摘要:

The complex interactions between accommodation and vergence have been described by dual interactive models which include influences of convergence accommodation and accommodative vergence. Using an SRI Eyetracker, we investigated changes in vergence and accommodation stimulated while looking through prisms or lenses, and while looking at real targets located at different distances. Our results suggest that both proximal accommodation and proximal vergence are stimulated when looking from a distant to a near real target. We suggest that models of convergence and accommodation interactions include proximal accommodation and proximal vergence before the crosslinks.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

A number of previous investigations have suggested that the relative contribution of proximal vergence to the overall vergence response may increase with age. Accordingly the present study has examined the proximal convergence/test distance (PC/T) ratio in a clinical population of 106 subjects ranging from 6 to 47 years of age. PC/T was assessed by comparing accommodative convergence/accommodative stimulus (AC/As) ratios obtained using the gradient and heterophoria techniques. With the former method, the stimulus to accommodation was produced optically using spherical lenses whereas in the latter technique, the change in accommodative stimulus arose from variation in actual target distance. The difference between these two measurements of AC/As reflected the output of proximal vergence. No significant variation in PC/T was observed with increasing age. It is proposed that increased disparity-vergence, rather than proximal vergence, is more likely to compensate for any age-related decline in AC.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

The concept of generalized inverse dioptric power is introduced. Its use is illustrated in solving Long's matrix generalization of Prentice's equation for the decentration required to produce a specified prismatic effect. The solution holds in all cases for which a solution exists, including for the pure cylinder, and the result gives all solutions when more than one exists.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

The matrix form of Prentice's equation is solved completely for dioptric power. Solutions in matrix form are presented. For any specified position on a lens and any specified prismatic effect at that position one can calculate a particular power consistent with the specified information. One can also determine the complete set of consistent powers. The purpose of the paper is essentially theoretical: it is to complete the mathematical basis for answering any conceivable question concerning Prentice's equation.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

It is important for clinicians to know the equivalent viewing power and corresponding equivalent viewing distance of the adjustable-focus telescopes they prescribe for low vision patients. Formulas are presented here to calculate the equivalent viewing power and distance of these telescopes. The optical effects of adjusting a telescope for near vision are discussed and compared to using a lens cap fitted on the objective to permit focusing on a near target. Commonly prescribed adjustable-focus telescopes of four representative magnifications were evaluated. The results are presented in the form of a nomogram which can be used for determining the equivalent viewing distance of these telescopes based on the front vertex focal distance of the focused device. Other considerations including sources of error and field of view are also addressed.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

This investigation was designed to compare the effects of three rigid gas permeable (RGP) contact lens solutions on the rabbit corneal epithelium. Boston Advance Conditioning Solution, Boston Conditioning Solution, and Allergan Wet-N-Soak, which are preserved with 0.0015% polyaminopropyl biguanide, 0.006% chlorhexidine gluconate, and 0.003% benzalkonium chloride, respectively, were evaluated by scanning (SEM) and transmission electron microscopy (TEM). Our results show that Boston Advance Conditioning Solution is significantly more toxic to the corneal epithelium than either Boston Conditioning Solution or Allergan Wet-NSoak Plus. This is presumably due to the presence and concentration of the preservative, 0.0015% polyaminopropyl biguanide. Although this study was conducted using rabbits, the results raise clinical concerns for human RGP contact lens wearers.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

The effects of rigid contact lens overall diameter changes on tear pump efficiency were studied for lenses of 7 overall diameters (8.2 to 10.0 mm in 0.3- mm steps) while maintaining all other parameters (including a 7.4-mm optic zone diameter) constant. Oxygen uptake rates were measured in the normal, open eye and after static (without blinking) and dynamic (with blinking once every 5 s) wear of oxygen-impermeable (polymethyl methacrylate) contact lenses. Differences between uptake rates obtained under static vs. dynamic conditions served as an index of tear pump efficiency for each lens. Tear pump efficiency increased with the use of progressively smaller lens diameters and subjects with large palpebral aperture sizes were also found to have better tear exchange (r=0.934, p=0.006 for the 9.1-mm overall diameter reference lens).

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

Refraction and its components were measured for 79 young adults, 29 of whom were youth-onset myopes, 26 were early adult-onset myopes, and 24 were emmetropes; By submitting the data to analysis of variance, it was found that: (1) corneal power was significantly greater for both groups of myopes than for the emmetropes; (2) lens power and thickness were not significantly different for the three groups of subjects; (3) anterior chamber depth was significantly greater for the youth-onset myopes than for the emmetropes; and (4) vitreous chamber depth and axial length were both significantly greater for youth-onset myopes than for early adult-onset myopes, and significantly greater for young adult-onset myopes than for emmetropes. When youth-onset myopes and early adult-onset myopes were matched on the basis of spherical equivalent refraction, no significant differences were found between the two groups for any of the components of refraction. It was concluded that the differences between youth-onset myopes and early adult-onset myopes found in the analysis of variance occurred only as a result of the fact that the youth-onset myopes had been myopic for longer periods of time and therefore were considerably more myopic.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

Screening the vision of pre-school populations has been suggested as a way to assure early identification of vision problems as well as early intervention, thereby enabling normal development of the visual system and learning process. In a masked investigation, 119 preschool children enrolled in Project Headstart with a mean age of 3.51 years (range 3 to 5 years) had their vision screened using the Modified Clinical Technique (MCT). Monocular visual acuities in this population were determined using preferential-looking (PL) cards. Visual acuity assessment of young children in screening settings is at best a challenge; yet, all 119 children were able to complete monocular acuity assessments on each eye. Acuities for nonreferrals (individuals who passed the screening) were significantly better than for referrals (individuals who failed the screening) at the 0.0167 level using a two-tailed Fisher's Exact Test with the Bonferroni Method for multiple comparisons. The effectivity of the screening technique as determined by phi-coefficient remained highest ( ?=+0.94) with the visual acuity referral criterion set at 6/12 (20/40) or less either eye and/or ≥ a 2-octave difference between the eyes. The resultant referral rate (17.6%) was higher than that predicted by extrapolation from the Orinda Study for this age group (12.3%) but expected because of the different socioeconomic group. PL cards enabled monocular visual acuity measurement on 100% of the children tested and measurably increased the effectiveness of the screening procedure.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

When luminance is lowered, both vergence and accommodation tend to shift toward different and individually characteristic resting postures, commonly referred to as dark-vergence and dark-focus. In order to determine the luminance level at which these two mechanisms correspond to a target distance, subjects viewed a light spot 0.4 mm in diameter, corresponding to 1.4 to 2.7 min arc, located at the individually determined darkfocus position. As the luminance of a binocularly viewed spot increased, accommodation did not change significantly from the target distance. However, fusional vergence gradually shifted from the dark-vergence to the target position. The critical luminance level at which the 2 responses were coupled for the 12 observers ranged from 0.01 to 0.45 cd/m2, with distinct individual differences. In a second experiment, the target was positioned at distances different from the dark-focus and luminance was set at 0.5 and 1.0 log unit higher than the subject's critical luminance level. At lower luminance levels, mononuclear focus tended to remain at the individual dark-focus, whereas the binocular focus tended to correspond to the target distance. These differences can be attributed to vergence accommodation. Previous studies on night myopia reported that a negative correction based on onehalf of the individual dark focus was superior to a full dark-focus correction. The present results suggest that this difference is due to vergence accommodation and that the optimum correction for night myopia depends, in part, on the critical luminance level for activation of fusional vergence.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID