A study of the Stiles-Crawford (S-C) function at 35 degrees in the temporal field and the stability of the foveal S-C function peak over time

Directional light-capture efficiency of the foveal and parafoveal photoreceptors at different luminance levels: an experimental and analytical study

A gradual drop in visibility with obliquely incident light on retinal photoreceptors is namely described by the Stiles-Crawford effect of the first kind and characterized by a directionality parameter. Using a digital micromirror device in a uniaxial flicker system, here we report on variations of this effect with luminance levels, wavelengths within the visible and near-infrared spectrum and retinal regions ranging from the fovea to 7.5° parafoveal. Results show a consistent directionality in mesopic and photopic conditions. Higher directionality is measured for longer wavelengths, and a decrease with retinal eccentricity is observed. Results are discussed in relation to an absorption model for the visual pigments taking the outer-segment packing and thickness of the neural retina into account. Good correspondence is found without enforcing photoreceptor waveguiding.

Directional model analysis of the spectral reflection from the fovea and para-fovea

Directional and nondirectional spectral reflection data from 0, 1, 2, 4, and 8 deg eccentricity, and the optic disk, were analyzed from 400 to 950 nm with an existing optical reflection model. The optical model, developed for the fovea, appeared to be also suitable for more eccentric locations. The optical densities of melanin and of the macular pigments zeaxanthin and lutein peaked in the fovea, in correspondence with literature data. The amplitude of the directional component, originating in the cone photoreceptors, had its maximum at 1 deg. The maximum of the directionality (peakedness) occurred at a slightly higher eccentricity.

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.

Variations in photoreceptor directionally across the central retina

Cones show a differential sensitivity to light coming from different portions of the pupil, typically being most sensitive to light from the center of the pupil. We measured the directional properties of the cones across the central 6 deg of the retina, using an optical imaging technique. We find that the cones in the center of the fovea have the broadest tuning. The width of the angular tuning changes rapidly from 0 deg to 1 deg retinal eccentricity, with cones at 1 deg being much more narrowly tuned that the cones in the center of the fovea. Directional tuning of the cones remains relatively constant from 1 deg to 3 deg retinal eccentricity. Receptoral disarray contributes minimally to the measured directional properties of the foveal cones, and there is no evidence of asymmetry between horizontal and vertical retinal locations. There are only small differences among the five subjects in the change in angular tuning of the cones with retinal location. We find that at the foveal center the directional tuning of the cones is limited by the diameter of the cone apertures.

A reflectometric technique for assessing photoreceptor alignment

Clinical studies of photoreceptor orientation are limited by the fact that psychophysical methods for measuring the Stiles-Crawford effect are time consuming and require excellent co-operation from the subject. We have developed a novel instrument, the photoreceptor alignment reflectometer (PAR), that determines photoreceptor alignment by measuring the distribution in the pupil of light reflected by one retinal location. This determination is accomplished in a measurement time of 4 sec and requires minimal co-operation from the subject. The technique is not significantly affected by reflections at the limiting membrane, or by changes in entrance and exit pupil configuration, or by location of bleaching light entry. The PAR was used to measure the orientation of foveal photoreceptors, their directionality, and the ratio of directional to diffuse flux in 20 normal subjects ranging in age from 20 to 60 yr.

Parametric representation of Stiles-Crawford functions: normal variation of peak location and directionality

Evidence suggests that the psychophysically determined Stiles-Crawford effect of the first kind (SCE) reflects waveguide properties of human photoreceptors. The peak of the SCE data set is assumed to reflect the principal alignment tendencies, and the spread (e.g., rho value, the curvature or width at half-height) is assumed to reflect the directionality (i.e., interreceptor differences in alignment) of the population of photoreceptors being tested. As such, disruption of the normal SCE can be used and/or has been used (1) to document early natural history of retinal pathology involving the photoreceptors, (2) to provide a firm rationale for therapeutic intervention, and (3) to provide a method for monitoring therapies designed to alter the natural course of retinal-disease processes. We report large-sample norms for foveal SCE peak location and spread (horizontal peak location, nasal 0.51 +/- 0.72, horizontal rho value 0.047 +/- 0.013, vertical peak location, superior 0.20 +/- 0.64, vertical rho value 0.053 +/- 0.012), compare these norms with values determined in other laboratories, and discuss the various mathematical forms used for the empirical description of SCE data sets.

Use of reflecto-modulometry to study the optical quality of the inner retina

The modulation of interference fringes reflected by the peripheral fundus is always lowered by passage through the outer segments of the photoreceptors. In addition, it also may be degraded by passage through a diseased inner retina or through the choroid (in cases where there is only light pigmentation of the fundus). The contributions of the inner retina and choroid can be differentiated by varying the spatial frequency of fringes, or by studying the directionality of the fundus reflection. Reflecto-modulometry seems to be an appropriate method for analysing pathological changes in the optical quality of the inner retina.

Perimetry of critical flicker frequency in human rod and cone vision

Photopic critical flicker frequency (CFF) to green and yellow-red targets became independent of visual field location when the decrease in the density of retinal ganglion cells and increase in their receptive-field size towards the retinal periphery were compensated for by increasing stimulus area in inverse proportion to the human cortical magnification factor squared (M-scaling) and by reducing stimulus luminance in inverse proportion to Ricco's area (F-scaling). In mesopic and scotopic vision CFF to green targets increased monotonically with eccentricity despite MF-scaling. Instead, CFF to MF-scaled yellow-red targets that predominantly stimulated cones was independent of eccentricity at all luminance levels tested.

Diffusion of the retinal layers of the living human eye

We measured the modulation depth of an interference pattern after double passage through the eye with different entry positions of the linearly polarized incident beam in the pupil plane. This shifting of the incident beam greatly modifies the mean irradiance and the modulation depth of the aerial image, whatever the direction of the polarizer. These measurements give a better understanding of the respective contributions of the retinal layers to the formation of the aerial image. Consequently, the modulation transfer function of the optics of the eye deduced from measurements with the aerial image must be considered cautiously.

On the problem of retinal directional sensitivity

When two physically similar pencils of light enter the eye, one through the pupillary centre and the other near the edge, the former appears brighter and, when monochromatic light is used, generally as thought it were of a shorter wavelength. This is usually attributed to the photoreceptors, notably the cones, being more sensitive to light travelling along them axially than to light with a radial component. However, allowance has also to be made for pre-retinal factors. One of these is the angle of incidence on the cornea; the Fresnel formulae show that, at perimetric angles less than 15 degrees, the ensuing asymmetry in the fractions of light lost by the two pencils needs compensating. Furthermore, a correction is required for differential light absorbance along different paths in the crystalline lens. Finally, the narrowness of the pencils necessitates the application of diffraction theory: the harmonic content of a light beam varies inversely with the distance between its point of pupillary entry and the optic axis, the effect varying across the spectrum. The above corrections suggest that there is little difference between the directional sensitivities of human rod and cone mechanisms; the above chromatic effect can be explained partly on the basis of pre-retinal factors, and earlier theoretical explanations are rendered more plausible.

Inferred positive phototropic activity in human photoreceptors

The Stiles-Crawford (S.-C.) function, a measure of the directional sensitivity of the retina, was used to infer the alignment characteristics of the sampled retinal elements. One assumes that the peak of the photopic S.-C. function reflects the central alignment tendency of renal elements sampled, and that the shape of the function reflects, among other factors, distributive qualities. Here two tests were performed to determine whether the function sampled reflected positive phototropic activity. The natural eye pupil was dilated and artificial pupils were substituted having specified eccentricity from the centre of the natural pupil. This was achieved with a displaced iris contact lens. After a series of complex experiments, it was finally shown that the peaks of the S.-C. function shifted towards the displaced aperture of the contact lens. As a second test, individuals were occluded uniocularly with a black patch for periods of time up to 10 days. This caused remarkable flattening of the measured S.-C. function. That flattening occurred in determinations of both photopic and scotopic S.-C. functions. Comparable effects were not seen in the second eye or if a diffuser was substituted for the black patch. Change and recovery in both experiments occurred within 3-5 days. On the basis of these experiments it is inferred that there is an active mechanism behaving in a positive phototropic manner present in the human retina.

Visual sensitivity, resolution, and Rayleigh matches following monocular occlusion for one week

Following three to four days of monocular light exclusion, the directional sensitivity of the retina was reduced at all areas tested. Changes in the shape of the Stiles-Crawford (SC) function were evident at both photopic and scotopic adaptation levels. The flattening of the SC functions was primarily a result of increased sensitivity to light entering through the periphery of the eye pupil. Here, concentration is on concomitant changes in other visual functions including an overall increase in visual sensitivity and a decrease in resolution of low-intensity gratings. Anomaloscope settings revealed a modest increase in red required for a yellow match.

Evidence for alteration in photoreceptor orientation

The effect of incident light, its point of origin, and its magnitude on receptor orientation was determined. A non-invasive test of vision, the Stiles-Crawford Effect (SCE), was used to investigate retinal directional sensitivity and alignment. The simple act of monocular black patching for a period of three to five days caused marked reduction in directional sensitivity and/or apparent dispersal of alignments. Recovery occurred in a comparable time period in normal adults 20 to 50 years of age. Special painted iris contact lenses with displaced pupillary apertures were worn to investigate the factors influencing alignment. Both light induced effects which caused alignment to shift toward a displaced aperture, and mechanical factors resulting in other alignment shifts were recorded. Thus, retinal receptors and/or associated anatomic components are apparently phototropic and subject to multiple forces influencing their directionality and/or alignment, and the milieu of the retinal receptor is active.

Monocular light exclusion for a period of days reduces directional sensitivity of the human retina

Single eyes of young adult observers were occluded for as long as 10 days. Directional sensitivity of the retina (the Stiles-Crawford effect of the first kind) under photopic conditions was dramatically reduced at every retinal location tested in all subjects. The maximum effect was observed within 3 to 5 days, and recovery took place at approximately the same rate after termination of patching.