Abstract
When interpreting object shape from shading the visual system exhibits a strong bias that illumination comes from above and slightly from the left. We asked whether such biases in the perceived direction of illumination might also influence its perceived intensity. Arrays of nine cubes were stereoscopically rendered where individual cubes varied in their 3D pose, but possessed identical triplets of visible faces. Arrays were virtually illuminated from one of four directions: Above-Left, Above-Right, Below-Left, and Below-Right (±24.4° azimuth; ±90° elevation). Illumination intensity possessed 15 levels, resulting in mean cube array luminances ranging from 1.31–3.45 cd/m2. A “reference” array was consistently illuminated from Above-Left at mid-intensity (mean array luminance = 2.38 cd/m2). The reference array's illumination was compared to that of matching arrays which were illuminated from all four directions at all intensities. Reference and matching arrays appeared in the left and right visual field, respectively, or vice versa. Subjects judged which cube array appeared to be under more intense illumination. Using the method of constant stimuli we determined the illumination level of matching arrays required to establish subjective equality with the reference array as a function of matching cube visual field, illumination elevation, and illumination azimuth. Cube arrays appeared significantly more intensely illuminated when they were situated in the left visual field (p = 0.017), and when they were illuminated from below (p = 0.001), and from the left (p = 0.001). An interaction of modest strength was that the effect of illumination azimuth was greater for matching arrays situated in the left visual field (p = 0.042). We propose that objects lit from below appear more intensely illuminated than identical objects lit from above due to long-term adaptation to downward lighting. The amplification of perceived intensity of illumination for stimuli situated in the left visual field and lit from the left is best explained by tonic egocentric and allocentric leftward attentional biases, respectively.
Highlights
There was a significant interaction between visual field and illumination azimuth [F(1, 19) = 4.74, p = 0.042, η2 = 0.200] such that the effect of matching cube illumination azimuth was significant for matching arrays situated in the LVF [t(19) = 4.31, p < 0.001], but not the RVF [t(19) = 0.99, p = 0.337]
EFFECT OF VISUAL FIELD Cube arrays situated in the left visual field appear significantly more intensely illuminated than identical arrays in the right visual field
EFFECT OF ILLUMINATION AZIMUTH Cube arrays lit from the left appear more intensely illuminated than identical arrays lit from the right
Summary
When interpreting the shape of ambiguous 3D surfaces the visual system exhibits a bias that directional illumination is mostly from above (Ramachandran, 1988; Sun and Perona, 1996a,b, 1998; Mamassian and Goutcher, 2001; Stone et al, 2009; de Montalembert et al, 2010; Morgenstern et al, 2011; Schofield et al, 2011; Andrews et al, 2013) and slightly from the left (Sun and Perona, 1998; Mamassian and Goutcher, 2001; Mamassian et al, 2003; McManus et al, 2004; Thomas et al, 2008; de Montalembert et al, 2010; Andrews et al, 2013). The light-fromabove bias is well illustrated by experiments demonstrating that discs with top-dark luminance gradients are seen as concavities while those with top-bright luminance gradients are perceived as convexities (Ramachandran, 1988). Search is typically efficient for items distinguished from distractors by a single feature These results imply that the attribute of being lit from below is a sufficiently uncommon stimulus attribute to be afforded status as a featural cue. Another potentially related phenomenon is the finding that discs possessing top-dark luminance gradients, which are seen as concavities due to the light-from-above assumption, appear to possess higher contrast than identical top-bright discs which are seen as convexities (Chacon, 2004). The mechanism resulting in the higher contrast appearance for the top-dark luminance gradients, remains unclear
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