摘要:

The effects of rigid contact lens axial edge lift changes on corneal oxygenation and tear exchange with the blink were studied for lenses of five axial edge lifts (0.05 to 0.13 mm in 0.02-mm steps). The overall and optic zone diameters were maintained at 8.8 and 7.4 mm, respectively, and all other parameters were held constant. Oxygen uptake rates were measured for the normal open eye after 5 min of static (without blinking) and dynamic (with blinking once every 5 s) contact lens wear. As expected, no significant differences were found in the oxygen uptake rates associated with the static wear of the contact lenses; however, under dynamic conditions, increasing edge lift was associated with a reduction in oxygen demand. The differences between data obtained under static and dynamic conditions increased with increasing axial edge lift, and subjects with small palpebral aperture sizes seemed to benefit more from increasing axial edge lift.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

Soft contact lenses presoaked in D2O or H217O were used to examine the movement of water immediately after their placement onto the eye. Two experiments were conducted: (1) the rate of D2O loss from these soft lenses was determined by assaying residual D2O with nuclear magnetic resonance spectroscopy; and (2) the movement of water into/inside the eye was followed by tracking the path of H217O using magnetic resonance imaging. D2O dissipation occurred in two phases: an initial rapid washout (1.25 ml/h) followed by a slower phase (0.13 ml/h). The latter was eliminated by previous contact lens wear. The H217O study showed that once water entered the anterior chamber, it rapidly exchanged with water in the ocular (predominantly iris) circulation (flow rate constant=0.1/min). It appears that exogenous water, in this case water from the soft lens, ultimately interacts with the ocular circulation. This is one aspect of cornea-soft lens interaction previously unreported.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

Two contrast sensitivity charts (Vistech and Pelli-Robson) have become available to the eye care practitioner. Their value as clinical tools for the assessment of visual function may be enhanced because of either an insensitivity to the effects of optical focus or a hypersensitivity to defocus. We compared the sensitivity to defocus of these charts to the traditional Snellen chart by examining the effect of up to ±5.00 D of spherical and astigmatic defocus on performance with each chart. In order to simulate the two types of clinical examination scenarios, tests were performed both with and without mydriatic/cycloplegic agents. The PelliRobson contrast sensitivity chart was very resistant to the effects of all types of optical defocus. As predicted, the high spatial frequencies on the Vistech chart were sensitive to defocus. However, although contrast sensitivities for the low frequencies were affected less, optical defocus produced significant decreases in low frequency Vistech contrast sensitivity. In addition, the Vistech chart was very insensitive to axis 180 blurring lenses. There was no indication that either contrast sensitivity chart was more sensitive to defocus than the standard Snellen chart.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

The history of magnetophosphenes and their closely related predecessor, electrophosphenes, is described from the mid-18th century to the present time. The current era of magnetic stimulation started in 1985 with the development of a practical capacitor-discharge electromagnetic stimulator by Barker and his colleagues at the University of Sheffield, and their application of it to the brain with Merton and Morton at the National Hospital, London. The safety of magnetostimulation of the brain is discussed as well as the advantages of magnetostimulation over electrostimulation. Principles of magnetostimulation of nerves and magnetic measurement are considered. Effects on motor and sensory systems of the brain are described including magnetic perceptual suppression in the visual cortex and other pioneering work of Amassian, Cracco and Maccabee at SUNY Health, Brooklyn. Magnetophosphenes from retinal and cortical magnetostimulation are distinguished. Now that visual cortical stimulation is possible with the strong magnetic pulses generated by capacitor-discharge instruments, the functional viability of the visual cortex may be tested directly and noninvasively.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

The validity and permanence of orthoptic treatment for vergence deficiencies requires investigation due to the subjective nature of determining success in most clinical cases, i.e., the amelioration of symptoms and increases in vergence ranges. The relation between Risley prism vergences, a subjective measure, and vergence tracking rate, an objective index, is investigated. The course of orthoptics progress is compared in cases of clinical vergence dysfunction. Vergence-deficient control subjects showed no significant change in either index. However, trained subjects demonstrated rapid increases in both indices. The persistence of the training effect was monitored for up to 9 months. No regression was observed in subjects who met all release criteria, but one subject who chose to terminate therapy early showed a slow regression in tracking rate and recurrence of symptoms. These data support the validity of vergence training and increase the plausibility of previous clinical reports of orthoptics success.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

This study was designed to provide information regarding two aspects of contrast sensitivity testing of aviator candidates: first to determine whether contrast sensitivity functions (CSF's) obtained with the Vistech Visual Contrast Test System (VCTS) are affected by cycloplegia and second to determine whether the VCTS provides useful CSF's under clinical screening conditions in a timely and simple manner. Contrast sensitivity thresholds were obtained at 5 spatial frequencies from 106 aviator candidates before and after administering a cycloplegic. CSF's obtained under cycloplegia were reduced by about 20%. The VCTS provided useful CSF's under military screening conditions, and added about 6 min to the time required for the standard flight physical.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

We provide a simple model for calculating the chromatic difference of magnification for the human eye. Spectral differences in image size are proportional to the eye's longitudinal chromatic aberration and the axial distance between the entrance pupil and nodal point. We verify that the model provides magnification estimates equal to previously published predictions, and we show the significant increase in this aberration produced by experimental use of artificial pupils.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

It is sometimes assumed that the prismatic effect in any astigmatic lens is orthogonal or perpendicular to the iso-thickness curves. This paper offers a formal proof of the validity of the assumption.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

摘要:

A hand-held noncontact tonometer (Pulsair, Keeler Ltd., UK) was tested in 42 children, from 5 months to 12 years of age. At least 1 intraocular pressure (IOP) reading was obtained in all of the 84 eyes studied, whereas with a Goldman applanation tonometer measurements could be obtained in only 26 eyes (14 subjects). Time required for examination with the Pulsair was quite variable and related to age (p<0.0001). Compliance in children was reported as “good” in the majority of cases; it was also age-related (p<0.005). Although its reliability could not be definitely evaluated, the results of this study indicate that this instrument may be a valuable choice for screening and follow-up purposes in childhood.

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID

ISSN:1040-5488    

出版商:OVID    

年代:1991

数据来源: OVID