Abstract
The directional component of the retinal reflection, i.e., the optical Stiles-Crawford effect (SCE), is well established to result from the waveguiding property of photoreceptors. Considerable uncertainty, however, remains as to which retinal reflections are waveguided and thus contribute. To this end we have developed a retina camera based on spectral-domain optical coherence tomography (SD-OCT) that axially resolves (approximately 5 microm) these reflections and permits a direct investigation of the SCE origin at near infrared wavelengths. Reflections from the photoreceptor inner/outer segments junction (IS/OS) and near the posterior tip of the outer segments (PTOS) were found highly sensitive to beam entry position in the pupil with a considerable decrease in brightness occurring with an increase in aperture eccentricity. Reflections from the retinal pigment epithelium (RPE) were largely insensitive. The average directionality (rho(oct) value) at 2 degree eccentricity across the four subjects for the IS/OS, PTOS, and RPE were 0.120, 0.270, and 0.016 mm(-2), respectively. The directionality for the IS/OS approached typical psychophysical SCE measurements, while that for the PTOS approached conventional optical SCE measurements. Precise measurement of the optical SCE was found to require significant A-scan averaging.
Highlights
It has been long established that the human visual system has reduced sensitivity to light rays that enter near the edge of the eye’s pupil compared to those entering near the center
The semilog ordinate used in the figure compresses the reflectivity, the strong dependence of the IS/OS and photoreceptor outer segments (PTOS) reflections on beam entry position is still readily apparent, even at the near-infrared wavelength of 842 nm
The axial resolution (~5 μm) and sensitivity to weak reflections (~94 dB with a ~27 dB dynamic range across the retina) of our SD-OCT greatly surpass those of established Stiles-Crawford effect (SCE) methods
Summary
It has been long established that the human visual system has reduced sensitivity to light rays that enter near the edge of the eye’s pupil compared to those entering near the center. This effect, referred to as the psychophysical Stiles-Crawford effect (SCE), was discovered by Stiles and Crawford, and results from the waveguiding property of photoreceptors[1,2,3]. The SCE has been used in preliminary studies to indicate the stage and degree of various retinal abnormalities. An added benefit of the SCE as a biomarker is its stability with age [15,16]
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