Eccentricity dependent changes of density, spacing and packing arrangement of parafoveal cones

Purpose To investigate the distribution of cone density and spacing as well as the preferred packing arrangement of the cone mosaic as a function of retinal eccentricity. Methods An adaptive optics retinal camera (rtx1, www.imagine-eyes.com) was used to obtain images of the parafoveal cone mosaic in nineteen healthy volunteers. Cone density and spacing were estimated for each subject (both eyes) using a sampling window of 80x80 pixels, at 250-, 450-, 650- and 1100-m eccentricities from the fovea along the nasal and temporal retina of both eyes. The inter-subject and intra-subject variation of cone density and spacing were calculated via Coefficient of Variation (CoV). Cone packing arrangement was assessed using Voronoi analysis; calculations were done over 1024x128 pixels image sections of the cone mosaic across the horizontal meridian from 200- to 1050-m eccentricities. Results The inter-subject variation of parafoveal cone density ranged between 10% and 15% (p<0.001) and the intra-subject variation of cone density was lower than 8% in all subjects, except for two. The cone spacing values showed a moderate inter-subject (CoV<7%; p<0.001) and a low intra-subject variation (CoV<4% in all subjects, except for two cases). In the parafoveal region, 40-50% of cones were hexagonally arranged; the percentage of non-hexagonal Voronoi tiles increased at greater eccentricities. Conclusion The use of multiple and complementary metric descriptors allows for a more detailed description of packing distribution and preferred arrangement of cone photoreceptors across the parafoveal retina.