Stiles-Crawford Effect Research Articles

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.

Can we do without mydriasis in multifocal ERG recordings?

The purpose of this study is to determine whether mfERG recordings with natural pupils approximate recordings obtained in mydriasis if the stimulus luminance is adjusted. If so, a bright monitor could obviate pupil dilation, rendering clinical management considerably easier. In 11 eyes of six subjects, we took four mfERG recordings with a resolution of 61 scaled hexagons per eye, two with natural pupils and two in mydriasis with a luminance of 150 cd/m(2) and 500 cd/m(2), respectively. Conditions will be abbreviated as "LoNat, HiNat, LoDil, and HiDil," where Lo/Hi stands for low vs. high luminance and Nat/Dil stands for natural vs. dilated pupils. Response densities and peak times were averaged for the central seven and the remaining 54 hexagons. An ANOVA found highly significant effects for both amplitude and peak time between the four recording conditions (P always <0.001). However, for the two "middle" conditions, HiNat and LoDil, amplitudes and peak times did not differ markedly. Retinal illuminances of the conditions LoNat and HiNat each differed from the conditions LoDil and HiDil, respectively, by a factor of 5.4. Amplitudes increased and peak times decreased in the order of conditions LoNat, HiNat, LoDil, HiDil. When data were corrected for the Stiles-Crawford effect, there was an inversion of effective retinal illuminances of the recordings HiNat and LoDil. The mfERG guidelines correspond to the condition LoDil; for all practical purposes, the amplitudes and peak times are the same when the HiNat condition is used. Thus, when clinical constraints render pupil dilation undesirable, the mfERG can be recorded with natural pupils when monitor luminance is increased by five times (if available).

Directional sensitivity of the retina: 75 years of Stiles-Crawford effect.

The reduction of the brightness when a light beam's entry into the eye is shifted from the centre to the edge of the pupil has from the outset been shown to be due to a change in luminous efficiency of radiation when it is incident obliquely on the retina. The phenomenon is most prominent in photopic vision and this has concentrated attention on the properties of retinal cones, where responsibility has yet to be assigned to factors such as differences in shape, fine structure and configuration, and membrane anchoring of photopigment molecules. Geometrical optics and waveguide formulations have been applied to the question of how light is guided in receptors, but details of their geometry and optical parameters even if they become available will make calculations complex and of only moderate generality. In practice, the diminution of oblique light helps visual performance by reducing deleterious influence of ocular aberrations and of glare caused by light scattering when the pupil is wide. Receptor orientation can come into play in ocular conditions due to mechanical disturbance and has been shown to have potentiality as a tool for clinical diagnosis. Currently, open questions include microanatomical and molecular differences between rods and cones, the coupling of the optical image of the eye with the transducing apparatus in the photoreceptors, possible phototropism and more convincing methods of estimating the actual spatial distribution of photon events as it affects visual resolution.

Directional and nondirectional spectral reflection from the human fovea

A model of the directional and nondirectional reflection spectrum of the human fovea is developed, incorporating reflectors, absorbers, and a wavelength-dependent optical Stiles-Crawford effect (OSCE). Data from 102 healthy subjects between 18 and 75 years obtained with the fundus reflection analyzer (FRA), an imaging spectrograph that measures the directional reflection profile of the human fovea in the pupil plane from 400 to 950 nm, were analyzed. Subgroups of young (<40 years) and old (>50 years) observers were defined. Mean results of the young group defined a template for directionality versus wavelength. For the whole group, mean reflection at 550 nm from the cones was 2.12%, from the retinal pigment epithelium 0.56%, and from the choroid 7.92%. Lens density, cone disc reflection, and blood layer thickness showed significant trends versus age. The model for the first time simultaneously describes the spectra of the directional and nondirectional reflection of the human fovea. Rayleigh scatter losses of the media and in preretinal layers were assumed zero in the nondirectional pathway. Mean density of the macular pigment of a subgroup (53 subjects, 19 to 75 years) correlated significantly with independent data from reflectance and autoflourescence images obtained by scanning laser ophthalmoscope (SLO) and data from flicker photometry.

Custom optimization of intraocular lens asphericity

To investigate the optimal amount of spherical aberration (SA) in an intraocular lens (IOL) to maximize optical quality. Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA. In 154 eyes of 94 patients aged 40 to 80 years, implantation of aspherical IOLs with different amounts of SA to produce residual ocular SA from -0.50 to +0.50 microm was simulated. Using the VOL-CT program (Sarver and Associates), corneal wavefront aberrations up to 6th order were computed from corneal topographic elevation data (Humphrey Atlas, Carl Zeiss, Inc.). Using the Zernike Tool program (Advanced Medical Optics, Inc.), the polychromatic point-spread function with Stiles-Crawford effect was calculated for the residual ocular higher-order aberrations (HOAs) (3rd to 6th order) for 6.0 mm and 4.0 mm pupils with defocus of 0.00 diopter (D), -0.50 D, and +0.50 D. Five parameters were used to quantify optical image quality, and the optimal ocular SA and IOL SA at which the maximal image quality was achieved in each eye were determined. Stepwise multiple regression analysis was performed to assess the predictors for optimal IOL SA for each eye. The optimal ocular SA and IOL SA for a 6.0 mm pupil varied widely between eyes. With defocus of 0.00 D, -0.50 D, and +0.50 D, most eyes achieved the best image quality at an ocular SA of -0.10 to 0.00 microm, +0.15 to +0.30 microm, and -0.40 to -0.20 microm, respectively. The IOL SA values that provided optimal visual quality in at least 10% of eyes were -0.45 to -0.20 microm with zero defocus, -0.15 to +0.10 microm with myopia of -0.50 D, and -0.75 microm to -0.45 microm with hyperopia of +0.50 D. The amount of optimal IOL SA could be predicted based on other HOAs of the cornea with multiple correlation coefficients up to 0.952. Among the Zernike terms, 4th-order SA, Z(4,0), made the greatest contribution to the optimal IOL SA, followed by the 6th-order SA, Z(6,0). The amount of IOL SA producing the best image quality varied widely between patients and could be predicted based on corneal HOAs. Selection of an aspherical IOL should be customized based on the full spectrum of corneal HOAs, not on 4th-order SA alone.

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.

A New Formula for Intraocular Lens Power Calculation After Refractive Corneal Surgery

When calculating the power of an intraocular lens (IOL) with conventional methods in eyes that have previously undergone refractive surgery, in most cases the power is inaccurate. To minimize these errors, a new IOL power calculation formula was developed. A theoretical formula empirically adjusted two variables: 1) the corneal power and 2) the anterior chamber depth (ACD). From the average curvature of the entrance pupil area, weighted according to the Stiles-Crawford effect, the corneal power is calculated by using a relative keratometric index that is a function of the actual corneal curvature, type of keratorefractive surgery, and induced refractive change. Anterior chamber depth is a function of the preoperative ACD, lens thickness, axial length, and the ACD constant. We used our formula in 20 eyes that previously underwent refractive surgery (photorefractive keratectomy [n = 6], laser subepithelial keratomileusis [n = 3], laser in situ keratomileusis [n = 6], and radial keratotomy [n = 5]) and compared our results to other formulas. Mean postoperative spherical equivalent refraction was +0.26 diopters (D) (standard deviation [SD] 0.73, range: -1.25 to +/- 1.58 D) using our formula, +2.76 D (SD 1.03, range: +0.94 to +4.47 D) using the SRK II, +1.44 D (SD 0.97, range: +0.05 to +4.01 D) with Binkhorst, 1.83 D (SD 1.00, range: -0.26 to +4.21 D) with Holladay I, and -2.04 D (SD 2.19, range: -7.29 to +1.62 D) with Rosa's method. With our formula, 60% of absolute refractive prediction errors were within 0.50 D, 80% within 1.00 D, and 93% within 1.50 D. In this first series of patients, we obtained encouraging results. With a greater number of cases, all statistical adjustments related to the different types of surgery should be improved.

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.

The Importance of the Phase Transfer Function to Visual Function and Visual Quality Metrics

To propose a vision quality metric that directly incorporates the phase transfer function. The process of retinal image formation from the pupil to the retina is presented to identify possible domains for calculating vision quality metrics. These domains are pupil plane metrics using ocular aberrations, and retinal plane metrics using the point spread function or optical transfer function. The importance of the phase transfer function to visual performance is demonstrated and a method is proposed for directly including phase in a vision quality metric based on ocular wavefront error. A family of vision quality metrics is proposed, which directly incorporates the Stiles-Crawford effect, contrast sensitivity function, and the phase transfer function. The phase transfer function is an important aspect of the quality of an image, and it may be directly included in metrics intended to quantify vision quality.

Assessment of just-noticeable differences for refractive errors and spherical aberration using visual simulation.

The aim of this study was to evaluate the threshold levels of aberration change that a typical reference eye is able to detect. The method involved simulation of the foveal vision of a typical eye in polychromatic light through optics affected by different levels of the various chosen monochromatic aberrations. The reference eye had the following monochromatic wavefront characteristics based on the aberrations of a population of young adults: no spherical defocus, astigmatism -0.37D oriented at 0 degrees celsius, coma -0.17 D/mm oriented at 270 degrees celsius, and spherical aberration -0.12 D/mm. Average amounts of longitudinal and transverse chromatic aberration were assumed, and allowance was made for the Stiles-Crawford effect. The pupil diameter of the simulated eye was kept fixed at 6 mm. Three observers each compared, 100 times, a simulated image as seen through the standard reference eye with a variant "aberrated" image. The varying parameter was the value of a chosen additional aberration affecting the variant image in the reference eye. The test was repeated for varying amounts of spherical defocus, astigmatic defocus, and spherical aberration. For each of these aberrations and each observer, the discrimination probability as a function of the aberration level in the variant image was determined. The just-noticeable difference in aberration (JNDA) was derived from each discrimination curve as the difference between the aberrations corresponding to discrimination probabilities of 75% and 25%. The JNDA values obtained were expressed in the form of root mean square (RMS) wavefront error thresholds. It was found that 0.04 microm of RMS aberration should be considered as the threshold of just-noticeable image change, in good agreement with the Maréchal criterion. The results imply that in normal viewing conditions (e.g., a 3-mm pupil size), optical corrections should be in the range of +/-0.15 D in sphere and cylinder from the target prescription if perceptible change in the quality of the perceived images is to be avoided. The design of conventional soft contact lenses of high negative power or positive power should aim to produce -0.07 D/mm of spherical aberration, with a tolerated interval between -0.15 to +0.01 D/mm for a 6-mm pupil size.

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.

Stiles-Crawford effect of the first kind (SCE-I) in post-photorefractive keratectomy and anisometropic subjects.

The Stiles-Crawford effect of the first kind (SCE-I) was measured on both post-photorefractive keratectomy (PRK) and anisometropic subjects at six different locations of retina. The eye that underwent the PRK procedure showed results that were typical of myopic eyes, although the vision in that eye has been maintained at 6/5 (or 20/15) since the operation. The anisometropic subject had one eye that was emmetropic and the other eye was myopic with a refractive error of -3.00 DS. The emmetropic eye showed the normal well-centered SCE-I functions across the retina, whereas the myopic eye of the same subject showed the nasal tilting of receptors in the nasal retina, which has been the typical finding among myopic subjects.