Spatial and temporal variation of rod photoreceptor reflectance in the human retina

Abstract

Using adaptive optics imaging tools to image the living retina, numerous investigators have reported temporal fluctuation in the reflectivity of individual cone photoreceptors. In addition, there is cone-to-cone (spatial) variation in reflectivity. As it has only recently become possible to image the complete rod photoreceptor mosaic in the living human retina, we sought to characterize the reflectivity of individual rods and compare their behavior to that of foveal/parafoveal cones. Across two subjects, we were able to successfully track the reflectance behavior of 1,690 rods and 1,980 cones over 12 hours. Rod and cone photoreceptors showed similar regional and temporal variability in their reflectance profiles, suggesting the presence of a common governing physiological process. Within the rod and cone mosaics, there was no sign of spatial clumping of reflectance profile behavior; that is, the arrangement of cells of a given archetypal reflectance profile within the mosaic was indistinguishable from random. These data demonstrate the ability to track the behavior of rod reflectivity over time. Finally, as these and other reflectance changes may be an indicator of photoreceptor function, a future extension of this method will be to analyze this behavior in patients with rod photoreceptor dysfunction (e.g., retinitis pigmentosa, Usher’s syndrome, and congenital stationary night blindness).

Adaptive optics scanning ophthalmoscopy with annular pupils

“…To quantify this change and compare it to the values predicted by the SCE, the imaging was repeated with constant PMT gain. Despite the photoreceptor reflectance fluctuations [11,41,42] and other sources of variability such as image registration, tear film and subject alignment, the observed reflectance amplitude values (shown in Table 2 ) are consistent with the SCE predictions. Table 2 Measured relative average image intensities for images collected with constant PMT gain compared against predicted values (in parenthesis) according to the SCE measured objectively at fixation (ρ = 0.11 mm ) [43].…”

“…annular) could be used to monitor the evolution of the multi-modal patterns, to determine if subtle transverse changes take place in the photoreceptor mosaic. This could be used to study the disc shedding and renewal process, in a similar way that the evolution of the fundamental modes has been used before [11,42,[50][51][52]. If the multi-modal intensity profiles are not affected by daily changes, they could potentially reveal early changes in photoreceptor disease such as swelling.…”

“…For larger pupils, however, retinal images are blurred by the monochromatic aberrations of the optics of the eye. Ophthalmic instruments that incorporate adaptive optics (AO) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] can compensate these aberrations, potentially reaching the classical Rayleigh resolution limit using larger diameter pupils. AO wavefront correction over a large pupil has resulted in an increase in transverse and axial resolution that has allowed in vivo visualization of multiple microscopic retinal structures in human and animal models, including but not limited to: the cone photoreceptor mosaic [1,6,16], the retinal pigment epithelial cell mosaic [8,17,18], leukocytes [19], the retinal capillary vasculature [19][20][21], fluorescently labeled retinal ganglion cells [8], the lamina cribrosa [22][23][24], and more recently, the rod photoreceptor mosaic [25,26].…”

Understanding the changes of cone reflectance in adaptive optics flood illumination retinal images over three years

“…In addition to their number and position, one of the most noticeable features of the photoreceptors is the spatial variability of their reflectance, which has been clearly detected with all AO imaging modalities [9][10][11][12][13][14][15][16][17][18]. From early observations, it has been shown that the cones change their reflectance with time even in healthy retinas [9].…”

“…It has been shown that the changes in reflectance occur independently between the cones [9] and that the cones change their brightness on time scales ranging from milliseconds [10,17] and seconds [11,15,16] to hours [9,12,13], days [14], months or years [18].…”