Abstract
Decades of experimental and theoretical investigations have established that photoreceptors capture light based on the principles of optical waveguiding. Yet considerable uncertainty remains, even for the most basic prediction as to whether photoreceptors support more than a single waveguide mode. To test for modal behavior in human cone photoreceptors in the near infrared, we took advantage of adaptive-optics optical coherence tomography (AO-OCT, λc = 785 nm) to noninvasively image in three dimensions the reflectance profile of cones. Modal content of reflections generated at the cone inner segment and outer segment junction (IS/OS) and cone outer segment tip (COST) was examined over a range of cone diameters in 1,802 cones from 0.6° to 10° retinal eccentricity. Second moment analysis in conjunction with theoretical predictions indicate cone IS and OS have optical properties consistent of waveguides, which depend on segment diameter and refractive index. Cone IS was found to support a single mode near the fovea (≤3°) and multiple modes further away (>4°). In contrast, no evidence of multiple modes was found in the cone OSs. The IS/OS and COST reflections share a common optical aperture, are most circular near the fovea, show no orientation preference, and are temporally stable. We tested mode predictions of a conventional step-index fiber model and found that in order to fit our AO-OCT results required a lower estimate of the IS refractive index and introduction of an IS focusing/tapering effect.
Highlights
Photoreceptor inner and outer segments (IS, OS) are highly evolved for capturing and guiding light
Best cone focus was achieved by systematically focusing the adaptive optics (AO)-optical coherence tomography (OCT) system, and analyzing the power spectra of the images
Peak energy at the cusp defines the plane of best focus, which in the figure occurs at the same system focus (0.15 D denoted as star in the legend) for the inner segment and outer segment junction (IS/OS) and cone outer segment tip (COST) layers
Summary
Photoreceptor inner and outer segments (IS, OS) are highly evolved for capturing and guiding light. The directional properties have been used in preliminary studies to indicate the stage and degree of various retinal abnormalities as listed in Gao, et al [7] Parallel to these experimental measures of photoreceptor directionality, extensive theory has been developed for modeling light propagation in photoreceptors based on the principles of optical waveguides, i.e., tiny optical fibers or light pipes [12,13,14,15,16,17,18]. These elegant models predict the directional sensitivity, and the existence of so‐called optical modes, which represent the spatial distribution of the light energy as it propagates through the photoreceptor. Even with these latest findings, uncertainty surrounds even the most basic question as to whether cone photoreceptor segments support just one mode (single mode behavior) or more than one mode (multimode behavior)
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