THE PHOTORECEPTOR MOSAIC

The organisation of the human photoreceptor mosaic reflects evolutiona ry strategies for optimising visual information under a wide range of stimulus conditions: (1) The rod population dominates (max. 170 000/mm (2) 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 t o 300 000 mm(2) in the fovea. A bundle of c. 300-500 foveolar cones ar e further distinguished by having their synaptic terminals located wit hin the capillary-free zone. Radial displacement (>350 mu m) of foveal tone terminals may result in the lesion of two sets of cone pathways by perifoveal laser treatment. Along the ora serrata peripheral cone d ensity (c. 4000) rises within a small rim (1 degrees) 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 arrangemen t are suggested. (3) Blue-sensitive (S-) cones constitute a regular an d independent submosaic of c. 7% across the periphery. An annular maxi mum (1000-5000/mm(2)) at c. 10 surrounds the foveola. There density de creases and irregular zones lacking S-cones result in tritan deficienc ies.