Lighting direction and visual field modulate perceived intensity of illumination

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.

Characterizing visual asymmetries in contrast perception using shaded stimuli

“…We did not find significant differences between the PSEs obtained in each location, paired t test, t(14) ¼ 0.525, p ¼ 0.6; the averaged PSEs in log units were Reference Left_visual_field ¼ À0.3039, SD ¼ 0.061, and Reference Right_visual_field ¼ À0.317, SD ¼ 0.047. Interestingly, when the reference stimuli were presented on the right side, the stimuli on the left side were perceived with slightly higher contrast in agreement with McCourt et al (2013) findings; however, in our case this difference was not significant.…”

“…Results of conditions 4 and 6 ( Figure 6) show that left-dark and top-dark stimuli were perceived with similar contrast. Previous findings, using 3D cubes presented in a stereo display, have found that arrays of top-dark cubes are perceived more intensely illuminated (higher brightness) than arrays of top-light cubes (McCourt et al, 2013). McCourt et al (2013) suggested that top-dark objects are perceived with higher contrast as a consequence of this increased perceived illumination and also because top-dark stimulus could be interpreted as a convex stimulus illuminated from below.…”

“…Previous findings, using 3D cubes presented in a stereo display, have found that arrays of top-dark cubes are perceived more intensely illuminated (higher brightness) than arrays of top-light cubes (McCourt et al, 2013). McCourt et al (2013) suggested that top-dark objects are perceived with higher contrast as a consequence of this increased perceived illumination and also because top-dark stimulus could be interpreted as a convex stimulus illuminated from below. However, our results from Experiment 4 show that topdark stimuli are perceived with higher contrast than top-light stimuli because there is an attenuation in the perceived contrast of the top-light stimuli while the perceived contrast in the top-dark (left-dark) stimuli was unaffected.…”

“…Continuous red line shows the difference for the three conditions tested in the control experiment [log 10 (0.5)-log 10 (0.4908); the averaged PSEs in log units was À0.309, SD ¼ 0.024; n ¼ 15, five subjects 3 three conditions]. Previous studies have shown that stimuli presented on the left visual field appear more intensely illuminated than the same stimuli presented on the right side (McCourt, Blakeslee, & Padmanabhan, 2013). Thus, we also tested the effect of the location in the visual field, left or right, of the reference stimulus on contrast perception.…”

Luminance gradient configuration determines perceived lightness in a simple geometric illusion

“…Interestingly, under the conditions tested, the spatial location of the gradient checks did not determine the lightness induction effect (but see McCourt et al, 2013 ). We tested two different spatial locations: gradients presented concurrently, in the same row (in the Test and Match checks, Configuration 5), and in parallel, in different rows (in the flankers, Configuration 6).…”