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A. D. Springer, A. E. Hendrickson; The Human Fovea Lacks a Single Center. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2138.
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Although the cone density gradient extending from the fovea to the periphery is well documented, the micro-topography of cone density within the fovea is uncertain. This study sought to determine whether the fovea has a single, very high density region, whose density diminishes progressively toward the parafovea.
Three published pictures of the human cone inner segments (IS) in retinal wholemounts (Curcio et al, 1990) were used for the analysis. A new method, independent of counting-window size, was used to determine foveal IS density (ISD). The method used the center-to-center distance (CTCD) between adjacent IS. Density was calculated for each CTCD value using the assumption that the IS were hexagonally shaped. The area of each IS was also measured. Thus, this method used the smallest counting-window possible (adjacent IS).
Mean foveal ISD for the 3 foveas were 9-21% higher than previously reported. IS CTCD were plotted as frequency histograms. The curves were skewed to the right, and the degree of skewedness (0.75-1.73) was correlated with ISD, which varied from 182-376k/mm2. The curves revealed the existence of much higher localized foveal ISD than did the mean ISD for an entire fovea. The IS areas were grouped into quartiles and the location of each quartile within the fovea was examined. Surprisingly, the IS quartile with the smallest IS areas were scattered, seemingly randomly, across the entire fovea. This was also the case for the other 3 quartiles having higher IS areas. All 3 foveas showed a similar pattern.
ISD is highest at the fovea and diminishes rapidly with distance from the fovea. Implicit in this well documented observation is that IS area increases with distance from the foveal center. In marked contrast, our micro-topographic analysis of IS revealed that the fovea does not have a single locus with a maximal ISD. Instead, the fovea has multiple rather than a single high ISD center. This finding may explain the observation that humans do not appear to have a single preferred fixation point (Putnam et al, 2005). A possible advantage of not having a fovea with a single high density locus is that it decreases the probability that a small foveal lesion would completely eliminate high visual acuity. The results also suggest that the earliest born cones, presumably at the center of the fovea, may not have the narrowest IS since narrow IS are scattered across the fovea.
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