Effect of Body Posture on the Interpretation of Cast Shadows

[Speaker] Koizumi, Tomomi:1
[Co-author] Ito, Hiroyuki:2, Sunaga, Shoji:2, Tomimatsu, Erika:2
1:Graduate School of Design, Kyushu University (Japan), 2:Faculty of Design, Kyushu University (Japan)

We investigated the light-from-above (or light-from-left) assumption in interpreting cast shadows, especially, which of the head or gravitational coordinate system the assumption depended on.
Our stimuli included five grey disks and four blurred shadows between the disks arranged vertically or horizontally. With four observation postures (sitting, lying-on-left, right and back), we measured relative strength of perceptual correspondence between the center disk and the adjacent cast shadows.
The results showed a light-from-above bias in the head coordinate system under the sitting, lying-on-left and lying-on-back conditions and a light-from-left bias only under the sitting condition. We conclude that the light-from-above (or light-from-left) assumption is effective when the head and gravitational coordinates are consistent. When the head is not upright, the assumption with the head coordinate system becomes less effective possibly because the visual system refers to both coordinate systems.
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