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Prior knowledge on the illumination position

Visual perception is fundamentally ambiguous because an infinite number of three-dimensional scenes are consistent with our retinal images. To circumvent these ambiguities, the visual system uses prior knowledge such as the assumption that light is coming from above our head. The use of such assumptions is rational when these assumptions are related to statistical regularities of our environment. In confirmation of previous visual search experiments, the present authors demonstrate, using 20 participants (aged 18-54 yrs) from the University of Glasgow, that the assumption on the illumination position is in fact biased to the above-left rather than directly above. This bias to the left reaches 26 degrees on average in a more direct shape discrimination task. Both right-handed and left-handed observers have a similar leftward bias. They discuss the possible origins of this singular bias on the illumination position. (PsycINFO Database Record (c) 2005 APA, all rights reserved)



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