Dynamic Cancellation of Perceived Rotation from the Venetian Blind Effect.

Geometric differences between the images seen by each eye enable the perception of depth. Additionally, depth is produced in the absence of geometric disparities with binocular disparities in either the average luminance or contrast, which is known as the Venetian blind effect. The temporal dynamics of the Venetian blind effect are much slower (1.3 Hz) than those for geometric binocular disparities (4-5 Hz). Sine-wave modulations of luminance and contrast disparity, however, can be discriminated from square-wave modulations at 1 Hz, which suggests a non-linearity. To measure this non-linearity, a luminance or contrast disparity modulation was presented at a particular frequency and paired with a geometric disparity modulation that cancelled the perceived rotation induced by the luminance or contrast modulation. Phases between the luminance or contrast and the geometric modulation varied in 50 ms increments from -200 and 200 ms. When phases were aligned, observers perceived little or no rotation. When not aligned, a perceived rotation was induced by a contrast or luminance disparity that was then cancelled by the geometric disparity. This causes the perception of a slight jump. The Generalized Difference Model, which is linear in time, predicted a minimal probability in cases when luminance or contrast disparities occurred before the geometric disparities due to the slower dynamics of the Venetian blind effect. The Gated Generalized Difference Model, which is non-linear in time, predicted a minimal probability for offsets of 0 ms. Results followed the Gated model, which further suggests a non-linearity in time for the Venetian blind effect.

Convexity Bias and Perspective Cues in the Reverse-Perspective Illusion.

The present experiment was designed to examine the roles of painted linear perspective cues, and the convexity bias that are known to influence human observers' perception of three-dimensional (3D) objects and scenes. Reverse-perspective stimuli were used to elicit a depth-inversion illusion, in which far points on the stimulus appear to be closer than near points and vice versa, with a 2 (Type of stimulus) × 2 (Fixation mark position) design. To study perspective, two types of stimuli were used: a version with painted linear perspective cues and a version with blank (unpainted) surfaces. To examine the role of convexity, two locations were used for the fixation mark: either in a locally convex or a locally concave part of each stimulus (painted and unpainted versions). Results indicated that the reverse-perspective illusion was stronger when the stimulus contained strong perspective cues and when observers fixated a locally concave region within the scene.

Ponzo's Illusion in 3D: Perspective Gradients Dominate Differences in Retinal Size.

A common form of the Ponzo illusion involves two test probes of equal size, embedded in a planar linear perspective painting depicting a three-dimensional (3D) scene, where the probe perceived to be farther is judged to be larger than the probe perceived closer to the viewer. In this paper, the same perspective 3D scene was painted on three surfaces: (a) A 2D surface incongruent with the 3D painted scene (flat perspective). (b) A 3D surface with a geometry congruent with the 3D scene (proper perspective). (c) A 3D surface with an opposite depth arrangement to the 3D scene (reverse perspective). This last stimulus was bistable and could be perceived veridically, as it physically existed, or as a depth-inverting illusion. For all experiments, observers relied on perspective gradients to estimate the size of a test probe placed within the scene; objects placed in a 'far' position as defined by perspective cues were perceived to be larger regardless of their physical distance. Further, illusion strength was tied to retinal size; small retinal-size differences (Experiments 1 and 2) did not affect illusion strength, whereas larger retinal-size differences (Experiment 3) did play a minor role.

The role of linear perspective cues on the perceived depth magnitude of the surfaces they are painted on: proper-, reverse-, and flat-perspective paintings.

Evidence from several studies suggests that perspective cues influence the perceived three-dimensional (3-D) layout of the surface they are painted on. The purpose of this study was to test how perspective cues influence the perceived depth magnitude. We rendered the same linear-perspective scene on three different surfaces: proper perspective, with a 3-D structure that was congruent with the painted scene; flat perspective, incongruent with the scene; reverse perspective, opposite to the scene, producing two competing stable percepts (veridical and illusory). We varied binocular disparity by using three different sizes for each type of stimulus. Observers assessed the magnitude of the perceived depth within each of these stimuli. Accuracy improved with increasing stimulus size that covaries with binocular-disparity magnitude. Generally, the magnitude of the perceived depth of stimuli painted with perspective cues was larger than the physical depth of the stimulus regardless of stimulus type (proper, reverse, flat). Further, depth magnitude tended to be larger when depth cues were congruent (proper) as compared with opposite (reverse) or incongruent (flat). There was no difference in perceived depth under the different percepts (veridical and illusory) for the reverse-perspective stimulus, suggesting that depth is assessed by the stimulus structure rather than by the percept obtained.

Recovering 3-D shape: roles of absolute and relative disparity, retinal size, and viewing distance as studied with reverse-perspective stimuli.

When viewing reverspective stimuli, data-driven signals such as disparity, motion parallax, etc, help to recover veridical three-dimensional (3-D) shape. They compete against schema-driven influences such as experience with perspective, foreshortening, and other pictorial cues that favor the perception of an illusory depth inversion. We used three scaled-size versions of a reverspective to study the roles of retinal size, binocular disparity, and viewing distance--that influences both vergence and accommodation--in recovering the true 3-D shape. Experiment 1 used three conditions, in each of which a parameter was kept fixed across the three stimulus sizes: (a) fixed retinal size, (b) fixed viewing distance, (c) fixed disparity. The predominance of the veridical percept was recorded. Generally, the illusion strength was the same when the viewing distance was fixed, despite significantly different disparities and retinal sizes; conversely, illusion strength changed significantly in fixed-disparity and fixed-retinal-size conditions. Experiment 2 confirmed the results of experiment 1b (roughly equal performances for fixed viewing distance, independent of size) for two additional distances. Viewing distance and "scaled disparity" (disparity divided by retinal size) are good predictors of the data trends. We propose that disparity scaling is supported by both mathematical and 3-D shape considerations.