The spatial arrangement of cones in the primate fovea

Abstract

THE retinae of Old World primates contain three classes of light-sensitive cone, which exhibit peak absorption in different spectral regions^1–4. But how are the different types of cone arranged in the hexagonal mosaic of the fovea? This question has often been answered with artists' impressions^5–7, but never with direct measurements. Staining for antibodies specific to the short-wave photopigment has revealed a sparse, semiregular array of cones⁸; but nothing is known about the arrangement of the more numerous long- and middle-wave cones. Are they randomly distributed, with chance aggregations of one type, as Hartridge postulated in these columns nearly 50 years ago^9,10? Or do they exhibit a regular alternation, recalling the systematic mosaics seen in some non-mammalian species^6,11? Or, conversely, is there positive clumping of particular cone types, as might be expected if local patches of cones were descended from a single precursor cell? We have made direct microspectrophotometric measurements of patches of foveal retina from Old World monkeys, and report here that the distribu tion of long- and middle-wave cones is locally random. These two cone types are present in almost equal numbers, and not in the ratio of 2:1 that has been postulated for the human fovea.