Approximation, torsion, and amodally-completed surfaces.

Consider a stereoscopic display simulating two rectangular patches, the lower frontoparallel and the upper slanted around the vertical axis. When the two patches are amodally completed and appear as the unoccluded parts of a smooth surface partially hidden by a foreground frontoparallel surface, either real or illusory, their relative slant is underestimated with respect to a baseline condition in which they are perceived as separate rectangles. Slant assimilation was studied in three experiments using with- vs. without-occluder displays and two methods, slant matching and speeded classification of twist direction. In Experiments 1 and 2 we found slant assimilation in with-occluder displays and slant contrast in without-occluder displays. In Experiment 3 we isolated a component of slant assimilation attributable to the mere presence of the occluder. Twist classification performance was impaired even when edge geometry hindered amodal completion, but the performance loss was larger when surface patches were amodally completed. To minimize the required amount of torsion, input fragments are misperceived, indicating that in limiting conditions amodal completion is mediated by approximation rather than interpolation. Slant assimilation decreases as twist angle increases, up to a limit above which the visual system does not support the formation of a smooth amodal surface with torsion.

Contour curvature polarity and surface interpolation.

Contour curvature polarity (i.e., concavity/convexity) is recognized as an important factor in shape perception. However, current interpolation models do not consider it among the factors that modulate the trajectory of amodally-completed contours. Two hypotheses generate opposite predictions about the effect of contour polarity on surface interpolation. Convexity advantage: if convexities are preferred over concavities, contours of convex portions should be more extrapolated than those of concave portions. Minimal area: if the area of amodally-completed surfaces tends to be minimized, contours of convex portions should be less extrapolated than contours of concave portions. We ran three experiments using two methods, simultaneous length comparison and probe localization, and different displays (pictures vs. random dot stereograms). Results indicate that contour polarity affects the amodally-completed angles of regular and irregular surfaces. As predicted by the minimal area hypothesis, image contours are less extrapolated when the amodal portion is convex rather than concave. The field model of interpolation [Fantoni, C., & Gerbino, W. (2003). Contour interpolation by vector-field combination. Journal of Vision, 3, 281-303. Available from http://journalofvision.org/3/4/4/] has been revised to take into account surface-level factors and to explain area minimization as an effect of surface support ratio.

Classical and inverted White's effects.

In classical White's effect, intermediate-luminance targets appear lighter when they interrupt the dark stripes of a grating and darker when they interrupt the light stripes. The effect is reversed when targets are of double-increment or double-decrement luminance, relative to the luminances of grating stripes. To find a common explanation for classical and inverted effects, we ran two experiments. In experiment 1, we utilised intermediate-target displays to show that perceived transparency dominates over occlusion only when the target luminance is close to the luminances of top regions. This result weakens transparency-based accounts of White's effect. In experiment 2, we varied grating contrast and target luminance to measure the classical effect in seven intermediate-target cases, as well as the inverted effect in four double-increment and four double-decrement cases. Both types of effect are explained by a common model, based on assimilation to the top region and contrast with the interrupted region, weighted by adjacency along the luminance continuum.

Paradoxical rest.

Paradoxical absence of motion in a rotating pattern is reported. The effect requires that the motion of the paradoxically stationary figure be underspecified by local motion signals, and that the paradoxically stationary figure be well segregated. This is consistent with proposals that figural segregation affects the integration of local motion signals.

Retinal vs. environmental orientation in the perception of the right angle.

The right angle is perceived as such only when its sides are vertical and horizontal (Goldmeier effect). We investigated if spontaneous verbal descriptions reflect this type of "oblique effect", and if the relevant frame of reference is retinal or environmental. We compared four conditions in which upright vs. tilted observers were asked to describe gravitationally normal vs. oblique angles. It is shown that the Goldmeier effect depends upon the environmental coordinates, which usually determine the orientation of the subjective frame of reference. We argue that the singularity of the right angle in the vertical/horizontal orientation depends on a phenomenological fact: only the normal right angle looks like a right angle.

Mental image reversal and verbal recoding: when ducks become rabbits.

A recent debate has concerned whether classical bistable configurations (e.g., duck/rabbit) can be reinterpreted using mental imagery. Research in this field indicates that image reversal is possible only when subjects change their specification of orientation. In a series of four experiments, we demonstrate that mental reversal of classical bistable configurations (CBCs) is impeded by verbally recoding the visual pattern at the time of input. When subjects were prevented from verbally recording visual stimuli in short-term memory, they fared systematically better in mentally reversing the CBC, even when they received no instructions to change their reference frame or specification of orientation. On this basis, we suggest a model of image reversal that takes into account the interaction between memory codes and provides a new perspective on verbal recording, verbal overshadowing, and mental discoveries.

Illusory figures based on local kinematics.

A new type of motion-induced illusory figure determined by local kinematic information is investigated. The new figure is induced by radial line patterns subjected to either figure motion (the lines change as if they were stationary and a triangle was rotating in front of them) or background motion (the lines change as if they were being rotated behind a stationary triangle). Although the two kinds of motion are equivalent from the viewpoint of relative displacements, perceptually they yield very different results. With background motion, observers tend to perceive rigid figures that have a triangular shape. With figure motion, observers report seeing deforming figures with shapes that vary depending on the number of lines in the display. We consider two alternative accounts for this asymmetry which we term the background superiority effect (BSE). The first account proposes that the effect is due to retinal persistence and to figure stability. Against this line of explanation, we demonstrate that observers also see rigid triangular shapes in displays where both the figure and the radial lines rotate (double motion displays). The second account proposes that the effect depends on the availability of local kinematic information constraining contour orientation. This second line of explanation is consistent with observers' reports of bowed edges in double motion displays rotating in phase or in counterphase. Candidate mechanisms for extracting local kinematic information are discussed.

Transparent layer constancy.

Perceived transparency was studied as a constancy problem. In the episcotister (E-) model of scission, luminances are partitioned into layer and background components; four luminances determine values of two layer parameters that specify constancy of a transparent layer on different backgrounds. The E-model was tested in an experiment in which 12 Ss matched 24 pairs of four-luminance patterns by adjusting two luminances of the comparison pattern. Both the standard and the comparison were perceived as a transparent layer on a checkerboard. The E-model predicts matches when layer values are identical in the two patterns. One parameter was constant, constraining the adjustment along the second dimension. Obtained values corresponded well with E-predictions. Alternative models based on local luminance or average contrast ratios accounted for less variability. Results indicate that transparency models should utilize luminance, not reflectance, as the independent variable.

Low-level and high-level processes in the perceptual organization of three-dimensional apparent motion.

An experiment is described in which a bistable motion display is shown in four combinations of monoptic and dichoptic viewing. When two triangles are replaced by one of them, one of two competing phenomena can be seen: local disappearance of the triangle that has not been replaced; a 3-D rotation of a rigid triangle from one location to another. The dependence of the first sensation on the duration of the interstimulus interval and on monocular availability of the local-identity information confirms previous evidence about the existence of two processes which mediate apparent motion. The localization of the high-level process, however, is questioned on the basis of a comparison of data obtained when motion information was monoptic or only dichoptic. As monoptic motion information seems more effective, the 'figural' higher process cannot be wholly located at the binocular level.

Monoptic and dichoptic signals do not cooperate in the perception of a bistable motion display.

Two new conditions of presentation of a bistable motion display are studied, where a competition between monoptic and monoptic plus dichoptic information is supposed to be involved. Data contradict the expectation that a dichoptic motion signal could cooperate with a monocular one. A different interpretation of the combination of monoptic and dichoptic signals is proposed. According to it, the clarity of rotational motions is based upon the evidence, even conflicting, which the two "peripheral" low-level processes separately provide.