83 research outputs found
The conscious awareness of visual space : a tripartite encoding model
No full textDhanraj Vishwanath was supported by a Leverhulme Foundation Grant RPG-2016-269.The prevailing model of 3D vision proposes that the visual system recovers a single internally consistent representation of physical 3D space based on a process of ideal-observer probabilistic inference. A significant challenge for this model has been in explaining our subjective awareness of visual space. Here it is argued that integrating phenomenological observations, empirical data, evolutionary logic, and neurophysiological evidence leads to the conjecture that the human conscious awareness of visual space is underwritten by multiple, sometimes mutually inconsistent, spatial encodings. By assessing four primary competencies in the conscious awareness of space, three major types of spatial encodings are conjectured. Among the most primitive of these is proposed to support the competency of the conscious awareness of distance at an ambulatory scale and is hypothesized to originate in medial temporal areas in allocortex. The second is proposed to support the competency of awareness of object layout and 3D shape without scale (operationally defined as relative depth) hypothesized to have evolved from primitive encodings that provide a depth-ordered segmentation of the visual field. The third encoding is proposed to support the competency of fine-grained awareness of intra- and inter-object distances and object scale in near space (operationally defined as scaled depth), likely involving the posterior parietal cortex and is conjectured to underlie the phenomenology of tangibility, spatial separation (negative space) and object realness that is often referred to as stereopsis. The first and third encoding are conjectured to contribute to the feeling of spatial immersion and presence.Peer reviewe
Toward a new theory of stereopsis
No full textHumans 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.Peer reviewe
Advancing a new theory of stereopsis: Reply to Rogers (2019)
No full textVishwanath (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.Peer reviewe
Saccadic localization in the presence of cues to three-dimensional shape
No full textSaccades directed to simple two-dimensional (2D) target shapes under instructions to look at the target as a whole land near the center of gravity (COG) of the shape with a high degree of precision (He Kowler, 1991; Kowler Blaser, 1995; McGowan, Kowler, Sharma, Chubb, 1998; Melcher Kowler, 1999; Vishwanath, Kowler, & Feldman, 2000). This pattern of performance has been attributed to the averaging of visual signals across the shape. Natural objects, however, are three-dimensional (3D), and the shape of the object can differ dramatically from its 2D retinal projection. This study examined saccadic localization of computer-generated perspective images of 3D shapes. Targets were made to appear either 2D or 3D by manipulating shading, context, and contour cues. Average saccadic landing positions (SDsimilar to10% eccentricity) fell at either the 2D or 3D COG, and occasionally in between, depending on the nature of the 3D cues and the subject. The results show that saccades directed to objects are not compelled to land at the 2D COG, but can be sensitive to other visual cues, such as cues to 3D structure. One way to account for these results, without abandoning the averaging mechanism that has accounted well for performance with simple 2D shapes, is for saccadic landing position to be computed based on averaging across a weighted representation of the shape in which portions projected to be located at a greater distance receive more weight.</p
The Utility of Defocus Blur in Binocular Depth Perception
No full textThe question of whether defocus blur is a quantitative cue for depth perception is a topic of renewed interest. A recent study suggests that relative defocus blur can be used in computing depth throughout the visual field, particularly in regions where disparity loses precision. However, elements of the study's experimental design and theoretical analysis appear to undermine this claim. First, the study did not provide evidence that blur can be used as a quantitative depth cue. It only measured blur discrimination thresholds, not perceived depth from for blur. Second, the study's conceptualization of the complementary use of blur and disparity, and related conjectures, are based on the specific viewing geometry and fixation distance tested. They do not appear to generalize to natural viewing situations and tasks. I suggest a different way in which defocus blur might affect depth perception. Because depth-of-focus blur is a cue to egocentric distance, it could contribute to quantitative depth perception by scaling depth relations specified by other relative depth cues
Induction of monocular stereopsis by altering focus distance : a test of Ames's hypothesis
No full textViewing 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.Peer reviewe
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