| Summary: | The "light-from-above" prior is invoked to simplify and expedite complex visual processing. This prior is observed in visual search and shape judgments with shaded stimuli, where perceived shape and ease of target identification are both affected by stimulus orientation. In addition, perceived surface reflectance varies with surface orientation in a manner consistent with assumed overhead lighting. Do the light-priors exhibited by these different tasks have the same underlying mechanism or even lighting direction? Some evidence has suggested that an "above-left" rather than "above" prior guides behavior in some tasks, but not others. In the current study, the "light-from-above" prior was measured using visual search, shape perception, and a novel reflectance-judgment task. There were substantial differences between observers. However, strong positive correlations were found between the light-priors measured using all three tasks. The data imply that a single mechanism is responsible for a light-from-above prior in "quick and dirty" visual search behavior, shape perception, and reflectance judgments. Furthermore, the data support the notion that perceived shape is the preattentive feature in visual search with shaded targets.
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