Neural compensation for long-term asymmetric optical blur to improve visual performance in keratoconic eyes

Assessment of a Clinical Test for Detection of Alteration in Visual Perception Due to Astigmatism

Purpose

Astigmatism blurs the retinal image of a circular spot along a particular orientation rendering it an elliptical shape. Astigmatic patients demonstrate adaptation to residual astigmatic blur that may affect their discrimination between oval and circular targets. The Wilkins Egg and Ball Test (WEBT) was created to detect altered visual perception due to residual astigmatic blur by discriminating a circle within a row of oval elements. This prospective, cross-sectional study examined the utility of WEBT in detecting uncorrected residual astigmatism on the perception of form symmetry in astigmatic and keratoconic participants as well as normal participants with induced astigmatism at four primary meridians.

Methods

The mean search time (sT) and number of errors (noE) of 33 non-astigmatic controls (mean age: 24±5, range: 18-43, 6 males), 23 astigmatic participants (mean age: 36±12, range: 18-43, 6 males) and 13 keratoconic participants (N=22 eyes, mean age: 36±12, range: 18-58, 6 males) were measured under baseline, and 2.00 DC induced cylinder at four primary meridians, and for uncorrected, spherical-correction only, and fully corrected conditions, respectively. Mean sT and noE were converted to Z-scores, combined for each condition, and compared using repeated measures ANOVA with post-hoc analysis.

Results

Combined Z-scores for the controls were significantly worse (p<0.001) for all induced cylinder conditions. The induced 180° condition was significantly better than 45° and 90° conditions (p=0.04), but not the 135° condition. For both astigmatic and keratoconic cohorts, Z-scores of the uncorrected condition were significantly worse than the fully corrected condition (both p<0.01), but the fully corrected and spherical-only conditions did not differ significantly (p=0.06 and p=0.05, respectively).

Conclusion

In accommodating young adults, WEBT detected altered visual perception due to overall blur, and moderate-high amounts of uncorrected induced astigmatism and keratoconus, but is not useful as a tool for detection of altered visual perception due to small residual astigmatic blur.

Effects of Neural Adaptation to Habitual Spherical Aberration on Depth of Focus

We investigated how long-term visual experience with habitual spherical aberration (SA) influences subjective depth of focus (DoF). Nine healthy cycloplegic eyes with habitual SAs of different signs and magnitudes were enrolled. An adaptive optics (AO) visual simulator was used to measure through-focus high-contrast visual acuity after correcting all monochromatic aberrations and imposing +0.5 μm and -0.5 μm SAs for a 6-mm pupil. The positive (n=6) and negative (n=3) SA groups ranged from 0.17 to 0.8 μm and from -1.2 to -0.12 μm for a 6-mm pupil, respectively. For the positive habitual SA group, the median DoF with positive AO-induced SA (2.18D) was larger than that with negative AO-induced SA (1.91D); for the negative habitual SA group, a smaller DoF was measured with positive AO-induced SA (1.81D) than that with negative AO-induced SA (2.09D). The difference in the DoF of individual participants between the induced positive and negative SA groups showed a quadratic relationship with the habitual SA. Subjective DoF tended to be larger when the induced SA in terms of the sign and magnitude was closer to the participant's habitual SA, suggesting the importance of considering the habitual SA when applying the extended DoF method using optical or surgical procedures.

Stability of the retinal image under normal viewing conditions and the implications for neural adaptation

Previous studies have demonstrated that the visual system adapts to the specific aberration pattern of an individual's eye. Alterations to this pattern can lead to reduced visual performance, even when the Root Mean Square (RMS) of the wavefront error remains constant. However, it is well-established that ocular aberrations are dynamic and can change with factors such as pupil size and accommodation. This raises an intriguing question: can the neural system adapt to continuously changing aberration patterns? To address this question, we measured the ocular aberrations in four subjects under various natural viewing conditions, which included changes in accommodative state and pupil size. We subsequently computed the associated Point Spread Functions (PSFs). For each subject, we examined the stability in the orientation of the PSFs and analyzed the cross-correlation between different PSFs. These findings were then compared to the characteristics of a distribution featuring PSF shapes akin to random variations. Our results indicate that the changes observed in the PSFs are not substantial enough to produce a PSF shape distribution resembling random variations. This lends support to the notion that neural adaptation is indeed a viable mechanism even in response to continuously changing aberration patterns.

Adaptive optics visual simulators: a review of recent optical designs and applications [Invited]

In their pioneering work demonstrating measurement and full correction of the eye's optical aberrations, Liang, Williams and Miller, [JOSA A14, 2884 (1997)10.1364/JOSAA.14.002884] showed improvement in visual performance using adaptive optics (AO). Since then, AO visual simulators have been developed to explore the spatial limits to human vision and as platforms to test non-invasively optical corrections for presbyopia, myopia, or corneal irregularities. These applications have allowed new psychophysics bypassing the optics of the eye, ranging from studying the impact of the interactions of monochromatic and chromatic aberrations on vision to neural adaptation. Other applications address new paradigms of lens designs and corrections of ocular errors. The current paper describes a series of AO visual simulators developed in laboratories around the world, key applications, and current trends and challenges. As the field moves into its second quarter century, new available technologies and a solid reception by the clinical community promise a vigorous and expanding use of AO simulation in years to come.

Suprathreshold Contrast Perception Is Altered by Long-term Adaptation to Habitual Optical Blur

Purpose

To investigate whether visual experience with habitual blur alters the neural processing of suprathreshold contrast in emmetropic and highly aberrated eyes.

Methods

A large stroke adaptive optics system was used to correct ocular aberrations. Contrast constancy was assessed psychophysically in emmetropic and keratoconic eyes using a contrast matching paradigm. Participants adjusted the contrasts of gratings at various spatial frequencies to match the contrast perception of a reference grating at 4 c/deg. Matching was done both with fully corrected and uncorrected ocular aberrations. Optical correction allowed keratoconus patients to perceive high spatial frequencies that they have not experienced for some time.

Results

Emmetropic observers exhibited contrast constancy both with their native aberrations and when their aberrations were corrected. Keratoconus patients exhibited contrast constancy with their uncorrected, native optics but they did not exhibit constancy during adaptive optics correction. Instead. they exhibited striking underconstancy: they required more contrast at high spatial frequencies than the contrast of the 4-c/deg stimulus to make them seem to have the same contrast.

Conclusions

The presence of contrast constancy in emmetropes and keratoconus patients viewing with their native optics suggests that they have learned to amplify neural signals to offset the effects of habitual optical aberrations. The fact that underconstancy was observed in keratoconus patients when their optics were corrected suggests that they were unable to learn the appropriate neural amplification because they did not have experience with fine spatial detail. These results show that even adults can learn neural amplification to counteract the effects of their own optical aberrations.

Improvement of neural contrast sensitivity after long-term adaptation in pseudophakic eyes

An adaptive optics (AO) system was used to investigate the effect of long-term neural adaptation to the habitual optical profile on neural contrast sensitivity in pseudophakic eyes after the correction of all aberrations, defocus, and astigmatism. Pseudophakic eyes were assessed at 4 and 8 months postoperatively for changes in visual performance. Visual benefit was observed in all eyes at all spatial frequencies after AO correction. The average visual benefit across spatial frequencies was higher in the pseudophakic group (3.31) at 4 months postoperatively compared to the normal group (2.41). The average contrast sensitivity after AO correction in the pseudophakic group improved by a factor of 1.73 between 4 and 8 months postoperatively. Contrast sensitivity in pseudophakic eyes was poorer, which could be attributed to long-term adaptation to the habitual optical profiles before the cataract surgery, in conjunction with age-related vision loss. Improved visual performance in pseudophakic eyes suggests that the aged neural system can be re-adapted for altered ocular optics.

Optical Evaluation of Intracorneal Ring Segment Surgery in Keratoconus

Purpose

The purpose of this study was to assess the impact of different intracorneal ring segments (ICRS) combinations on corneal morphology and visual performance on patients with keratoconus.

Methods

A total of 124 eyes from 96 patients who underwent ICRS surgery were analyzed and classified into 7 groups based on ICRS disposition and the diameter of the surgical zone (5- and 6-mm). Pre- and postoperative complete ophthalmological examinations were conducted. Corneal geometry, volume, and symmetry were studied. Zernike polynomials were used to build a virtual ray-tracing model to evaluate optical aberrations and the Visual Strehl (VS).

Results

ICRS induced significant flattening across the cornea, being more pronounced on the anterior (+0.38 mm, P < 0.001) than on the posterior (+0.15 mm, P < 0.001) corneal radius. Asphericity experienced a larger change for a 6-mm surgical zone diameter (from −1.23 ± 1.1 to −1.86 ± 1.2, P < 0.001) than for a 5-mm zone (from −1.99 ± 1.1 to −2.10 ± 1.5, P = 0.536). Mean astigmatism was reduced by 2.05 D (P < 0.001). Combination four was the most effective in reducing astigmatism. Coma decreased by 30% on average and combination one produced an average reduction by 51% (P < 0.05). Patients experienced significant improvement in visual performance, best corrected visual acuity increased from 0.57 ± 0.21 to 0.69 ± 0.21 and VS changed from 0.049 ± 0.02 to 0.065 ± 0.041.

Conclusions

ICRS combinations implanted within 5 mm diameter zone are more effective in flattening the cornea, whereas those implanted on 6 mm diameter are as effective in reducing astigmatism and are a good choice if the asymmetry and the intended flattening are smaller. Combinations with asymmetrical implants are the best option to regularize corneal surface.

Translational Relevance

This study uses methods and metrics of optical research applied to daily clinical practice.

Assessing the visual image quality provided by refractive corrections during keratoconus progression

PURPOSE

To expand the SyntEyes keratoconus (KTC) model to assess the Visual Image Quality (VIQ) of sphero-cylindrical spectacle and rigid contact lens corrections as keratoconus progresses.

METHODS

The previously published SyntEyes KTC eye model to determine best sphero-cylindrical spectacle and rigid contact lens correction in keratoconic eyes was expanded to include the natural progression of keratoconus, thus allowing the assessment of corrected VIQ with disease progression.

RESULTS

As keratoconus progresses, the pattern of visual Strehl ratio (VSX) in correction space for spectacles alters from a typical hourglass into a shell pattern. The former would guide the subjective refraction towards the optimal correction while the latter is relatively insensitive to large dioptric steps. In 15 out of the 20 SyntEyes, the shell pattern eventually produces two foci on different sides of the correction space separated by a clinically significant dioptric difference with a similar, albeit lower VIQ. Wearing the best possible spectacle corrections provided an average gain of up to 3.5 lines of logMAR visual acuity compared to the uncorrected cases, which increased to 5.5 lines for the best rigid contact lens correction. Continuing to wear a spectacle correction as the disease progresses often leads to a VIQ that is almost as bad as the uncorrected case. Continuing to wear a rigid contact lens correction as the disease progresses maintains a relatively high level of VIQ, albeit in the low range for typically well-corrected normal eyes.

CONCLUSIONS

The results reflect the clinical experience that subjective refraction is difficult in highly-aberrated keratoconic eyes, the benefit of spectacle correction is short lived and that rigid contact lenses provide better and more stable VIQ with disease progression. Other aspects, such as the presence and behaviour of the second focus in some cases, remain to be confirmed clinically.

SLM-based interferometer for assessing the polychromatic neural transfer function of the eye

A novel interferometric instrument for measuring neural transfer function (NTF) of the eye is presented. The device is based on a liquid-crystal-on-silicon spatial light modulator (SLM), which is used to create two laterally separated wavefronts in the pupil plane of the eye that interfere on the retina. The phase mask on the SLM, consisting of two diffraction gratings mixed in a checkerboard pattern and acting as a shearing interferometer, allows independent control of spatial frequency, orientation, and contrast of the fringes, as well as the field of view in a wide polychromatic spectrum. Coupled with a supercontinuum source, the system is able to produce achromatic fringes on the retina. The instrument was successfully tested in six normal subjects in four light conditions: polychromatic light and monochromatic blue, green and red light respectively (central wavelengths - 450, 550 and 650 nm). On average, the NTF in polychromatic light was approximately 20% higher than for green and red light, although not statistically significant due to high intersubject variability. Due to all-digital control of the interference fringes, the device is optically simple and virtually unsusceptible to vibrations, allowing its use in a non-laboratory environment. The study also contributes to color vision research, allowing to evaluate contrast sensitivity function without monochromatic or chromatic aberrations.

Contrast sensitivity and higher-order aberrations in Keratoconus subjects

This study analyzes the relationship between contrast-sensitivity and higher-order aberrations (HOA) in mild and subclinical-keratoconus in subjects with good visual-acuity (VA). Keratoconus group (including subclinical-keratoconus) and controls underwent autokeratometry, corneal-tomography, autorefraction and HOA measurement. Contrast-sensitivity was tested using a psychophysical two-alternative forced-choice Gabor patches in three blocks (6, 9, 12 cycles/deg). Controls were compared to the keratoconus group and to a keratoconus subgroup with VA of 0.00 LogMar group ("keratoconus-0.00VA"). Spearman correlation tested association between HOA and contrast-sensitivity. Twenty-two keratoconus subjects (38 eyes: 28 keratoconus, 10 subclinical-keratoconus, 20 keratoconus-0.00VA) and 35 controls were included. There was a significant difference between control and keratoconus, and between control and keratoconus-0.00VA, for keratometry, cylinder, thinnest and central corneal thickness (p < 0.001). Controls showed lower HOA and higher contrast-sensitivity for all spatial-frequencies (p < 0.001). Most HOA were negatively correlated with contrast-sensitivity for all spatial-frequencies for keratoconus group and for 9 and 12 cycles/deg for keratoconus-0.00VA. Keratoconus subjects with good VA showed reduction in contrast-sensitivity and increased HOAs compared to controls. HOA and contrast-sensitivity are inversely correlated in subjects with mild keratoconus despite good VA. This suggests that the main mechanism underlying the decreased vision quality in keratoconus is the increase of HOA.

Optics and neural adaptation jointly limit human stereovision

Stereovision is the ability to perceive fine depth variations from small differences in the two eyes' images. Using adaptive optics, we show that even minute optical aberrations that are not clinically correctable, and go unnoticed in everyday vision, can affect stereo acuity. Hence, the human binocular system is capable of using fine details that are not experienced in everyday vision. Interestingly, stereo acuity varied considerably across individuals even when they were provided identical perfect optics. We also found that individuals' stereo acuity is better when viewing with their habitual optics rather than someone else's (better) optics. Together, these findings suggest that the visual system compensates for habitual optical aberrations through neural adaptation and thereby optimizes stereovision uniquely for each individual. Thus, stereovision is limited by small optical aberrations and by neural adaptation to one's own optics.

Functional reallocation of sensory processing resources caused by long-term neural adaptation to altered optics

The eye’s optics are a major determinant of visual perception. Elucidating how long-term exposure to optical defects affects visual processing is key to understanding the capacity for, and limits of, sensory plasticity. Here, we show evidence of functional reallocation of sensory processing resources following long-term exposure to poor optical quality. Using adaptive optics to bypass all optical defects, we assessed visual processing in neurotypically-developed adults with healthy eyes and with keratoconus – a corneal disease causing severe optical aberrations. Under fully-corrected optical conditions, keratoconus patients showed altered contrast sensitivity, with impaired sensitivity for fine spatial details and better-than-typical sensitivity for coarse spatial details. Both gains and losses in sensitivity were more pronounced in patients experiencing poorer optical quality in their daily life and mediated by changes in signal enhancement mechanisms. These findings show that adult neural processing adapts to better match the changes in sensory inputs caused by long-term exposure to altered optics.

Anterior, posterior, and nonkeratometric contributions to refractive astigmatism in pseudophakes

PURPOSE

To investigate the relationship between measured anterior (ACA) and posterior (PCA) keratometric astigmatism and postoperative refractive astigmatism (RA) and to quantify noncorneal astigmatism (NCA) contributions to RA.

SETTING

Penn State College of Medicine, Hershey, Pennsylvania, USA.

DESIGN

Retrospective consecutive case series.

METHODS

Consecutive eyes underwent preoperative biometry (IOLMaster 700) and tomography/topography using a dual Scheimpflug-placido disk-based device (Galilei G4), cataract surgery with implantation of a monofocal intraocular lens (IOL), and postoperative manifest refractions. RA was compared with keratometric astigmatism using the following methods: IOLMaster, SimK, CorT, SimK + measured PCA, total corneal power at the corneal plane (TCP2), and CorT(Total). An ocular residual astigmatism (ORA) vector was calculated between RA and each measured astigmatism.

RESULTS

Analysis was based on 296 eyes. ORA centroids were 0.28 @ 179, 0.45 @ 001, 0.37 @ 001, 0.19 @ 003, 0.19 @ 001, and 0.23 @ 178 diopter (D) for the 6 aforementioned methods, respectively (P < .000001 [ORAx, ORAy]). Based on TCP2 measurements, eyes with against-the-rule ACA and with-the-rule (WTR) ACA had ORA centroids of 0.09 @ 082 and 0.58 @ 001 D (P < .000001 [ORAx, ORAy]), respectively. ORA was nonzero and not entirely explained by the cornea, especially in those with WTR ACA.

CONCLUSIONS

Total keratometric astigmatism did not explain all ocular astigmatism. Noncorneal contributions were significant, especially in eyes with WTR ACA.

Orientation-specific long-term neural adaptation of the visual system in keratoconus

Eyes with the corneal ectasia keratoconus have performed better than expected (e.g. visual acuity) given their elevated levels of higher-order aberrations that cause rotationally asymmetric retinal blur. Adapted neural processing has been suggested as an explanation but has not been measured across multiple meridional orientations. Using a custom Maxwellian-view laser interferometer to bypass ocular optics, sinusoidal grating neural contrast sensitivity was measured in six eyes (three subjects) with keratoconus and four typical eyes (two subjects) at six spatial frequencies and eight orientations using a two-interval forced-choice paradigm. Total measurement duration was 24 to 28 hours per subject. Neural contrast sensitivity functions of typical eyes agreed with literature and generally showed the oblique effect on a linear-scale and rotational symmetry on a log-scale (rotational symmetry was quantified as the ratio of the minor and major radii of an ellipse fit to all orientations within each spatial frequency; typical eye mean 0.93, median 0.93; where a circle = 1). Mean sensitivities of eyes with keratoconus were 20% to 60% lower (at lower and higher spatial frequencies respectively) than typical eyes. Orientation-specific neural contrast sensitivity functions in keratoconus showed substantial rotational asymmetry (ellipse radii ratio: mean 0.84; median 0.86) and large meridional reductions. The visual image quality metric VSX was used with a permutation test to combine the asymmetric optical aberrations of the eyes with keratoconus and their measured asymmetric neural functions, which illustrated how the neural sensitivities generally mitigated the detrimental effects of the optics.

Effect of rigid gas permeable contact lens on keratoconus progression: a review

The prevalence of keratoconus is 1/2000 in the general population and is high in adolescents. Keratoconus is a progressive disease, which has a great impact on patients' quality of life and mental health. It can be managed by surgical and non-surgical means, rigid gas permeable (RGP) contact lens as its main non-surgical method is widely used in clinic. The efficacy of wearing RGP contact lens has been confirmed to some extent, but some studies have found that wearing RGP contact lens has adverse effects, which may promote disease progression. In this paper, the advantages and disadvantages of RGP contact lens in controlling keratoconus were reviewed to provide more suggestions and references for the clinical application of RGP contact lens.

Investigation of the impact of blur under mobile attentional orientation using a vision simulator

It is well-known that correction of blur can improve visual perception. However, it is unclear how the beneficial effect of correction is affected by the regions of correction and the spatial uncertainty introduced by the retinal stimulation. The purpose of this study was two-fold: first, to compare the impacts of blur correction between isoeccentric locations of the visual field; and second, to evaluate the effect of spatial cueing in each corrected location on performing a simple task. Five subjects were asked to complete a simple detection task of a small dark spot stimulus presented randomly at four cardinal retinal locations (eccentricity: 5°) under manipulation of attention via an exogenous cue. Both clear and blurred targets were randomly displayed across the visual field and viewed monocularly through a vision simulator, used to minimize peripheral ocular aberrations. Results confirmed the advantage of clear vs/ blurred images under spatial uncertainty. It was also found that the visual benefit from blur correction is unequal at isoeccentric locations, even for a simple detection task. While manipulation of attention in the presence of spatial uncertainty significantly modulated response time (RT) performance, no differential effect was observed for clear and blurred stimuli, suggesting that attention has only a small effect on the optical benefit for a simple detection task when the display is depleted (no distractor). Those observations highlight the importance of field performance asymmetries in optical interventions and may offer useful implications for the design of extrafoveal refractive correction. Further studies are needed to elucidate how the focus of attention interacts with the perceived gain of vision correction.

A computational analysis of retinal image quality in eyes with keratoconus

Higher-order aberrations (HOA’s) are exaggerated in eyes with keratoconus but little is known about their impact on the retinal image quality (IQ) of these eyes. This computational study determined changes in IQ [peak IQ, best focus and depth of focus (DOF)] of 12 subjects with manifest keratoconus in both eyes (KCE cohort), 9 subjects with very asymmetric ectasia (VAE cohort) with and without their Rigid Gas Permeable contact lenses (RGP CL’s) and 20 age-matched controls, using a HOA-based through-focus analysis performed on the logNS IQ metric over 5 mm pupil diameter following cycloplegia. All IQ parameters were significantly worse in the KCE cohort with their native HOA’s, relative to controls and in the ectatic eye of the VAE cohort, relative to the fellow non-ectatic eye (p ≤ 0.008 for all). Reduction in HOA’s of these eyes with RGP CL’s resulted in a significant improvement in all IQ parameters but they all remained significantly poorer than controls (p ≤ 0.02 for all). The inter-subject variability of best focus and the DOF range were inversely related to peak IQ in these eyes (r = 0.85; p < 0.001). These results provide the optical basis for two clinical observations on keratoconus: (1) optical performance of keratoconic eyes are significantly better with RGP CL’s than with spectacles or unaided conditions and (2) the endpoint of subjective refraction is elusive in keratoconic eyes, relative to healthy controls or to the non-ectatic eye in bilaterally asymmetric ectasia.

Comparison of Wavefront-guided and Best Conventional Scleral Lenses after Habituation in Eyes with Corneal Ectasia

SIGNIFICANCE

Visual performance with wavefront-guided (WFG) contact lenses has only been reported immediately after manufacture without time for habituation, and comparison has only been made with clinically unrefined predicate conventional lenses. We present comparisons of habitual corrections, best conventional scleral lenses, and WFG scleral lenses after habituation to all corrections.

PURPOSE

The purpose of this study was to compare, in a crossover design, optical and visual performance of eyes with corneal ectasias wearing dispensed best conventional scleral lens corrections and dispensed individualized WFG scleral lens corrections.

METHODS

Ten subjects (20 eyes) participated in a randomized crossover study where best conventional scleral lenses and WFG scleral lenses (customized through the fifth radial order) were worn for 8 weeks each. These corrections, as well as each subject's habitual correction and normative data for normal eyes, were compared using (1) residual higher-order aberrations (HORMS), (2) visual acuity (VA), (3) letter contrast sensitivity (CS), and (4) visual image quality (logarithm of the visual Strehl ratio, or logVSX). Correlations were performed between Pentacam biometric measures and gains provided by WFG lenses.

RESULTS

Mean HORMS was reduced by 48% from habitual to conventional and 43% from conventional to WFG. Mean logMAR VA improved from habitual (+0.12) to conventional (-0.03) and further with WFG (-0.09); six eyes gained greater than one line with WFG over conventional. Area under the CS curve improved by 26% from habitual to conventional and 14% from conventional to WFG. The percentage of the eyes achieving normal levels were as follows: HORMS, 40% for conventional and 85% for WFG; VA, 50% for conventional and 85% for WFG; and CS, 60% for conventional and 90% for WFG. logVSX improved by 16% from habitual to conventional and 25% further with WFG. Reduction in aberrations with WFG lenses best correlated with posterior cornea radius of curvature.

CONCLUSIONS

Visual performance was superior to that reported with nonhabituated WFG lens wear. With WFG lenses, HORMS and logVSX significantly improved, allowing more eyes to reach normal levels of optical and visual performance compared with conventional lenses.

Visual and Refractive Long-Term Outcomes Following Standard Cross-Linking in Progressive Keratoconus Management

Purpose

To analyze the effectiveness and stability of the refractive, topographic and visual outcomes of the standard cross-linking (SCXL) in keratoconus (KC) management.

Patients and methods

This study was designed as a retrospective non-comparative study that included 28 KC patients (n=49 eyes) who performed SCXL as a single procedure to treat KC and completed five-year follow-up period. The topographic, refractive and visual data were recorded preoperatively and at 12, 24, 36 and 60 months postoperatively.

Results

Forty eyes (81.6%) showed achieved postoperative spherical equivalent (SE) refraction better than the attempted refraction. Ten eyes (20.4%) improved by <1 D, 23 eyes (46.9%) improved from 1 D to <2 D and 7 eyes (14.3%) improved by ≥2 D. Both uncorrected distant visual acuity (UDVA) and corrected distant visual acuity (CDVA) showed statistically significant improvement from preoperative 1.34±0.29 (mean±SD) and 0.74±0.23 LogMAR to postoperative 0.99±0.32 and 0.50±0.22 LogMAR (P<0.0001) respectively. Both Kmax and SE refraction showed statistically significant and stable improvement from preoperative 51.95±1.90 and −7.90±3.14 D to postoperative 50.19±1.96 and −6.35±2.49 D (P<0.0001) respectively. Two eyes (4%) showed KC progression at the end of 5th follow-up year.

Conclusion

SCXL had good effectiveness and stability that halted KC progression over 5-year follow-up period. It had also unexpected improvement in the KC refractive components mainly the spherical and SE components.

LogMAR and Stereoacuity in Keratoconus Corrected with Spectacles and Rigid Gas-permeable Contact Lenses

SIGNIFICANCE

This study showed an improvement in three-dimensional depth perception of subjects with bilateral and unilateral keratoconus with rigid gas-permeable (RGP) contact lens wear, relative to spectacles. This novel information will aid clinicians to consider RGP contact lenses as a management modality in keratoconic patients complaining of depth-related difficulties with their spectacles.

PURPOSE

The aim of this study was to systematically compare changes in logMAR acuity and stereoacuity from best-corrected spherocylindrical spectacles to RGP contact lenses in bilateral and unilateral keratoconus vis-à-vis age-matched control subjects.

METHODS

Monocular and binocular logMAR acuity and random-dot stereoacuity were determined in subjects with bilateral (n = 30; 18 to 24 years) and unilateral (n = 10; 18 to 24 years) keratoconus and 20 control subjects using standard psychophysical protocols.

RESULTS

Median (25th to 75th interquartile range) monocular (right eye) and binocular logMAR acuity and stereoacuity improved significantly from spectacles to RGP contact lenses in the bilateral keratoconus cohort (P < .001). Only monocular logMAR acuity of affected eye and stereoacuity improved from spectacles to RGP contact lenses in the unilateral keratoconus cohort (P < .001). There was no significant change in the binocular logMAR acuity from spectacles to RGP contact lenses in the unilateral keratoconus cohort. The magnitude of improvement in binocular logMAR acuity and stereoacuity was also greater for the bilateral compared with the unilateral keratoconus cohort. All outcome measures of cases with RGP contact lenses remained poorer than control subjects (P < .001).

CONCLUSIONS

Binocular resolution and stereoacuity improve from spectacles to RGP contact lenses in bilateral keratoconus, whereas only stereoacuity improves from spectacles to RGP contact lenses in unilateral keratoconus. The magnitude of improvement in visual performance is greater for the binocular compared with the unilateral keratoconus cohort.

Customised aberration-controlling corrections for keratoconic patients using contact lenses

Technological advancements in the design of soft and scleral contact lenses have led to the development of customised, aberration-controlling corrections for patients with keratoconus. As the number of contact lens manufacturers producing wavefront-guided corrections continues to expand, clinical interest in this customisable technology is also increasing among both patients and practitioners. This review outlines key issues surrounding the measurement of ocular aberrations for patients with keratoconus, with a particular focus on the possible factors affecting the repeatability of Hartmann-Shack aberrometry measurements. This review also discusses and compares the relative successes of studies investigating the design and fitting of soft and scleral customised contact lenses for patients with keratoconus. A series of key limitations that should be considered before designing customised contact lens corrections is also described. Despite the challenges of producing and fitting customised lenses, improvements in visual performance and comfortable wearing times, as provided by these lenses, could help to reduce the rate of keratoplasty in keratoconic patients, thereby significantly reducing clinical issues related to corneal graft surgery. Furthermore, enhancements in optical correction, provided by customised lenses, could lead to increased independence, particularly among young adult keratoconic patients, therefore leading to improvements in quality of life.

Binocular cross-correlation analyses of the effects of high-order aberrations on the stereoacuity of eyes with keratoconus

Stereoacuity losses are induced by increased magnitudes and interocular differences in high-order aberrations (HOAs). This study used keratoconus as a model to investigate the impact of HOAs on disparity processing and stereoacuity. HOAs and stereoacuity were quantified in subjects with keratoconus (n = 21) with HOAs uncorrected (wearing spectacles) or minimized (wearing rigid gas-permeable contact lenses) and in control subjects without keratoconus (n = 5) for 6-mm pupil diameters. Disparity signal quality was estimated using metrics derived from binocular cross-correlation functions of stereo pairs convolved with point-spread functions from these HOAs. Metrics computed for all subjects were compared with stereoacuities. The effects of contrast losses and phase shifts on disparity signal quality were studied independently by manipulating the amplitude and phase components of optical transfer functions. The magnitudes, orientations, interocular relationships in magnitude, and shape of the point-spread function affected the cross-correlation metrics that determine disparity signal quality. Stereoacuity covaries strongly with cross-correlation metrics and moderately with image-quality metrics. Both phase distortions and contrast losses due to HOAs significantly influence computations of binocular disparity. HOA-induced stereoacuity reductions are attributable to disparity blur and noise from image properties that reduce the height and kurtosis of the peak stimulus disparity match of the cross-correlation. Phase distortions and contrast losses due to HOAs are both partly responsible for the greater stereoacuity losses seen with spectacles compared to rigid gas-permeable contact lenses in keratoconus.

Visual acuity in patients with keratoconus: a comparison with matched regular myopic astigmatism

PURPOSE

To compare uncorrected distance visual acuity (UDVA) and best-corrected distance visual acuity (CDVA) between patients with keratoconus (KC) and matched patients with regular myopic astigmatism.

METHODS

This retrospective study included consecutive patients diagnosed with KC between 2008 and 2013 at Care-Vision Laser Centers, Tel-Aviv, Israel, and matched patients with regular myopic astigmatism. Data included were central corneal thickness (CCT), spherical equivalent (SE), cylinder (CYL), mean keratometric power, maximum keratometric power (K_max), UDVA, CDVA, and defocus equivalent (DEQ).

RESULTS

The KC group included 734 patients with a mean age of 33.8 ± 9.5 years. The matched, control group included 1462 patients with a mean age of 33.2 ± 9.7 years (p = 0.14). The mean SE and CYL of the KC group were - 3.34 ± 3.29D and - 3.01 ± 1.99D, respectively, compared to - 3.34 ± 2.92D (p = 0.98) and - 2.97 ± 1.35 (p = 0.58). Mean K (46.8 ± 3.3D versus 44.0 ± 1.8D, p < 0.0001) and K_max (48.4 ± 4.0D versus 45.3 ± 2.0D, p < 0.0001) were statically significant higher in the KC group. CCT was significantly thinner in the KC group (444 ± 49 versus 527 ± 40 μm, p < 0.0001). The KC group had a better UDVA than the non-KC group (1.10 ± 0.68 versus 1.22 ± 0.64 logMAR, p < 0.0001). CDVA was significantly lower in the KC group (p < 0.001).

CONCLUSIONS

For defocus equivalents above 6D, the KC group had better UDVA than the non-KC group in spite of worse CDVA.

Response to short-term deprivation of the human adult visual cortex measured with 7T BOLD

Sensory deprivation during the post-natal ‘critical period’ leads to structural reorganization of the developing visual cortex. In adulthood, the visual cortex retains some flexibility and adapts to sensory deprivation. Here we show that short-term (2 hr) monocular deprivation in adult humans boosts the BOLD response to the deprived eye, changing ocular dominance of V1 vertices, consistent with homeostatic plasticity. The boost is strongest in V1, present in V2, V3 and V4 but absent in V3a and hMT+. Assessment of spatial frequency tuning in V1 by a population Receptive-Field technique shows that deprivation primarily boosts high spatial frequencies, consistent with a primary involvement of the parvocellular pathway. Crucially, the V1 deprivation effect correlates across participants with the perceptual increase of the deprived eye dominance assessed with binocular rivalry, suggesting a common origin. Our results demonstrate that visual cortex, particularly the ventral pathway, retains a high potential for homeostatic plasticity in the human adult.

The world around us changes all the time, and the brain must adapt to these changes. This process, known as neuroplasticity, peaks during development. Abnormal sensory input early in life can therefore cause lasting changes to the structure of the brain. One example of this is amblyopia or ‘lazy eye’. Infants who receive insufficient input to one eye – for example, because of cataracts – can lose their sight in that eye, even if the cataracts are later removed. This is because the brain reorganizes itself to ignore messages from the affected eye.

Does the adult visual system also show neuroplasticity? To explore this question, Binda, Kurzawski et al. asked healthy adult volunteers to lie inside a high-resolution brain scanner with a patch covering one eye. At the start of the experiment, roughly half of the brain’s primary visual cortex responded to sensory input from each eye. But when the volunteers removed the patch two hours later, this was no longer the case.

Some areas of the visual cortex that had previously responded to stimuli presented to the non-patched eye now responded to stimuli presented to the patched eye instead. The patched eye had also become more sensitive to visual stimuli. Indeed, these changes in visual sensitivity correlated with changes in brain activity in a pathway called the ventral visual stream. This pathway processes the fine details of images. Groups of neurons within this pathway that responded to stimuli presented to the patched eye were more sensitive to fine details after patching than before.

Visual regions of the adult brain thus retain a high degree of neuroplasticity. They adapt rapidly to changes in the environment, in this case by increasing their activity to compensate for a lack of input. Notably, these changes are in the opposite direction to those that occur as a result of visual deprivation during development. This has important implications because lazy eye syndrome is currently considered untreatable in adulthood.

Comparative analysis of the visual performance after implantation of the toric implantable collamer lens in stable keratoconus: a 4-year follow-up after sequential procedure (CXL+TICL implantation)

Aims

To report on 4-year postoperative visual performance with the toric implantable collamer lens (TICL) for stable keratoconus after sequential procedure (corneal collagen crosslinking plus TICL implantation).

Methods

Forty eyes of 24 patients with stable keratoconus with myopia between 0.00 and −18.00 dioptres (D) and astigmatism between 1.25 and 8.00 D were evaluated in this prospective interventional study (https://clinicaltrials.gov/ct2/show/NCT02833649). We evaluated refraction, visual outcomes, astigmatic changes analysed by Alpins vector, contrast sensitivity, aberrometry, modulation transfer functions (MTFs), defocus curve, and operative and postoperative complications.

Results

At 4-year follow-up, 45% had 20/20 vision or better and 100% had 20/40 or better uncorrected visual acuity (UCVA). Vector analysis of refractive astigmatism shows that the surgically induced astigmatism (SIA) (3.20±1.46 D) was not significantly different from the target induced astigmatism (TIA) (3.14±1.42 D) (p=0.620). At 4 years postoperatively, none of the eyes showed a decrease in UCVA, in contrast to 24 eyes in which UCVA was increased by ≥1 lines, with contrast sensitivity and improvement in total aberrations and MTF value at 5 per degree (*p=0.004) after TICL implantation. The cumulative 4-year corneal endothelial cell loss was ≤5%. No patients reported dissatisfaction. At the end of follow-up, the vault was 658±54.33m (range, 500–711) and the intraocular pressure was 11.7±2.08 mm Hg. Occurrences of glare and night-driving troubles diminished after TICL surgery.

Conclusion

The results from this standardised clinical investigation support TICL implantation from clinical and optical viewpoints in patients with stable keratoconus.

Trial registration number

NCT02833649, Pre-results

Mechanisms of Visual Disturbance in Dry Eye

Dry eye has been believed to be a chronic, symptomatic ocular surface disease that affects vision in a limited manner, because it is difficult to detect visual or optical changes using standard visual acuity testing. In practice, common visual complaints associated with dry eye include fluctuating vision with blinking, blurred vision, glare, and eye fatigue. This review discusses our past and current understanding of visual disturbances in dry eye and the various tools available for assessing visual function or optical quality. Tear film instability and ocular surface damage are key factors in the core mechanism underlying dry eye. The mechanisms of these vision-related subjective symptoms of dry eye based on visual function, particularly wavefront aberrations or light scattering, and the core mechanism of dry eye are described. Tear film instability is associated with decreased stability of postblink higher-order aberrations, leading to "fluctuating vision with blinking" and with increased ocular forward light scattering, leading to "glare." Ocular surface damage in the overlying optical zone (in the central corneal regions), is associated with increased higher-order aberrations and increased corneal backward light scattering that can lead to "blurred vision". "Eye fatigue" occurs when patients with dry eye struggle to see things under such visual symptoms.

Vision science and adaptive optics, the state of the field

Adaptive optics is a relatively new field, yet it is spreading rapidly and allows new questions to be asked about how the visual system is organized. The editors of this feature issue have posed a series of question to scientists involved in using adaptive optics in vision science. The questions are focused on three main areas. In the first we investigate the use of adaptive optics for psychophysical measurements of visual system function and for improving the optics of the eye. In the second, we look at the applications and impact of adaptive optics on retinal imaging and its promise for basic and applied research. In the third, we explore how adaptive optics is being used to improve our understanding of the neurophysiology of the visual system.

Neural adaptation to peripheral blur in myopes and emmetropes

In the presence of optical blur at the fovea, blur adaptation can improve visual acuity (VA) and perceived image quality over time. However, little is known regarding blur adaptation in the peripheral retina. Here, we examined neural adaptation to myopic defocus at the fovea and parafovea (10° temporal retina) in both emmetropes and myopes. During a 60-min adaptation period, subjects (3 emmetropes and 3 myopes) watched movies with +2 diopters of defocus blur through a 6-mm artificial pupil in two separate, counter-balanced sessions for each retinal location. VA was measured at 10-min intervals under full aberration-corrected viewing using an adaptive optics (AO) vision simulator. By correcting subjects' native optical aberrations with AO, we bypassed the influence of the individual subjects' optical aberrations on visual performance. Overall, exhibited a small but significant improvement after the 60-min of adaptation at both the fovea (mean±SE VA improvement: -0.06±0.04 logMAR) and parafovea (mean±SE VA improvement: -0.07±0.04 logMAR). Myopic subjects exhibited significantly greater improvement in parafoveal VA (mean±SE VA improvement: 0.10±0.02 logMAR), than that of emmetropic subjects (mean±SE VA improvement: 0.04±0.03 logMAR). In contrast, there was no significant difference in foveal VA between the two refractive-error groups. In conclusion, our results reveal differences in peripheral blur adaptation between refractive-error groups, with myopes displaying a greater degree of adaptation.

Enhanced neural function in highly aberrated eyes following perceptual learning with adaptive optics

Highly aberrated keratoconic (KC) eyes do not elicit the expected visual advantage from customized optical corrections. This is attributed to the neural insensitivity arising from chronic visual experience with poor retinal image quality, dominated by low spatial frequencies. The goal of this study was to investigate if targeted perceptual learning with adaptive optics (AO) can stimulate neural plasticity in these highly aberrated eyes. The worse eye of 2 KC subjects was trained in a contrast threshold test under AO correction. Prior to training, tumbling 'E' visual acuity and contrast sensitivity at 4, 8, 12, 16, 20, 24 and 28 c/deg were measured in both the trained and untrained eyes of each subject with their routine prescription and with AO correction for a 6mm pupil. The high spatial frequency requiring 50% contrast for detection with AO correction was picked as the training frequency. Subjects were required to train on a contrast detection test with AO correction for 1h for 5 consecutive days. During each training session, threshold contrast measurement at the training frequency with AO was conducted. Pre-training measures were repeated after the 5 training sessions in both eyes (i.e., post-training). After training, contrast sensitivity under AO correction improved on average across spatial frequency by a factor of 1.91 (range: 1.77-2.04) and 1.75 (1.22-2.34) for the two subjects. This improvement in contrast sensitivity transferred to visual acuity with the two subjects improving by 1.5 and 1.3 lines respectively with AO following training. One of the two subjects denoted an interocular transfer of training and an improvement in performance with their routine prescription post-training. This training-induced visual benefit demonstrates the potential of AO as a tool for neural rehabilitation in patients with abnormal corneas. Moreover, it reveals a sufficient degree of neural plasticity in normally developed adults who have a long history of abnormal visual experience due to optical imperfections.

The effect of astigmatism axis on visual acuity

PURPOSE

To evaluate the effect of astigmatism axis on uncorrected distance visual acuity (UDVA) in emmetropic eyes that underwent laser refractive surgery.

METHODS

This retrospective study included patients who underwent laser in situ keratomileusis or photorefractive keratectomy between January 2000 and December 2015 at the Care-Vision Laser Centers, Tel Aviv, Israel. Eyes with a 3-month postoperative spherical equivalent between -0.5 D and 0.5 D were included in this study. Eyes with ocular comorbidities and planned ametropia were excluded. Study eyes were divided into 3 groups according to the steep astigmatic axis: with the rule (WTR) (60-120), oblique (31-59 or 121-149), and against the rule (ATR) (0-30 or 150-180). The UDVA of these 3 groups was compared. The oblique group was divided into oblique ATR and oblique WTR, which were compared with each other.

RESULTS

A total of 17,416 consecutive eyes of 8,708 patients were studied. The WTR eyes (n = 10,651) had significantly better UDVA (logMAR 0.01 ± 0.08) than the oblique (n = 3,141, logMAR 0.02 ± 0.09) and ATR eyes (n = 3,624, logMAR 0.02 ± 0.10) (p<0.001). The oblique WTR group had significantly better UDVA than the oblique ATR group (p<0.001). The UDVA of the oblique and ATR groups was similar. Stepwise multiple regression analysis showed that the group accounted for 15% of the UDVA variance (p = 0.04).

CONCLUSIONS

The astigmatic axis has a small but significant effect on UDVA in emmetropic eyes; WTR was better than oblique and ATR astigmatism. Therefore, when correcting astigmatism, it may be preferable to err towards WTR astigmatism.

The Long-term Clinical Outcome after Corneal Collagen Cross-linking in Korean Patients with Progressive Keratoconus

Purpose

To evaluate the long-term clinical effectiveness and safety of corneal collagen cross-linking (CXL) in progressive keratoconus compared with untreated contralateral eyes.

Methods

In this retrospective study, nine eyes of nine patients with progressive keratoconus who received CXL (treatment group) and nine untreated contralateral eyes with keratoconus (control group) were included. All patients were followed for at least 5 years and assessed with best-corrected visual acuity, maximum keratometry, mean keratometry, corneal astigmatism, and corneal thickness. Clinical data were collected preoperatively and at 1, 3, 6, 12, 24, 36, 48, and 60 months, postoperatively.

Results

Mean best-corrected visual acuity improved significantly from 0.58 ± 0.37 logarithm of minimum angle of resolution preoperatively to 0.39 ± 0.29 logarithm of minimum angle of resolution at 5 years after corneal CXL (p = 0.012). There was significant flattening of the maximum keratometry and mean keratometry from preoperative values of 63.39 ± 10.89 and 50.87 ± 6.27 diopter (D) to postoperative values of 60.89 ± 11.29 and 49.54 ± 7.23 D, respectively (p = 0.038, 0.021). Corneal astigmatism decreased significantly from 7.20 ± 1.83 D preoperatively to 5.41 ± 1.79 D postoperatively (p = 0.021). The thinnest corneal thickness decreased from 434.00 ± 54.13 to 365.78 ± 71.58 µm during 1 month after treatment, then increased to 402.67 ± 52.55 µm at 5 years, which showed a statistically significant decrease compared to the baseline (p = 0.020). In the untreated contralateral eyes, mean keratometry increased significantly at 2 years compared with the baseline (p = 0.043).

Conclusions

CXL seems to be an effective and safe treatment for halting the progression of keratoconus over a long-term follow-up period of up to 5 years in progressive keratoconus.

Optical and neural anisotropy in peripheral vision

Optical blur in the peripheral retina is known to be highly anisotropic due to nonrotationally symmetric wavefront aberrations such as astigmatism and coma. At the neural level, the visual system exhibits anisotropies in orientation sensitivity across the visual field. In the fovea, the visual system shows higher sensitivity for cardinal over diagonal orientations, which is referred to as the oblique effect. However, in the peripheral retina, the neural visual system becomes more sensitive to radially-oriented signals, a phenomenon known as the meridional effect. Here, we examined the relative contributions of optics and neural processing to the meridional effect in 10 participants at 0°, 10°, and 20° in the temporal retina. Optical anisotropy was quantified by measuring the eye's habitual wavefront aberrations. Alternatively, neural anisotropy was evaluated by measuring contrast sensitivity (at 2 and 4 cyc/deg) while correcting the eye's aberrations with an adaptive optics vision simulator, thus bypassing any optical factors. As eccentricity increased, optical and neural anisotropy increased in magnitude. The average ratio of horizontal to vertical optical MTF (at 2 and 4 cyc/deg) at 0°, 10°, and 20° was 0.96 ± 0.14, 1.41 ± 0.54 and 2.15 ± 1.38, respectively. Similarly, the average ratio of horizontal to vertical contrast sensitivity with full optical correction at 0°, 10°, and 20° was 0.99 ± 0.15, 1.28 ± 0.28 and 1.75 ± 0.80, respectively. These results indicate that the neural system's orientation sensitivity coincides with habitual blur orientation. These findings support the neural origin of the meridional effect and raise important questions regarding the role of peripheral anisotropic optical quality in developing the meridional effect and emmetropization.

Advances in understanding the molecular basis of the first steps in color vision

Serving as one of our primary environmental inputs, vision is the most sophisticated sensory system in humans. Here, we present recent findings derived from energetics, genetics and physiology that provide a more advanced understanding of color perception in mammals. Energetics of cis-trans isomerization of 11-cis-retinal accounts for color perception in the narrow region of the electromagnetic spectrum and how human eyes can absorb light in the near infrared (IR) range. Structural homology models of visual pigments reveal complex interactions of the protein moieties with the light sensitive chromophore 11-cis-retinal and that certain color blinding mutations impair secondary structural elements of these G protein-coupled receptors (GPCRs). Finally, we identify unsolved critical aspects of color tuning that require future investigation.

Astigmatism impact on visual performance: meridional and adaptational effects

PURPOSE

Astigmatic subjects are adapted to their astigmatism and perceptually recalibrate upon its correction. However, the extent to which prior adaptation to astigmatism affects visual performance, whether this effect is axis dependent, and the time scale of potential changes in visual performance after astigmatism correction are not known. Moreover, the effect of possible positive interactions of aberrations (astigmatism and coma) might be altered after recalibration to correction of astigmatism.

METHODS

Visual acuity (VA) was measured in 25 subjects (astigmats and non-astigmats, corrected and uncorrected) under induction of astigmatism and combinations of astigmatism and coma while controlling subject aberrations. Astigmatism (1.00 diopter) was induced at three different orientations, the natural axis, the perpendicular orientation, and 45 degrees for astigmats and at 0, 90, and 45 degrees for non-astigmats. Experiments were also performed, adding coma (0.41 μm at a relative angle of 45 degrees) to the same mentioned astigmatism. Fourteen different conditions were measured using an 8-Alternative Forced Choice procedure with Tumbling E letters and a QUEST algorithm. Longitudinal measurements were performed up to 6 months. Uncorrected astigmats were provided with proper astigmatic correction after the first session.

RESULTS

In non-astigmats, inducing astigmatism at 90 degrees, produced a statistically lower reduction in VA than at 0 or 45 degrees, whereas in astigmats, the lower decrease in VA occurred for astigmatism induced at the natural axis. Six months of astigmatic correction did not reduce the insensitivity to astigmatic induction along the natural axis. Differences after orientation of astigmatism were also found when adding coma to astigmatism.

CONCLUSIONS

The impact of astigmatism on VA is greatly dependent on the orientation of the induced astigmatism, even in non-astigmats. Previous experience to astigmatism plays a significant role on VA, with a strong bias toward the natural axis. In contrast to perceived isotropy, the correction of astigmatism does not shift the bias in VA from the natural axis of astigmatism.

Change in visual acuity is well correlated with change in image-quality metrics for both normal and keratoconic wavefront errors

We determined the degree to which change in visual acuity (VA) correlates with change in optical quality using image-quality (IQ) metrics for both normal and keratoconic wavefront errors (WFEs). VA was recorded for five normal subjects reading simulated, logMAR acuity charts generated from the scaled WFEs of 15 normal and seven keratoconic eyes. We examined the correlations over a large range of acuity loss (up to 11 lines) and a smaller, more clinically relevant range (up to four lines). Nine IQ metrics were well correlated for both ranges. Over the smaller range of primary interest, eight were also accurate and precise in estimating the variations in logMAR acuity in both normal and keratoconic WFEs. The accuracy for these eight best metrics in estimating the mean change in logMAR acuity ranged between ±0.0065 to ±0.017 logMAR (all less than one letter), and the precision ranged between ±0.10 to ±0.14 logMAR (all less than seven letters).

Persistent biases in subjective image focus following cataract surgery

We explored the perception of image focus in patients with cataracts, and how this perception changed following cataract removal and implantation of an intraocular lens. Thirty-three patients with immature senile cataract and with normal retinal function were tested before surgery and 2 days after surgery, with 18 of the patients retested again at 2 months following surgery. The subjective focus of natural images was quantified in each session by varying the slope of the image amplitude spectra. At each time, short-term adaptation to the spectral slope was also determined by repeating the measurements after exposure to images with blurred or sharpened spectra. Despite pronounced acuity deficits, before surgery images appeared "best-focused" when they were only slightly blurred, consistent with a strong compensation for the acuity losses. Post-operatively, the image slopes that were judged "in focus" before surgery appeared too sharp. This bias remained strong at 2 months, and was independent of the rapid blur aftereffects induced by viewing filtered images. The focus settings tended to renormalize more rapidly in patients with higher post-operative acuity, while acuity differences were unrelated to the magnitude of the short-term blur aftereffects. Our results suggest that subjective judgments of image focus are largely compensated as cataracts develop, but potentially through a very long-term form of adaptation that results in persistent biases after the cataract is removed.

Just-noticeable differences for wavefront aberrations obtained with a staircase procedure

PURPOSE

To explore the magnitude of aberrations that alter the subjective visual quality measurably. The suitability of a staircase procedure for threshold determination was investigated.

METHODS

Spectacle correction (correction A) was achieved in 28 healthy eyes by an adaptive optics set-up, and additional astigmatism, coma, or trefoil was induced (correction B). Subjects observed a radial test pattern with noncyclopleged eyes using an artificial 5-mm pupil. They compared corrections A and B and identified the subjectively better correction. The magnitude of the aberration for each presentation varied according to the "Best PEST" staircase procedure. Just-noticeable difference (JND) was found after 30 presentations. Participants were divided into three groups, and the experiment was repeated under the following conditions: without modifications (n = 9), with a letter chart (n = 10), and with full correction of all aberrations (n = 9).

RESULTS

Median JND was 0.091 μm for astigmatism, 0.059 μm for coma, and 0.108 μm for trefoil, with a large interindividual spread. Repeatability coefficient across the three aberrations was 0.095 μm. A significant correlation was found between JNDs and the residual aberrations (without the generated aberration) in astigmatism and coma. Differences between radial test pattern and letter chart and between spectacle correction and full correction were not significant.

CONCLUSIONS

JNDs were lower for coma than for astigmatism and trefoil, depending on the magnitude of other residual aberrations, and showed large variations among subjects. Thus, the decision to correct more pronounced aberrations (eg, by wavefront-guided LASIK) should not depend on wavefront measurement results alone. A staircase procedure for determination of JNDs provides repeatable results.

Adaptive Optics for Visual Simulation

A revision of the current state-of-the-art adaptive optics technology for visual sciences is provided. The human eye, as an optical system able to generate images onto the retina, exhibits optical aberrations. Those are continuously changing with time, and they are different for every subject. Adaptive optics is the technology permitting the manipulation of the aberrations, and eventually their correction. Across the different applications of adaptive optics, the current paper focuses on visual simulation. These systems are capable of manipulating the ocular aberrations and simultaneous visual testing though the modified aberrations on real eyes. Some applications of the visual simulators presented in this work are the study of the neural adaptation to the aberrations, the influence of aberrations on accommodation, and the recent development of binocular adaptive optics visual simulators allowing the study of stereopsis.

Perceptual adaptation to the correction of natural astigmatism

BACKGROUND

The visual system adjusts to changes in the environment, as well as to changes within the observer, adapting continuously to maintain a match between visual coding and visual environment. We evaluated whether the perception of oriented blur is biased by the native astigmatism, and studied the time course of the after-effects following spectacle correction of astigmatism in habitually non-corrected astigmats.

METHODS AND FINDINGS

We tested potential shifts of the perceptual judgments of blur orientation in 21 subjects. The psychophysical test consisted on a single interval orientation identification task in order to measure the perceived isotropic point (astigmatism level for which the image did not appear oriented to the subject) from images artificially blurred with constant blur strength (B=1.5 D), while modifying the orientation of the blur according to the axis of natural astigmatism of the subjects. Measurements were performed after neutral (gray field) adaptation on naked eyes under full correction of low and high order aberrations. Longitudinal measurements (up to 6 months) were performed in three groups of subjects: non-astigmats and corrected and uncorrected astigmats. Uncorrected astigmats were provided with proper astigmatic correction immediately after the first session. Non-astigmats did not show significant bias in their perceived neutral point, while in astigmatic subjects the perceived neutral point was significantly biased, typically towards their axis of natural astigmatism. Previously uncorrected astigmats shifted significantly their perceived neutral point towards more isotropic images shortly (2 hours) after astigmatic correction wear, and, once stabilized, remained constant after 6 months. The shift of the perceived neutral point after correction of astigmatism was highly correlated with the amount of natural astigmatism.

CONCLUSIONS

Non-corrected astigmats appear to be naturally adapted to their astigmatism, and astigmatic correction significantly changes their perception of their neutral point, even after a brief period of adaptation.

Visual performance with wave aberration correction after penetrating, deep anterior lamellar, or endothelial keratoplasty

PURPOSE

To investigate the contribution ocular aberrations have on visual performance by quantifying improvements in best-corrected visual acuity (VA) and contrast sensitivity (CS) obtained with higher-order aberration (HOA) correction after penetrating (PK), deep anterior lamellar (DALK), or Descemet's stripping automated endothelial keratoplasty (DSAEK).

METHODS

Sixteen eyes were evaluated from 14 subjects who underwent PK (n = 5), DALK (n = 6), or DSAEK (n = 5) greater than 1 year prior to study enrollment. Ocular aberrations were measured and an adaptive optics system was used to correct ocular lower-order aberration (LOA) and HOA. VA and CS were measured for each subject with LOA or full-aberration correction. CS was measured at each of three spatial frequencies: 4, 8, and 12 cycles/deg.

RESULTS

All keratoplasty groups had more aberration than that of a normal myopic population and experienced significant VA gains with full-aberration correction (P < 0.0013). PK subjects had better VA than that of DSAEK subjects with LOA correction (logMAR VA 0.03 ± 0.05 vs. 0.25 ± 0.05; P = 0.0870). After HOA correction this trend persisted (P = 0.1734). DSAEK subjects also experienced less VA benefit from full-aberration correction than that of PK and DALK subjects. All keratoplasty groups demonstrated similar CS benefits from full-aberration correction despite differing higher-order root-mean-square magnitudes.

CONCLUSIONS

PK eyes had better logMAR VA than that of DSAEK eyes with LOA correction, whereas DALK eyes performed intermediate between the two. When full correction was applied, the same trend persisted. The findings suggest that factors other than aberration contribute to decrements in VA with DSAEK compared with PK.

Effects on vision with glare after correction of monochromatic wavefront aberrations

PURPOSE

To investigate effects of optical aberration correction on vision with glare.

METHODS

Correction of aberrations up to the 6th Zernike order (closed-loop correction) was compared with conventional spectacle correction in 42 healthy eyes. To create these corrections, an adaptive optics system including a thin-film transistor (TFT) monitor for displaying optotypes with additional glare sources was used. Employing both corrections, visual acuity and contrast sensitivity (CS) were tested alternately with and without glare. Disability glare was computed as the difference between log CS without and with glare. Individuals were also asked to rate subjectively the quality of three images displayed on the TFT monitor.

RESULTS

Significant improvements of CS without and with glare were found with the closed-loop correction (0.147 and 0.198 log CS, respectively), whereas no significant difference in visual acuity was found in either correction. Correlations were determined between reduction of total root-mean-square error and increase of CS with glare (Pearson correlation coefficient r=0.42) and decrease of disability glare (r=-0.33). Visual acuity was correlated with the visual Strehl ratio based on the optical transfer function (r=0.46). Subjective comparison of the images showed improvements more clearly. Depending on the image, in 57% to 78% of the eyes, closed-loop correction was rated better than spectacle correction. The subjective glare effect was reduced as well.

CONCLUSIONS

Investigation of vision with glare seems to be a reasonable additional test to evaluate the visual outcome of a customized correction.

Vision is adapted to the natural level of blur present in the retinal image

Background

The image formed by the eye's optics is inherently blurred by aberrations specific to an individual's eyes. We examined how visual coding is adapted to the optical quality of the eye.

Methods and Findings

We assessed the relationship between perceived blur and the retinal image blur resulting from high order aberrations in an individual's optics. Observers judged perceptual blur in a psychophysical two-alternative forced choice paradigm, on stimuli viewed through perfectly corrected optics (using a deformable mirror to compensate for the individual's aberrations). Realistic blur of different amounts and forms was computer simulated using real aberrations from a population. The blur levels perceived as best focused were close to the levels predicted by an individual's high order aberrations over a wide range of blur magnitudes, and were systematically biased when observers were instead adapted to the blur reproduced from a different observer's eye.

Conclusions

Our results provide strong evidence that spatial vision is calibrated for the specific blur levels present in each individual's retinal image and that this adaptation at least partly reflects how spatial sensitivity is normalized in the neural coding of blur.

Visual perception changes and optical stability after intracorneal ring segment implantation: comparison between 3 months and 1 year after surgery

PURPOSE

To prospectively evaluate intracorneal ring segment (ICRS) implantation on quality of life (QoL) of patients with keratoconus changes and identify factors responsible.

METHODS

Sixty-nine eyes of 42 keratoconus patients were implanted with the Keraring (Mediphacos, Belo Horizonte, Brazil). Best corrected visual acuity (BCVA), refraction, and steep keratometry were analyzed 3 months and 1 year after surgery. All patients self-administered the National Eye Institute Refractive Error Quality of life instrument at 2 time points: after having worn best correction for at least 30 days since evaluation (mean 4 months after surgery) and 1 year after surgery. To analyze if the use of the appropriate correction at 1 year follow up had any impact on visual acuity and V-QoL, patients were divided into 2 groups: group A (appropriate correction) and B (not appropriate correction).

RESULTS

After 1 year, QoL changes related to scales 'clarity of vision', 'near vision', and 'far vision'. Keratometric values, sphere, and spherical equivalent did not differ significantly between 3 months and 1 year postoperative. Cylinder increase was statistically but not clinically significant. Binocular BCVA did not change 1 year after surgery in group A and showed a clinically significant impairment in group B. A year after surgery, 18 patients did not use correction suggested by a physician 3 months after surgery. QoL was not statistically different 1 year after surgery between group A and group B.

CONCLUSION

Our findings show that the way keratoconic patients see is difficult to analyze using only quantitative and 1-visit metrics. They highlight the importance of patients' self perception and performing longitudinal analysis to consider neural compensation to optical changes from surgery.