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Yun Chen, Weizhong Lan, Frank Schaeffel; Foveal blue scotoma correlated with the shape of the foveal pit. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3000.
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Humans perceive foveal blue scotomas as a black spot when looking through a blue cut-off filter (i.e. BG12, cut-off at 480 nm) at a computer screen on which the brightness is sinusoidally modulated at about 1Hz. Since there is large inter-individual variability in the diameters of the perceived blue scotomas, we have studied whether differences in the shape of the foveal pit may partially account for this variability.
9 subjects were asked to draw foveal blue scotomas, one eye after the other, on a plastic foil that was attached on a TFT display in a dark room. The procedure was repeated four times and an average drawing was generated by the public software ImageJ (http://rsb.info.nih.gov/ij/). Foveal shape was measured in the horizontal plane by high-resolution OCT. Differences in retinal thickness were measured between the center of the fovea and 4 parafoveal positions (0.4 and 0.8mm away from the center). In addition, the distribution of the macular pigment was measured by a Non-Mydriatic Retinal Camera. Both foveal shapes and macular pigment distributions were correlated to the dimensions of the perceived blue scotomas.
Two versions of blue scotomas were observed in our subjects: a round small spot with a diameter of about 30 minutes of arc, or a disc of about 120arcmin, sometimes surrounded by star-shaped radial extensions. The dimensions of the blue scotomas in both eyes were highly correlated (horizontal diameters: R=0.969, p< 0.01; vertical: R=0.976, p< 0.01). In OCT scans, the average thickness of the central fovea was 0.253 mm±0.023 mm in the right eyes and 0.257 mm±0.039 mm in the left. Strikingly, the dimensions of the perceived blue scotomas were significantly correlated to the differences in retinal thickness between foveal center and at 0.4 mm or 0.8 mm away (correlation coefficients ranging from R=0.609 to R=0.741 in all measured positions, p<0.05 in all cases; both eyes pooled, df=17, p<0.01). No correlations were found between the diameters of the blue scotomas and the distribution of the macular pigment.
Based on their diameters, the perceived blue scotomas in the 9 subjects fell into two categories. The large inter-individual variability was partially explained by differences in the shapes of the foveal pit. The steeper the slopes of the foveal pit, the larger the blue scotomas. An interesting question is how foveal shape and blue cone distributions interact during development.
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