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Arthur Lugtigheid, Andrew Welchman; A surprising influence of retinal size on disparity-defined distance judgments. Journal of Vision 2010;10(7):63. doi: 10.1167/10.7.63.
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© ARVO (1962-2015); The Authors (2016-present)
From simple geometry, the retinal projection of an object in the environment depends on both its size and distance from an observer. Thus, given a sensed retinal size, the brain should not know the distance of the object: the retinal measurement is compatible with infinite combinations of physical sizes and distances. A number of previous studies have demonstrated that, in the absence of visual cues to distance, an object with a larger retinal image is perceived as closer than an object with a smaller retinal size. However, when binocular disparity information is available, observers should be able to judge the distance between two objects accurately. Here we report the seemingly surprising result that the retinal size influences observers' judgments of disparity-defined distance. Our stimuli consisted of large and small discs surrounded by a peripheral reference volume of textured cubes that provided a continuous reference frame to support reliable disparity estimates. Observers judged which of two sequentially-presented stimuli of different retinal sizes was closer to them. Disparity-defined depth was varied parametrically to measure psychometric functions. Our results showed a shift in the PSE of around 5 cm, so that large objects were seen as closer than small objects when disparity-defined distance was the same. In contrast, there was no bias when two objects of equal size were presented. Varying the ratio of object sizes, and testing objects placed at different distances revealed that bias increased as (i) the viewing distance increased; and (ii) the ratio of the object sizes increased. We propose that the retinal size of an object is probabilistically related to its distance in the environment and this information is combined with disparity when making judgments of distance.
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