Individual variations in human cone photoreceptor packing density: Variations with refractive error

PURPOSE. To measure the variation in human cone photoreceptor packing density across the retina, both within an individual and between individuals with different refractive errors. METHODS. A high-resolution adaptive optics scanning laser ophthalmoscope was used to image the cones of 11 human eyes. Five subjects with emmetropia and six subjects with myopia were tested (+0.50 to -7.50 D). For each subject, four approximately 10° x 1.5° strips of cone images were obtained. Each strip started at the fovea and proceeded toward the periphery along the four primary meridians. The position of each cone within the sampling windows was digitized manually by the investigator. From these cone counts, the density of the cones was calculated for a set of fixed distances from the fovea at locations throughout the image. RESULTS. Cone photoreceptor packing density decreased from 27,712 cells/mm² to 7,070 cells/mm² from a retinal eccentricity of 0.30 to 3.40 mm along the superior meridian in five emmetropic eyes. Cone photoreceptor packing density (cells per square millimeter) was significantly lower in myopic eyes than in emmetropic eyes. At a given location, there was considerable individual variation in cone photoreceptor packing density, although more than 20% of the variance could be accounted for by differences in axial length. CONCLUSIONS. The results provide a baseline analysis of individual difference in cone photoreceptor packing density in healthy human eyes. As predicted by retinal stretching models, cone photoreceptor packing density is lower in highly myopic eyes than in emmetropic eyes.