Crawford Effect Research Articles

Influences of accommodation and myopia on the foveal Stiles–Crawford effect

We determined the foveal Stiles–Crawford effect (SCE) as a function of up to 8D accommodation stimulus in six young emmetropes and six young myopes using a psychophysical two-channel Maxwellian system in which the threshold luminance increment of a 1 mm spot entering through variable positions in the pupil was determined against a background formed by a 4 mm spot entering the pupil centrally. The SCE became steeper in both groups with increasing accommodation stimulus, but with no systematic shift of the peak. Combining the data of both groups gave significant increases in directionality of 15–20% in horizontal and vertical pupil meridians with 6D of accommodation. However, additional experiments indicated that much of this was an artefact of higher order aberrations and accommodative lag. Thus, there appears to be little changes in orientation or directionality in the SCE with accommodation stimulus levels up to 6 D, but it is possible that changes may occur at very high accommodation levels.

Integration of the Stiles–Crawford effect of the first kind

In 1932/1933, Walter Stanley Stiles and Brian Hewson Crawford, sought to measure the area of the entrance pupil of their subject's eyes. They assumed all portions of the entrance pupil of the eye contributed equally to visual excitation. Their pupillometer did not function as hypothesized. Using photopic stimuli, they found light-rays entering the pupil center had greater capability to stimulate vision than rays penetrating peripheral portions of the pupil. They deduced the reason for this failure, and discovered the Stiles–Crawford effect of the first kind (SCE-1), or ‘the directional sensitivity of the retina’. This important finding resulted in numerous fine studies, but integration of excitation across the entrance pupil of the eye, per se, has been rarely considered. At a later date, Enoch, then Drum, addressed this problem. We address integration and specification of visual stimuli, consider confounds, such as the effects of aberration-induced blur, and the means of mitigating confounds encountered. We consider use of adaptive optics for these purposes.

Traction centered nasal to the optic nerve head in high myopic observers

Measuring the Stiles–Crawford effect of the first kind (SCE-1), we determined effects of tractional strains caused by serial Opto-Kinetic Nystagmus (OKN) stimuli in myopic observers (near-sighted or short-sighted) just nasal to the optic disc. Responses were recorded in high myopes with and without myopic supertraction and in low myopes. SCE-1 was measured before, just after, and following recovery from serial OKN stimuli at 22○ Temporal Visual Field (TVF). Previously, this site was identified as near the locus of greatest retinal traction in high myopic observers. Testing was photopic and within the linear portion of the Weber function. A modest increase was found in measured SCE-1 thresholds (= sensitivity reduced) immediately after serial OKN stimuli. SCE-1 functions were flatter; their peaks were transiently altered, and shifted in the opposite direction to OKN-induced saccades. Recovery occurred within a few minutes. Transients in photoreceptor alignments and sensitivity (triggered by OKN-return saccadic eye movements) occurred near 22○ TVF in the nasal retina in all subjects measured. Differences existed between short and longer-term photoreceptor alignments.

Stiles–Crawford effect of the first kind: influence of the outer segment disc structure of human retina cones

This study analyses the way that the disc structure of the cone outer segment of the human retina affects light propagation through the cones, and how it influences the Stiles–Crawford effect of the first kind (SCE I). For this, we have used the finite-difference time-domain method (FDTD), enabling a numerical solution of Maxwell's equations. The intensity patterns at the cone endface are then compared with those found using the cone model with a homogeneous outer segment used in previous studies. The results for the new model, based on segments of optical fibre, show greater confinement of light propagating through the cone, and higher light intensity when the light strikes the entry face of the cone at certain angles.

The virtual pupil

Stiles and Crawford showed over 75 years ago that the sensitivity of the retina varies with the place of entry of the light into the pupil, and thus with the direction of incidence of light upon the retina. The effect is a property of the cones. Rods have a very limited directional sensitivity with a common centre of symmetry with the photopic SC-function. One reason, amongst others, for the existence of the Stiles–Crawford effect is supposed to be the suppression of stray light in the ocular media. The light from, for example, the sun or the bright sky, will be scattered in the ocular media, and cause a haze over the visual scene, thereby decreasing contrast. Thanks to the Stiles–Crawford effect, this scattered light will not be perceived. In order to judge the weight, with respect to other reasons, of this stray light theory, a quantitative study of the amount of stray light is required. This suggestion is based on indications that the scattering in the ocular media is mainly forward scattering, and that stray light coming from the retina seems not to contribute much either. Indirect evidence in favour of the stray light hypothesis is that in nighttime conditions, the visual surround is dark, disturbing stray light does not exist, and the rods therefore do not need directional sensitivity. This is indeed the case, and therefore the pupil size is fully used for the few incoming light quanta. At mesopic levels, the disturbing effects of a large pupil, such as spherical and chromatic aberrations, will be limited for the cones by their directional sensitivity, in the same way as a small pupil does at high luminance levels. It is therefore proposed to consider the directional sensitivity of cones as a virtual pupil.

Effect of wavelength and the Stiles–Crawford effect on the Nonius horopter

This study examines the effect of wavelength on the Nonius horopter. We have investigated whether the observed change in directionality is due to chromostereopsis and the Stiles–Crawford effect (SCE). The Nonius horopter was measured foveally in five subjects as a function of four different wavelengths. Illumination and pupil condition were varied. Chromostereopsis was also recorded. The result for the different luminance conditions reveals that SCE modifies the curves under photopic conditions, and the direction of the horopter corresponds to the direction of chromostereopsis. We conclude that the changes in the horopter curves are due to both optical and neural factors.

The Stiles–Crawford effect of the first kind and the full-field electroretinogram (ERG)

Electroretinography (ERG) can be used to quantify the directional sensitivity of the retina to incident stimuli. We have developed a rapid technique to derive a global measurement of the Stiles–Crawford effect of the first kind (SCE I) using the full-field flash ERG using artificial pupils. Three opaque haptic contact lenses were designed to direct stimuli along the photoreceptor axes with a single central aperture or obliquely through double apertures of equivalent area (4.5 mm2) separated horizontally by 3.0 and by 5.5 mm. ERGs recorded using an HK loop electrode to a series of white flashes ranging from sub-threshold to 2000 cd s/m2 were presented in a ganzfeld bowl. Sensitivity was compared using the luminance:response functions for the ERG a-wave and b-wave amplitudes. The b-waves of the ERGs showed a shift in sensitivity between the central mid and outer apertures requiring stronger flashes for threshold, semi-saturation and saturation for the more peripheral apertures. The ERG a-waves did not demonstrate a significant sensitivity shift. Small apertures are suitable for generating oblique stimulation of photoreceptors for ERG recordings. The method described gives a global measure of the SCE I that may be useful when time or cooperation is limited. Apertures reduce the available light and positioning is less precise than with Maxwellian view systems. Thus, physical apertures in contact lenses may not be suitable for positioning stimuli on small retinal regions.

Encroachment of the retina and choroid onto the nasal portion of the optic nerve head in high myopia: visual and anatomical manifestations

Myopia (short-sightedness/near-sightedness) is widespread and is increasing rapidly worldwide! We study a common, but rarely considered complication of high myopia known as nasal myopic super-traction (or super-involution), or encroachment of the retina and choroid onto the optic nerve head (disc) , and associated anatomical, physiological, and visual manifestations. This paper provides the foundation for associated extended SCE-1 studies. Here, histological records are presented; optical coherence tomographic (OCT) findings of this disorder are introduced; and perimetric data were obtained using fundus camera perimetry. We relate these data to visual functions measured upon and adjacent to the optic nerve head of the eye (the disc). In a separate paper in this issue (Chang et al., J. Mod. Opt. 2009, 20, 2272), the Stiles–Crawford effect of the first kind (SCE-1) was used to study photo-receptor alignments, and associated transient changes in photoreceptor alignments recorded using the same test subjects. We sought to understand better nasal myopic super-traction and its complications. We considered associated affects on the retinal pigment epithelium (RPE), nerve fiber bundles on the optic nerve head, etc. We assessed visual functions upon and about the optic nerve head. We demonstrated unique OCT properties in the disc-area in high myopia, evidence of disc hyperemia, reported the presence of maintained on-the-disc visual responses in over-ride tissues, and recorded reductions in visual sensitivity in retinal tissues translated onto/upon the disc. On-the-disc retinal and choroidal over-rides in high myopia remain viable, but exhibit reduced visual sensitivity; reveal time-varying alterations in photoreceptor alignments; and there exists evidence for on-going physical changes occurring in these over-ride tissues.

Guided light and diffraction model of human-eye photoreceptors

The photoreceptors of the living human eye are known to exhibit waveguide-characteristic features. This is evidenced by the Stiles-Crawford effect observed for light incident near the pupil rim, and by the directional component of light reflected off the retina in the related optical Stiles-Crawford effect. We describe a model for the coupling of light to/from photoreceptors on the basis of waveguide theory that includes diffraction between the eye pupil and the photoreceptor apertures, and we show that valuable insight can be gained from a Gaussian approximation to the mode field. We apply this knowledge to a detailed study of the relationship between the Stiles-Crawford effect and its optical counterpart.

FDTD analysis of the light propagation in the cones of the human retina: an approach to the Stiles–Crawford effect of the first kind

In this work, we analyse the propagation of light in the human retinal cones by the finite-difference time-domain method (FDTD) in cylindrical coordinates, for systems with axial symmetry or body of revolution (BOR). This work constitutes a preliminary approach to the study of the Stiles?Crawford effect of the first kind (SCE I), as a cone has been simulated, while the SCE I is a consequence of a large number of photoreceptors. The FDTD method enables a numerical resolution of Maxwell's equations. With the computational implementation of these equations we have obtained, in the cone endface, the intensity patterns for different angles with which the light strikes the input side of the photoreceptor, and different wavelengths of incident light. In view of the results, the characteristics of fibre optics presented by the retinal cones remain valid, as does the goodness of the method and of the computational cone model that we have proposed. This work demonstrates, in short, the computational validity of our cone model as well as the power and versatility of the FDTD-BOR algorithm for the analysis and comprehension of the phenomenon of light propagation in retinal photoreceptors. It therefore offers broad possibilities in the field of research on visual processes, serving as support for analysis and interpretation of experimental measurements.

Investigating the light absorption in a single pass through the photoreceptor layer by means of the lipofuscin fluorescence

Reflection densitometry has been widely used to measure the density difference of the bleachable cone photopigments in human eyes. Most such measurements make a series of assumptions concerning the amount of scattered light to derive an estimate of the true cone photopigment density from the density difference measurements. The current study made three types of measurements of the light returning from the eye before and after bleaching: the amount of light returning in the “directed” reflection, which is a double-pass estimate of the cone photopigment density; the amount of light in undirected or diffuse reflection; and the amount of fluorescence from lipofuscin in the RPE, which provides a single-pass measurement of optical density difference. For a 1 deg foveally fixated field, the density difference estimates for the three measurements were 0.68, 0.21, and 0.22 respectively. The lipofuscin fluorescence was found to be unguided. The background density difference was non-negligible and very close to the single pass estimate from fluorescence. These measurements each involve potentially different pathways of light through the retina, and therefore place different constraints on models of these pathways. A simple model comparing the directional and the fluorescence optical densities produced retinal coverage estimates around 70–75%. Estimates of the shape factor of the single pass optical Stiles–Crawford effect were evaluated from the dark-adapted and bleached fluorescence measurements. The values were closer to those obtained from psychophysical methods than to the double pass optical Stiles–Crawford shape factors obtained directly from retinal reflectometry.

Spectral and directional reflectance of the fovea in diabetes mellitus: Photoreceptor integrity, macular pigment and lens

Our aim was to assess the integrity of the photoreceptors in the fovea, and to measure the optical density of the macular pigment and the eye lens in patients with diabetes mellitus, and to compare the results with those of a group of healthy subjects. The directional and spectral properties of the light reflected from a 1.9 deg field centered on the fovea were measured simultaneously, in a single one second flash, with the Foveal Reflection Analyzer. The directional characteristics, i.e., the optical Stiles–Crawford effect, provided information on the integrity of the foveal photoreceptors. Model analysis of the spectral reflectance yielded optical densities of the macular pigment and the lens. The amplitude of the directional reflectance in diabetic eyes was significantly lower compared to controls ( P < 0.001). This indicates that the integrity of the photoreceptors in the fovea was altered in diabetics. Surprisingly, the directionality (a measure for the peakedness) was similar in diabetics and controls ( P = 0.3). The density of macular pigment was not different from that in controls ( P = 0.3). The optical density of the lens increased with age in both groups, but the rate of increase was larger in the diabetics ( P < 0.05). Possibly, the lens optical density increasing at a higher rate with age reflects changes preceding cataract formation.

Absorption of the eye lens and macular pigment derived from the reflectance of cone photoreceptors

We measured the amplitude of the directional component of the bleached fundus reflectance, the so-called optical Stiles-Crawford effect, as a function of wavelength. The directional reflectance originates from within the outer segments of the photoreceptors. Thus only two anterior absorbers are of importance: macular pigment and the crystalline lens. Analysis of spectra obtained in pseudophakes established that the cone photoreceptors act as spectrally neutral reflectors. The reflectance spectra, expressed in density units, resembled the macular pigment density spectrum. Studying age effects in the lens of normal subjects resulted in a description of the optical density of the lens in terms of a "young" and an "aged" template. The young template represents the pigment O-beta-glucoside of 3-hydroxykynurenine, which dominates the light absorption in young eyes and decreases with age. The aged template represents the pigments accumulating in the lens with age. The total optical density increased with age, but it was lower in the wavelength region 500-650 nm than was previously assumed on the basis of psychophysical studies. Analysis of the spectra also provided precise individual estimates of the optical density of macular pigment. Finally, we observed a decrease in the photoreceptor reflectivity with age, possibly reflecting a degradation of the photoreceptors.

Small foveal targets for studies of accommodation and the Stiles–Crawford effect

The properties of small monochromatic targets as accommodative stimuli are not well understood. We used a dynamic optometer to record accommodation responses to monochromatic disc targets (1.0–27.3 min arc) and to a Maltese cross. Accommodation responded adequately to points as small as 13.6 min arc. The response to these small targets is relevant to the question of whether the Stiles–Crawford (SC) effect could provide a stimulus to accommodation. Previous studies have used pupil apodizing filters to neutralise the natural SC function and so determine how visual performance or accommodation is influenced by the SC effect. However, these filters cannot correct for known inhomogeneities in the SC function across the retina for extended targets. Therefore, we calculated the SC function inhomogeneities across the retinal image of a smaller 13.6-min arc target. Unfortunately, even this small target is too large to permit a homogenous SC function across its extent. Alternatives to the apodizing filter approach are discussed.

Defective cone photoreceptor cytoskeleton, alignment, feedback, and energetics can lead to energy depletion in macular degeneration.

Macular degeneration (MD) is a puzzling disease characterized by disturbance, and then complete loss, of fine detailed vision in the central macular region of the human retina, as a result of disturbed function and then death of photoreceptor cells. This review describes a possible pathomechanism for MD that involves a causal relationship between mutated genes, altered photoreceptor cytoskeletal proteins, defective cone photoreceptor alignment, and disturbed visual feedback mechanisms that leads to energy depletion and apoptosis of macular cone photoreceptors. MD may be associated with mutations in genes encoding certain photoreceptor proteins (ATP-binding cassette transporter retina, retinitis pigmentosa GTPase regulator, retinal degeneration slow/peripherin) that are components of, or interact with, microtubule-containing cytoskeletal systems at the connecting cilium of rods and cones and at the multiple incisures of rod outer segments (OSs). Vertebrate photoreceptors are directionally sensitive: the longitudinal axis of each cell is actively aligned towards the entrance pupil of the eye, and the cells are most sensitive to light travelling along this axis (as in the Stiles-Crawford effect). The mechanisms responsible for photoreceptor alignment involve movements made by photoreceptors in response to the direction of incident light, using their cytoskeletons. A model is proposed for photoreceptor alignment whereby light absorption in the OS causes fast membrane and slower cytoplasmic changes that spread from the OS to the inner segment myoid, where they activate feedback-controlled motor functions by cytoskeletal elements (microtubules and microfilaments) to produce a differential local bending that adjusts photoreceptor orientation. The fovea at the center of the human macula has specialized features that enable it to provide uniquely high visual acuity, if its cones are accurately aligned. Accordingly, it is proposed that some gene defects in MD cause disturbances at photoreceptor connecting cilia that lead to gradual defects in photoreceptor alignment; misalignment of central macular cones will be initially perceived as blurring, distortions, and decreased acuity of central vision. Although photoreceptors throughout the retina use their cytoskeletons for alignment, the accurate alignment of foveal cones is particularly important because their signals contain fine resolution information that is used in visual feedback systems, e.g., for adjusting accommodation and eye movements. Vision involves multiple feedback loops that are interdependent, e.g., the accuracy of alignment of foveal cones influences how effectively changes in accommodation bring images into focus, and the state of accommodation in turn influences how light entering the eye is projected onto foveal cones. It is proposed that in MD gene defects that disturb the cytoskeleton and alignment of photoreceptors lead to a disturbance in the normal signalling within these feedback systems, causing the mechanisms controlling the alignment of cones in the center of the macula to become progressively more disturbed and the cells to unnecessarily expend energy. These disturbances can lead to local energy depletion within the metabolically fragile central macular cones that trigger them to die, producing central areas of blindness. If this line of reasoning is correct, it may be possible to treat the local energy depletion within macular cones in MD by energy supplementation.

Evidence for transient forces/strains at the optic nerve head in myopia: repeated measurements of the Stiles-Crawford effect of the first kind (SCE-I) over time.

Probable transient changes in photoreceptor alignments, inferred from the measurements of the Stiles-Crawford effect of the first kind (SCE - I), were demonstrated in myopic eyes with elongated axial length (regardless of the magnitude of refractive error) at three retinal locations; the fovea (point of fixation), and 22 degrees and 27 degrees in the nasal retina. The changes were much bigger at 22 degrees and 27 degrees in the nasal retina (which are located beyond the optic nerve head) than at the fovea. These transient effects were revealed by repeatedly testing the same retinal locations over a period of time. Time intervals between the subsequent/repeated measurements ranged from less than an hour to several months. In some locations, the changes were recorded in less than an hour. Collectively, these were very meaningful changes. Generally bigger effects were recorded in the horizontal SCE - I than in the vertical SCE - I.

The relationship between the Stiles-Crawford effect of the first kind (SCE-I) and myopia.

The Stiles-Crawford effect of the first kind (SCE-I) was measured on both emmetropic and myopic subjects at six different retinal locations. The results revealed a number of significant discrepancies in receptor alignment between the groups of different refractive errors. In myopic subjects, the receptors in the nasal retina (i.e. between the fovea and the optic nerve head) were found to be aligned nasally towards the optic nerve head, whereas the receptors in the temporal retina were aligned towards the centre of the exit pupil. In emmetropic subjects, the receptors across the retina were finely tuned towards the centre of the exit pupil. The magnitude of the receptor displacement in myopic subjects was found to be directly associated with the length of the eyeball.

The reference axis in corneal refractive surgeries: Visual axis or the line of sight?

A reference axis for proper centration of the corneal working zone in refractive surgical techniques is required. This is difficult to define due to the fact that the human eye is not an aligned optical system. This theoretical note critically reviews and discusses the two commonly used reference axes; line of sight and the coaxially sighted corneal reflex. Taking into consideration the Stiles—Crawford effect, it is concluded that the coaxially sighted corneal reflex is remarkably close to the visual axis (or nodal axis) and is the preferred reference axis.

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.

The Stiles–Crawford effect and subjective measurement of aberrations

We considered the influence that the Stiles–Crawford effect (SCE) has on the measurement of subjective monochromatic and transverse aberration measurements. The SCE was measured with a two channel Maxwellian-viewing system. Transverse aberrations were measured using a vernier alignment technique in three subjects, with the natural SCE operating, with the SCE neutralised by filters optically conjugate with the eye's pupil, and for one subject with filters that shifted the SCE by more than 2 mm. As pupil diameter increased from 1 to 5 mm diameter, without the filters the slope of the transverse aberration versus position in the pupil decreased, e.g. for chromatic aberration this decreased by approximately 90%. The filters had little influence on transverse aberration. The results indicate that subjects do not use the centroid of the image of a blurred line target for alignment, but may rely very much on other cues.