The viewpoint-dependency of veridicality: psychophysics and modelling

Sensitivity to Nonaccidental Configurations of Two-Line Stimuli

According to Recognition-By-Components theory, object recognition relies on a specific subset of three-dimensional shapes called geons. In particular, these configurations constitute a powerful cue to three-dimensional object reconstruction because their two-dimensional projection remains viewpoint-invariant. While a large body of literature has demonstrated sensitivity to changes in these so-called nonaccidental configurations, it remains unclear what information is used in establishing such sensitivity. In this study, we explored the possibility that nonaccidental configurations can already be inferred from the basic constituents of objects, namely, their edges. We constructed a set of stimuli composed of two lines corresponding to various nonaccidental properties and configurations underlying the distinction between geons, including collinearity, alignment, curvature of contours, curvature of configuration axis, expansion, cotermination, and junction type. Using a simple visual search paradigm, we demonstrated that participants were faster at detecting targets that differed from distractors in a nonaccidental property than in a metric property. We also found that only some but not all of the observed sensitivity could have resulted from simple low-level properties of our stimuli. Given that such sensitivity emerged from a configuration of only two lines, our results support the view that nonaccidental configurations could be encoded throughout the visual processing hierarchy even in the absence of object context.

Towards a new kind of experimental psycho-aesthetics? Reflections on the Parallellepipeda project

Experimental psycho-aesthetics-the science aimed at understanding the factors that determine aesthetic experience-is reviewed briefly as background to describe the Parallellepipeda project, a cross-over project between artists and scientists in Leuven. In particular, I sketch how it started and developed further, with close interactions between the participating artists and scientists. A few examples of specific research projects are mentioned to illustrate the kind of research questions we address and the methodological approach we have taken. We often found an effect of providing participants with additional information, a difference between novice and expert participants, and a shift with increasing experience with an artwork, in the direction of tolerating more complexity and acquiring more order from it. By establishing more connections between parts of an artwork and more associations to the artwork, it becomes a stronger Gestalt, which is more easily mastered by the viewer and leads to increased appreciation. In the final part of the paper, I extract some general lessons from the project regarding a possible new way of doing psycho-aesthetics research, which is able to solve some of the problems of traditional experimental psycho-aesthetics (eg, trade-off between experimental control and ecological validity).

Detection of symmetry and repetition in one and two objects. Structures versus strategies

Symmetry is usually easier to detect within a single object than in two objects (one-object advantage), while the reverse is true for repetition (two-objects advantage). This interaction between regularity and number of objects could reflect an intrinsic property of encoding spatial relations within and across objects or it could reflect a matching strategy. To test this, regularities between two contours (belonging to a single object or two objects) had to be detected in two experiments. Projected three-dimensional (3-D) objects rotated in depth were used to disambiguate figure-ground segmentation and to make matching based on simple translations of the two-dimensional (2-D) contours unlikely. Experiment 1 showed the expected interaction between regularity and number of objects. Experiment 2 used two-objects displays only and prevented a matching strategy by also switching the positions of the two objects. Nevertheless, symmetry was never detected more easily than repetition in these two-objects displays. We conclude that structural coding, not matching strategies, underlies the one-object advantage for symmetry and the two-objects advantage for repetition.

The Effects of Mirror Reflections and Planar Rotations of Pictures on the Shape Percept of the Depicted Object

Mirror reflections and planar rotations of a picture do not result in any variations concerning the internal geometrical layout of the objects depicted in the picture. We examined to what extent these picture plane transformations gave rise to perceptual differences. A large set of pictures was generated by mirror-reflecting and rotating a set of six original photographs in the picture plane. We externalised the percepts of the depicted objects by using a direct perceptual method: the gauge-figure method. Participants had to adjust a gauge figure so that it seemed to be painted on the surface of the depicted object. From an extensive set of settings collected this way, we computed for each picture the three-dimensional interpretation—or pictorial relief—of the depicted object. On the basis of this set of pictorial reliefs, we addressed the effects of mirror reflections and rotations of pictures on the shape percept of the depicted object. Mirror-reflecting a picture around the horizontal axis resulted in large differences in pictorial reliefs, whereas mirror-reflecting pictures around the vertical axis resulted in only small differences in pictorial reliefs. Clockwise 90°, 180°, and 270° rotation affected the pictorial relief significantly. In all cases, the differences between the pictorial reliefs could be resolved by affine transformations, and could thus be ascribed to different solutions of the depth ambiguities inherent in pictures.

Multiple routes to object matching from different viewpoints: mental rotation versus invariant features

Previous research has shown that object recognition from different viewpoints often yields strong effects of viewpoint. However, for some objects and experimental paradigms almost complete viewpoint invariance is obtained. This suggests the existence of multiple routes to object recognition. In this study we further strengthen this notion by designing two different conditions using the same experimental paradigm (simultaneous matching) and highly similar objects (multiblock figures). In the first condition (involving a handedness violation), strong effects of viewpoint were obtained. In the second condition (involving an invariance violation), the effects of viewpoint were negligible. This result illustrates that asking under what circumstances object recognition is viewpoint dependent or independent is more fruitful than attempting to show that object recognition is either viewpoint dependent or independent.

Matching multicomponent objects from different viewpoints: mental rotation as normalization?

Novel multicomponent objects were created, and 3 distractors were created for each object by changing the relations between the parts of the object. In a set of 5 experiments, target objects were presented as a motion sequence of multiple views or as a single view. Participants were asked to determine whether an image of an object, viewed from another viewpoint, was the same as the target object. The axis of rotation was aligned with one of the environmental axes or with the main axis of the object. The effects of viewpoint on performance imply that the matching of objects is viewpoint dependent and requires a process of normalization to undo the differences between the perceptual description and the stored object descriptions. The lack of a systematic effect of the axis of rotation, however, suggests that this normalization is best understood as not involving a 3-D transformation of stored 3-D object models.