Abstract
The rod photoreceptors are implicated in a number of devastating retinal diseases. However, routine imaging of these cells has remained elusive, even with the advent of adaptive optics imaging. Here, we present the first in vivo images of the contiguous rod photoreceptor mosaic in nine healthy human subjects. The images were collected with three different confocal adaptive optics scanning ophthalmoscopes at two different institutions, using 680 and 775 nm superluminescent diodes for illumination. Estimates of photoreceptor density and rod:cone ratios in the 5°–15° retinal eccentricity range are consistent with histological findings, confirming our ability to resolve the rod mosaic by averaging multiple registered images, without the need for additional image processing. In one subject, we were able to identify the emergence of the first rods at approximately 190 μm from the foveal center, in agreement with previous histological studies. The rod and cone photoreceptor mosaics appear in focus at different retinal depths, with the rod mosaic best focus (i.e., brightest and sharpest) being at least 10 μm shallower than the cones at retinal eccentricities larger than 8°. This study represents an important step in bringing high-resolution imaging to bear on the study of rod disorders.
Highlights
The human photoreceptor mosaic is multifaceted—among other things, providing exquisite resolution of spatial detail, single-photon sensitivity, and discrimination of millions of hues
Foveal fixation was confirmed by examining the appearance of the cone mosaic while the subject was fixating at the center of the raster scan
Note that most of the rod photoreceptor mosaic can be resolved in the logarithmic image, despite the 80% increase in the fullwidth at half-maximum (FWHM) of the PSF with respect to the image displayed with a linear grayscale
Summary
The human photoreceptor mosaic is multifaceted—among other things, providing exquisite resolution of spatial detail, single-photon sensitivity, and discrimination of millions of hues. These functional capabilities are a result of the presence of two interleaved mosaics of photoreceptors : rods and cones. Despite the majority of our daily vision being driven by the cone photoreceptors, nearly 95% of the total photoreceptor population in the human retina comprises rods [1]. Lacking are tools with which to assess rod structure in the living retina; such tools could enable researchers to elucidate the sequence of events that lead to vision loss in these conditions, as well as allow for improved monitoring of the efficacy of current and new treatments
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