both horizontal and vertical components. Fixation eccentricity was monitored by using the border enhancement method, as described in previous experiments. It was found that fixation eccentricity increased as the eyes were directed toward stimuli oriented in directions other than straight ahead. When vertical and horizontal components of oblique gaze were combined, the fixation eccentricity was found to be greater than with either component alone. When differential magnification between left and right retinal images was introduced, the fixation eccentricity increased further, the increment being approximately constant for all directions of gaze.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

The results of this study indicate that the axial length is not strongly associated with the Hirschberg formula, and therefore the axial length does not have to be considered when measuring an ocular deviation with the Hirschberg test. This study does demonstrate an association between the radius of curvature of the cornea and the Hirschberg formula. For patients with keratometer readings below 46 D, the use of the standard Hirschberg formula, 1 mm = 22δ, is appropriate. For patients with keratometer readings greater than 46 D, and with deviations greater than about 30δ, the use of the Hirschberg formula, 1 mm = 27δ, is more appropriate. When performing the Hirschberg test on infants up to about 2 months of age, the use of the higher Hirschberg formula would probably be more appropriate because this age group often has higher keratometer readings.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

Contrast sensitivity using Vistech plates was measured repeatedly in normal subjects to determine predictive power. The contrast sensitivity measures were quite stable from the first administration, and practice effects were not observed. Test-retest reliability was lowest for the lower spatial frequencies, but was adequate throughout if more than one measurement was made. Threshold scores for all five spatial frequencies were generally not independent and factor analysis revealed that only two spatial frequency domains were relatively independent: high and low. The correlations between adjacent spatial frequencies were highest, and correlations decreased as distance between spatial frequencies increased. To detect subtle differences all three Vistech plates should be used to arrive at a threshold for each spatial frequency and, if time permits, more measures should be taken at the lower spatial frequencies to compensate for the lower reliability. This method is believed to be adequate for repeated-measures clinical and experimental studies.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

The corneal epithelium can be affected adversely by some preservatives used in contact lens solutions. To test the effect of H2O2at the epithelial surface, the rabbit cornea was perfused with various concentrations of H2O2, and the effect on stromal and Anterior Bright Band (ABB) thickness measured for 150 min. The study compares stromal swelling and ABB swelling for a single 10-min pulse with that for a 150-min sustained dose. With the 10-min pulse dose there was no significant swelling up to 235 ppm H2O2, the highest dose tested. With the sustained dose, significant stromal swelling began between 72 and 153 ppm H2O2. These results indicate that residual H2O2present at the epithelial surface after neutralization in current contact lens disinfection systems does not have a high enough concentration to affect corneal function adversely.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

Human anisometropia and changes in it were followed over a 3-year period in 238 schoolchildren who had uncomplicated school myopia in both eyes. Anisometropia of the spherical equivalent (ASFE) type increased in 27%, decreased in 6%, and remained unchanged in 67% of these children. When mean myopia increased from −1.43 to −3.06 D, mean ASFE increased from 0.30 to 0.51 D and the anisometropia of astigmatism (AAST) from 0.15 to 0.23 D. The faster the increase in myopia the greater was the increase in ASFE (r = 0.133, N = 238, p = 0.020). The higher the spherical equivalent at the end of the study the higher was the ASFE (r = 0.135, N = 238, p = 0.019). The initial refractive error or the amount or the axis of astigmatism did not have any prognostic value for the changes in ASFE. The higher the ASFE at the end of the study the higher also was the AAST (r = 0.136, N = 238, p = 0.018). The change in ASFE was independent of the wearing of spectacles. The distribution of the anisometropia of the spherical equivalent and its change followed a nearly normal distribution with the peak of eyes at about ±0 refraction.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

The contrast sensitivity function (CSF), although containing more information than traditional measures of acuity, has found difficulty gaining clinical acceptance. The hesitancy of clinicians to adopt the CSF stems, in part, from the fact that it is not as readily interpreted as is acuity. In order to facilitate such interpretation, five indices of spatial vision which are derivable from the CSF were examined in a sample of 287 persons aged 5 to 85 years. All indices were found to be both age-sensitive and strongly related to each other, but bandwidth of the CSF was chosen as a practical index for clinical settings. In a second study, acuity and CSF bandwidth were measured under 0 to ±1 D optical blur. It was found that the correction providing best acuity also maximized CSF bandwidth, and that bandwidth was more sensitive to optical blur than was acuity. Results support the assertion that CSF bandwidth is a readily interpreted index of spatial vision that can be measured efficiently within the context of clinical refraction.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

The authors have presented in two previous articles the graphic solutions resembling Tscherning ellipses, for spherical as well as for aspherical ophthalmic lenses free of astigmatism or power error. These solutions were exact, inasmuch as they were based on exact ray tracing, and not third-order theory as frequently done. In this paper sphero-cylindrical lenses are now analyzed, also using exact ray tracing. The functional dependence of the astigmatism and the power error for these lenses is described extensively.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

Although previous investigators have attempted to calculate the longitudinal spherical aberration inherent in soft and rigid contact lenses both on and off the eye, the use of inappropriate assumptions on which to base their calculations has left the problem unresolved. In this study, the longitudinal spherical aberration of both soft and rigid contact lenses was calculated surface by surface both in air and on the eye using a two-dimensional, exact ray tracing program. The erroneous assumptions made by previous investigators were avoided by using an elliptical model for the anterior corneal surface and assuming that the posterior surfaces of soft lenses aligned exactly with the anterior corneal surface after flexure onto the eye. The results demonstrated that, with a 6-mm pupil, contact lenses induce significant levels of spherical aberration in the ocular system for soft lenses of back vertex power greater than +3.00 D or −6.00 D and for rigid lenses of powers more positive than −3.00 D. It is suggested that visual disturbance due to induced spherical aberration has not been a major clinical problem in the past because these conditions fall outside those experienced by a large proportion of the contact lens-wearing public.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

Previous studies of visual “crowding” in children have been restricted to the use of high contrast optotypes. In this study, visual acuity was measured in a group of 30 normally sighted children using high (96%), medium (11%), and low (4%) contrast Snellen charts and isolated letter cards. The crowding effect was found to be less for low contrast letters than for high contrast letters.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID

摘要:

Oxygen uptake rates were measured on 6 human corneas fitted with a standardized rigid contact lens in order to determine the relation between vertical palpebral aperture height and exchange efficiency of tears in such systems. The contribution provided by tear exchange to the oxygen available to the cornea was isolated through the use of lenses of negligible transmissivity (PMMA). Measurements of corneal oxygen uptake were made on the normal open eye, after 5 min of static (with no blinking) open eye contact lens wear, and after 5 min of dynamic (with blinking every 5 s) contact lens wear. The difference between the oxygen demand associated with static conditions (SC's) and that associated with dynamic conditions (DCs) is an index of the tear pump efficiency for each subject. A statistically significant correlation (p ≤ 0.0001) was found between the vertical palpebral aperture height and the tear bulk-flow of oxygen to the cornea, confirming that small palpebral apertures can restrict lens motion.

ISSN:1040-5488    

出版商:OVID    

年代:1990

数据来源: OVID