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Tony Redmond, Usha Halai, David F. Garway-Heath, Roger S. Anderson; The Effect of Stray Light on the Critical Summation Area in the Peripheral Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5503.
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We have previously suggested the absence of an age-related change in the critical summation area (Ricco’s area) owes to increased intraocular straylight (enlarged point-spread function) masking any expected increase in Ricco’s area resulting from age-related neural decline (Redmond et al, IOVS 2010;51:6533-6539). Here we test this hypothesis
Achromatic spatial summation curves for circular stimuli were measured three times, and averaged, for three psychophysically-experienced healthy observers (aged 27, 30 and 45) in four retinal locations, at 15 degrees from the fovea. Ricco’s area was determined using two-phase regression analysis. Experiments were repeated using a white opacity filter, which was initially characterised in terms of wide-angle scatter and corresponding simulated age using the Oculus C-Quant instrument (Oculus, Wetzlar, Germany). Changes in Ricco’s area were compared to those expected for a decline in ganglion cell density over the same simulated age-range.
The filter increased average intraocular straylight by 0.96 log units (a simulated age increase of 76 years). Introduction of the filter led to a reductin in sensitivity to all stimulus sizes and decreased the average Ricco’s area by -0.21 log deg2 (p<0.05). Assuming that Ricco’s area covers a constant number of retinal ganglion cells, our previous psychophysical data on age-related decline in ganglion cell density predicts an age-related increase in Ricco’s area of 0.22 log deg2 (Redmond et al, IOVS 2010;51:6533-6539).
Ricco’s area decreases as intraocular straylight increases. This change in Ricco’s area appears to be equal and opposite to that expected for a decline in ganglion cell density over a given age range, and may account in part for the absence of an observable age-related enlargement of Ricco’s area under normal photopic conditions. Measurements of straylight may be beneficial clinically for greater precision in the interpretation of perimetric results and for analysis of the structure/function relationship with age and in glaucoma.
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