Contrast sensitivity and the Stiles--Crawford effect

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.

Subjective measurement of the Stiles-Crawford effect with different field sizes

The Stiles-Crawford effect of the first kind (SCE) is the phenomenon in which light entering the eye near the center of the pupil appears brighter than light entering near the edge. Previous investigations have found an increase in the directionality (steepness) of the effect as the testing location moves from the center of the visual field to parafoveal positions, but the effect of central field size has not been considered. The influence of field size on the SCE was investigated using a uniaxial Maxwellian system in which stimulus presentation was controlled by an active-matrix liquid crystal display. SCE directionality increased as field size increased from 0.5° to 4.7° diameter, although this was noted in four mild myopes and not in two emmetropes. The change with field size was supported by a geometric optics absorption model.

Higher order aberrations of the eye: Part one

This article is the first in a series of two articles that provide a comprehensive literature review of higher order aberrations (HOAs) of the eye. The present article mainly explains the general principles of such HOAs as well as HOAs of importance, and the measuring apparatus used to measure HOAs of the eye. The second article in the series discusses factors contributing to variable results in measurements of HOAs of the eye.Keywords: Higher order aberrations; wavefront aberrations; aberrometer

Influence of spherical aberration, stimulus spatial frequency, and pupil apodisation on subjective refractions

PURPOSE

To test competing hypotheses (Stiles Crawford pupil apodising or superior imaging of high spatial frequencies by the central pupil) for the pupil size independence of subjective refractions in the presence of primary spherical aberration.

METHODS

Subjective refractions were obtained with a variety of test stimuli (high contrast letters, urban cityscape, high and low spatial frequency gratings) while modulating pupil diameter, levels of primary spherical aberration and pupil apodisation. Subjective refractions were also obtained with low-pass and high-pass stimuli and using 'darker' and 'sharper' subjective criteria.

RESULTS

Subjective refractions for stimuli containing high spatial frequencies focus a near paraxial region of the pupil and are affected only slightly by level of Seidel spherical aberration, degree of pupil apodisation and pupil diameter, and generally focused a radius of about 1-1.5 mm from the pupil centre. Low spatial frequency refractions focus a marginal region of the pupil, and are significantly affected by level of spherical aberration, amount of pupil apodisation, and pupil size. Clinical refractions that employ the 'darker' or 'sharper' subjective criteria bias the patient to use lower or higher spatial frequencies, respectively.

CONCLUSIONS

In the presence of significant levels of spherical aberration, the pupil size independence of subjective refractions occurs with or without Stiles Crawford apodisation for refractions that optimise high spatial frequency content in the image. If low spatial frequencies are optimised by a subjective refraction, spherical refractive error varies with spherical aberration, pupil size, and level of apodisation. As light levels drop from photopic to scotopic, therefore, we expect a shift from pupil size independent to pupil size dependent subjective refractions. Emphasising a 'sharper' criterion during subjective refractions will improve image quality for high spatial frequencies and generate pupil size independent refractions.

Pupil size and LASIK: a review

PURPOSE

To provide a literature review on the evidence both for and against pupil size as an independent predictor of adverse visual outcomes after LASIK.

METHODS

Peer-reviewed publications on the effect of pupil size on LASIK outcomes since 2002 are reviewed. Particular attention was paid to the following attributes of each publication: type of study, number of patients or eyes, mean age, mean level of myopia, mean pupil size, testing conditions, ablation zone diameter, presence or absence of blend zones, and mean follow-up period.

RESULTS

Among the 19 studies examined, none correlates a persistent relationship between pupil size and night vision complaints (NVCs) beyond 3 months when LASIK was performed with a 6.0-mm optical zone or larger ablation. The studies that did explicitly determine a correlation either included some or all patients with ablation zones smaller than 6.0 mm or did not specify ablation diameter at all. Among the studies that had drawn more mixed conclusions, the studies either covered short follow-up intervals (1 to 3 months) or showed a progressive improvement in NVCs over time in a relatively small patient cohort.

CONCLUSIONS

As keratorefractive technology continues to evolve, the role of pupil size warrants further investigation; however, based on the literature reviewed herein, modern LASIK has negated the role of the low light pupil in predicting adverse visual outcomes after LASIK outside of the early postoperative period.

Subjective blur limits for higher order aberrations

PURPOSE

We compared subjective blur limits for defocus and the higher-order aberrations of coma, trefoil, and spherical aberration.

METHODS

Spherical aberration was presented in both Zernike and Seidel forms. Black letter targets (0.1, 0.35, and 0.6 logMAR) on white backgrounds were blurred using an adaptive optics system for six subjects under cycloplegia with 5 mm artificial pupils. Three blur criteria of just noticeable, just troublesome, and just objectionable were used.

RESULTS

When expressed as wave aberration coefficients, the just noticeable blur limits for coma and trefoil were similar to those for defocus, whereas the just noticeable limits for Zernike spherical aberration and Seidel spherical aberration (the latter given as an "rms equivalent") were considerably smaller and larger, respectively, than defocus limits.

CONCLUSIONS

Blur limits increased more quickly for the higher order aberrations than for defocus as the criterion changed from just noticeable to just troublesome and then to just objectionable.

Potential signal to accommodation from the Stiles-Crawford effect and ocular monochromatic aberrations

The purpose of this study is to determine if cues within the blurred retinal image due to the Stiles-Crawford (SC) effect and the eye's monochromatic aberrations can drive accommodation with a small pupil (3 mm) that is typical of bright photopic conditions.The foveal, psychophysical SC function (17 min arc) and ocular monochromatic aberrations were measured in 21 visually normal adults. The retinal image of a 10.2 min arc disc was simulated for spherical defocus levels of -1 D, 0 D and +1 D in each of four conditions consisting of combinations of the presence or absence of the individual SC function and monochromatic aberrations with a 3 mm pupil. Accommodation was recorded in eleven participants as each viewed the simulations through a 0.75-mm pinhole.The SC effect alone did not provide a significant cue to accommodation. Monochromatic aberrations provided a statistically significant but rather small cue to monocular accommodation.

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.

Photoreceptor waveguides and effective retinal image quality

Individual photoreceptor waveguiding suggests that the entire retina can be considered as a composite fiber-optic element relating a retinal image to a corresponding waveguided image. In such a scheme, a visual sensation is produced only when the latter interacts with the pigments of the outer photoreceptor segments. Here the possible consequences of photoreceptor waveguiding on vision are studied with important implications for the pupil-apodization method commonly used to incorporate directional effects of the retina. In the absence of aberrations, it is found that the two approaches give identical predictions for an effective retinal image only when the pupil apodization is chosen twice as narrow as suggested by the traditional Stiles-Crawford effect. In addition, phase variations in the retinal field due to ocular aberrations can delicately alter a waveguided image, and this may provide plausible justification for an improved visual sensation as compared with what should be expected on the grounds of a retinal image only.

Refraction and aberration across the horizontal central 10 degrees of the visual field

PURPOSE

The purpose of this study was to measure refraction and aberrations across the horizontal central visual field.

METHODS

Cycloplegic refraction was measured on eight subjects at 13 points across the horizontal central 10 degrees of the retina using a Hartmann-Shack wavefront sensor. Refractions were converted into mean sphere (M), 90 degrees to 180 degrees astigmatism (J180), and 45 degrees to 135 degrees astigmatism (J45) components. For five subjects, higher-order aberrations were determined at the center and edges of the field.

RESULTS

Subtle changes in refraction were found to exist across the central 10 degrees of the retina, with changes in mean best sphere varying by up to half a diopter across this region and with smaller changes in astigmatism. Horizontal coma, but no other higher-order aberrations, varied systemically across the visual field; it varied linearly with angle but at different rates for the different subjects.

CONCLUSION

Subtle changes in cycloplegic refraction exist across the horizontal central 10 degrees of the retina. The results indicate the need for correct alignment when measuring objective refraction.

Aberrations and myopia

It has been suggested that high levels of axial aberration or specific patterns of peripheral refraction could play a role in myopia development. Possible mechanisms involving high levels of retinal image blur caused by axial aberrations include form deprivation through poor retinal image quality in distance vision, enhanced accommodative lags favouring compensatory eye growth, and an absence of adequate directional cues to guide emmetropization. In addition, in initially emmetropic eyes, hyperopia in the retinal periphery may result in local compensatory eye growth, which induces axial myopia. Evidence in support of these ideas is reviewed and it is concluded that, for any fixed pupil diameter, evidence for higher levels of axial aberration in myopes in comparison with other refractive groups is weak, making involvement of axial aberrations in myopization through image degradation at the fovea unlikely. If, however, some potential myopes had unusually large pupil diameters, their effective aberration levels and associated retinal blur would be larger than those of the rest of the population. There is stronger evidence in favour of differences in patterns of peripheral refraction in both potential and existing myopes, with myopes tending to show relative hyperopia in the periphery. These differences appear to be related to a more prolate eyeball shape. Longitudinal studies are required to confirm whether the retinal defocus associated with the peripheral hyperopia can cause patterns of eyeball growth which lead to axial myopia.

Noticeable, troublesome and objectionable limits of blur

We investigated limits at which induced blur becomes noticeable, troublesome and objectionable. We used 15 cyclopleged subjects, a Badal optometer with lines of three high contrast letters as targets, 3-6 mm artificial pupils, and 0.0-0.7 logMAR letter sizes. For 0.0 logMAR size, mean "noticeable" blur limits were +/-0.33D, +/-0.30D and +/-0.28D at 3 mm, 4 mm and 6 mm, respectively, but increased by about 70% for 0.7 logMAR letters. All limits reduced by about 17% as pupil size increased from 3 mm to 6 mm. Letter size had a significant influence on all blur limits (1.6-2.1 times), but blur direction had no significant effect. Magnitudes of "troublesome" and 'objectionable" limits were 1.6-1.8 times and 2.1-2.5 times relative to "noticeable" limits, respectively. Our results suggest criteria for troublesome and objectionable blur are relatively unaffected by letter size.

Gaussian weighting of ocular wave-front measurements

The measurement of ocular wave-front error gives insight into the optical performance of the eye and possibly a means for assessing visual performance. The visual system responds not only to the quality of the optical image formed on the retina but also to the processing that occurs in the retina and the brain. To develop a metric of visual performance based on wave-front error measurements, these latter processes must somehow be incorporated. In representing the wave-front error in terms of Zernike polynomials, it appears that terms with lower angular frequency have a greater deleterious effect on visual performance than higher-angular-frequency terms. A technique for weighting the pupil function of the eye with a Gaussian filter is demonstrated. It is further demonstrated that the variance of the Gaussian-weighted wave-front error is well correlated with visual performance.

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.