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Susan Kelly, Yi Pang; Suppression of the Implicit Surface. Invest. Ophthalmol. Vis. Sci. 2013;54(15):571. doi: https://doi.org/.
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A number of authors have reported that human observers can accurately judge the physical distance of visual targets if they are located on a flat ground surface. The correlation between distance judgments measured with the blind-walking technique correlate highly with a target’s physical distance in lighted conditions. However distance judgments of self-illuminated targets in darkness are very inaccurate. Ooi and colleagues have proposed that in both darkness and light, observers judge an object’s distance as the intersection between its angle of declination and the perceived slant of the ground surface. They report that in darkness the human visual system relies on a default ground surface called the implicit surface that tilts upwards towards the horizon with a slant of around 12 deg. Our experiments have revealed another aspect of this implicit surface; it is triggered by the absence of light in the visual field and suppressed by the presence of light in the visual field.
Forty-five visually normal young adults observed a small self-illuminated target in darkness. Subjects were instructed to blind-walk to the remembered location of the target under one of the following conditions: 1) blind-walk in darkness (n=28); 2) blind-walk in presence of white light (n=10) and 3) blind-walk in presence of red light (n=7).
The blind-walking estimates of a target’s distance are much more accurate if the blind-walking takes place in a lighted room versus a dark room (F=18.85, p<0.001) even though in both cases subjects are blind-folded when walking; the only difference between these conditions is the presence or absence of light in the visual field. Pair-wise comparison showed no difference in estimates made in white versus red room illumination (p=0.21) but both differed significantly from the dark condition (p<0.01).
The distance perception of targets viewed in darkness is dramatically affected by the presence or absence of room lighting; when subjects blind-walk to the remembered target location in darkness they rely on the implicit surface and their distance estimates are inaccurate. However, if they blind-walk to the remembered target location with room illumination present then there is no default to the implicit surface and distance perceptions are accurate. Neither the photoreceptors nor the neural pathway responsible for suppressing the implicit surface are known.
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