Predicting the effects of optical defocus on human contrast sensitivity

Absolute prediction of relative changes in contrast sensitivity with aberrations using a single metric of retinal image quality

Metrics of retinal image quality predict optimal refractive corrections and correlate with visual performance. To date, they do not predict absolutely the relative change in visual performance when aberrations change and therefore need to be a-posteriori rescaled to match relative measurements. Here we demonstrate that a recently proposed metric can be used to predict, in an absolute manner, changes in contrast sensitivity measurements with Sloan letters when aberrations change. Typical aberrations of young and healthy eyes (for a 6 mm pupil diameter) were numerically introduced, and we measured the resulting loss in contrast sensitivity of subjects looking through a 2 mm diameter pupil. Our results suggest that the metric can be used to corroborate measurements of visual performance in clinical practice, thereby potentially improving patient follow-ups.

Predicting the effects of defocus blur on contrast sensitivity with a model-based metric of retinal image quality

We measure the effect of defocus blur on contrast sensitivity with Sloan letters in the 0.75-2.00 arc min range of letter gaps. We compare our results with the prediction of the Dalimier and Dainty model [J. Opt. Soc. Am. A25, 2078 (2008)JOAOD60740-323210.1364/JOSAA.25.002078] and propose a new metric of retinal image quality that we define as the model limit for very small letters. The contrast sensitivity is measured for computationally blurred Sloan letters (0, 0.25, and 0.50 diopters for a 3 mm pupil) of different sizes (20/40 to 20/15 visual acuity), and subjects look through a small (2 mm) diaphragm to limit the impact of their own aberration on measurements. Measurements and model predictions, which are normalized by the blur-free condition, weakly depend on letter size and are in good agreement with our metric of retinal image quality. Our metric relates two approaches of modeling visual performance: complete modeling of the optotype classification task and calculation of retinal image quality with a descriptive metric.

Defocused contrast sensitivity function in peripheral vision

PURPOSE

Human peripheral detection performance is affected by optical factors such as defocus and higher order aberrations. From optical theory, we would expect defocus to produce local depressions (notches) in the contrast sensitivity function (CSF). However, such notches have not been observed in peripheral vision, and it is unknown whether human peripheral vision can detect local depressions (notches) in the CSF, such as those produced by monochromatic defocus when all monochromatic ocular aberrations are corrected. The purpose of the study was to identify such notches.

METHODS

Participants were three adult emmetropes. Following full adaptive optics correction, on-axis and 20° nasal visual field detection CSFs in monochromatic light were measured for the right eye with a 7 mm diameter pupil, both without and with ±2 D defocus, and with separate determinations for horizontal and vertical gratings. Defocused CSFs were compared with predictions based on theoretical modulation transfer functions.

RESULTS

Notches in the monochromatic defocused CSFs were identified for peripheral vision at optically predicted spatial frequencies with other monochromatic ocular aberrations corrected, provided that there was adequate spatial frequency sampling. The spatial frequencies of notches were similar to those predicted from optical theory, but their depths (0.3 to 0.9 log unit) were smaller than predicted.

CONCLUSION

With fine spatial frequency sampling, notches were identified in defocused monochromatic CSFs when all other monochromatic ocular aberrations were corrected, both on-axis and at 20° eccentricity. Unless recognised as such, notches may contribute to noise in through-focus detection measurements of peripheral visual performance.

Functional integration of eye tissues and refractive eye development: Mechanisms and pathways

Refractive eye development is a tightly coordinated developmental process. The general layout of the eye and its various components are established during embryonic development, which involves a complex cross-tissue signaling. The eye then undergoes a refinement process during the postnatal emmetropization process, which relies heavily on the integration of environmental and genetic factors and is controlled by an elaborate genetic network. This genetic network encodes a multilayered signaling cascade, which converts visual stimuli into molecular signals that guide the postnatal growth of the eye. The signaling cascade underlying refractive eye development spans across all ocular tissues and comprises multiple signaling pathways. Notably, tissue-tissue interaction plays a key role in both embryonic eye development and postnatal eye emmetropization. Recent advances in eye biometry, physiological optics and systems genetics of refractive error have significantly advanced our understanding of the biological processes involved in refractive eye development and provided a framework for the development of new treatment options for myopia. In this review, we summarize the recent data on the mechanisms and signaling pathways underlying refractive eye development and discuss new evidence suggesting a wide-spread signal integration across different tissues and ocular components involved in visually guided eye growth.

Genome-wide analysis of retinal transcriptome reveals common genetic network underlying perception of contrast and optical defocus detection

Background

Refractive eye development is regulated by optical defocus in a process of emmetropization. Excessive exposure to negative optical defocus often leads to the development of myopia. However, it is still largely unknown how optical defocus is detected by the retina.

Methods

Here, we used genome-wide RNA-sequencing to conduct analysis of the retinal gene expression network underlying contrast perception and refractive eye development.

Results

We report that the genetic network subserving contrast perception plays an important role in optical defocus detection and emmetropization. Our results demonstrate an interaction between contrast perception, the retinal circadian clock pathway and the signaling pathway underlying optical defocus detection. We also observe that the relative majority of genes causing human myopia are involved in the processing of optical defocus.

Conclusions

Together, our results support the hypothesis that optical defocus is perceived by the retina using contrast as a proxy and provide new insights into molecular signaling underlying refractive eye development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-021-01005-x.

The Impact of IOL Abbe Number on Polychromatic Image Quality of Pseudophakic Eyes

Purpose

The human eye exhibits large amounts (2.5 diopters) of longitudinal chromatic aberration (LCA). Its impact on polychromatic image quality, however, has been shown experimentally and by computer modeling to be small or absent. We hypothesized that modest changes in pseudophakic LCA created by higher and lower Abbe number materials will have little or no impact on polychromatic image quality in pseudophakic eyes.

Materials and Methods

Using published chromatic and monochromatic aberration data from pseudophakic eyes and higher and lower Abbe number materials (37 and 55), we computed monochromatic point spread functions for 21 wavelengths across the visible spectrum. After weighting by either the RGB spectra of a liquid crystal display or by a flat white spectrum, they were weighted by the human spectral sensitivity function (V_λ) before being added to generate polychromatic PSFs.

Results

In the absence of monochromatic aberrations, the reduced LCA due to higher Abbe number intraocular lens (IOL) materials resulted in a reduction of 0.08 diopters in the mean defocus generated by LCA. At the retinal plane, the higher Abbe number pseudophakic model produced improvements in polychromatic modulation transfer functions (MTFs) similar to those generated by a 0.05 diopter reduction in spherical defocus. When monochromatic aberrations were added to make the model more representative of actual pseudophakic eyes, the differences in image quality became sub-threshold for human vision or disappeared completely.

Conclusion

The anticipated gains in polychromatic image quality from employing higher Abbe number IOL materials with reduced LCA do not materialize in plausible aberrated models of pseudophakic eyes.

The Effect of a Spectral Filter on Visual Quality in Patients with an Extended-Depth-Of-Focus Intraocular Lens

PURPOSE

Wavelength dependence of diffractive intraocular lenses (IOLs) was recognized in vitro but not yet assessed in vivo. By examining pseudophakic patients who had extended-depth-of-focus diffractive implants, this spectral effect on their vision was measured clinically and the lens was tested in vitro.

DESIGN

Cross-sectional study with laboratory investigation.

METHODS

Twelve pseudophakic patients (23 eyes) with a Symfony lens (Johnson & Johnson Vision) were measured monocularly under red and white light at far, intermediate, and near distances. Corrected distance visual acuity (CDVA), distance-corrected intermediate visual acuity (DCIVA), and distance corrected near visual acuity (DCNVA) were assessed. Contrast sensitivity was examined at several spatial frequencies. The in vitro lens modulation transfer function was measured under different spectral conditions by using an IOL metrology device.

RESULTS

CDVA was comparable under red and white light. DCIVA and DCNVA were significantly better under white light by 0.06 and 0.09, respectively. Contrast sensitivity was slightly better with a red filter at far distance but was worse at intermediate distance, although differences were significant only at 1 frequency. Near contrast sensitivity was better under polychromatic than red light, which was significant at 3 frequencies. The in vitro analysis confirmed Symfony's wavelength dependence: performance was improved at far distance but was worse at intermediate and near distances.

CONCLUSIONS

Symfony's spectral dependence was observed to affect visual acuity and contrast sensitivity. Although the red filter did not improve distance vision, it caused visual deterioration at near distance. One should take this effect into account when optimizing the reading performance of patients with diffractive IOLs.

TuebingenCSTest - a useful method to assess the contrast sensitivity function

Since contrast sensitivity (CS) relies on the accuracy of stimulus presentation, the reliability of the psychophysical procedure and observer's attention, the measurement of the CS-function is critical and therefore, a useful threshold contrast measurement was developed. The Tuebingen Contrast Sensitivity Test (TueCST) includes an adaptive staircase procedure and a 16-bit gray-level resolution. In order to validate the CS measurements with the TueCST, measurements were compared with existing tests by inter-test repeatability, test-retest reliability and time. The novel design enables an accurate presentation of the spatial frequency and higher precision, inter-test repeatability and test-retest reliability compared to other existing tests.

Demonstrating correction of low levels of astigmatism with realistic scenes

PURPOSE AND METHOD

Modern standard visual acuity tests are primarily designed as diagnostic tools for use during subjective refraction and normally bear little relation to real-world situations. We have developed a methodology to create realistic rendered scenes that demonstrate potential vision improvement in a relevant and engaging way. Low-cylindrical refractive error can be made more noticeable by optimizing the contrast and spatial frequencies, and by testing four different visual perception skills: motion tracking, pattern recognition, visual clutter differentiation and contrast sensitivity. Using a 1.00DC lens during iteration, we created a range of still and video scenes before optimizing to a selection 3-D rendered street scenes. These were assessed on everyday relevance, emotional and visual engagement and sensitivity to refractive correction for low-cylinder astigmats (0.75-1.00DC, n=74) wearing best spherical equivalent correction and then with astigmatism corrected.

RESULTS AND CONCLUSIONS

The most promising visual elements involved or combined optimized textures, distracting patterns behind text, faces at a distance, and oblique text. 91.9% of subjects (95% CI: 83.2, 97.0) reported an overall visual improvement when viewing the images with astigmatic correction, and 96% found the images helpful to determine which type of contact lens to use. Our method, which combines visual science with design thinking, takes a new approach to creating vision tests. The resultant test scenes can be used to improve patient interaction and help low cylinder astigmats see relevant, every-day benefits in correcting low levels (0.75 & 1.00DC) of astigmatism.

The impact of higher-order aberrations on the strength of directional signals produced by accommodative microfluctuations

It has been proposed that the accommodation system could perform contrast discrimination between the two dioptric extremes of accommodative microfluctuations to extract directional signals for reflex accommodation. Higher-order aberrations (HOAs) may have a significant influence on the strength of these contrast signals. Our goal was to compute the effect HOAs may have on contrast signals for stimuli within the upper defocus limit by comparing computed microcontrast fluctuations with psychophysical contrast increment thresholds (Bradley & Ohzawa, 1986). Wavefront aberrations were measured while subjects viewed a Maltese spoke stimulus monocularly. Computations were performed for accommodation or disaccommodation stimuli from a 3 Diopter (D) baseline. Microfluctuations were estimated from the standard deviation of the wavefronts over time at baseline. Through-focus Modulation Transfer, optical contrast increments (ΔC), and Weber fractions (ΔC/C) were derived from point spread functions computed from the wavefronts at baseline for 2 and 4 cycles per degree (cpd) components, with and without HOAs. The ΔCs thus computed from the wavefronts were compared with psychophysical contrast increment threshold data. Microfluctuations are potentially useful for extracting directional information for defocus values within 3 D, where contrast increments for the 2 or 4 cpd components exceed psychophysical thresholds. HOAs largely reduce contrast signals produced by microfluctuations, depending on the mean focus error, and their magnitude in individual subjects, and they may shrink the effective stimulus range for reflex accommodation. The upper defocus limit could therefore be constrained by discrimination of microcontrast fluctuations.

Immature visual neural system in children reflected by contrast sensitivity with adaptive optics correction

This study aimed to explore the neural development status of the visual system of children (around 8 years old) using contrast sensitivity. We achieved this by eliminating the influence of higher order aberrations (HOAs) with adaptive optics correction. We measured HOAs, modulation transfer functions (MTFs) and contrast sensitivity functions (CSFs) of six children and five adults with both corrected and uncorrected HOAs. We found that when HOAs were corrected, children and adults both showed improvements in MTF and CSF. However, the CSF of children was still lower than the adult level, indicating the difference in contrast sensitivity between groups cannot be explained by differences in optical factors. Further study showed that the difference between the groups also could not be explained by differences in non-visual factors. With these results we concluded that the neural systems underlying vision in children of around 8 years old are still immature in contrast sensitivity.

Effect of blur and subsequent adaptation on visual acuity using letter and Landolt C charts: differences between emmetropes and myopes

PURPOSE

To compare the effects of defocus induced blur and blur adaptation on visual acuity and to evaluate any differences between emmetropes and myopes using letter and Landolt C logMAR charts.

METHODS

The sample consisted of 26 volunteers, with a mean age of 27 ± 3 years, comprising 13 emmetropes (spherical equivalent range: -0.63 to +0.50 D) and 13 myopes (spherical equivalent range: -0.75 to -5.00 D). Monocular visual acuity (VA) was measured in each eye using letter and Landolt C logMAR charts under the following conditions: (1) with the distance refractive correction, (2) immediately after exposure to +2.00 D defocus and (3) following 60 min of binocular adaptation to +2.00 D blur. Objective refraction at the beginning and at the end of the experimental procedure was evaluated. Averaged VA data between the two eyes were used for analysis.

RESULTS

Deterioration in VA with +2.00 D defocus was greater in the emmetropes compared to myopes for both charts. The mean difference between the two refractive groups was more pronounced for the Landolt-C (0.17 logMAR) compared to the letter chart (0.10 logMAR). The reduction in VA with blur was related to the amount of the refractive error. Following 60 min of adaptation, a significant improvement in VA was observed in both groups that did not differ between the two charts. The improvement in VA following adaptation using the letter chart was linearly correlated with spherical equivalent refractive correction.

CONCLUSIONS

Myopes show higher tolerance to retinal defocus compared to emmetropes, which could be attributed to previous blur experience. The effect of blur on VA is more pronounced using Landolt C optotypes than with letters. Prolonged exposure to blur results in equally improved performance for both refractive groups.

Accommodation, pattern glare, and coloured overlays

We manipulated the accommodative response using positive and negative lenses to study any association between symptoms of pattern glare and accommodation. Two groups of eighteen young adults were selected from seventy-eight on the basis (i) that their rate of reading increased by 5% or more with an overlay compared to their rate without it, and (ii) that they reported more than 2 symptoms of pattern glare (group 1) or had no such increment in reading speed and reported fewer than 3 symptoms (group 2). Under double-masked conditions participants observed at 0.4 m a pattern of stripes while measurements of accommodation were made using an open field autorefractor with and without positive and negative trial lenses (0.75 D), and with and without a coloured overlay. Pattern glare was also assessed with and without the trial lenses. Without lenses, the mean accommodative response in group 1 was 1.55 D, a lag of 0.95 D +/- 0.24 D relative to the demand. The lag decreased by 0.43 D (p < 0.0001) when the chosen overlay was used, an effect that was not shown in group 2 even when lag increased with negative trial lenses (p = 0.13). In both groups, pattern glare scores were reduced by the trial lenses, but were unaffected by the sign of the lenses. This suggests that symptoms of pattern glare are not strongly associated with accommodative response.

Effects of accommodation training on accommodation and depth of focus in an eye implanted with a crystalens intraocular lens

PURPOSE

To investigate objective measures of the effects of accommodative training of a pseudophakic eye implanted with a Crystalens AT-52SE (eyeonics Inc) intraocular lens (IOL) on reading performance, accommodation, and depth of focus.

METHODS

Objective dynamic measures of accommodation, pupil size, and depth of focus were quantified from wavefront measures before and after 1 week of accommodative training that began 29 months after implantation of an accommodating IOL in one patient. Depth of focus was estimated from 50% cut-off of peak performance levels for defocus curves that were computed from the image quality metric VSOTF based on ocular wavefront aberrations.

RESULTS

The patient reported improved near vision reading performance after completing the training procedure. After training, there was a shift in conjugate focus in the hyperopic direction, yet the depth of focus increased significantly for near objects. Simulated retinal images and the calculated modulation transfer function of the eye both demonstrated improved quality for near vision after training.

CONCLUSIONS

The subjective report of improved near vision after training was correlated with improvement of objective measures. Depth of focus increased for near objects with attempts to accommodate after training. This change was linked to increases in aberrations and pupil size and occurred despite the conjugate focus shifting in the hyperopic direction. These results demonstrate that accommodative training may be useful in improving near vision in patients with accommodating IOLs.

Spatial vision meets spatial cognition: examining the effect of visual blur on human visually guided route learning

Visual navigation is a task that involves processing two-dimensional light patterns on the retinas to obtain knowledge of how to move through a three-dimensional environment. Therefore, modifying the basic characteristics of the two-dimensional information provided to navigators should have important and informative effects on how they navigate. Despite this, few basic research studies have examined the effects of systematically modifying the available levels of spatial visual detail on navigation performance. In this study, we tested the effects of a range of visual blur levels--approximately equivalent to various degrees of low-pass spatial frequency filtering--on participants' visually guided route-learning performance using desktop virtual renderings of the Hebb-Williams mazes. Our findings show that the function of blur and time to finish the mazes follows a sigmoidal pattern, with the inflection point around +2 D of experienced defocus. This suggests that visually guided route learning is fairly robust to blur, with the threshold level being just above the limit for legal blindness. These findings have implications for models of route learning, as well as for practical situations in which humans must navigate under conditions of blur.

A relationship between tolerance of blur and personality

PURPOSE

To determine whether tolerance of dioptric spherical defocus is related to measures of personality. Clinical observations suggest that there is individual variability in tolerance of blur.

METHODS

A computer-controlled Badal optometer was used to measure just-noticeable blur and just-objectionable blur responses to positive lens defocus. Blur tolerance was defined as the difference between these two responses. A personality battery was administered consisting of the NEO-FFI (Neuroticism-Extroversion-Openness-Five Factor Inventory) and the California Adult Q-sort (general measures), as well as individual measures (hypothesis-driven scales) of perfectionism, neuroticism, highly sensitive person, ego resiliency, need for structure, and negative emotionality. Ninety-nine normally sighted subjects (median age, 21 years, median refractive error 0.6 DS) completed both parts of the study.

RESULTS

Within-subject blur tolerance measures with three different pupil sizes correlated highly (r(s) = 0.79-0.86), implying good repeatability. There was a wide range of individual blur tolerance (0.0-2.7 D). The personality questionnaires exhibited acceptable reliability (Cronbach's α = 0.67-0.91). Two perfectionism scales correlated significantly with blur tolerance (r = 0.25 and 0.27). The 15 questionnaire items that correlated most with blur tolerance were factor analyzed and yielded two conceptually meaningful factors (both α = 0.76). The "low self confidence" and "disorganization" factors correlated positively with blur tolerance (r = 0.38 and 0.36, respectively) and their composite correlated with blur tolerance (r = 0.46).

CONCLUSIONS

These results provide the first evidence of a relationship between personality and tolerance of blur. Tolerance of blur may be related to perception of image quality. If so, personality may influence refractive error correction and development and other choices that are made when presented with degraded images.

Changes in through-focus spatial visual performance with adaptive optics correction of monochromatic aberrations

We determined the influence of adaptive optics correction on through-focus illiterate-E visual acuity and through-focus contrast sensitivity under monochromatic conditions. In two subjects, adaptive optics improved high and low (12%) contrast in-focus visual acuity by 0.1 to 0.15logMAR, but resulted in more rapid and more symmetrical deterioration in visual acuity away from in-focus. In one subject, adaptive optics improved in-focus contrast sensitivity and resulted in more symmetrical and greater loss of contrast sensitivity about the peak sensitivity because of correction of higher-order aberrations. The results show that full correction of higher-order aberrations may worsen spatial visual performance in the presence of some defocus.

Effect of positive and negative defocus on contrast sensitivity in myopes and non-myopes

This study investigated the effect of lens induced defocus on the contrast sensitivity function in myopes and non-myopes. Contrast sensitivity for up to 20 spatial frequencies ranging from 1 to 20 c/deg was measured with vertical sine wave gratings under cycloplegia at different levels of positive and negative defocus in myopes and non-myopes. In non-myopes the reduction in contrast sensitivity increased in a systematic fashion as the amount of defocus increased. This reduction was similar for positive and negative lenses of the same power (p = 0.474). Myopes showed a contrast sensitivity loss that was significantly greater with positive defocus compared to negative defocus (p = 0.001). The magnitude of the contrast sensitivity loss was also dependent on the spatial frequency tested for both positive and negative defocus. There was significantly greater contrast sensitivity loss in non-myopes than in myopes at low-medium spatial frequencies (1-8 c/deg) with negative defocus. Latent accommodation was ruled out as a contributor to this difference in myopes and non-myopes. In another experiment, ocular aberrations were measured under cycloplegia using a Shack-Hartmann aberrometer. Modulation transfer functions were calculated using the second order term for defocus as well as the fourth order Zernike term for spherical aberration. The theoretical maximal contrast sensitivity based on aberration data predicted the measured asymmetry in contrast sensitivity to positive and negative defocus that was observed in myopic subjects. The observed asymmetry in contrast sensitivity with positive and negative defocus in myopes may be linked to the altered accommodative response observed in this group.

The influence of the Stiles-Crawford peak location on visual performance

We investigated the influence of the Stiles-Crawford peak location on visual acuity, contrast sensitivity and phase transfer with 6 mm diameter pupils in two subjects. Apodising filters were used to move the peak. One subject (SM) had her natural peak 0.9 mm below pupil centre, and visual performance was measured for both this peak position and when the peak was moved to the same distance above pupil centre. The other subject (DAA) had a more centred peak and visual performance was measured for this peak position and when the peak was moved both 2.3 mm temporally and 2.6 mm nasally. Measurements of contrast sensitivity and phase transfer were compared with predictions based on aberration measurements. The peak position had definite influence on performance, but this was mainly noticeable when subjects were defocused e.g. SM's visual acuity was reduced by 0.13 log units under the peak-shifted condition at -2D (hypermetropic) defocus.

Contrast sensitivity and the Stiles--Crawford effect

We investigated the influence of the Stiles-Crawford effect (SCE) of the first kind on the contrast sensitivity function using the apodisation model of the SCE. The SCE was measured for the right eyes of two subjects using an increment threshold technique involving a two-channel Maxwellian-viewing system. Filters made of photographic film neutralised or doubled the SCE. Contrast sensitivities were measured with a 6 mm pupil diameter, defocus to +/-2D, and three SCE conditions (normal, neutralised and doubled). Modulation transfer functions were derived after measuring transverse aberrations with a vernier alignment technique, and were used to predict contrast sensitivity functions (CSFs). The measured CSFs were, in general, reasonable matches with the predicted CSFs. In particular, both demonstrated definite undulations ("notches") as defocus level increased. The influences of the SCE-modifying filters were generally of similar magnitude and direction to predictions, thus supporting the apodisation model of the SCE. The magnitudes of SCE influence between SCE-neutralised and SCE-doubled conditions were usually small at about 0.2-0.3 log unit, with a maximum influence of 0.5 log unit. Influences of the SCE were greater for myopic than for hypermetropic defocus. As measured by the CSF and an apodisation model, this study is in agreement with previous theoretical work and one experimental study in indicating that the SCE plays a minor role in improving spatial visual performance.

Apodization by the Stiles-Crawford effect moderates the visual impact of retinal image defocus

Previous optical modeling of the human eye with large pupils has predicted a larger impact of defocus on the human contrast sensitivity function and modulation transfer function than is observed experimentally. Theory predicts that aberrations and the Stiles-Crawford effect (SCE) should both lead to increased depth of focus, resulting in higher contrast sensitivities and veridical (not phase-reversed) perception over a larger range of spatial frequencies in defocused retinal images. Using a wave optics model, we examine these predictions quantitatively and compare them with psychophysical experiments that measure the effect of defocus on contrast sensitivity and perceived phase reversals. We find that SCE apodization has its biggest effect on defocused image quality when defocus and spherical aberration have the same sign. A model including typical amounts of spherical aberration and pupil apodization provides a dramatically improved prediction of the effects of defocus on contrast sensitivity with large pupils. The SCE can significantly improve defocused image quality and defocused vision, particularly for tasks that require veridical phase perception.