Local photoreceptor alignment measured with a scanning laser ophthalmoscope

Abstract

The aim of this study was to develop a fast test for local photoreceptor alignment. Photoreceptor alignment is an important indicator of retinal integrity. Digitized images of fundus reflectance were obtained for 20-30 pupil entry positions with a custom built scanning laser ophthalmoscope (SLO). The data permitted the calculation of curve peakedness of the optical Stiles-Crawford effect (SCE) as a function of retinal location. We found that the peakedness is low in the central 0.5 deg, reaches a maximum at an eccentricity of 1-2 deg and gradually drops with increasing eccentricity. These data are in conformity with the anatomy of foveal cones. Additionally, the psychophysical SCE was measured with red light and an 8 deg stimulus. The mean peak position of the SCE in the pupil plane for both methods was similar, but the optical SCE was clearly steeper. The SLO provides a fast, reliable and objective way to determine local receptor alignment in the central retina.

Variations in photoreceptor directionality across the central retina

“…He confirmed this prediction psychophysically, and since then it has been well established that ρ varies with retinal eccentricity. 3,7,15 Enoch and Hope 7 measured ρ from 0 to 10 deg in the temporal retina. They found that ρ was minimum at the center of the fovea, increased rapidly out to 2-5 deg eccentricity, and then decreased very slowly.…”

“…Starr et al, 31 using a 2-min test target and careful control of the optical correction of the eye, found that for the very center of the retina the cones' directional sensitivity was flat across the pupil. Recently Delint et al 15 used an optical method based on a scanning laser ophthalmoscope to measure ρ as a function of retinal position. In agreement with Enoch and Hope 7 and Gorrand and Delori, 13 de Lint et al 15 found that ρ was small in the center of the fovea and increased to either side.…”

“…Recently Delint et al 15 used an optical method based on a scanning laser ophthalmoscope to measure ρ as a function of retinal position. In agreement with Enoch and Hope 7 and Gorrand and Delori, 13 de Lint et al 15 found that ρ was small in the center of the fovea and increased to either side. However, they observed that beyond 2 deg, ρ decreased very rapidly: at 3-4 deg retinal eccentricity ρ reached a value similar to that measured in the center of the fovea.…”

“…However, it is also possible to use optical techniques to measure photoreceptor directionality. [11][12][13][14][15] The optical techniques are based on the principle that light entering the inner segments of the cones is guided along the outer segment and is absorbed by the photopigment, multiply scattered into the deeper retinal layers, or singly scattered or reflected back along the outer segments, thereby retracing its path. 13,16 This light is then radiated from the inner segments back toward the pupil.…”

Model for cone directionality reflectometric measurements based on scattering

“…Different groups have used slightly different approaches. For example, Van Blockland 10 measured the scattering diagrams as the exit pupil was scanned along one axis; Gorrand and Delori 11 scanned both the entry pupil (for illumination) and the exit pupil (for light collection); Burns et al 6,9 imaged the entire distribution of the light returning from the retina through the pupil for any given pupil entry position; and Delint et al 12 analyzed digitized images of the fundus reflectance obtained with a scanning laser ophthalmoscope for different pupil entry locations.…”