Cubes in Light: Lighting Direction and Visual Field Modulate the Perceived Intensity of Illumination
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 the direction of lighting influences the perceived intensity of illumination (brightness) of objects. 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 (±24.4o azimuth; ±90o elevation). Simulated illumination intensity possessed 15 levels, where mean cube luminance ranged from 1.31-3.45 cd/m2. The “reference” array was always illuminated from above-left at mid-intensity (2.38 cd/m2); matching arrays were illuminated from all four directions. Reference and matching arrays appeared in the left and right visual field, respectively, or vice versa. Using the method of constant stimuli we determined the virtual illumination level of the 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 (brighter) when they were situated in the left visual field (p = .017), and when they were illuminated from below (p = .001), and from the left (p = .001). Interactions of modest strength included that the effect of illumination elevation was greater for arrays situated in the right visual field (p = .05), and the effect of illumination azimuth was greatest for arrays situated in the left visual field (p = .042). We propose that objects lit from below appear more intensely illuminated than ones lit from above because of long-term adaptation to downward lighting. The amplification of perceived intensity of illumination for stimuli situated in the left visual field is likely due to attentional enhancement.