Abstract
Assumed lighting direction in cast-shadow interpretation was investigated. Experiment 1 used an ambiguous object–shadow-matching task to measure bias in shadow-matching direction. The shadow-matching bias was largest when the lighting direction was on average 38.3° left from above (a median of 25.1°). Experiment 2 tested the effect of body posture (head orientation) on cast-shadow interpretation using stimuli aligned in a head-centrically vertical or horizontal orientation. The below-shadow (light-from-above) bias in the head-centric frame was robust across the sitting upright, reclining-on-the-left-side, reclining-on-the-right-side, and supine conditions. A right-shadow (light-from-left) bias in the head-centric frame was found for the sitting upright and reclining-on-the-right-side conditions. In the reclining-on-the-left-side condition, shadow biases to the gravitational below direction and head-centric right direction may have cancelled each other out. These results are consistent with findings from previous shape-from-shading studies, suggesting that the same light-source assumption is applied to shading and shadow interpretations.
Highlights
Shading and cast shadows are known as pictorial depth cues
These characteristics may be common to both shading and cast-shadow processing and suggest that the lighting assumption is shared by both processes
The mechanism underlying the tilt of assumed lighting direction from the top remains to be elucidated in both the shape-from-shading and cast-shadow-matching paradigms
Summary
Shading and cast shadows are known as pictorial depth cues. Such cues allow the observer to recover the three-dimensional structure of the world. This study investigated lighting direction assumptions used in the interpretation of ambiguous cast-shadow displays and the frame of reference such assumptions are based on. Koizumi, Ito, Sunaga, and Ogawa (2017) demonstrated the existence of a below-shadow bias (i.e., the light-from-above assumption) using ambiguous cast-shadow stimuli in which disks and shadows were vertically aligned As they did not include obliquely aligned stimuli, they failed to test a possible light-from-above-left assumption for the cast-shadow interpretation. The light-from-above assumption in the shape-from-shading paradigm can be explained by the position of the sun, which is usually above the observer’s head during the day (Ramachandran, 1988b). This assumption can sometimes be inappropriate when interpreting a cast shadow. To the best of our knowledge, this is the first study on this topic
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