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Wendy Adams; Frames of reference for the light-from-above prior in visual search and shape judgements. Journal of Vision 2007;7(9):836. doi: 10.1167/7.9.836.
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© ARVO (1962-2015); The Authors (2016-present)
Faced with highly complex and ambiguous visual input, human observers must rely on prior knowledge and assumptions to efficiently determine the structure of their surroundings. One of these assumptions is the ‘light-from-above’ prior. In the absence of explicit light-source information, the visual system assumes that the light source is roughly overhead. A simple, low-cost strategy, would place this ‘light-from-above’ prior in a retinal frame of reference. A more complex, but optimal strategy would be to assume that the light source is gravitationally up, and compensate for observer orientation. Evidence to support one or other strategy from psychophysics and neurophysiology has been mixed. This study pits the gravitational and retinal frames against each other in two different visual tasks that relate to the light-from-above prior. Observers performed either a visual search task with shape-from-shading (SFS) stimuli, or made convex / concave shape judgments of simple SFS objects at various head orientations. In both tasks the retinal frame-of-reference dominated.Visual search behaviour with SFS stimuli was modulated purely by stimulus orientation relative to the retina. However, the gravitational frame of reference had a significant effect on shape judgements, with a 30% correction for observer orientation. These results are consistent with current neurophysiological data on SFS if we re-frame compensation for observer orientation as a cue-combination problem.
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