Experiential3D: Four Illusions Challenge Our 3D Visual Experience

Four illusions suggest that many 3D visual processes are cognitive rather than perceptual: 1. LINTON STEREO ILLUSION: A back circle (at 50cm) and a front circle (at 40cm) appear to move rigidly together in depth (whilst their angular size is fixed) when their separation is kept constant in retinal disparity (moving to 40cm and 33cm) rather than physically constant (40cm and 30cm). This suggests that perceived stereo depth reflects retinal disparities, and depth constancy is cognitive rather than perceptual. 2. LINTON SCALE ILLUSION: Increasing interpupillary distance using a telestereoscope makes scenes seem miniature. We develop a VR telestereoscope that decouples increases of vergence and vertical disparities from increases in horizontal disparities. When horizontal disparities are increased, but vergence and vertical disparity are normal, the scene looks miniature. When vergence and vertical disparity are increased, but horizontal disparities are normal, the scene looks normal. This suggests that visual scale relies on a cognitive association between accentuated stereo shape and closer distances. 3. LINTON UN-HOLLOW FACE and LINTON MORPHING FACE ILLUSIONS: Using VR, we argue perceived depth is not inverted in the Hollow-Face illusion: First, objects placed in the hollow of the Hollow-Face illusion can be seen veridically in depth against the Hollow-Face illusion. Second, if we put points on the tip and base of the nose, and gradually switch the position of the two eyes (morphing from hollow to protruding), the change in relative depth between these points is seen veridically. This suggests that depth cue integration is cognitive rather than perceptual. 4. LINTON SIZE CONSTANCY and LINTON SHAPE CONSTANCY ILLUSIONS: Placing a solid rectangular frame around instances of pictorial size constancy, stereo size constancy, and stereo shape constancy all demonstrate that these constancies can be experienced without affecting perceived angular size, suggesting that these constancies are cognitive rather than perceptual.