Assessing the variation of photoreceptor reflectance in cone dominant species

Abstract

Purpose : Spatial and temporal variability in individual photoreceptor reflectance has been observed in humans using adaptive optics ophthalmoscopy. Here we examined if there is similar variability in photoreceptor reflectance in two cone dominant animal models, the northern tree shrew (Tupaia belangeri), and the thirteen-lined ground squirrel (Ictidomys tridecemlineatus).

Methods : Under isoflurane anesthesia, confocal (30μm pinhole, 0.622 ADD) adaptive optics scanning light ophthalmoscope (AOSLO) images were acquired using 790nm light in two thirteen-lined ground squirrels (1M, 1F) and two northern tree shrews (1M, 1F). Videos were recorded at two retinal locations for each animal at six-minute intervals until five timepoints per location were acquired. Videos from each timepoint were processed as previously described to produce a single .tif image for each time point.¹ These images were aligned using a SIFT algorithm, then averaged using ImageJ to produce an image for identifying cones for analysis. Cones were identified using custom software, from which the Voronoi domain for each cone was derived. An eroded Voronoi-based mask (excludes the outermost 2-pixel edge of the Voronoi domain) was used to calculate the reflectance of each cone at each timepoint. These values were normalized to the mean pixel intensity from each timepoint. The variance over time was measured for each cell, and those with a variance of less than 0.05 were considered to have stable reflectance. The spatial variation of the cone reflectance was found by taking the standard deviation of all the cone reflectance values at a given timepoint.

Results : A total of 1,440 and 2,395 cones were analyzed for the northern tree shrews and thirteen-lined ground squirrels, respectively. There was spatial variation in cone reflectance in both species: the standard deviation was 59% of the mean cone reflectance for the northern tree shrew, and 47% of the mean for the thirteen-lined ground squirrel. In addition, 36% of the analyzed northern tree shrew cones showed temporal variance, compared to 32% for the thirteen-lined ground squirrel.

Conclusions : Despite substantial anatomical differences between cone photoreceptors in these species and humans, they display similar spatial and temporal variability in reflectance when imaged using confocal AOSLO. These data can help constrain models describing the cellular origins of such reflectance signals.

¹PMID: 28392976

This is a 2020 ARVO Annual Meeting abstract.