Correction and manipulation of peripheral refractive errors are indispensable for people with central vision loss and in optical interventions for myopia control. This study investigates further enhancements of peripheral vision by compensating for monochromatic higher-order aberrations (with an adaptive optics system) and chromatic aberrations (with a narrowband green filter, 550 nm) in the 20° nasal visual field. Both high-contrast detection cutoff and contrast sensitivity improved with optical correction. This improvement was most evident for gratings oriented perpendicular to the meridian due to asymmetric optical errors. When the natural monochromatic higher-order aberrations are large, resolution of 10% contrast oblique gratings can also be improved with correction of these errors. Though peripheral vision is mainly limited by refractive errors and neural factors, higher-order aberration correction beyond conventional refractive errors can still improve peripheral vision under certain circumstances.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
In this study, we investigate the effect of optical correction on peripheral vision. For peripheral high-contrast resolution acuity, the absolute limiting factor is the neural sampling density of the retina, and correcting the optical errors has no beneficial effect [1–4]. However, other aspects of peripheral vision such as detection and low-contrast resolution are affected by optical errors and are hence improved with optical correction of defocus and astigmatism [5,6]. This is of crucial importance for people with central visual field loss, for whom enhanced peripheral optical quality has resulted in a general vision improvement, including high-contrast resolution acuity [7,8]. Any additional improvements with further optical correction will be beneficial for these subjects. Another field, where manipulation of peripheral optical quality is interesting, is myopia control [9,10]. Hence, it is important to investigate how different levels of optical correction will affect peripheral vision. Therefore, we evaluated peripheral vision under monochromatic and polychromatic visual conditions, both with the natural monochromatic higher-order aberrations and with the correction of the same.
Refractive errors vary across the visual field [11–13]; hence, foveal refractive correction will not be optimal for peripheral vision in most cases. Several earlier studies on healthy eyes and eyes with central visual field loss have shown improved peripheral vision when compensating for the refractive errors of the particular angle [14–16]. Naturally, the benefit of the correction depends on the magnitude of the refractive errors, and the studies mentioned earlier have reported significant improvements in 20° peripheral vision when correcting for 1.00 D or more of defocus and 1.50 D or more of astigmatism. In contrast with the aforementioned studies, where peripheral refractive errors were corrected with spectacle lenses, this work concerns further levels of peripheral optical correction. These levels include correction for the monochromatic higher-order aberrations and the chromatic aberrations of the eye, in addition to the refractive error correction. Many of these aberrations increase with eccentricity, particularly the off-axis astigmatism, coma, and transverse chromatic aberrations [13,17–20]. As these aberrations are asymmetric aberrations, they reduce the contrast of certain object orientations. Thus, lines oriented perpendicular to the peripheral visual field meridian are affected more than parallel lines [21,22]. However, the oblique gratings will be affected more equally by the asymmetric optical errors; therefore, we performed the vision measurements with both combinations of grating orientations. In peripheral vision, lines oriented parallel with the meridian (radially) are seen better than the perpendicular due to a neural effect and are referred to as meridional bias [22–26]. Hence, even though the correction of asymmetric optical errors will have a major effect on perpendicular orientations, the anisotropy can still be present due to the underlying neural differences. Zheleznyak et al. demonstrated this neural anisotropy by compensating for both monochromatic and chromatic aberrations in the periphery .
Only a few previous studies have investigated and compared peripheral vision with and without the correction of monochromatic higher-order aberrations and chromatic aberrations. In the 20° peripheral visual field, the correction of monochromatic aberrations resulted in improved low-contrast resolution [7,16,27], and transverse chromatic aberration has been shown to affect detection acuity [21,28]. Winter et al. demonstrated that better detection acuity could be achieved by providing the appropriate prism power that compensated for the ocular transverse chromatic aberration in that particular eccentricity . Cheney et al. found improved vision at 20° eccentricity by avoiding the transverse chromatic aberration through monochromatic interference patterns on the retina . However, no study on peripheral vision has compared the effect of correcting separately the monochromatic higher-order aberrations, chromatic aberrations, and correcting both at the same time.
The present study compares peripheral vision under four different levels of optical image quality: with and without correction of monochromatic aberrations with adaptive optics and in polychromatic and monochromatic light to avoid the chromatic aberrations.
Vision in the 20° nasal visual field was evaluated with different levels of optical correction using trial lenses and an adaptive optics system to improve and monitor the image quality on the peripheral retina in real time . The subjects fixated foveally at a Maltese cross mounted on the wall with the left eye and were aligned so that the optical axis of the adaptive optics instrument coincided with the 20° nasal visual field of the right eye. Fixation was monitored during vision measurements, and the subjects were realigned if necessary. The peripheral testing stimuli presented on a calibrated CRT monitor were hence seen by the right eye through the adaptive optics system. Four different optical conditions were evaluated, the descriptions of which are given below.
A. Levels of Optical Correction
Level 1 (Ref-W): The first level of correction was the refractive correction in white light. Refractive errors were corrected with trial lenses, while the vision measurements were performed using the full RGB spectrum of the monitor (white). Hence, both the natural monochromatic higher-order aberrations and chromatic aberrations were present.
Level 2 (Ref-G): In this level, the chromatic aberrations were avoided in addition to the refractive correction. Refractive errors were corrected with trial lenses, while the vision measurements were performed with a narrow bandwidth green filter (peak 550 nm, FWHM 25 nm) introduced in the adaptive optics system. Hence, only the natural monochromatic higher-order aberrations were present.
Level 3 (AO-W): The third level of correction was the full monochromatic aberration correction. This was provided by the adaptive optics system running in a closed-loop along with the refractive correction by the trial lenses. The vision measurements were performed with the full RGB spectrum of the monitor (white). In this level, only the natural chromatic aberrations were present.
Level 4 (AO-G): The fourth and final level of correction was performed with full monochromatic adaptive optics correction along with the refractive correction by the trial lenses. The vision evaluations were performed with the narrowband green filter in front of the monitor (peak 550 nm, FWHM 25 nm). This correction provided a close to diffraction limited green image on the retina.
B. Stimuli and Procedure
The stimuli were sinusoidal Gabor gratings () of different orientations presented on a calibrated 10-bit gray-scale monitor. The luminance of the screen was attenuated to always expose the subject to in both white and green conditions. The vision evaluations utilized a forced choice algorithm in a Bayesian psychophysical procedure: the Psi-method, as described by Kontsevich and Tyler . As in our previous studies of peripheral vision [6,22,28], the psychophysical algorithms were implemented in MATLAB and Psychophysics toolbox . All the measurements were performed with natural pupils. The refractive errors were obtained from the second-order Zernike coefficients of the wavefront measurements and corrected with trial lenses.
In total, seven subjects (one D myope, one D hyperope, and five emmetropes with age range 26–46 years) with no known ocular diseases were included in the study. All subjects had foveal visual acuity of 0.0 logMAR or better. High-contrast (100%) detection cutoffs as well as contrast sensitivity (CS) for three spatial frequencies were evaluated for one emmetrope (subject S1) and one myope (subject S2) under all four optical conditions. Subject S2 and the other five subjects performed 10% contrast resolution cutoff measurements under all four optical conditions. The study protocol was reviewed by the regional ethics committee and followed the tenets of the Declaration of Helsinki; written informed consent was obtained from all subjects before the measurements.
The high-contrast detection acuity cutoffs and CS measurements were performed on subjects S1 and S2 with Gabor gratings oriented either parallel (radial) or perpendicular (azimuthal) to the field meridian (i.e., with the lines lying horizontally or standing vertically). The stimulus presentation time was 500 milliseconds. The stimuli were presented in a two-interval forced choice algorithm, and the subject’s task was to identify the interval in which the grating was shown and respond with the corresponding key on a keypad. Each measurement had 100 trials with 50 trials for each grating orientation. The lapse rate was set to 5%, and the threshold was estimated at 72.5% correct responses. The stimuli were adjusted to take spectacle magnification provided by the trial lenses into consideration. This compensation for the spherical and cylindrical power of the trial lenses was implemented separately for the parallel and perpendicular gratings, as the spectacle magnification was different in different meridians. Detection CS was evaluated for three spatial frequencies: 1.0, 1.9, and 3.6 cycles per degree (cpd). All measurements were done with three repetitions, and test conditions were randomized.
In the second part of the study, the 10% contrast resolution acuity cutoffs of subjects S2–S7 were measured with obliquely oriented Gabor gratings leaning either to the left or right in a standard two alternative forced choice method with 40 trials. The cutoff was estimated for resolution of the combined grating orientations and compensated for the spectacle magnification by the mean spherical equivalent of the trial lenses. Except for that, the same settings as in the first experiment were used; all four levels of optical corrections were tested in random order with three repetitions.
During all visual evaluations, the peripheral wavefronts of the right eye were recorded continuously by the wavefront sensor in the adaptive optics system. The examiner also monitored the wavefront recordings manually to check for any large deviations during the measurements. This wavefront data were analyzed after the procedure to calculate the modulation transfer function (MTF) during every second of the measurement and then compute the time-average MTF over each measurement. The MTF calculations included both lower- and higher-order aberration Zernike terms.
A. High-Contrast Detection Acuity Cutoff with Parallel- and Perpendicular-Oriented Gratings
For both parallel and perpendicular gratings, the high-contrast detection acuity cutoffs showed changes with different levels of optical correction in both S1 and S2 (Fig. 1). However, the detection acuity for perpendicular gratings showed a larger improvement than parallel gratings in both subjects.
B. Detection Contrast Sensitivity with Parallel- and Perpendicular-Oriented Gratings
The detection CS curves for subjects S1 and S2 are shown in Fig. 2 separately for gratings parallel and perpendicular to the field meridian (left and right graph, respectively). Here, CS was measured by varying the contrast of gratings of 1, 1.9, and 3.6 cpd. In addition to the CS values, the detection acuity cutoffs (from Fig. 1, corresponding to 0 logCS) are also plotted to complete the curve. The CS values are plotted as separate curves for the four optical conditions. As can be seen, CS for the three spatial frequencies tested were consistently best for the fully corrected state (AO-G, represented with solid green line). Though both subjects exhibited a different magnitude of improvements in CS with different levels of optical correction, the maximum improvement was seen for the perpendicular gratings for the highest spatial frequency tested. The comparison between logCS values for the first and final levels of correction (Ref-W versus AO-G) is shown in Table 1.
C. Low-Contrast Resolution Cutoff with Oblique Gratings
Figure 3 shows the resolution results for six subjects under the four different optical conditions. The bars show the low-contrast resolution acuity cutoffs of subjects S2-S7 tested with a combination of oblique gratings. As can be seen, there is no consistent trend of improved resolution with improved image quality. Averaging over the six subjects shows no changes in low-contrast resolution with correction (): logMAR for Ref-W, logMAR for Ref-G, logMAR for AO-W, and logMAR for AO-G. On the individual level, subject S2 shows clear improvement (no overlapping of the individual data points) in the fully corrected case (AO-G) compared with the least-corrected case (Ref-W) whereas subject S7 shows slightly worse acuity for the fully corrected case (AO-G) compared with the least corrected case (Ref-W), and the other subjects show no such changes.
We also analyzed the time-average monochromatic MTF from the wavefront data recorded during the vision measurements. The MTF values for 3 cpd, which are close to the low-contrast resolution acuity (corresponding to 1.0 logMAR), and the corresponding pupil sizes are shown in Table 2. As expected, image quality improved with the correction of the monochromatic higher-order aberrations (compare Ref with AO) for all subjects. Furthermore, it can be seen that the monochromatic image quality was generally similar between the white and the green conditions. The MTF values suggest that the AO correction worked as expected and resulted in the desired level of optical correction during the vision measurements. Subject S2 had a substantial improvement with AO correction (fourfold improvement while comparing Ref-W with AO-G conditions), whereas other subjects only had a small change (around 1.25-fold). This is due to the fact that the natural monochromatic higher-order aberrations were larger in S2, and the other subjects had already a better image quality (compared with S2) with the refractive correction. This shows that the improvement in image quality was correlated to the improvement in low-contrast resolution cutoff, as S2 showed an improvement of 0.14 logMAR, while no significant improvement was seen in other subjects.
The individual MTF values are the time-average of three repetitions given for 3 cpd (corresponding to 1.0 logMAR). The individual pupil radii are the time-average of three repetitions of the minor axis of the elliptical pupil in millimeters. The last row shows the average and the standard deviation over all six subjects and repetitions. The four correction levels, i.e., Ref-W, Ref-G, AO-W, and AO-G, are the same as described in Fig. 1.
In order to evaluate the effect of different levels of optical correction on peripheral vision, we controlled the monochromatic higher-order aberrations with an adaptive optics system, and the chromatic aberrations were avoided by performing the vision evaluation under monochromatic conditions. This methodology is similar to two of the earlier studies that looked into the effect of aberrations on foveal vision [31,32] to achieve close to diffraction limited optical quality. Furthermore, the method is less complex than correcting for the chromatic aberrations with achromatic lenses or prisms [28,33,34]. Comparison between the monochromatic and the polychromatic condition is valid because both conditions were performed at the same photopic luminance, and both stimulated the long and medium wavelength cones. The two studies mentioned above demonstrated improved foveal visual function with monochromatic aberration correction and further improvement by avoiding chromatic aberrations, in monocular and binocular foveal vision, respectively. The present study did not find the same improvements for the peripheral low-contrast resolution when tested with oblique gratings. Note that we did not include the worst-case scenario, that is, leaving the peripheral refractive errors uncorrected, as it is already established that refractive correction improves low-contrast resolution [15,16].
Peripheral high-contrast detection acuity is sensitive to optical errors due to aliasing. The different levels of optical corrections gave different magnitudes of improvements for the parallel and perpendicular gratings (Figs. 1 and 2). This ambiguous improvement could be explained by the complex interaction between aliasing and meridional bias due to neural orientation preference [21,22]. On the other hand, the detection CS measurements, which were all performed in the nonaliasing zone, showed that the CS for perpendicular gratings improved more than that of the parallel gratings for the highest spatial frequency tested. This is consistent with the optical theory that both coma and transverse chromatic aberrations affect the image quality of perpendicular gratings more than parallel.
Unlike the parallel and perpendicular gratings, the two oblique grating orientations will be equally affected by asymmetric optical aberrations. In addition, the neural differences seen between the parallel and perpendicular grating acuity (the meridional bias) are not observed when using the two oblique gratings . In the second half of the present study, the low-contrast oblique grating resolution acuity therefore did not show any consistent improvement across the different levels of optical correction. Measurements with only perpendicular gratings in 10% contrast would have given different results, as the corrections will lead to a better image quality due to the nature of coma and transverse chromatic aberration. This can be seen from CS measurements on S1 and S2, at 10% contrast level (corresponding to log CS of 1); the improvements with different levels of optical corrections are greater for the perpendicular gratings than the parallel gratings. Even though these are detection CS measurements, the 10% contrast level is too low to produce any aliasing; hence, detection and resolution should be the same .
The present study demonstrates that the corrections beyond refractive errors can have an effect on peripheral vision. It is most evident for perpendicular gratings because of the large peripheral asymmetric aberrations and when the monochromatic higher-order aberrations are large (such as subject S2 in the current study). The effect on visual quality in real-life situations, when all object orientations are present, is likely to be small for subjects with average levels of higher-order aberrations. Nevertheless, improving the peripheral optical image quality is beneficial for subjects with central visual field loss. Earlier studies have demonstrated enhanced peripheral resolution by correcting the monochromatic lower- and higher-order aberrations [7,8,14]. The only remaining error that needed to be evaluated was the chromatic aberration. Even though the current study demonstrates no general improvements in peripheral vision by eliminating chromatic aberrations in subjects with normal central vision, it might still bring additional improvements in subjects with central visual field loss. Similarly, the effects can be different in further peripheral angles beyond 20°, as the optical errors increase with eccentricity. Another area where the manipulation of peripheral image quality can be meaningful is myopia development and control [9,10]. Many optical myopia control interventions alter the asymmetric blur in periphery and thereby change the image quality [35,36]. The results of this study indicate that, if large optical aberrations are induced by the intervention, it can reduce the visual performance in the periphery.
In conclusion, corrections of peripheral monochromatic higher-order aberrations and chromatic aberrations in a 20° visual field have a beneficial effect mainly on the visibility of perpendicular gratings. When the naturally occurring monochromatic higher-order aberrations are large, optical correction can improve the low-contrast resolution also for other grating orientations.
European Union's Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie Grant, MYFUN Project (H2020 Marie Skłodowska-Curie Actions (MSCA)) (675137).
1. L. Lundström, S. Manzanera, P. M. Prieto, D. B. Ayala, J. Gustafsson, P. Unsbo, and P. Artal, “Effect of optical correction and remaining aberrations on peripheral resolution acuity in the human eye,” Opt. Express 15, 1357–1362 (2007). [CrossRef]
2. L. N. Thibos, F. E. Cheney, and D. J. Walsh, “Retinal limits to the detection and resolution of gratings,” J. Opt. Soc. Am. A 4, 1524–1529 (1987). [CrossRef]
3. L. N. Thibos, D. L. Still, and A. Bradley, “Characterization of spatial aliasing and contrast sensitivity in peripheral vision,” Vis. Res. 36, 249–258 (1996). [CrossRef]
4. D. R. Williams and N. J. Coletta, “Cone spacing and the visual resolution limit,” J. Opt. Soc. Am. A 4, 1514–1523 (1987). [CrossRef]
5. Y. Z. Wang, L. N. Thibos, and A. Bradley, “Effects of refractive error on detection acuity and resolution acuity in peripheral vision,” Invest. Ophthalmol. Visual Sci. 38, 2134–2143 (1997).
6. R. Rosén, L. Lundström, and P. Unsbo, “Influence of optical defocus on peripheral vision,” Invest. Ophthalmol. Visual Sci. 52, 318–323 (2011). [CrossRef]
7. K. Baskaran, R. Rosén, P. Lewis, P. Unsbo, and J. Gustafsson, “Benefit of adaptive optics aberration correction at preferred retinal locus,” Optom. Vis. Sci. 89, 1417–1423 (2012). [CrossRef]
8. L. Lundström, J. Gustafsson, and P. Unsbo, “Vision evaluation of eccentric refractive correction,” Optom. Vis. Sci. 84, 1046–1052 (2007). [CrossRef]
9. J. Huang, D. Wen, Q. Wang, C. McAlinden, I. Flitcroft, H. Chen, S. M. Saw, H. Chen, F. Bao, Y. Zhao, L. Hu, X. Li, R. Gao, W. Lu, Y. Du, Z. Jinag, A. Yu, H. Lian, Q. Jiang, Y. Yu, and J. Qu, “Efficacy comparison of 16 interventions for myopia control in children: A network meta-analysis,” Ophthalmology 123, 697–708 (2016). [CrossRef]
10. World Health Organization, The Impact of Myopia and High Myopia: Report of the Joint World Health Organization–Brien Holden Vision Institute Global Scientific Meeting on Myopia (University of New South Wales, 2015).
11. D. A. Atchison, N. Pritchard, and K. L. Schmid, “Peripheral refraction along the horizontal and vertical visual fields in myopia,” Vision Res. 46, 1450–1458 (2006). [CrossRef]
12. A. Mathur and D. A. Atchison, “Peripheral refraction patterns out to large field angles,” Optom. Vis. Sci. 90, 140–147 (2013). [CrossRef]
13. L. Lundström and R. Rosén, “Peripheral aberrations,” in Handbook of Visual Optics, Volume One: Fundamentals and Eye Optics, P. Artal, ed., 1st ed. (CRC Press, 2017), Chap. 21.
14. P. Lewis, A. P. Venkataraman, and L. Lundström, “Contrast sensitivity in eyes with central scotoma: effect of stimulus drift,” Optom. Vis. Sci. 95, 354–361 (2018). [CrossRef]
15. D. A. Atchison, A. Mathur, and S. R. Varnas, “Visual performance with lenses correcting peripheral refractive errors,” Optom. Vis. Sci. 90, 1304–1311 (2013). [CrossRef]
16. P. Lewis, K. Baskaran, R. Rosén, L. Lundström, P. Unsbo, and J. Gustafsson, “Objectively determined refraction improves peripheral vision,” Optom. Vis. Sci. 91, 740–746 (2014). [CrossRef]
17. D. A. Atchison, “The Glenn A. fry award lecture 2011: peripheral optics of the human eye,” Optom. Vis. Sci. 89, E954–E966 (2012). [CrossRef]
18. L. Lundström, J. Gustafsson, and P. Unsbo, “Population distribution of wavefront aberrations in the peripheral human eye,” J. Opt. Soc. Am. A 26, 2192–2198 (2009). [CrossRef]
19. Y. U. Ogboso and H. E. Bedell, “Magnitude of lateral chromatic aberration across the retina of the human eye,” J. Opt. Soc. Am. A 4, 1666–1672 (1987). [CrossRef]
20. S. Winter, R. Sabesan, P. Tiruveedhula, C. Privitera, P. Unsbo, L. Lundström, and A. Roorda, “Transverse chromatic aberration across the visual field of the human eye,” J. Vis. 16(14):9 (2016). [CrossRef]
21. F. E. Cheney, L. N. Thibos, and A. Bradley, “Effect of ocular transverse chromatic aberration on detection acuity for peripheral vision,” Ophthalmic Physiolog. Opt. 35, 70–80 (2015). [CrossRef]
22. A. P. Venkataraman, S. Winter, R. Rosén, and L. Lundström, “Choice of grating orientation for evaluation of peripheral vision,” Optom. Vis. Sci. 93, 567–574 (2016). [CrossRef]
23. M. S. Banks, A. B. Sekuler, and S. J. Anderson, “Peripheral spatial vision: limits imposed by optics, photoreceptors, and receptor pooling,” J. Opt. Soc. Am. A 8, 1775–1787 (1991). [CrossRef]
24. J. Rovamo, V. Virsu, P. Laurinen, and L. Hyvärinen, “Resolution of gratings oriented along and across meridians in peripheral vision,” Invest. Ophthalmol. Visual Sci. 23, 666–670 (1982).
25. L. Zheleznyak, A. Barbot, A. Ghosh, and G. Yoon, “Optical and neural anisotropy in peripheral vision,” J. Vis. 16(5), 1–11 (2016). [CrossRef]
26. Y. L. Yap, D. M. Levi, and S. A. Klein, “Peripheral hyperacuity: isocentric bisection is better than radial bisection,” J. Opt. Soc. Am. A 4, 1562–1567 (1987). [CrossRef]
27. R. Rosén, L. Lundström, and P. Unsbo, “Adaptive optics for peripheral vision,” J. Mod. Opt. 59, 1064–1070 (2012). [CrossRef]
28. S. Winter, M. T. Fathi, A. P. Venkataraman, R. Rosén, A. Seidemann, G. Esser, L. Lundström, and P. Unsbo, “Effect of induced transverse chromatic aberration on peripheral vision,” J. Opt. Soc. Am. A 32, 1764–1771 (2015). [CrossRef]
29. L. L. Kontsevich and C. W. Tyler, “Bayesian adaptive estimation of psychometric slope and threshold,” Vis. Res. 39, 2729–2737 (1999). [CrossRef]
30. D. H. Brainard, “The psychophysics toolbox,” Spat. Vis. 10, 433–436 (1997). [CrossRef]
31. C. Schwarz, C. Canovas, S. Manzanera, H. Weeber, P. M. Prieto, P. Piers, and P. Artal, “Binocular visual acuity for the correction of spherical aberration in polychromatic and monochromatic light,” J. Vis. 14(2):8 (2014). [CrossRef]
32. G. Yoon and D. R. Williams, “Visual performance after correcting the monochromatic and chromatic aberrations of the eye,” J. Opt. Soc. Am. A 19, 266–275 (2002). [CrossRef]
33. N. López-Gil and R. Montés-Micó, “New intraocular lens for achromatizing the human eye,” J. Cataract Refractive Surg. 33, 1296–1302 (2007). [CrossRef]
34. Y. Benny, S. Manzanera, P. M. Prieto, E. N. Ribak, and P. Artal, “Wide-angle chromatic aberration corrector for the human eye,” J. Opt. Soc. Am. A 24, 1538–1544 (2017). [CrossRef]
35. R. Rosén, B. Jaeken, A. Lindskoog Petterson, P. Artal, P. Unsbo, and L. Lundström, “Evaluating the peripheral optical effect of multifocal contact lenses,” Ophthalmic Physiolog. Opt. 32, 527–534 (2012). [CrossRef]
36. Q. Ji, Y. Yoo, H. Alam, and G. Yoon, “Through-focus optical characteristics of monofocal and bifocal soft contact lenses across the peripheral visual field,” Ophthalmic Physiolog. Opt. 38, 326–336 (2018). [CrossRef]