Reference frame effects on shape perception in two versus three dimensions

Similarity and Dissimilarity in Perceptual Organization: On the Complexity of the Gestalt Principle of Similarity

The main purpose of this work is to explore the Gestalt principle of similarity and to demonstrate that the use of this term alone is not sufficient to understand the dynamics of grouping fully and correctly. More generally, this work aims to show that the Gestalt principle of similarity alone is not sufficient for a full understanding of perceptual organization occurring both in the classical and mostly in the new phenomena here presented. Limits and incompleteness of the similarity principle have suggested the basic, more general and stronger role of dissimilarity in perceptual grouping under a large variety of conditions. Dissimilarity was shown as a basic principle of figure–ground segregation, as a tool useful to create at will new groups and visual objects within patterns where they are totally invisible, as an attribute that is able to accentuate different shape components within the same object, as a way to distort shapes and create visual illusions, but also to reduce or annul them and, finally, to decompose, ungroup and reshape single objects. The results demonstrated the necessity to introduce a principle of dissimilarity that is complementary to similarity as already studied by Gestalt psychologists.

Visualization of needle access pathway and a five-DoF evaluation

It is a common practice nowadays to plan needle access pathways to the volumetric organs before performing surgeries. An enormous amount of needle access planning systems has been proposed in recent years. Recent works mainly focus on the system usability or target accessibility. Visualization of the planned access pathways has drawn little attention and its effect on insertion quality is left unattended. We aim to address this problem by introducing an all-round evaluation framework that links up with human motions and computer graphics. Our evaluation framework provides an objective and quantitative analysis of the illustrativeness of the needle access pathway visualization techniques to an extent of five degrees of freedom. Our experimental results show that the visualization method adopted greatly influences insertion accuracy. Based on this finding, we propose a new visualization technique that intuitively conveys placement and orientation information. We also show that our method better conveys pathway orientation and thus enables a higher quality of insertion.

Mach's square-or-diamond phenomenon in microgravity during parabolic flight

In a classic demonstration, Ernst Mach showed that the same figure could be perceived as a square or as a diamond depending on the orientation of the subject relative to gravity. Such phenomenon is based on the use of a geocentric reference frame for object perception. If the central nervous system perceives an object with respect to the gravitationally defined vertical, what will happen if this reference frame is removed? We investigated the Mach phenomenon in subjects placed in short-term microgravity during parabolic flight. Subjects were presented with a square with a corner pointing upwards, and asked whether they perceived it as a diamond or square with the head upright or tilted 45 degrees in roll both in normal gravity and when free-floating in microgravity during parabolic flight. The addition of a rectangular frame around the figure was also investigated. In contrast to the normal gravity condition, with the head tilted the subjects still perceived a diamond figure in microgravity, indicating that they had switched from a geocentric to an egocentric reference frame. Also in contrast to the normal gravity condition, adding a rectangular frame around the figure did not significantly change the perception of the object in microgravity, suggesting that an intrinsic reference determined by the axis of elongation or symmetry of the object does not easily override an egocentric reference frame like it does for a geocentric reference frame.

Attentional frames, frame curves and figural boundaries: the inside/outside dilemma

Attended and unattended regions of the image array are viewed often as binary complements to one another, with a well-defined boundary between them. A simple experiment shows otherwise: if the contour of a simple convex shape is perturbed to create a distinctive texture, it is typically the outside of the contour that provides the basis for similarity judgement, not the inside. The introduction of the appropriate task, however, can make the inside part of the contour become more salient. A similar result occurs for concave shapes, such as a C, where notions of containment are not easily specified. These observations suggest that figure boundaries are difficult to define objectively and that the setting of an attentional reference frame plays a key role in object description. We propose that this frame is part of a virtual, transparent blackboard or "clearboard", bound to the scene, not to the image.