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T. Redmond, D. F. Garway-Heath, M. B. Zlatkova, R. S. Anderson; Elevated Perimetric Thresholds in Early Glaucoma Are Matched to an Enlargement of the Area of Complete Spatial Summation. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5507.
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© ARVO (1962-2015); The Authors (2016-present)
Ricco’s law of complete spatial summation states that, for a range of small stimuli, stimulus area x intensity = k at threshold. The area of complete spatial summation (Ricco’s area) is the largest stimulus size for which this law holds and has previously been shown to alter in size with differing adaptive states and with retinal eccentricity, possibly as a noise-compensation mechanism. We wished to investigate if Ricco’s area changes in early glaucoma to account for the decreased visual signal/noise ratio that may accompany retinal ganglion cell loss.
We measured spatial summation functions at four retinal locations (10 degrees eccentricity) in 24 patients with early glaucoma (13 POAG, 11 NTG; average TD at test locations: -1.4dB) and 26 age-similar controls (of an initial estimate of 55 per group), using achromatic stimuli and S-cone selective stimuli. Ricco’s area was determined by two-phase regression analysis. Peripheral achromatic grating resolution acuity (PGRA) was measured in the same locations to estimate local functional ganglion cell sampling density.
PGRA was significantly reduced in glaucoma patients, compared with controls (p<0.001). A lateral shift of the averaged spatial summation curve (and enlargement of Ricco’s area) was observed for glaucoma patients, relative to controls for both stimulus types (achromatic: 0.65 log units, p < 0.01; chromatic: 0.26 log units, p < 0.01), with negligible vertical shift (0.01 log units; p > 0.5). When this lateral shift was accounted for, spatial summation curves for patients and controls overlapped indistinguishably.
Elevated perimetric thresholds in early glaucoma are completely matched to an enlargement of Ricco’s area to maintain a constant threshold at Ricco’s area. This finding suggests an increase in signal pooling by second stage spatial filters in response to ganglion cell loss owing to the disease. Furthermore, our findings suggest that perimetric stimuli should be designed to modulate in size (either entirely, or in conjunction with contrast), in order to boost the glaucoma signal within measurement noise.
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