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Tobias Otte, Lothar Spillmann, Kai Hamburger, Florian Brüning, Andreas Mader, Svein Magnussen; Filling-in of the blind spot: How much information is needed?. Journal of Vision 2005;5(8):715. doi: https://doi.org/10.1167/5.8.715.
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© ARVO (1962-2015); The Authors (2016-present)
The blind spot is a fairly large (5° x 7°, centered 16° temporally) area of the retina that contains no photoreceptors. In binocular vision the absence of information is compensated by the other eye, which receives input from the corresponding region of the visual field, but even in monocular vision the blind spot is not subjectively noticeable as the color and texture of surrounding regions are perceptually filled in. It is debated whether this filling-in reflects higher-order cognitive processes or is generated by neural mechanisms at cortical areas where the retinal topography is preserved. We have obtained evidence for the latter hypothesis in a series of experiments on the minimum information required for filling-in. The blind spots of 6 trained observers were carefully mapped on a computer screen, and individually fitted frames of color or texture, having the same shape as the blind spot but varying in width, were generated in Adobe Photoshop. Frame and background luminances were 35.0 and 4.0 cd/m2, respectively. With narrow frames, due to the influence of eye movements and Troxler fading, filling-in might be short-lived and partial, i.e. color or texture invaded part of the blind spot but left minor areas unfilled or “foggy”. Observers rated the area subtended by the filling-in on a ten-step scale. With red, green and blue frames, frame widths broader than 0.26° produced complete filling-in (> 90%) on all trials, and complete filling-in was occasionally observed with frames as narrow as 0.06°, the latter producing a mean filling-in of 80%. With texture, complete filling-in (> 90%) of dot patterns and horizontal as well as vertical gratings (1.5 and 2.3 c/deg) was observed with frame widths of 0.43° and broader, and occasionally with frames of 0.22°, that produced a mean filling-in of about 70%. We suggest that filling-in is generated by local mechanisms of the cortex, analogous perhaps to the mechanisms generating the Craik-O'Brien-Cornsweet illusion.
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