Humans can obtain an unambiguous perception of depth and 3-dimensionality with one 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 by-product 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 phenomenal characteristic associated with stereopsis: the impression of interactability. 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. StereopsisVishwanath 3

A picture viewed from its center of projection generates the same retinal image as the original scene, so the viewer perceives the scene correctly. When a picture is viewed from other locations, the retinal image specifies a different scene, but we normally do not notice the changes. We investigated the mechanism underlying this perceptual invariance by studying the perceived shapes of pictured objects viewed from various locations. We also manipulated information about the orientation of the picture surface. When binocular information for surface orientation was available, perceived shape was nearly invariant across a wide range of viewing angles. By varying the projection angle and the position of a stimulus in the picture, we found that invariance is achieved through an estimate of local surface orientation, not from geometric information in the picture. We present a model that explains invariance and other phenomena (such as perceived distortions in wide-angle pictures).

The 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.

This article presents an experimental study on the naturally biased association between shape and color. For each basic geometric shape studied, participants were asked to indicate the color perceived as most closely related to it, choosing from the Natural Color System Hue Circle. Results show that the choices of color for each shape were not random, that is, participants systematically established an association between shapes and colors when explicitly asked to choose the color that, in their view, without any presupposition, they saw as the most naturally related to a series of given shapes. The strongest relations were found between the triangle and yellows, and the circle and square with reds. By contrast, the parallelogram was connected particularly infrequently with yellows and the pyramid with reds. Correspondence analysis suggested that two main aspects determine these relationships, namely the "warmth" and degree of "natural lightness" of hues.

The tendency of fish to perceive the Ebbinghaus illusion was investigated. Redtail splitfins (Xenotoca eiseni, family Goodeidae) were trained to discriminate between two disks of different sizes. Then, fish were presented with two disks of the same size surrounded by disks of large or small size (inducers) arranged to produce the impression (to a human observer) of two disks of different sizes (in the Ebbinghaus illusion, a central disk surrounded by small inducers appears bigger than an identical one surrounded by large inducers). Fish chose the stimulus that, on the basis of a perception of the Ebbinghaus illusion, appeared deceptively larger or smaller, consistent with the condition of training. These results demonstrate that redtail splitfins tend to perceive this particular illusion. The results are discussed with reference to other related illusions that have been recently observed to be experienced by fish (such as the Navon effect), and with regard to their possible evolutionary implications.

A central function of vision is determining the layout and size of objects in the visual field, both of which require knowledge of egocentric distance (the distance of an object from the observer). A wide range of visual cues can reliably signal relative depth relations among objects, but retinal signals directly specifying distance to an object are limited. A potential source of distance information is the pattern of blurring on the retina, since nearer fixation generally produces larger gradients of blur on the extra-foveal retina. While prior studies implicated blur as only a qualitative cue for relative depth ordering, we find that retinal blur gradients can act as a quantitative cue to distance. Surfaces depicted with blur gradients were judged as significantly closer than those without, with the size of the effect modulated by the degree of blur, as well as the availability of other extra-retinal cues to distance. Blur gradients produced substantial changes in perceived distance regardless of relative depth relations of the surfaces indicated by other cues, suggesting that it operates as a robust cue to distance, consistent with the empirical relationship between blur and fixation distance.

Saccades 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 (SD approximately 10% 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.

Blur in images can create the sensation of depth because it emulates an optical property of the eye; namely, the limited depth of field created by the eye's lens. When the human eye looks at an object, this object appears sharp on the retina, but objects at different distances appear blurred. Advances in gaze-tracking technologies enable us to reproduce dynamic depth of field in regular displays, providing an alternative way of conveying depth. In this paper we investigate gazecontingent depth of field as a method to produce realistic 3D images, and analyze how effectively people can use it to perceive depth. We found that GC DOF increases subjective perceived realism and depth and can contribute to the perception of ordinal depth and distance between objects, but it is limited in its accuracy.