Abstract
Viewing a real scene or a stereoscopic image (e.g., 3D movies) with both eyes yields a vivid subjective impression of object solidity, tangibility, immersive negative space and sense of realness; something that is not experienced when viewing single pictures of 3D scenes normally with both eyes. This phenomenology, sometimes referred to as stereopsis, is conventionally ascribed to the derivation of depth from the differences in the two eye’s images (binocular disparity). Here we report on a pilot study designed to explore if dissociable neural activity associated with the phenomenology of realness can be localized in the cortex. In order to dissociate subjective impression from disparity processing, we capitalized on the finding that the impression of realness associated with stereoscopic viewing can also be generated when viewing a single picture of a 3D scene with one eye through an aperture. Under a blocked fMRI design, subjects viewed intact and scrambled images of natural 3-D objects, and scenes under three viewing conditions: (1) single pictures viewed normally with both eyes (binocular); (2) single pictures viewed with one eye through an aperture (monocular-aperture); and (3) stereoscopic anaglyph images of the same scenes viewed with both eyes (binocular stereopsis). Fixed-effects GLM contrasts aimed at isolating the phenomenology of stereopsis demonstrated a selective recruitment of similar posterior parietal regions for both monocular and binocular stereopsis conditions. Our findings provide preliminary evidence that the cortical processing underlying the subjective impression of realness may be dissociable and distinct from the derivation of depth from disparity.
Highlights
A fundamental insight of early enquiries into visual space perception was that only certain types of visual stimulation give rise to a perceptual state in which space and objects appear visually “real” and in vivid “depth relief.” Renaissance artists correctly observed that viewing realistically rendered and optically correct perspective images of 3D scenes produced an accurate impression of 3-dimensionality, but that they lacked the phenomenological sense of object solidity and realness that is obtained in natural scenes (Wade et al, 2001)
The sensation of realness associated with binocular stereopsis is one of the central attributes of human perceptual experience of the visual world and has been the subject of enquiry since the very beginning of empirical science
We measured the amplitude of the BOLD signal while participants passively viewed images of natural scenes or objects under three different viewing conditions (BP, stereoscopic anaglyph (SA), and monocular-aperture pictorial (MaP))
Summary
A fundamental insight of early enquiries into visual space perception was that only certain types of visual stimulation give rise to a perceptual state in which space and objects appear visually “real” and in vivid “depth relief.” Renaissance artists correctly observed that viewing realistically rendered and optically correct perspective images (paintings) of 3D scenes produced an accurate impression of 3-dimensionality, but that they lacked the phenomenological sense of object solidity and realness that is obtained in natural scenes (Wade et al, 2001). Since the Renaissance, it has been widely observed and reported that this same phenomenology of solidity, tangibility, negative space, and realness can be obtained under conditions in which neither binocular disparity nor motion parallax are present (Claparède, 1904; Ames, 1925; Schlosberg, 1941; Michotte, 1948; Koenderink et al, 1994; Koenderink, 1998; Vishwanath and Hibbard, 2013; da Vinci, cited in Wheatstone, 1838; Wade et al, 2001; Wijntjes, 2017); an observation that has been recently confirmed empirically in naïve observers, when viewing single images monocularly through an aperture (Vishwanath and Hibbard, 2013) This suggests that the visual phenomenology of stereopsis is not tied to binocular disparity or parallax processing but derives from a more general visual property or process. We provide an initial exploratory investigation to determine if cortical processes underlying the subjective visual phenomenology associated with real and stereoscopic 3D scenes (stereopsis) can be dissociated, localized, and distinguished from disparity processing
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