Induction of monocular stereopsis by altering focus distance : a test of Ames's hypothesis

Viewing a real 3-dimensional scene or a stereoscopic image with both eyes generates a vivid phenomenal impression of depth known as stereopsis. Numerous reports have highlighted the fact that an impression of stereopsis can be induced in the absence of binocular disparity. A method claimed by Ames (1925) involved altering accommodative (focus) distance while monocularly viewing a picture. This claim was tested on naïve observers using a method inspired by the observations of Gogel and Ogle on the equidistance tendency. Consistent with Ames’s claim, most observers reported that the focus manipulation induced an impression of stereopsis comparable to that obtained by monocular-aperture viewing.Publisher PDFPeer reviewe

Advancing a new theory of stereopsis: Reply to Rogers (2019)

Vishwanath (2014) presented analyses and proposed conjectures aimed at a unified understanding of both qualitative and quantitative aspects of stereopsis in pictorial and natural (real-world) 3-dimensional (3D) vision. A recent commentary by Rogers (2019) conceded the key argument in the article, that stereopsis can be induced in the absence of binocular disparity and motion parallax but criticized the wider analyses and conjectures. Rogers argued that a focus on visual appearance and qualitative aspects of 3D perception is unproductive and that the analysis of pictorial space perception adds little to our wider understanding of 3D vision. I argue here that the critique is not persuasive as it misconstrues the distinction between qualitative and quantitative aspects of perception and its claims regarding pictorial depth perception rely on introspections that often do not align with the empirical record. I reaffirm that an integrative focus on both qualitative and quantitative aspects of both pictorial and natural 3D perception is crucial for advancing an understanding of the complex phenomenon of stereopsis.PostprintPeer reviewe

A shape-level flanker facilitation effect in contour integration and the role of shape complexity

This work by funded by an EPSRC doctoral training grant at the University of St Andrews.The detection of an object in the visual field requires the visual system to integrate a variety of local features into a single object. How these local processes and their global integration is influenced by the presence of other shapes in the visual field is poorly understood. The detectability (contour integration) of a central target object in the form of a two dimensional Gaborized contour was compared in the presence or absence of nearby surrounding objects. A 2-AFC staircase procedure added orientation jitter to the constituent Gabor patches to determine the detectability of the target contour. The set of contours was generated using shape profiles of everyday objects and geometric forms. Experiment 1 examined the effect of three types of congruencies between the target and two flanking contours (contour shape, symmetry and familiarity). Experiment 2 investigated the effect of varying the number and spatial positions of the flankers. In addition, a measure of shape complexity (reciprocal of shape compactness) was used to assess the effects of contour complexity on detection. Across both experiments the detectability of the target contour increased when the target and flanker had the same shape and this was related to both the number of flankers and the complexity of the target shapes. Another factor that modulated this shape-level flanker facilitation effect was the presence of symmetry. The overall results are consistent with a contour integration process in which the visual system incorporates contextual information to extract the most likely smooth contour within a noise field.PostprintPeer reviewe

Localization of shapes: eye movements and perception compared

AbstractThe localization of spatially extended objects is thought to be based on the computation of a default reference position, such as the center of gravity. This position can serve as the goal point for a saccade, a locus for fixation, or the reference for perceptual localization. We compared perceptual and saccadic localization for non-convex shapes where the center of gravity (COG) was located outside the boundary of the shape and did not coincide with any prominent perceptual features. The landing positions of single saccades made to the shape, as well as the preferred loci for fixation, were near the center of gravity, although local features such as part boundaries were influential. Perceptual alignment positions were also close to the center of gravity, but showed configural effects that did not influence either saccades or fixation. Saccades made in a more naturalistic sequential scanning task landed near the center of gravity with a considerably higher degree of accuracy (mean error <4% of saccade size) and showed no effects of local features, constituent parts, or stimulus configuration. We conclude that perceptual and oculomotor localization is based on the computation of a precise central reference position, which coincides with the center of gravity in sequential scanning. The saliency of the center of gravity, relative to other prominent visual features, can depend on the specific localization task or the relative configuration of elements. Sequential scanning, the more natural of the saccadic tasks, may provide a better way to evaluate the “default” reference position for localization. The fact that the reference position used in both oculomotor and perceptual tasks fell outside the boundary of the shapes supports the importance of spatial pooling, in contrast to local features, in object localization

Color determinants of surface stratification

Although color plays a crucial role in the demarcation of surfaces in the visual field, its role in depth perception is not well understood. Certain special effects of color on depth perception that arise from optical factors such as chromatic aberration (chromostereopsis) have been studied, but less is known about the role of perceptual factors of color in determining depth relations. The present study explores the role that the different attributes of color such as hue, chroma, and lightness play in the stratification of surfaces in depth. In two experiments, subjects manipulated specific dimensions of colors (hue, chroma, lightness, and whiteness) while making judgments of coplanarity of either two or more abutting surfaces. The results demonstrate that for surfaces to appear coplanar, their lightness has to be proportional to the natural (intrinsic) lightness of the hues. No meaningful effects of chroma, whiteness, or blackness were found in depth stratification. The results suggest a primary role of the natural lightness of hues in depth perception.PostprintPeer reviewe

Dissociating neural activity associated with the subjective phenomenology of monocular stereopsis : an EEG study

Support for DV and MU was provided by the Leverhulme Trust Research Project Grant (Grant Reference RGP-2016-269).The subjective phenomenology associated with stereopsis, of solid tangible objects separated by a palpable negative space, is conventionally thought to be a by-product of the derivation of depth from binocular disparity. However, the same qualitative impression has been reported in the absence of disparity, e.g., when viewing pictorial images monocularly through an aperture. Here we aimed to explore if we could identify dissociable neural activity associated with the qualitative impression of stereopsis, in the absence of the processing of binocular disparities. We measured EEG activity while subjects viewed pictorial (non-stereoscopic) images of 2D and 3D geometric forms under four different viewing conditions (Binocular, Monocular, Binocular aperture, Monocular aperture). EEG activity was analysed by oscillatory source localization (beamformer technique) to examine power change in occipital and parietal regions across viewing and stimulus conditions in targeted frequency bands (alpha: 8–13Hz & gamma: 60–90Hz). We observed expected event-related gamma synchronization and alpha desynchronization in occipital cortex and predominant gamma synchronization in parietal cortex across viewing and stimulus conditions. However, only the viewing condition predicted to generate the strongest impression of stereopsis (monocular aperture) revealed significantly elevated gamma synchronization within the parietal cortex for the critical contrasts (3D vs. 2D form). These findings suggest dissociable neural processes specific to the qualitative impression of stereopsis as distinguished from disparity processing.PostprintPeer reviewe

Eye fixation during multiple object attention is based on a representation of discrete spatial foci

We often look at and attend to several objects at once. How the brain determines where to point our eyes when we do this is poorly understood. Here we devised a novel paradigm to discriminate between different models of spatial selection guiding fixation. In contrast to standard static attentional tasks where the eye remains fixed at a predefined location, observers selected their own preferred fixation position while they tracked static targets that were arranged in specific geometric configurations and which changed identity over time. Fixations were best predicted by a representation of discrete spatial foci, not a polygonal grouping, simple 2-foci division of attention or a circular spotlight. Moreover, attentional performance was incompatible with serial selection, suggesting that attentional selection and fixation share the same spatial representation. Together with previous findings on fixational microsaccades during covert attention, our results suggest a more nuanced definition of overt vs. covert attention.Publisher PDFPeer reviewe

Identifying cortical substrates underlying the phenomenology of stereopsis and realness : a pilot fMRI study

Funding information: support for DV and MU was provided by the Leverhulme Trust Research Project Grant (Grant Reference RPG-2016- 269).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 localised in the cortex. In order to dissociate subjective impression from disparity processing, we capitalised 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); (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.Publisher PDFPeer reviewe

Toward a new theory of stereopsis

Humans can obtain an unambiguous perception of depth and 3-dimensionality with 1 eye or when viewing a pictorial image of a 3-dimensional scene. However, the perception of depth when viewing a real scene with both eyes is qualitatively different: There is a vivid impression of tangible solid form and immersive negative space. This perceptual phenomenon, referred to as “stereopsis,” has been among the central puzzles of perception since the time of da Vinci. After Wheatstone’s invention of the stereoscope in 1838, stereopsis has conventionally been explained as a byproduct of binocular vision or visual parallax. However, this explanation is challenged by the observation that the impression of stereopsis can be induced in single pictures under monocular viewing. Here I propose an alternative hypothesis that stereopsis is a qualitative visual experience related to the perception of egocentric spatial scale. Specifically, the primary phenomenal characteristic of stereopsis (the impression of “real” separation in depth) is proposed to be linked to the precision with which egocentrically scaled depth (absolute depth) is derived. Since conscious awareness of this precision could help guide the planning of motor action, the hypothesis provides a functional account for the important secondary phenomenal characteristics associated with stereopsis: the impression of interactability and realness. By linking stereopsis to a generic perceptual attribute, rather than a specific cue, it provides a potentially more unified account of the variation of stereopsis in real scenes and pictures and a basis for understanding why we can perceive depth in pictures despite conflicting visual signals.PostprintPeer reviewe

From pictures to reality:modelling the phenomenology and psychophysics of 3D perception

The dominant inferential approach to human 3D perception assumes a model of spatial encoding based on a physical description of objects and space. Prevailing models based on this physicalist approach assume that the visual system infers an objective, unitary and mostly veridical representation of the external world. However, careful consideration of the phenomenology of 3D perception challenges these assumptions. I review important aspects of phenomenology, psychophysics and neurophysiology which suggest that human visual perception of 3D objects and space is underwritten by distinct and dissociated spatial encodings that are optimized for specific regions of space. Specifically, I argue that 3D perception is underwritten by at least three distinct encodings for (1) egocentric distance perception at the ambulatory scale, (2) exocentric distance (scaled depth) perception optimized for near space, and (3) perception of object shape and layout (unscaled depth). This tripartite division can more satisfactorily account for the phenomenology, psychophysics and adaptive logic of human 3D perception