摘要:

In the author's theory of color vision the blue and the green fundamental fall on a tritanopic confusion line. This provides the basis for comparing the mixture data of a tritanope with those of a normal. At the red end of the spectrum the data conform very well to what is predicted by theory. At the blue end the data point to the possibility that the pigment responsible for the red response may differ from the pigment involved at the red end of the spectrum.

ISSN:0093-7002    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

Following a suggestion made by Percival in regard to dissociated phorias, we corrected vertical associated phoria in several patients who had disparities in both vertical and horizontal fixation. The principal objective result of this correction was a flattening of the slope of the type I horizontal forced-vergence curve. This result may be significant particularly because the slope has been identified as being a good prognosticator of patients likely to be symptomatic. Attention to a concurrent vertical component may offer a convenient way to normalize a steep slope on horizontal fixation disparity curves.

ISSN:0093-7002    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

Mechanisms accounting for the fact that the eye tends to become and remain emmetropic have been proposed for the developing eye, but no such mechanisms have been proposed for the adult eye. In the study reported here, refractive component data published by Sorsby and his co-workers were reanalyzed in terms of the variations in these components with age. Included in the analysis are component data for one eye of each of 271 subjects from age 4 to age 70 years, whose ocular refraction was between piano and +2.00 D. The results of this analysis show that the axial length of the eye appears to decrease during the adult years of life, concurrently with a decrease in anterior chamber depth and an increase in the refractive power of both the cornea and the lens. It is proposed that a reduction in the axial length of the adult eye serves as an emmetropizing mechanism, occurring in harmony with the increase in the refracting power of the eye, which would otherwise cause the refraction of the eye to move in the myopic direction.

ISSN:0093-7002    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

The origins and implications of frequency-doubling (i.e., the dominance of the 4th-harmonic response instead of the reversal response) in the visual evoked potential (VEP) were investigated. Previous research demonstrated that frequency-doubling occurs in humans when sinusoidal but not square-wave luminance modulation is used to elicit the VEP, but little else is known about this intriguing phenomenon. Six adult humans viewed luminance and chromatic gratings that were presented using three different spatiotemporal combinations: 4 c/deg-3 Hz, 4 c/deg-6 Hz, and 12 c/deg-3 Hz. VEP's were recorded monopolarly from Ozusing an ear reference and were subjected to a Fourier analysis. The results showed that frequency-doubling occurred only using sinusoidal luminance contrasts at 4 c/deg-3 Hz, although the relative strength of higher harmonic responses was greater for both sine- and square-wave luminance contrasts relative to their chromatic counterparts. Many hypotheses can be put forth to account for VEP frequency-doubling (none of which is entirely satisfactory); however, a plausible view is that this phenomenon is related to the magnocellular-parvocellular distinction. In general, the presence of frequency-doubling and other harmonic distortions may preclude the scalp-recorded steady-state VEP to suprathreshold luminance contrasts from describing accurately the spatiotemporal transfer characteristics of the human visual system.

ISSN:0093-7002    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

A number of factors have been suggested to account for the reduced swelling response of the peripheral cornea. The osmotic swelling was determined in 25 adult cats by dripping deionized water onto the cornea for 30 min. The central cornea swelled 62 ± 19 Mm compared to 32 ± 21 Mm for the peripheral cornea. Before treatment, the cat was found to have a very uniform thickness profile, measuring 592 ± 42 μm centrally, and 585 ± 46 ± peripherally. The difference in central and peripheral swelling was therefore not due to topographical differences in baseline thickness. Endothelial cell density (ECD) determined with specular microscopy was 2920 cells/mm2 centrally, compared to 3185 cells/mm2 peripherally. Differences in cell density between the center and periphery are not responsible for the reduced swelling response of the corneal periphery inasmuch as swelling response was not correlated with cell density either in the center of the cornea or in the periphery (r = 0.179 and r = 0.221, respectively). By selecting an animal with a uniform thickness profile and by using an osmotic stimulus, this study has shown that the most likely cause for the reduced swelling response near the limbus is a difference in structural and or hydration characteristics of the cornea at the limbus.

ISSN:0093-7002    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

A letter counting task was presented on a Macintosh computer screen using two different versions of 24 point Times Roman print. One version, called “grainy,” had 12 matrix units per font height and the other, called “smooth,” had 24. A mixed group of low vision subjects and a normally sighted group had speed and accuracy measured as they performed the test task from two different viewing distances. For both population groups, the remote test distances were arranged individually so that the letters subtended an angular size scarcely larger than threshold. The close distances were fairly representative of practical working distances. It was shown that, for both groups of subjects, smooth letters allowed faster performance for the closer working distances only. Smoothing the letters helped accuracy of performance at the far distances, and at the close distances, smoothing improved the accuracy for the low vision group but not for the normals. The implications are discussed.

ISSN:0093-7002    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

Contour interaction was investigated in 12 preschool children 3 to 4 years of age, and compared to the results obtained from 5 normal adults tested under an identical paradigm. Observers viewed the display from a distance at which they could identify the location of a gap (up or down) in an isolated C correctly on 90 to 95% of the trials. The isolated C and C's with bars tangentially located at various positions above and below the test optotype were intermixed randomly. Percent correct was plotted as a function of the angular subtense of the gap width of the test optotype. A significant decrease in performance was found when the bars were positioned at 0.71 to 1.42 times the angular subtense of the gap for both the preschool children and the adults. The results suggest that preschool children demonstrate contour interaction that is quantitatively similar to adults. Because the spacing of letters on standard acuity charts is typically larger than the range over which contour interaction occurs, the poorer acuity often measured with charts compared to isolated letter presentation in preschool children suggests that factors other than contour interaction (perhaps attentional factors) are involved.

ISSN:0093-7002    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

Contrast sensitivity derangement may accompany keratoconus even in the presence of normal or near normal Snellen visual acuity. This has been demonstrated with computer-driven contrast sensitivity test devices. The purpose of this study was to evaluate the utility and efficacy of rapid screening devices for contrast sensitivity testing. However, such devices are often cumbersome and difficult to use clinically. We tested 12 patients with keratoconus on 2 simple chart systems designed to test contrast sensitivity in a rapid and clinically useful manner: the Vistech chart and the Regan multi-contrast visual acuity charts. Both devices detected the contrast sensitivity abnormalities present in early keratoconus, but some patients [with ≥ 6/12 (20/40) visual acuity] were unable to respond to areas of the charts corresponding to high spatial frequency and/or low contrast tasks. Such simple wall charts may be useful in measuring the visual abnormalities in early keratoconus, in monitoring the progression of the disease, and in evaluating various treatment options.

ISSN:0093-7002    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

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.

ISSN:0093-7002    

出版商:OVID    

年代:1987

数据来源: OVID

摘要:

Hyperopes fitted with contact lenses often exhibit more plus acceptance compared to full spectacle correction beyond that accounted for by vertex distance considerations. The objective of this investigation was to attempt to identify the possible factors involved with this phenomenon. Eight hyperopes, one myope, and one control hyperope between 20 and 30 years of age were fitted with polymethyl meth-acrylate lenses, and over-refractions performed on different days with and without corneal anesthesia. The result was that every test hyperope accepted more plus without anesthetic than with anesthetic, by an average of 0.725 D, which was significant (p = 0.0000). One possible mechanism appears to be neurosensory feedback between corneal sensation and the level of accommodative tonus of the ciliary body.

ISSN:0093-7002    

出版商:OVID    

年代:1987

数据来源: OVID