The photoreceptor mosaic

The organisation of the human photoreceptor mosaic reflects evolutionary strategies for optimising visual information under a wide range of stimulus conditions: (1) The rod population dominates (max. 170,000/mm2 at c. 30 degrees sup.) except for the central 2 degrees and along the ora serrata. (2) Density of cone inner/outer segments reaches up to 300,000 mm2 in the fovea. A bundle of c. 300-500 foveolar cones are further distinguished by having their synaptic terminals located within the capillary-free zone. Radial displacement (> 350 microns) of foveal cone terminals may result in the lesion of two sets of cone pathways by perifoveal laser treatment. Along the ora serrata peripheral cone density (c. 4000) rises within a small rim (1 degree) to up to 20,000, but may be considerably decreased by cystoid degenerations. For the L- and M-cone subpopulations ratios of 2:1 to 1:1 and random arrangement are suggested. (3) Blue-sensitive (S-) cones constitute a regular and independent submosaic of c. 7% across the periphery. An annular maximum (1000-5000/mm2) at c. 1 degree surrounds the foveola. There density decreases and irregular zones lacking S-cones result in tritan deficiencies.