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Gustafsson, Jörgen
Publications (10 of 57) Show all publications
Lewis, P., Baskaran, K., Rosen, R., Lundström, L., Unsbo, P. & Gustafsson, J. (2014). Objectively Determined Refraction Improves Peripheral Vision. Optometry and Vision Science, 91(7), 740-746
Open this publication in new window or tab >>Objectively Determined Refraction Improves Peripheral Vision
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2014 (English)In: Optometry and Vision Science, ISSN 1040-5488, E-ISSN 1538-9235, Vol. 91, no 7, p. 740-746Article in journal (Refereed) Published
Abstract [en]

Purpose. The purpose of this study was twofold: to verify a fast, clinically applicable method for determining off-axis refraction and to assess the impact of objectively obtained off-axis refractive correction on peripheral low-contrast visual acuity. Methods. We measured peripheral low-contrast resolution acuity with Gabor patches both with and without off-axis correction at 20 degrees in the nasal visual field of 10 emmetropic subjects; the correction was obtained using a commercial open-field Hartmann-Shack wavefront sensor, the COAS-HD VR aberrometer. Off-axis refractive errors were calculated for a 5-mm circular pupil inscribed within the elliptical wavefront by COAS using the instruments' inbuilt "Seidel sphere" method. Results. Most of the subjects had simple myopic astigmatism, at 20 degrees in the nasal visual field ranging from -1.00 to -2.00 DC, with axis orientations generally near 90 degrees. The mean uncorrected and corrected low-contrast resolution acuities for all subjects were 0.92 and 0.86 logMAR, respectively (an improvement of 0.06 logMAR). For subjects with a scalar power refractive error of 1.00 diopters or more, the average improvement was 0.1 logMAR. The observed changes in low-contrast resolution acuity were strongly correlated with off-axis astigmatism (Pearson r = 0.95; p < 0.0001), the J(180) cross-cylinder component (Pearson r = 0.82; p = 0.0034), and power scalar (Pearson r = -0.75; p = 0.0126). Conclusions. The results suggest that there are definite benefits in correcting even moderate amounts of off-axis refractive errors; in this study, as little as -1.50 DC of off-axis astigmatism gave improvements of up to a line in visual acuity. It may be even more pertinent for people who rely on optimal peripheral visual function, specifically those with central visual field loss; the use of open-field aberrometers could be clinically useful in rapidly determining off-axis refractive errors specifically for this patient group who are generally more challenging to refract.

Keywords
visual acuity, low-contrast resolution acuity, off-axis refractive errors, peripheral vision, off-axis astigmatism, macular degeneration, AMD
National Category
Ophthalmology
Research subject
Natural Science, Optometry
Identifiers
urn:nbn:se:lnu:diva-36833 (URN)10.1097/OPX.0000000000000301 (DOI)000338778400010 ()2-s2.0-84903754538 (Scopus ID)
Available from: 2014-09-10 Created: 2014-09-10 Last updated: 2017-12-05Bibliographically approved
Lewis, P., Holm, V., Baskaran, K. & Gustafsson, J. (2013). Dynamic Stimulus Presentation Facilitates Peripheral Resolution Acuity. Paper presented at ARVO 2013, Life-changing research, Seattle, Wash., May 5 – 9, 2013. Investigative Ophthalmology and Visual Science, 54, Article ID e-abstract 574.
Open this publication in new window or tab >>Dynamic Stimulus Presentation Facilitates Peripheral Resolution Acuity
2013 (English)In: Investigative Ophthalmology and Visual Science, ISSN 0146-0404, E-ISSN 1552-5783, Vol. 54, article id e-abstract 574Article in journal, Meeting abstract (Other academic) Published
Abstract [en]

Purpose: Peripheral high-contrast resolution is sampling limited; the center to center spacing between ganglion cells ultimately limiting visual performance (Thibos et al., 1987). Although retinal image motion in the fovea has a detrimental effect on visual acuity, previous studies have suggested that retinal image motion may be advantageous in the peripheral visual field (Bex et al., 2003; Brown, 1972; Macedo et al., 2010). The aim of this study was to evaluate the effect of drift motion on peripheral resolution acuity.

Methods: Peripheral high-contrast resolution acuity in a group of 26 subjects (age 23.5 ± 3.2 years) was initially determined using a 2-alternative forced-choice Bayesian algorithm; the threshold value defined as the spatial frequency resulting in a 75% correct response rate. The stimuli used to measure static visual acuity were stationary Gabor-patches with a visible diameter of 2° and were presented at 20° in the nasal visual field. We determined the percentage correct response rate for varying velocities using drifting Gabor patches of the same spatial frequency as determined during measurement of static visual acuity. The sine-wave gratings drifted within the Gaussian envelope at one of 10 angular velocities, varying from 0.2 to 2.0 degrees/second in 0.2 degrees/second steps.

Results: Results showed an overall improvement in the subjects’ performance for all velocities. There was a significant difference in the percentage of correct responses between static stimulus presentation and for velocities of between 0.4 to 1.2 degrees/second (p < 0.05, One-way repeated measures ANOVA with Bonferroni post hoc tests). The average “correct response” for static stimulus presentation was 76 ± 2 %, improving to at least 85 % for velocities between 0.4 to 1.2 degrees/second. At velocities greater than 1.2 degrees/second performance was still better than for static stimulus presentation, but showed a gradual decline with increasing speed.

Conclusions: In line with previous studies stimulus motion has a positive effect on peripheral high-contrast resolution acuity. Presenting moving stimuli may benefit patients who rely on peripheral visual function, such as those with central visual field loss subsequent to AMD.

Keywords
Dynamic, visual, acuity
National Category
Ophthalmology
Research subject
Natural Science, Optometry
Identifiers
urn:nbn:se:lnu:diva-30839 (URN)
Conference
ARVO 2013, Life-changing research, Seattle, Wash., May 5 – 9, 2013
Available from: 2013-12-02 Created: 2013-12-02 Last updated: 2017-12-06Bibliographically approved
Baskaran, K., Rosen, R., Lewis, P., Unsbo, P. & Gustafsson, J. (2012). Benefit of Adaptive Optics Aberration Correction at Preferred Retinal Locus. Optometry and Vision Science, 89(9), 1417-1423
Open this publication in new window or tab >>Benefit of Adaptive Optics Aberration Correction at Preferred Retinal Locus
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2012 (English)In: Optometry and Vision Science, ISSN 1040-5488, E-ISSN 1538-9235, Vol. 89, no 9, p. 1417-1423Article in journal (Refereed) Published
Abstract [en]

PURPOSE.: To investigate the effect of eccentric refractive correction and full aberration correction on both high- and low-contrast grating resolution at the preferred retinal locus (PRL) of a single low-vision subject with a long-standing central scotoma. METHODS.: The subject was a 68-year-old women with bilateral absolute central scotoma due to Stargardt disease. She developed a single PRL located 25 degrees nasally of the damaged macula in her left eye, this being the better of the two eyes. High- (100%) and low-contrast (25 and 10%) grating resolution acuity was evaluated using four different correction conditions. The first two corrections were solely refractive error corrections, namely, habitual spectacle correction and full spherocylindrical correction. The latter two corrections were two versions of adaptive optics corrections of all aberrations, namely, habitual spectacle correction with aberration correction and full spherocylindrical refractive correction with aberration correction. RESULTS.: The mean high-contrast (100%) resolution acuity with her habitual correction was 1.06 logMAR, which improved to 1.00 logMAR with full spherocylindrical correction. Under the same conditions, low-contrast (25%) acuity improved from 1.30 to 1.14 logMAR. With adaptive optics aberration correction, the high-contrast resolution acuities improved to 0.89/0.92 logMAR and the low-contrast acuities improved to 1.04/1.06 logMAR under both correction modalities. The low-contrast (10%) resolution acuity was 1.34 logMAR with adaptive optics aberration correction; however, with purely refractive error corrections, she was unable to identify the orientation of the gratings. CONCLUSIONS.: Correction of all aberrations using adaptive optics improves both high- and low-contrast resolution acuity at the PRL of a single low-vision subject with long-standing absolute central scotoma

Keywords
REFRACTIVE ERROR, RESOLUTION ACUITY, VISION
National Category
Ophthalmology
Research subject
Natural Science, Optometry
Identifiers
urn:nbn:se:lnu:diva-21527 (URN)10.1097/OPX.0b013e318264f2a7 (DOI)22842306 (PubMedID)2-s2.0-84865730607 (Scopus ID)
Available from: 2012-09-05 Created: 2012-09-05 Last updated: 2017-12-07Bibliographically approved
Lewis, P., Baskaran, K., Rosén, R., Unsbo, P. & Gustafsson, J. (2012). The Effect of Refractive-Correction on Peripheral Low-Contrast Resolution Acuity. Paper presented at ARVO 2012, May 06-09, 2012 Fort Lauderdale, Florida.
Open this publication in new window or tab >>The Effect of Refractive-Correction on Peripheral Low-Contrast Resolution Acuity
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2012 (English)Conference paper, Poster (with or without abstract) (Other academic)
Abstract [en]

Purpose: High-contrast resolution acuity in the peripheral retina has been shown to be sampling-limited, however, a recent study indicates that peripheral low-contrast resolution acuity (PLCRA) is degraded by imposition of optical defocus. On the other hand, it has not been investigated whether refractive errors present in the peripheral field of normal, emmetropic eyes are sufficient to degrade PLCRA. The aim of this study was to evaluate the effect of peripheral refractive-correction on PLCRA in young emmetropes.Methods: Low-contrast (10 %) resolution acuity was measured off-axis (20° nasal visual field) on the right eyes of 10 emmetropic subjects (age 22 ± 2 years), both with and without peripheral refractive correction. Central and peripheral refractive errors were acquired using an open-field COAS-HD VR aberrometer. Stimuli, consisting of low-contrast Gabor patches with a visible diameter of 2°, were presented on a CRT monitor situated 3.0 meters from the subjects. Resolution thresholds were determined using a 2-alternative forced-choice Bayesian algorithm.Results: All the subjects had off-axis astigmatism (against-the-rule) at 20° in the nasal visual field ranging from -1.00 DC to -2.00 DC; of which four had -1.00 DC, three had -1.25 DC, one had -1.50 DC and the remaining two had -2.00 DC. Two of the subjects with -1.25 DC had -0.50 DS to -0.75 DS spherical errors; the rest had purely cylindrical errors. The mean uncorrected and corrected low-contrast resolution acuities for all subjects were 0.92 logMAR and 0.86 logMAR respectively. This shows an improvement in resolution acuity of 0.06 logMAR (p = 0.028) after correction of off-axis refractive errors. However when grouped according to the amount of astigmatism, the -1.00 DC group showed no difference in acuity with correction, the -1.25 DC, -1.50 DC and -2.00 DC groups improved on average by 0.05, 0.14 and 0.16 logMAR respectively.Conclusions: Correction of off-axis refractive errors had a positive effect on low-contrast resolution thresholds in the peripheral visual field on the emmetropic subjects in this study. Moreover, the effects become appreciable only for those subjects having larger degrees of astigmatism. This suggests that optical factors influence peripheral visual tasks involving low-contrast. There may be benefits in correcting even moderate amounts of off-axis refractive errors, especially for people who rely on optimal peripheral visual function.

National Category
Natural Sciences
Research subject
Natural Science, Optometry
Identifiers
urn:nbn:se:lnu:diva-19441 (URN)
Conference
ARVO 2012, May 06-09, 2012 Fort Lauderdale, Florida
Available from: 2012-06-04 Created: 2012-06-04 Last updated: 2017-02-17Bibliographically approved
Kristiansson, H., Baskaran, K., Lewis, P., Rosén, R., Unsbo, P. & Gustafsson, J. (2011). Effect of Optical Defocus on Peripheral Resolution Acuity in Old Healthy Emmetropes. Paper presented at American Academy of Optometry 2011 Boston.
Open this publication in new window or tab >>Effect of Optical Defocus on Peripheral Resolution Acuity in Old Healthy Emmetropes
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2011 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Background:

A recent study by Rosén et al found peripheral low contrast resolution acuity, but not high contrast acuity, to be affected by defocus in young healthy eyes. Since aging causes considerable degradation in peripheral optics even in healthy subjects we wanted to see if, older subjects were also sensitive to defocus in low contrast acuity.

 

Purpose:

The aim of this study was to evaluate the effect of optical defocus on high and low contrast resolution acuity in the peripheral visual field of healthy older emmetropes.

 

Subjects:

High- and low-contrast resolution acuity was evaluated under spherical defocus in the 20° nasal visual field of four healthy older emmetropic subjects. The off-axis refractive error at the 20° nasal visual field was measured by a COAS-HD VR aberrometer and was corrected accordingly for each subject.

 

Methods:

Resolution thresholds for visual acuity (VA) were obtained using stimuli consisting of high- (100%) and low- (10%) contrast gratings that were presented on a CRT monitor situated 1.0 meter from the subject. Stimuli, 3° in diameter were presented for 300 ms using a 2AFC paradigm. Two repeated measurements, for both high and low contrast, were obtained for each point of defocus in 1.0 D steps up to ±4 D at 45mm vertex distance. The results are corrected to effective defocus at the corneal plane.

 

Results:

Defocus had no visible effect on high contrast VA, although there was a slight decrease in VA with higher amounts of positive defocus. However, defocus was found to have a significant effect on low contrast VA. Moreover, low contrast resolution was more sensitive to positive defocus than the negative defocus.

 

Conclusions:

Defocus has an impact on low contrast resolution whereas no such effect was found for high contrast resolution. These results are similar to those obtained by Rosén et al1 in young eyes. These results suggest that low contrast optotypes could possibly be used for determining subjective refraction in low vision subjects.

Keywords
Defocus, Periphery, Resolution, Grating Acuity
National Category
Ophthalmology
Research subject
Natural Science, Optometry
Identifiers
urn:nbn:se:lnu:diva-15010 (URN)
Conference
American Academy of Optometry 2011 Boston
Available from: 2011-10-18 Created: 2011-10-18 Last updated: 2017-02-17Bibliographically approved
Baskaran, K., Lewis, P., Rosen, R., Unsbo, P. & Gustafsson, J. (2011). Effects of Optical Defocus on Resolution Acuity in Preferred Retinal Locus. In: Invest Ophthalmol Vis Sci 2011;52: E-Abstract 1900.: . , 52(6)
Open this publication in new window or tab >>Effects of Optical Defocus on Resolution Acuity in Preferred Retinal Locus
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2011 (English)In: Invest Ophthalmol Vis Sci 2011;52: E-Abstract 1900., 2011, Vol. 52, no 6Conference paper, Published paper (Refereed)
Abstract [en]

PurposeResolution acuity in the peripheral visual field is primarily limited by retinal sampling. In healthy eyes, the correction of peripheral refractive errors does not produce significant visual benefits other than improved detection and low contrast acuity. However, studies (Lundstrom L et al, Optom Vis Sci, 2007;84:1046-52) have shown that peripheral refractive corrections improve resolution acuity in subjects with central visual field loss (CFL) who have an established preferred retinal locus (PRL). The aim of this study was to evaluate the effect of optical defocus on high contrast resolution acuity in the PRL. MethodsResolution acuity was evaluated under spherical defocus in the PRL of three low vision subjects (mean age 75 years) with long standing CFL (due to age-related macular degeneration). Off-axis refractive error at the PRL was measured by an open-field COAS-HD VR aberrometer and was corrected accordingly. The PRL for subject 1 was located at 10{degrees} in the temporal visual field (left eye), subject 2 at 20{degrees} in the nasal visual field (right eye) and subject 3 at 15{degrees} in the inferior visual field (left eye). Stimuli consisting of high-contrast Gabor patches with a visible diameter of 3{o} were presented on a CRT monitor situated 1.0 meter from the subject. Resolution thresholds for static visual acuity (SVA) and dynamic visual acuity (DVA) were obtained using an adaptive Bayesian algorithm. Fixation was aided using illuminated concentric rings covering {+/-}25{degrees} in the visual field. Defocus was altered in 1D steps up to {+/-}4D. When measuring DVA, the sine-wave gratings drifted within the Gaussian envelope at an angular velocity of 1{degrees}/sec. ResultsResolution thresholds for both SVA and DVA in the PRL varied significantly with the amount of optical defocus. The results show a 2 - 3 line decrease (logMAR) in SVA and DVA with 4 D positive and negative defocus. There was no significant difference between SVA and DVA with increasing defocus. In the absence of defocus, SVA was significantly better than DVA in the PRL. ConclusionsDefocus as low as one dioptre has an impact on both static and dynamic high contrast resolution acuity for CFL subjects using a PRL. The results of this study suggest that, for CFL subjects using a PRL, resolution acuity is not only sampling limited but also influenced by the optics of the eye.

National Category
Medical and Health Sciences
Research subject
Natural Science, Optometry
Identifiers
urn:nbn:se:lnu:diva-12319 (URN)
Available from: 2011-06-10 Created: 2011-06-10 Last updated: 2017-02-17Bibliographically approved
Baskaran, K., Unsbo, P. & Gustafsson, J. (2011). Influence of age on peripheral ocular aberrations.. Optometry and Vision Science, 88(9), 1088-1098
Open this publication in new window or tab >>Influence of age on peripheral ocular aberrations.
2011 (English)In: Optometry and Vision Science, ISSN 1040-5488, E-ISSN 1538-9235, Vol. 88, no 9, p. 1088-1098Article in journal (Refereed) Published
Abstract [en]

PURPOSE.: To compare peripheral lower and higher order aberrations across the horizontal (±40°) and inferior (-20°) visual fields in healthy groups of young and old emmetropes. METHODS.: We have measured off-axis aberrations in the groups of 30 younger (24 ± 3 years) and 30 older (58 ± 5 years) emmetropes. The aberrations of OD were measured using the COAS-HD VR Shack-Hartmann aberrometer in 10° steps to ±40° horizontally and -20° inferiorly in the visual field. The aberrations were quantified with Zernike polynomials for a 4 mm pupil diameter. The second-order aberration coefficients were converted to their respective refraction components (M, J45, and J180). Mixed between-within subjects, analysis of variance were used to determine whether there were significant differences in the refraction and aberration components for the between-subjects variable age and the within-subjects variable eccentricity. RESULTS.: Peripheral refraction components were similar in both age groups. Among the higher order coefficients, horizontal coma (C3) and spherical aberration (C4) varied mostly between the groups. Coma increased linearly with eccentricity, at a more rapid rate in the older group than in the younger group. Spherical aberration was more positive in the older group compared with the younger group. Higher order root mean square increased more rapidly with eccentricity in the older group. CONCLUSIONS.: Like the axial higher order aberrations, the peripheral higher order aberrations of emmetropes increase with age, particularly coma and spherical aberration.

Keywords
peripheral aberrations; off-axis refraction; aging; COAS-HD VR
National Category
Ophthalmology
Research subject
Natural Science, Optometry
Identifiers
urn:nbn:se:lnu:diva-13993 (URN)10.1097/OPX.0b013e3182234630 (DOI)21666523 (PubMedID)2-s2.0-80052457355 (Scopus ID)
Available from: 2011-09-04 Created: 2011-09-04 Last updated: 2017-12-08Bibliographically approved
Lewis, P., Rosén, R., Unsbo, P. & Gustafsson, J. (2011). Naso-temporal Asymmetry of Peripheral Static and Dynamic Visual Acuity. In: : . Paper presented at EOS Topical Meeting: 5th European Meeting on Visual and Physiological Optics (EMVPO), 22 August 2010 - 24 August 2010, Stockholm, Sweden.
Open this publication in new window or tab >>Naso-temporal Asymmetry of Peripheral Static and Dynamic Visual Acuity
2011 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Summary

Static and dynamic visual acuity was evaluated in the peripheral visual field on normally sighted emmetropes. The results show a significant asymmetry for both static and dynamic visual acuity between the nasal and temporal visual fields.

 

Introduction

It is well known that visual performance thresholds decrease rapidly with increasing retinal eccentricity1. This reduction in performance can be attributed to both optical factors and reduced neural sampling2-3; the latter being the predominant limiting factor in the peripheral retina3.  Previous studies have shown that slowly moving stimuli are more easily resolved than stationary stimuli in the peripheral retina4. There is little evidence published regarding resolution thresholds for moving stimuli in more than a few limited directions in the visual field.     

In this study, static visual acuity (SVA) and dynamic visual acuity (DVA) thresholds were measured at 10° intervals both nasally and temporally on healthy, young emmetropes. DVA was measured at angular velocities of 1 °/s and 2 °/s using drifting Gabor patches.  

 

Discussion

Static and dynamic visual acuity was measured on the right eye of emmetropic subjects. Results for SVA showed significantly better resolution in the temporal visual field compared with the nasal visual field at eccentricities 20° and beyond. The mean difference in acuity at 20° was approximately 0.2 LogMAR and at 30°, 0.3 LogMAR. The difference between the thresholds for DVA showed a similar naso-temporal asymmetry; the reduction in DVA paralleling the decrease in SVA for eccentricities 10° and beyond.  No significant differences were observed between averaged results of SVA and DVA for the eccentricities tested in this study.

 

Conclusions

The results of this study confirm previous research conducted by Frisén (1987) showing better resolution for static stimuli presented in the temporal visual field compared to the nasal visual field. We have found that this is also true for DVA.

Series
Journal of Modern Optics, ISSN 0950-0340, E-ISSN 1362-3044
National Category
Ophthalmology
Research subject
Natural Science, Optometry
Identifiers
urn:nbn:se:lnu:diva-10203 (URN)
Conference
EOS Topical Meeting: 5th European Meeting on Visual and Physiological Optics (EMVPO), 22 August 2010 - 24 August 2010, Stockholm, Sweden
Note

Ej belagd 20190117

Available from: 2011-01-18 Created: 2011-01-18 Last updated: 2019-01-17Bibliographically approved
Lewis, P., Rosén, R., Unsbo, P. & Gustafsson, J. (2011). Resolution of static and dynamic stimuli in the peripheral visual field.. Vision Research, 51(16), 1829-1834
Open this publication in new window or tab >>Resolution of static and dynamic stimuli in the peripheral visual field.
2011 (English)In: Vision Research, ISSN 0042-6989, E-ISSN 1878-5646, Vol. 51, no 16, p. 1829-1834Article in journal (Refereed) Published
Abstract [en]

In a clinical setting, emphasis is given to foveal visual function, and tests generally only utilize static stimuli. In this study, we measured static (SVA) and dynamic visual acuity (DVA) in the central and peripheral visual field on healthy, young emmetropic subjects using stationary and drifting Gabor patches. There were no differences between SVA and DVA in the peripheral visual field; however, SVA was superior to DVA in the fovea for both velocities tested. In addition, there was a clear naso-temporal asymmetry for both SVA and DVA for isoeccentric locations in the visual field beyond 10° eccentricity. The lack of difference in visual acuity between static and dynamic stimuli found in this study may reflect the use of drift-motion as opposed to displacement motion used in previous studies.

Keywords
DVA; Dynamic visual acuity; Periphery
National Category
Ophthalmology
Research subject
Natural Science, Optometry
Identifiers
urn:nbn:se:lnu:diva-13814 (URN)10.1016/j.visres.2011.06.011 (DOI)21722661 (PubMedID)2-s2.0-79961028317 (Scopus ID)
Available from: 2011-08-18 Created: 2011-08-18 Last updated: 2017-12-08Bibliographically approved
Lundström, L., Rosén, R., Baskaran, K., Jaeken, B., Gustafsson, J., Artal, P. & Unsbo, P. (2011). Symmetries in peripheral ocular aberrations. Journal of Modern Optics, 58(19-20 SI), 1690-1695
Open this publication in new window or tab >>Symmetries in peripheral ocular aberrations
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2011 (English)In: Journal of Modern Optics, ISSN 0950-0340, E-ISSN 1362-3044, Vol. 58, no 19-20 SI, p. 1690-1695Article in journal (Refereed) Published
Abstract [en]

A mirror symmetry in the aberrations between the left and right eyes has previously been found foveally, but while a similar symmetry for the peripheral visual field is likely, it has not been investigated. Nevertheless, the peripheral optical quality is often evaluated in only one eye, because it is more time efficient than analyzing the whole visual field of both eyes. This study investigates the correctness of such an approach by measuring the peripheral wavefront aberrations in both eyes of 22 subjects out to +/- 40 degrees horizontally. The largest aberrations (defocus, astigmatism, and coma) were found to be significantly correlated between the left and right eyes when comparing the same temporal or nasal angle. The slope of the regression line was close to +/- 1 (within 0.05) for these aberrations, with a negative slope for the horizontally odd aberrations, i.e. the left and right eyes are mirror symmetric. These findings justify that the average result, sampled in one of the two eyes of many subjects, can be generalized to the other eye as well.

National Category
Medical and Health Sciences
Research subject
Natural Science, Optometry
Identifiers
urn:nbn:se:lnu:diva-11156 (URN)10.1080/09500340.2011.564317 (DOI)2-s2.0-84856927274 (Scopus ID)
Available from: 2011-03-17 Created: 2011-03-17 Last updated: 2017-12-11Bibliographically approved
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