Crawford Effect Research Articles

Visual Axis and Stiles-Crawford Effect Peak Show a Positional Correlation in Normal Eyes: A Cohort Study.

The purpose of this study was to locate the visual axis and evaluate its correlation with the Stiles-Crawford effect (SCE) peak. Ten young, healthy individuals (20 eyes) were enrolled. An optical system was developed to locate the visual axis and measure SCE. To locate the visual axis, 2 small laser spots at 450 nm and 680 nm were co-aligned and delivered to the retina. The participants were asked to move a translatable pinhole until these spots were perceived to overlap each other. The same system assessed SCE at 680 nm using a bipartite, 2-channel (reference and test) Maxwellian-view optical system. The peak positions were estimated using a two-dimensional Gaussian fitting function and correlated with the visual axis positions. Both the visual axis (x = 0.24 ± 0.35 mm, y = -0.16 ± 0.34 mm) and the SCE peak (x = 0.27 ± 0.35 mm, y = -0.15 ± 0.31 mm) showed intersubject variability among the cohort. The SCE peak positions were highly correlated in both the horizontal and vertical meridians to the visual axes (R2 = 0.98 and 0.96 for the x and y coordinates, respectively). Nine of the 10 participants demonstrated mirror symmetry for the coordinates of the visual axis and the SCE peak between the eyes (R2 = 0.71 for the visual axis and 0.76 for the SCE peak). The visual axis and SCE peak locations varied among the participants; however, they were highly correlated with each other for each individual. These findings suggest a potential mechanism underlying the foveal cone photoreceptor alignment.

Uniaxial flicker analysis of the psychophysical Stiles–Crawford effects

Purpose: We report on a semi-automated system for frequency analysis of the Stiles–Crawford effect of the first kind (SCE-I) using flicker methodology designed to gain insight into the temporal dynamics of the perceived visibility for alternating pupil entrance points. We describe the system and its calibration in detail and discuss psychophysical measurement data obtained for the two authors. Methods: A uniaxial system is used for SCE-I characterization of two emmetropic subjects as a function of flicker frequency for narrow wavelength bands chosen in the range of 450–700 nm using a fibre-guided tungsten–halogen lamp as light source. The flicker is realized using two orthogonally mounted galvanometric scanning mirrors that allow linear trajectories at any angle across the pupil. A fast tuneable liquid-crystal neutral density filter is used for brightness adjustment and another liquid-crystal filter is used for wavelength adjustment at each pupil point allowing simultaneous hue-shift determination for the Stiles–Crawford effect of the second kind (SCE-II). Results: Validation of the system is realized with a CCD camera, a spectrometer and a powermeter, and the data obtained are used in the software to calibrate all subsequent human subject measurements. The psychophysical data obtained show a strong frequency dependence of the Gaussian SCE-I with a characteristic directionality parameter, ρ, that is found to increase from 0.03 to 0.06/mm2 with flicker in the range of 1–10 Hz. The simultaneously determined hue shift could not be determined beyond 1 Hz due to the longer time required for a subjective determination. Conclusion: We have reported on a fast uniaxial system for temporal characterization of the SCE-I. The psychophysical results obtained show that accurate specification of frequency in flicker analysis is mandatory when comparing SCE-I visibility and directionality curves obtained with those obtained using quasi-static bipartite fields. A uniaxial design offers unique advantages over that of common two-channel systems by completely eliminating spectral errors or brightness differences in the two branches that otherwise will impose on those of the visual system and degrade the psychophysical data. Future work with more subjects will be used to narrow the uncertainty and the causes of the effects observed.

Directionality of individual cone photoreceptors in the parafoveal region

The pointing direction of cone photoreceptors can be inferred from the Stiles–Crawford Effect of the First Kind (SCE-I) measurement. Healthy retinas have tightly packed cones with a SCE-I function peak either centered in the pupil or with a slight nasal bias. Various retinal pathologies can change the profile of the SCE-I function implying that the arrangement or the light capturing properties of the cone photoreceptors are affected. Measuring the SCE-I may reveal early signs of photoreceptor change before actual cell apoptosis occurs. In vivo retinal imaging with adaptive optics (AO) was used to measure the pointing direction of individual cones at eight retinal locations in four control human subjects. Retinal images were acquired by translating an aperture in the light delivery arm through 19 different locations across a subject’s entrance pupil. Angular tuning properties of individual cones were calculated by fitting a Gaussian to the reflected intensity profile of each cone projected onto the pupil. Results were compared to those from an accepted psychophysical SCE-I measurement technique. The maximal difference in cone directionality of an ensemble of cones, ρ¯, between the major and minor axes of the Gaussian fit was 0.05 versus 0.29mm−2 in one subject. All four subjects were found to have a mean nasal bias of 0.81mm with a standard deviation of ±0.30mm in the peak position at all retinal locations with mean ρ¯ value decreasing by 23% with increasing retinal eccentricity. Results show that cones in the parafoveal region converge towards the center of the pupillary aperture, confirming the anterior pointing alignment hypothesis.

The Stiles–Crawford Effect: spot-size ratio departure in retinitis pigmentosa

The Stiles–Crawford effect of the first kind is the retina’s compensative response to loss of luminance efficiency for oblique stimulation manifested as the spot-size ratio departure from the perfect power coupling for a normal human eye. In a retinitis pigmentosa eye (RP), the normal cone photoreceptor morphology is affected due to foveal cone loss and disrupted cone mosaic spatial arrangement with reduction in directional sensitivity. We show that the flattened Stiles–Crawford function (SCF) in a RP eye is due to a different spot-size ratio departure profile, that is, for the same loss of luminance efficiency, a RP eye has a smaller departure from perfect power coupling compared to a normal eye. Again, the difference in spot-size ratio departure increases from the centre towards the periphery, having zero value for axial entry and maximum value for maximum peripheral entry indicating dispersal of photoreceptor alignment which prevents the retina to go for a bigger compensative response as it lacks both in number and appropriate cone morphology to tackle the loss of luminance efficiency for oblique stimulation. The slope of departure profile also testifies to the flattened SCF for a RP eye. Moreover, the discrepancy in spot-size ratio departure between a normal and a RP eye is shown to have a direct bearing on the Stiles–Crawford diminution of visibility.

Retinal response to departure from perfect power coupling: implications for the Stiles–Crawford effect

Maximum power transfer occurs in an oscillatory system driven by an external periodic force matching the natural frequency. Here, we analyze the role of matching in the biological phenomenon of waveguiding of light in a photoreceptor cone. When the fraction of light power that is not able to be coupled to a fundamental mode of a foveal cone is computed, it is found that the mismatch is proportional to the degree of departure from the perfect matching for one hundred percent coupling. Thus, Stiles–Crawford effect of the first kind is shown to be the response of the retina to departure from perfect matching. We show by correlating how the diminution in luminous efficiency recorded in psychophysical measurements for beams entering the pupil mm from the point of optimum efficiency in Stiles–Crawford effect of the first kind is due to a departure of either six or one-sixth times from this perfect match. The Stiles–Crawford function is recreated by interpreting perfect matching for axial entry of the beam and proportionate mismatching for corresponding degree of peripheral entries. Now the fundamental question is: Does a biological system respond by decreasing the efficiency by same amount irrespective of increase or decrease so long the product of departures from the value for optimum efficiency is unity?

Functional optical coherence tomography reveals transient phototropic change of photoreceptor outer segments.

Dynamic near infrared microscopy has revealed transient retinal phototropism (TRP) correlated with oblique light stimulation. Here, by developing a hybrid confocal microscopy and optical coherence tomography (OCT), we tested sub-cellular source of the TRP in living frog retina. Dynamic confocal microscopy and OCT consistently revealed photoreceptor outer segments as the anatomic source of the TRP. Further investigation of the TRP can provide insights in better understanding of Stiles-Crawford effect (SCE) on rod and cone systems, and may also promise an intrinsic biomarker for early detection of eye diseases that can produce photoreceptor dysfunction.

The Dependence of Spatial Distribution of Excitement of Receptors on Their Stiles-Crawford Effect

The spatial distribution of excitement of receptors depends on the magnitude of amplitude of light integrated over the wavefront passing through the pupil as conditioned by the receptor's acceptance lobe. Because the peripheral zones of the pupil are less efficient in eliciting visual excitation than the pupil's centre, otherwise known as Stiles-Crawford effect of the first kind (SCE I), the spatial distribution of excitement of receptors will differ depending on their Stiles- Crawford effect. Assuming the funneling of light into the receptors through a single entering aperture the SCE I is integrated into the calculation of retinal diffraction images, in the manner of an apodization effect. And, in the absence of aberrations, the pupil apodization is chosen twice as narrow as suggested by the SCE I to compute modulation in the retinal light distributions. But using coherent illumination, a periodic target of rectangular profile and incorporating SCE I as pupil apodization we got a result where neither the intensity nor the modulation changes indicating absence of SCE I for a full pupil admitting a single coherent beam.

Dynamic near-infrared imaging reveals transient phototropic change in retinal rod photoreceptors

Stiles-Crawford effect (SCE) is exclusively observed in cone photoreceptors, but why the SCE is absent in rod photoreceptors is still a mystery. In this study, we employed dynamic near infrared light imaging to monitor photoreceptor kinetics in freshly isolated frog and mouse retinas stimulated by oblique visible light flashes. It was observed that retinal rods could rapidly (onset: ∼10 ms for frog and 5 ms for mouse; time-to-peak: ∼200 ms for frog and 30 ms for mouse) shift toward the direction of the visible light, which might quickly compensate for the loss of luminous efficiency due to oblique illumination. In contrast, such directional movement was negligible in retinal cones. Moreover, transient rod phototropism could contribute to characteristic intrinsic optical signal (IOS). We anticipate that further study of the transient rod phototropism may not only provide insight into better understanding of the nature of vision but also promise an IOS biomarker for functional mapping of rod physiology at high resolution.

Defocus-corrected analysis of the foveal Stiles–Crawford effect of the first kind across the visible spectrum

The Stiles–Crawford effect of the first kind describes a gradually diminished visibility of light that enters the eye towards the pupil rim. Although of retinal origin, it is commonly described by a Gaussian pupil apodization whose width is determined by a directionality parameter that depends on retinal eccentricity, wavelength and spatial coherence of the light. As the measurements are done psychophysically they are prone to subjective variations and difficult to obtain across the visible spectrum. In this work, requirements for accurate refractive correction when determining the directionality parameter at any given wavelength are discussed and we show that a current-controlled tunable liquid-polymer lens provides a convenient means to accomplish this without requiring mechanical readjustments. This may be the most convenient way to combat defocus across the visible spectrum in the analysis of the Stiles–Crawford effect as demonstrated through experiments and with a detailed Zemax eye-and-system analysis. The results obtained are discussed in relation to myopia and a reduced directionality for highly myopic eyes.

Retinal light distributions, the Stiles–Crawford effect and apodization

Analysis of retinal image formation for beams of coherent and incoherent radiation emphasizes the role of the Poynting vector's inclination with respect to the retinal image plane. Coherent beams interfere and give rise to a single Poynting vector that highlights the unique direction of incidence of energy flow, whereas multiple incoherent beams, especially incoherent extended sources in the pupil, generate electro-magnetic disturbances in the image plane each characterized by Poynting vectors of their own. As a result, the Stiles-Crawford diminution of luminant efficiency adds differently depending on the coherence of the entering light. Two practical considerations follow: first, in performing diffraction calculations for the retinal image from known wavefronts in the pupil plane, apodization terms should not be factored in, and, second, in principle, for perfect imaging in standard target viewing, Stiles-Crawford integration with increasing pupil diameter is not expected.

The Stiles–Crawford effect: a theoretical revisit

In 1973, Snyder and Pask published an explanation of the Stiles–Crawford effect based on optical-waveguide theory, but used only one set of experimental data for comparison. Since then there have been many new experiments and comments on the theory. In this paper we examine the assumptions made in the original theory, introduce some refinements, address some of the comments and examine how well the theory explains the experimental results obtained since 1973. We conclude that the waveguide theory, with some minor improvements, provides the theoretical basis for understanding the Stiles–Crawford effect.

Exploring the Stiles–Crawford effect of the first kind with coherent light and dual Maxwellian sources

The visual impact of light obliquely incident on the retina is diminished due to the Stiles-Crawford effect of the first kind. It is normally analyzed by scanning a small Maxwellian source across the eye pupil while making subjective visibility comparisons to a static reference field that enters the eye near the pupil center. Here, we propose an alternative characterization method with two coherent Maxwellian point sources located at opposing sides of the pupil. This produces interference fringes at the retina with an underlying phase gradient. Altering the power ratio of the two point sources makes tuning of the wavefront inclination at the retina feasible. Thus, the Stiles-Crawford effect of the first kind can be examined without scanning the incident light across the pupil. In this paper, a spatial light modulator with holographic phase maps has been used to generate two Maxwellian point sources at the pupil that project a given phase variation onto the retina. We found that the effective obliqueness of light at the retina is determined by the weighted center-of-mass of the field amplitude at the pupil. Alternative techniques to generate the two secondary point sources may improve the accuracy of the method.

Influence of Stiles–Crawford effect on visual performance after laser in situ keratomileusis

We evaluated the impact of the Stiles-Crawford effect (SCE) on visual performance following laser in situ keratomileusis procedures. This prospective study included 71 eyes of 36 consecutive myopic patients (mean age, 20.94±3.69 years). Ocular aberrations and contrast sensitivity were measured one month after surgery. The SCE was modeled optically as a filter placed in front of the eye, and then the modulation transfer functions (MTFs) and the predicted log contrast sensitivity were calculated from the measured wavefront aberration data. Then the visual Strehl ratio for MTF (VSMTF) was calculated. The results indicated that the computed MTF with SCE were superior to that without SCE. The predicted contrast sensitivity functions were underestimated about 20% when the SCE was not taken into account for the scotopic pupil. Moreover, the measured contrast sensitivity was not significantly different from that with SCE at 6, 12, and 18 cycles/deg spatial frequencies. According to the obtained VSMTF ratio, optical qualities of all eyes were underestimated with the range from 5% (0.02 log unit) to 65% (0.22 log unit) without SCE, and the average value is 41% (0.15 log unit). When only taking higher-order aberrations into account, the predicted postoperative visual performance would be different from the real values. The evaluation of postoperative visual performance from wavefront aberrations should consider not only the compensation relationship between defocus and spherical aberration but also the SCE.

Single- and multimode characteristics of the foveal cones: the super-Gaussian function

A Super-Gaussian (SG) function has been introduced to describe the Stiles–Crawford effect and to match apodization changes caused by single- and multimode characteristics of the cone photoreceptors. The SG function has been validated by fitting it to the apodization at the pupil predicted by waveguide models and experimentally using a liquid-filled crystal fibre simulating the retina structure. Experiments of the Stiles–Crawford effect were performed for the author's eyes to examine the variation with wavelength throughout the visible spectral region of its characteristic directionality parameter and the visibility dependence on pupil point. The SG function describes a multimodal characteristic of the foveal cones at short wavelengths.

Spatial and spectral characterisation of the first and second Stiles–Crawford effects using tuneable liquid-crystal filters

A semi-automated bipartite Maxwellian illumination system was used to examine the Stiles–Crawford effects of the first and second kind for the fovea of the authors’ eyes. The illumination was realised with a tungsten–halogen source in combination with tuneable liquid-crystal spectral filters. Visibility and hueshift dependencies were examined spatially and spectrally. They have been found to be in fair agreement with former studies in terms of overall appearance but with a notable reduction in directionality for the blue spectral range. The observations are discussed in relation to recent theory that relates the Stiles–Crawford effect of the second kind to the Stiles–Crawford effect of the first kind via a pigment-specific colour visibility function. The results confirm a direct relationship between the two effects and a reduction in hueshift for narrowband illumination. Differences in the green spectral range remain, however, that may be a consequence of a different directionality of S-cones as compared to the M- and L-cones.

Effect of Pupil Size on Visual Acuity in a Laboratory Model of Pseudophakic Monovision

To investigate the effect of pupil size on visual acuity in pseudophakic monovision. For the simulation, a modified Liou-Brennan model eye was used. The model eye was designed to include a centered optical system, corneal asphericity, an iris pupil, a Stiles-Crawford effect, an intraocular lens, and chromatic aberration. Calculation of the modulation transfer function (MTF) was performed with ZEMAX software. Visual acuity was estimated from the MTF and the retinal threshold curve. The sizes of the entrance pupil were 2.0, 2.5, 3.0, and 4.0 mm. Decreasing pupil diameter and increasing myopia progressively improved near visual acuity. For an entrance pupil size of 2.5 mm and a refractive error of -1.50 diopters, the logMAR value (Snellen; metric) in the non-dominant eye at 40 cm was 0.06 (20/23; 6/6.9). Knowledge of the patient's pupil diameter at near fixation can assist surgeons in determining the optimum degree of myopia for successful monovision.

Evaluation of Image Quality Metrics for the Prediction of Subjective Best Focus

Seven existing and three new image quality metrics were evaluated in terms of their effectiveness in predicting subjective cycloplegic refraction. Monochromatic wavefront aberrations (WA) were measured in 70 eyes using a Shack-Hartmann based device (Complete Ophthalmic Analysis System; Wavefront Sciences). Subjective cycloplegic spherocylindrical correction was obtained using a standard manifest refraction procedure. The dioptric amount required to optimize each metric was calculated and compared with the subjective refraction result. Metrics included monochromatic and polychromatic variants, as well as variants taking into consideration the Stiles and Crawford effect (SCE). WA measurements were performed using infrared light and converted to visible before all calculations. The mean difference between subjective cycloplegic and WA-derived spherical refraction ranged from 0.17 to 0.36 diopters (D), while paraxial curvature resulted in a difference of 0.68 D. Monochromatic metrics exhibited smaller mean differences between subjective cycloplegic and objective refraction. Consideration of the SCE reduced the standard deviation (SD) of the difference between subjective and objective refraction. All metrics exhibited similar performance in terms of accuracy and precision. We hypothesize that errors pertaining to the conversion between infrared and visible wavelengths rather than calculation method may be the limiting factor in determining objective best focus from near infrared WA measurements.

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 11 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.

75th anniversary of the Stiles–Crawford effect(s): a celebratory special symposium: October 23 2008, Rochester, New York

There are rather few articles which, so to speak, serve to change the landscape in a scientific field. One of those was the discovery of the ‘directional sensitivity of the retina’ by Walter Stanley Stiles and Brian Hewson Crawford (first reported in 1933). Subsequently, their findings were subdivided by Hansen into two logical components, ‘the Stiles–Crawford Effects of the First and Second Kinds (SCE-1 and SCE-2)’. The former (SCE-1) dealt with aspects of their research which addressed alterations in perceived brightness of a visual stimulus; the second (SCE-2) was associated with the perceived hue and saturation of these visual stimuli. These discoveries arose out of a failed attempt by W.S. Stiles and B.H. Crawford to measure properly the areas of the entrance pupils of their experimental subjects as part of a research program which addressed problems of glare, e.g. disability glare in illuminating engineering. Their research was conducted at the National Physical Laboratory (NPL), which is located in Teddington, Middlesex, England. These two fine scientists properly deduced the reason for the failure of their experimental design, and they effectively described and defined a new feature of the visual system which was largely ascribed to the retina. In time, it was realized that this phenomenon was associated in large measure with the waveguide/fiber-optics properties of photoreceptors, and that this was a feature shared by virtually all vertebrate species. This paper is divided into two parts. In the first part, Enoch describes, as best he can, the culture and working conditions at NPL during 1959/60 when he served as a post-doctoral fellow with W.S. Stiles. And in the second part of this paper, the authors describe the findings of W.S. Stiles and B.H. Crawford at the time of their discovery. Today, we celebrate the 75th anniversary of that research. The organizing committee for this program (alphabetically) is David Atchison, Jay M. Enoch, Vasudevan Lakshminarayanan, and Pieter Walraven. Our group of speakers today will follow with discussions of aspects of subsequent work which has evolved from the initial discoveries made by the late W.S. Stiles and B.H. Crawford.

On the spectral relation between the first and second Stiles–Crawford effect

The first Stiles–Crawford effect describes the psychophysical visibility dependence on pupil point for a narrow beam of light incident on the eye. Its distribution function commonly attributed to photoreceptor waveguiding is characterized by a point of maximum visibility typically near the pupil centre and a wavelength-dependent directionality parameter. The second Stiles–Crawford effect describes a minor hue shift of obliquely incident light commonly attributed to pigment screening that accompanies observations of the first Stiles–Crawford effect. Here the first Stiles–Crawford effect and the wavelength dependence of its directionality parameter are examined in terms of photoreceptor waveguiding. Its distribution function is analyzed to derive characteristic features of the second Stiles–Crawford effect directly from the spectrum of the directionality parameter. The results obtained are independent of the exact physical mechanisms underlying both effects but do support a major role played by waveguiding. Finally, the influence of bleaching is discussed in relation to the model and to pigment screening.