Abstract
A horizontally moving grating viewed through a diamond-shaped aperture appears to move either upwards or downwards if binocular depth cues (disparity and monocular half-occlusions) are used to simulate partial occlusion of that grating (Duncan, Albright and Stoner, 2000, J Neurosci 20:5885). For these ‘barber diamond’ stimuli, the grating is perceived to move towards (and slide under) the occluding panels.
We asked whether depth from shadows is as effective as binocular cues in biasing perceived direction. We created barber diamond movies in which the depth order of the surrounding panels was based on either binocular cues or shadows. In movies with shadows, all image features had zero disparity. Subjects (n=11) viewed all movies through anaglyph glasses and reported the perceived direction of the gratings by adjusting the orientation of a bar.
Reports in the direction of near panels significantly outnumbered those in the direction of the far panels (binomial proportions test, p < 0.05) for all subjects for both shadowed and binocularly cued movies. The directional bias for the two types of depth cues was indistinguishable for each subject (binomial proportions test, p < 0.05 in all cases). These results indicate that shadows can be at least as powerful as binocular cues in biasing perceived direction of motion. Since directional responses of area MT neurons have been shown to reflect the perceived direction for barber diamond stimuli with binocular depth cues, this raises the question of whether the effect of shadows is also present in that cortical area.