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Effects of visual blur and contrast on spatial and temporal precision in manual interception. Exp Brain Res 2021; 239:3343-3358. [PMID: 34480594 PMCID: PMC8542000 DOI: 10.1007/s00221-021-06184-8] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2021] [Accepted: 07/22/2021] [Indexed: 12/04/2022]
Abstract
The visual system is said to be especially sensitive towards spatial but lesser so towards temporal information. To test this, in two experiments, we systematically reduced the acuity and contrast of a visual stimulus and examined the impact on spatial and temporal precision (and accuracy) in a manual interception task. In Experiment 1, we blurred a virtual, to-be-intercepted moving circle (ball). Participants were asked to indicate (i.e., finger tap) on a touchscreen where and when the virtual ball crossed a ground line. As a measure of spatial and temporal accuracy and precision, we analyzed the constant and variable errors, respectively. With increasing blur, the spatial and temporal variable error, as well as the spatial constant error increased, while the temporal constant error decreased. Because in the first experiment, blur was potentially confounded with contrast, in Experiment 2, we re-ran the experiment with one difference: instead of blur, we included five levels of contrast matched to the blur levels. We found no systematic effects of contrast. Our findings confirm that blurring vision decreases spatial precision and accuracy and that the effects were not mediated by concomitant changes in contrast. However, blurring vision also affected temporal precision and accuracy, thereby questioning the generalizability of the theoretical predictions to the applied interception task.
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Comparison between optical and digital blur using near visual acuity. Sci Rep 2021; 11:3437. [PMID: 33564011 PMCID: PMC7873285 DOI: 10.1038/s41598-021-82965-z] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/2020] [Accepted: 01/27/2021] [Indexed: 11/17/2022] Open
Abstract
In a low-cost laboratory setup, we compared visual acuity (VA) for stimuli rendered with Zernike aberrations to an equivalent optical dioptric defocus in emmetropic individuals using a relatively short observing distance of 60 cm. The equivalent spherical refractive error of + 1, + 2 or + 4 D, was applied in the rendering of Landolt Rings. Separately, the refractive error was introduced dioptrically in: (1) unchanged Landolt Rings with an added external lens (+ 1, + 2 or + 4 D) at the subject's eye; (2) same as (1) but with an added accommodation and a vertex distance adjustment. To compare all three approaches, we examined VA in 10 healthy men. Stimuli were observed on a PC CRT screen. For all three levels of refractive error, the pairwise comparison did not show a statistically significant difference between digital blur and accommodation-plus-vertex-distance-adjusted dioptric blur (p < 0.204). The best agreement, determined by Bland–Altman analysis, was measured for + 4 D and was in line with test–retest limits for examination in the clinical population. Our results show that even for a near observing distance, it is possible to use digitally rendered defocus to replicate dioptric blur without a significant change in VA in emmetropic subjects.
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Sudhir RR, Farhath Pattan H, Rathore M, Kuppuswamy Parthasarathy M, Padmanabhan P, Lakshminarayanan V. The effect of refractive surgery on blur thresholds. Indian J Ophthalmol 2020; 68:2998-3001. [PMID: 33229685 PMCID: PMC7856997 DOI: 10.4103/ijo.ijo_1512_20] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
Purpose: The aim of this study was to measure blur thresholds before and after refractive surgery. Methods: In this prospective cohort study conducted in a tertiary eye hospital in South India. Blur thresholds were measured for 30 young adult myopic patients 1 month prior to and after refractive surgery. Patients were asked to report three stages of blur, namely Detectable Blur (DB), Bothersome Blur (BB), and Non-resolvable Blur (NB). Blur was created by adding plus lenses (in steps of 0.12D) over their optimal subjective refraction. The blur judgments were made both monocularly and binocularly when looking through a 3 mm artificial pupil at one line above the best-corrected visual acuity. Results: A total of 30 participants were included in this study (mean age = 25.5 ± 3.8 (20–36) years; 77% female). The mean binocular preoperative blur of this group was: DB = 0.39 ± 0.26D, BB = 0.74 ± 0.28D and NB = 1.04 ± 0.42D. The corresponding mean binocular blur one-month post-operatively was DB = 0.46 ± 0.28D, BB = 0.83 ± 0.35D, and NB = 1.21 ± 0.44D. Although there was a marginal increase in the blur thresholds postoperatively, the difference was not statistically significant (DB: P = 0.320; BB: P = 0.229; NB: P = 0.054). Conclusion: All three blur thresholds showed an insignificant minimal increase at 1 month post-operatively suggesting that patients adapt to the induced blur following refractive surgery. A longer follow up would reveal how the adaptation to blur would change with time.
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Affiliation(s)
- Rachapalle Reddi Sudhir
- Department of Cornea & Refractive Surgery, Medical Research Foundation, Sankara Nethralaya, Chennai, Tamil Nadu, India
| | - Hadiya Farhath Pattan
- Department of Cornea & Refractive Surgery, Medical Research Foundation, Sankara Nethralaya, Chennai, Tamil Nadu, India
| | - Mehal Rathore
- Elite School of Optometry, Unit of Medical Research Foundation, Chennai, Affiliated to SASTRA University, Thanjavur, Tamil Nadu, India
| | | | - Prema Padmanabhan
- Department of Cornea & Refractive Surgery, Medical Research Foundation, Sankara Nethralaya, Chennai, Tamil Nadu, India
| | - Vasudevan Lakshminarayanan
- School of Optometry and Vision Science, Departments of Physics, ECE and Systems Design Engineering, University of Waterloo, Canada
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Straßer T, Kurtenbach A, Langrová H, Kuehlewein L, Zrenner E. The perception threshold of the panda illusion, a particular form of 2D pulse-width-modulated halftone, correlates with visual acuity. Sci Rep 2020; 10:13095. [PMID: 32753676 PMCID: PMC7403154 DOI: 10.1038/s41598-020-69952-6] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/22/2020] [Accepted: 07/17/2020] [Indexed: 11/17/2022] Open
Abstract
To call attention to the danger of extinction of the panda bear, the Lithuanian artist Ilja Klemencov created the artwork “They can disappear”. The illustration is composed of black-and-white zigzagged lines, which form the famous panda logo of the World Wild Fund For Nature (WWF) when seen from a distance. If one is too close to the artwork, it is difficult to spot the bear, however, if one steps back or takes off one’s glasses the panda suddenly appears. This led us to ask if the ability to see the panda is related to the visual acuity of the observer and if therefore, the panda illusion can be used to assess the spatial resolution of the eye. Here we present the results of the comparison between visual acuity determined using the Landolt C and that predicted from the panda illusion in 23 healthy volunteers with artificially reduced visual acuity. Furthermore, we demonstrate that the panda illusion is based on a 2D pulse-width modulation, explain its technical history, and provide the equations required to create the illusion. Finally, we explain why the illusion indeed can be used to predict visual acuity and discuss the neural causes of its perception with best-corrected visual acuity.
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Affiliation(s)
- Torsten Straßer
- Institute for Ophthalmic Research, Centre for Ophthalmology, University of Tuebingen, Elfriede-Aulhorn-Straße 7, 72076, Tuebingen, Germany.
| | - Anne Kurtenbach
- Institute for Ophthalmic Research, Centre for Ophthalmology, University of Tuebingen, Elfriede-Aulhorn-Straße 7, 72076, Tuebingen, Germany
| | - Hana Langrová
- Institute for Ophthalmic Research, Centre for Ophthalmology, University of Tuebingen, Elfriede-Aulhorn-Straße 7, 72076, Tuebingen, Germany.,University Eye Hospital, Hradec Králové, Czech Republic
| | - Laura Kuehlewein
- Institute for Ophthalmic Research, Centre for Ophthalmology, University of Tuebingen, Elfriede-Aulhorn-Straße 7, 72076, Tuebingen, Germany.,University Eye Hospital Tuebingen, Elfriede-Aulhorn-Straße 5, 72076, Tuebingen, Germany
| | - Eberhart Zrenner
- Institute for Ophthalmic Research, Centre for Ophthalmology, University of Tuebingen, Elfriede-Aulhorn-Straße 7, 72076, Tuebingen, Germany.,Werner Reichardt Centre for Integrative Neuroscience (CIN), Otfried-Mueller-Str. 25, 72076, Tuebingen, Germany
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Effect of Simulated and Real Spherical and Astigmatism Defocus on Visual Acuity and Image Quality Score. Optom Vis Sci 2020; 97:36-44. [PMID: 31895276 DOI: 10.1097/opx.0000000000001463] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
Abstract
SIGNIFICANCE Image simulation is a useful and efficient tool to explore the impact of spherical and astigmatic blur on visual acuity (VA) and image gradation. It could help to design new optical corrections more efficiently and rapidly. PURPOSE The purpose of this study was to compare the effects of simulated (convolution by an artificial eye) and real spherical and astigmatic defocus on VA and image gradation. METHODS Experiments were performed under highly controlled conditions: dynamic correction of the subjects' aberrations at 1 Hz and application of an artificial pupil. In experiment 1, Landolt C VA was measured in various conditions of spherical and astigmatism defocus. The amounts of spherical or positive astigmatic defocus oriented at 45° that gives a Landolt C VA of 0.0, 0.2, and 0.5 logMAR were measured in experiment 2. In experiment 3, the subjects scored the quality of the perceived image (three high-contrast 0.4 logMAR letters) with a five-item continuous grading scale. RESULTS Simulated blur was always more detrimental than optical blur. We measured a difference of 0.08 ± 0.03 and 0.11 ± 0.05 logMAR between both conditions, respectively, in presence of spherical and astigmatism defocus. An average ± standard deviation difference of 0.16 ± 0.06 D (i.e., spherical defocus) and 0.24 ± 0.15 D (i.e., astigmatism defocus) was observed between simulated and real optics blur to provide a given VA. The differences of image quality score between both conditions were, respectively, 15.13 ± 9.63 and 13.33 ± 4.83 for spherical and astigmatism defocus. Most of the differences were statistically significant. CONCLUSIONS We observed a difference of about 20 and 35% between simulated and real optics blur, respectively, in presence of spherical and astigmatism blur. However, the difference between both methods remains equal to or below the clinically significant difference.
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Heinrich SP, Strübin I. Use of diffusing filters for artificially reducing visual acuity when testing equipment and procedures. Doc Ophthalmol 2019; 140:83-93. [PMID: 31489519 DOI: 10.1007/s10633-019-09715-5] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2019] [Accepted: 08/27/2019] [Indexed: 10/26/2022]
Abstract
PURPOSE When evaluating ophthalmological devices and procedures, for instance those for visual electrophysiology, it is often desirable to perform tests with reduced acuity. Doing this with individuals with actual visual impairments has a number of disadvantages, such as considerable recruitment efforts, especially when a specific acuity range is targeted, and little control about the actual perceptual characteristics of the impairment, which are normally not fully known. Lenses with positive diopters or blurring filters that are placed in front of the eyes of visually normal observers promise a simple solution to the problem. However, defocus results in considerable spurious resolution, and previous studies suggest that the frequently used Bangerter occluders are not optimal for the purpose. The present study therefore reviews a number of other options and tests a selection of filters with respect to their effect on acuity and contrast sensitivity with the aim of identifying filters that primarily degrade acuity while mostly sparing contrast sensitivity. METHODS First, we screened several filters for potential usefulness. The Freiburg Acuity and Contrast Test was then used to measure visual acuity and contrast sensitivity with a subset of three filters (Luminit LSD 0.5° and 1°, and LEE 420) and, for comparison, with a Bangerter occluder with a nominal acuity grade of 0.1. A qualitative comparison of the filters' effect on the checkerboard-reversal VEP was also performed. RESULTS With both Luminit filters, variability in acuity across participants was relatively small, and at least with the 0.5° version, contrast sensitivity was relativity little affected. The LEE filter and the Bangerter occluder resulted in more variability and, compared to the effect on acuity, a relatively strong reduction in contrast sensitivity. Comparing the Luminit 0.5° and 1° filters, the reduction of acuity was not proportional to physical stimulus degradation. The effect on VEP responses was consistent with the psychophysical data. CONCLUSIONS The Luminit filters, which have a Gaussian light diffusion profile, appear to be a good choice for artificial reduction of acuity.
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Affiliation(s)
- Sven P Heinrich
- Eye Center, Medical Center, University of Freiburg, Killianstr. 5, 79106, Freiburg, Germany. .,Faculty of Medicine, University of Freiburg, Freiburg, Germany.
| | - Isabell Strübin
- Eye Center, Medical Center, University of Freiburg, Killianstr. 5, 79106, Freiburg, Germany
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Suchkov N, Fernández EJ, Artal P. Wide-range adaptive optics visual simulator with a tunable lens. JOURNAL OF THE OPTICAL SOCIETY OF AMERICA. A, OPTICS, IMAGE SCIENCE, AND VISION 2019; 36:722-730. [PMID: 31044998 DOI: 10.1364/josaa.36.000722] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/06/2023]
Abstract
An adaptive optics visual simulator (AOVS) with an extended dioptric range was developed, allowing measuring and correcting aberrations in a majority of highly ametropic eyes. In the instrument, a tunable lens is used for defocus correction, while a liquid-crystal-on-silicon spatial light modulator is used for compensating or inducing any other aberration. The instrument incorporates a digital projector, which uses a micromirror array to display the stimuli. A motorized diaphragm enables operation for any physiological pupil size. A full description of the instrument and its calibration are provided, together with the results obtained in seven highly myopic subjects with refraction of -7.2±1.8 D (mean±SD). Refraction obtained with the instrument was compared to the standard refraction prescribed by trial lenses. When using the refraction obtained by the AOVS, the visual acuity (VA) exhibited an average increase of 0.21 (decimal scale). The visual impact of correcting high-order aberrations is presented in three subjects, whose VAs slightly improved with the correction. High myopes are able to benefit from the improved refraction assessment. The new instrument creates a possibility for a wide number of new experiments, especially for eyes exhibiting large refractive errors, where previous AO instruments failed to operate.
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Abstract
Optical blur from defocus is quite frequently considered as equivalent to low-pass filtering. Yet that belief, although not entirely wrong, is inaccurate. Here, we wish to disentangle the concepts of dioptric blur, caused by myopia or mis-accommodation, from blur due to low-pass filtering when convolving with a Gaussian kernel. Perhaps surprisingly-if well known in optometry-the representation of a blur kernel (or point-spread function) for dioptric blur is, to a good approximation and disregarding diffraction, simply a cylinder. Its projection onto the retina is classically referred to as a blur circle, the diameter of which can easily be deduced from a light-ray model. We further give the derivation of the relationship between the blur-disk's diameter and the extent of blur in diopters, as well as the diameter's relation to the near or far point, and finally its relationship to visual acuity.
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Affiliation(s)
- Hans Strasburger
- Institute of Medical Psychology, Ludwig-Maximilians-Universität, München, Germany; Department of Medical Psychology and Medical Sociology, Georg-August University, Göttingen, Germany
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Symmetric visual response to positive and negative induced spherical defocus under monochromatic light conditions. Vision Res 2017; 143:52-57. [PMID: 29277449 DOI: 10.1016/j.visres.2017.12.003] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2017] [Revised: 11/30/2017] [Accepted: 12/12/2017] [Indexed: 11/20/2022]
Abstract
The purpose of the study was to investigate the sign-dependent response to real and simulated spherical defocus on the visual acuity under monochromatic light conditions. The investigation included 15 myopic participants with a mean spherical equivalent error of -2.98 ± 2.17 D. Visual acuity (VA) was tested with and without spherical defocus using the source method (simulated defocus) and the observer method (lens-induced defocus) in a range of ±3.0 D in 1.0 D steps. VA was assessed using Landolt C's, while the threshold was determined with an adaptive staircase procedure. Monochromatic light conditions were achieved using band pass filters with a wavelength of 450 ± 2 nm, 530 ± 2 nm and 630 ± 2 nm. Results showed that the reduction of VA was significantly different under blue lighting conditions, when compared to the green and red light conditions. No significant difference in the reduction of the VA was found between the positive and the negative sign of defocus for all lighting conditions. The agreement for the VA between the source and observer method was significantly dependent on the wavelength as well as on the level of defocus. To conclude, under monochromatic light conditions, myopes show a symmetric sign-dependency regarding the influence of spherical defocus on visual acuity. The observed results indicate that the human visual system is capable of integrating the chromatic differences in refraction to distinguish between the signs of defocus.
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Eisenbarth W, Richert J, MacKeben M. Testing macular letter recognition - reliability and influence of refraction errors. Clin Exp Optom 2016; 99:322-7. [PMID: 27087542 DOI: 10.1111/cxo.12360] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/23/2015] [Revised: 07/10/2015] [Accepted: 07/30/2015] [Indexed: 11/28/2022] Open
Abstract
PURPOSE The goal was the validation of the Macular Mapping Test (MMT) for clinical use. We studied its susceptibility to blur caused by refractive errors and its test-retest reliability. METHODS We tested letter recognition in 33 target locations in the central visual field (10° radius) at two contrast levels, 10 and 100 per cent. Healthy subjects were either young (mean: 25.7 years) or elderly (mean: 67.0 years). A third group (patients with age-related macular degeneration, mean age: 76.4 years) were tested with their habitual optical correction and subsequently with optimal correction. The influence of refractive errors on performance was measured only for the healthy subgroups. All visual acuities were measured at 6.0 and 0.4 metres. Outcome measure was the 'general field score' (GFS), a single number expressing the overall level of letter recognition performance. RESULTS The general field score versus refractive error showed a mean loss of 3.9 points per dioptre and 5.9 points per dioptre in young subjects at 100 and 10 per cent contrast, respectively. In elderly subjects, mean losses were 2.6 points per dioptre and 5.5 points per dioptre for 100 and 10 per cent contrast, respectively. The general field score ratio (GFS at 10 per cent divided by GFS at 100 per cent) shows a decrease of eight per cent for the young group and 11 per cent for the elderly group. Performance increased after improvement of the refractive status in the AMD group: At 100 per cent contrast, the general field score increase from 21.5 to 24.1 (p < 0.008) was significant but not at 10 per cent contrast. Mean increase of acuity with the optimal correction was 0.32 to 0.46 decimal for distance (p = 0.018) and 0.28 to 0.44 decimal near (p = 0.018). Test-retest reliability was good but dependent on contrast. CONCLUSIONS At both contrasts, performance declined with increasing blur caused by refractive errors. The slope of this decline is more pronounced at the lower contrast. In healthy subjects, optical blur beyond 1.00 D can cause significant performance losses.
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Affiliation(s)
- Werner Eisenbarth
- Department of Optometry/Ophthalmic Optics, Munich University of Applied Sciences, Munich, Germany.
| | - Johann Richert
- Department of Optometry, University of Applied Sciences, Aalen, Germany
| | - Manfred MacKeben
- The Smith-Kettlewell Eye Research Institute, San Francisco, California, USA
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Heinrich SP, Bach M. Resolution acuity versus recognition acuity with Landolt-style optotypes. Graefes Arch Clin Exp Ophthalmol 2013; 251:2235-41. [PMID: 23828648 DOI: 10.1007/s00417-013-2404-6] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2012] [Revised: 05/17/2013] [Accepted: 06/11/2013] [Indexed: 11/25/2022] Open
Abstract
BACKGROUND International standards define acuity as the reciprocal of the threshold gap size of a Landolt C optotype. However, the literature is inconsistent as to what type of acuity is measured with Landolt Cs. The present study addresses this question more directly than previous studies by quantifying the effect of an inherent luminance artifact in Landolt-style optotypes. METHODS Two groups of modified optotypes were used. In the first group, each optotype had a single gap structure with the same average luminance. Between optotypes, the gap structures differed in their degree of fineness. In the second group of optotypes, a standard gap was always present, defining the orientation of the optotype. Additional gap structures of the same average luminance, but different fineness, were inserted at the remaining potential gap locations, thereby balancing luminance across potential gap locations. Visual acuity measures were obtained for each optotype variant, using a computer-based test employing a staircase procedure. RESULTS Similar acuity values were obtained for all optotypes of the first group, and for standard Landolt Cs, irrespective of the fineness of the gap structure. With luminance-balanced optotypes of the second group, measured acuity was halved, compared to standard optotypes. CONCLUSIONS The results support the view that it is recognition acuity, rather than resolution acuity, which is measured with standard Landolt-style optotypes, with the imbalanced luminance distribution serving as a cue. Luminance-balanced optotypes may help to obtain a more veridical estimate of resolution acuity, although recognition acuity may be more relevant in daily living.
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Affiliation(s)
- Sven P Heinrich
- Sektion Funktionelle Sehforschung, Univ.-Augenklinik Freiburg, Killianstr. 5, 79106, Freiburg, Germany,
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Watanabe K, Negishi K, Kawai M, Torii H, Kaido M, Tsubota K. Effect of experimentally induced astigmatism on functional, conventional, and low-contrast visual acuity. J Refract Surg 2013; 29:19-24. [PMID: 23311738 DOI: 10.3928/1081597x-20121211-01] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2012] [Accepted: 10/23/2012] [Indexed: 11/20/2022]
Abstract
PURPOSE To report the changes in functional visual acuity (FVA) in eyes with experimentally induced astigmatism. METHODS This study included 26 right eyes from 26 healthy participants (mean age: 27.0±4.4 years). After confirming best correction under cycloplegia, the cylinder power ×90° and ×180° (against-the-rule [ATR] and with-the-rule [WTR] astigmatism, respectively) were added with 0.50-diopter (D) steps, from 0.50 to 2.50 D. Conventional visual acuity (VA), FVA as measured by the FVA Measurement System (NIDEK Co Ltd), and 10% low-contrast visual acuity (LCVA) were measured with 3-mm artificial pupil under each condition. RESULTS The mean corrected logMAR VA, FVA, LCVA were -0.18±0.00 (20/13), -0.10±0.08 (20/16), and 0.06±0.06 (20/23), respectively. The VA, FVA, and LCVA had a significant linear negative correlation with the addition of cylinder irrespective of the axes. A significant decrease in FVA was noted on addition of ⩾0.50 D WTR astigmatism and ⩾1.00 D ATR astigmatism. With the addition of 1.00 D of cylindrical lens, 96% of eyes with WTR and ATR astigmatism could maintain 20/20 in VA testing, whereas only 50% and 62% of eyes with WTR and ATR astigmatism, respectively, could maintain 20/20 in FVA testing. CONCLUSIONS Astigmatism may be associated with deterioration of visual function even when conventional VA of 20/20 is attained. Functional VA testing seems to be useful in evaluating the masked astigmatic visual impairment that cannot be detected by conventional VA testing.
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Affiliation(s)
- Kazuhiro Watanabe
- Department of Opthalmology, Keio University School of Medicine, Tokyo, Japan
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