Luigi Burigana

Organization by rules in finite sequences

Abstract The overall content of a given sequence divides into a structural part, which is common to sequences with the same organization, and a metrical part, which differentiates it from structurally equivalent sequences. Of particular interest here are regularities in sequences, as notable factors of perceptual and/or cognitive organization. Accordingly, the structural part or structuration of a sequence is conceived as a configuration of rules, each qualifying some structural unit. Analytically, structurations thus defined amount to sets of relations between the components of a sequence, so that in a fixed structuration those components are to a certain extent mutually dependent and involve redundancy; a suitable measure of the efficiency of the given structuration may be derived from the number of degrees of freedom allowed by such a relational network. Sections 3–6 of this paper present a formal investigation of dependence relations induced by structurations, relying mainly on a direct correspondence between this type of relations and the algebraic notions of closure-operators and closure-systems (Moore families). Last, the suggested principles of structural appraisal of sequences are illustrated by some examples.

Perceptual Evaluation of Motion Editing for Realistic Throwing Animations

Animation budget constraints during the development of a game often call for the use of a limited set of generic motions. Editing operations are thus generally required to animate virtual characters with a sufficient level of variety. Evaluating the perceptual plausibility of edited animations can therefore contribute greatly towards producing visually plausible animations. In this article, we study observers’ sensitivity to manipulations of overarm and underarm biological throwing animations. In the first experiment, we modified the release velocity of the ball while leaving the motion of the virtual thrower and the angle of release of the ball unchanged. In the second experiment, we evaluated the possibility of further modifying throwing animations by simultaneously editing the motion of the thrower and the release velocity of the ball, using dynamic time warping. In both experiments, we found that participants perceived shortened underarm throws to be particularly unnatural. We also found that modifying the throwers motion in addition to modifying the release velocity of the ball does not significantly improve the perceptual plausibility of edited throwing animations. In the third experiment, we modified the angle of release of the ball while leaving the magnitude of release velocity and the motion of the thrower unchanged, and found that this editing operation is efficient for improving the perceptual plausibility of shortened underarm throws. Finally, in Experiment 4, we replaced the virtual human thrower with a mechanical throwing device (a ramp) and found the opposite pattern of sensitivity to modifications of the release velocity, indicating that biological and physical throws are subject to different perceptual rules. Our results provide valuable guidelines for developers of games and virtual reality applications by specifying thresholds for the perceptual plausibility of throwing manipulations while also providing several interesting insights for researchers in visual perception of biological motion.

Contribution of surface material and size to the expected versus the perceived weight of objects.

Because the perceived weight of objects may be affected by various nonweight properties, such as their size and the density of their surface material, relative weight is sometimes misperceived (the size–weight illusion and the material–weight illusion, respectively). A widely accepted explanation for weight illusions is provided by the so-called expectation model, according to which the perceived weight stems from the contrast between the actual and expected weights. In the present study, we varied both the surface material and the size of stimuli, while keeping constant their physical weights. In Experiment 1, the participants lifted the stimuli by grasping them on opposite sides, whereas in Experiment 2 they lifted them by using a string that was attached to their top surface. We used a variant of the random conjoint measurement paradigm to obtain subjective interval scales of the contributions of surface material and size to the expected and the perceived weight of the stimuli. Inconsistently with the predictions from the expectation model, we found, in both experiments, that the surface material contributed more than the size to the expected weight, whereas the size contributed more than the surface material to the perceived weight. The results support the hypothesis that perceived weight may depend on implicit, rather than explicit, weight expectations.

Properties of the size-weight illusion as shown by lines of subjective equality.

We studied the size-weight illusion through comparative judgments. The experiment had two direct aims: to verify whether the relative contribution of size to apparent heaviness can differ for different stimulus sets, and to verify whether that contribution can differ for different methods of comparing two objects (consecutive vs. simultaneous weighing). Thirty university students participated. Results show that the relative contribution of size depends on stimulus set, but is independent of the method used for comparing objects. The first finding implies that a linear model cannot describe the integration of size and weight information in the illusion; the second finding is evidence for the low-level character of the integration process.

Intuitive understanding of the relationship between the elasticity of objects and kinematic patterns of collisions

Horizontal collisions have long been used as a tool for exploring people’s intuitive understanding of elementary physical laws. Here, we explored intuitive understanding of the relationship between the kinematic patterns of collisions and the elasticity of the colliding objects. In Experiment 1A, we manipulated the simulated materials of two virtually colliding spheres and asked the participants to judge whether the simulated collisions appeared “natural” or “unnatural.” We did the same in Experiments 1B and 2, but asked the participants to adjust the velocities until the collisions appeared to be “perfectly natural.” In Experiment 3, we removed pictorial cues to the materials of the colliding spheres and asked the participants to rate the bounciness of the materials, in view of the kinematics of simulated collisions. Overall, the results showed that observers intuitively understood that collisions between more elastic objects subtend a higher coefficient of restitution than collisions between objects with lesser elasticity. The results also highlighted some discrepancies between the intuitive and Newtonian physics of collisions. Observers were somewhat insensitive to violations of the principle of energy conservation, and their responses were influenced by irrelevant kinematic features of the collisions, such as the collision type and precollision velocity. We discuss our experimental results in relation to salient theoretical perspectives on intuitive physics.

Constraints in vision: Outline of a set-theoretic approach

Abstract The idea is advanced that the highly complex relationship between stimuli and percepts in vision may be formally described as an extensive system of relational constraints, involving both stimulus and perceptual variables; some guidelines are also suggested for working out this idea and applying it to salient general questions of vision research. The paper starts by defining the basic notions of located variable and located constraint ( 2 Variables and valuations , 3 Constraints on variables ) with respect to a presumed homogeneous set of observational situations. Then the most important notions of a system of constraints and of pairing between a system of constraints and an assignment of values to stimulus variables are introduced ( 4 Systems of constraints , 5 Pairing of constraints and valuations ). A set-theoretic way of representing the combined predictive power of such pairing is suggested, and some elementary properties of the concept are described. Lastly, a few comments are made on the possible use of the constraint paradigm in discussing some critical aspects of contemporary research on vision and some reasons are given for the suggested set-theoretic approach ( 6 Constraints in perceptual science , 7 Epilogue: reasons for the set-theoretic approach ).

“Invariants” in Koffka’s Theory of Constancies in Vision: Highlighting Their Logical Structure and Lasting Value

Summary By introducing the concept of “invariants”, Koffka (1935) endowed perceptual psychology with a flexible theoretical tool, which is suitable for representing vision situations in which a definite part of the stimulus pattern is relevant but not sufficient to determine a corresponding part of the perceived scene. He characterised his “invariance principle” as a principle conclusively breaking free from the “old constancy hypothesis”, which rigidly surmised point-to-point relations between stimulus and perceptual properties. In this paper, we explain the basic terms and assumptions implicit in Koffka’s concept, by representing them in a set-theoretic framework. Then, we highlight various aspects and implications of the concept in terms of answers to six separate questions: forms of invariants, heuristic paths to them, what is invariant in an invariant, roots of conditional indeterminacy, variability vs. indeterminacy, and overcoming of the indeterminacy. Lastly, we illustrate the lasting value and theoretical power of the concept, by showing that Koffka’s insights relating to it do occur in modern perceptual psychology and by highlighting its role in a model of perceptual transparency.

Modules in spatial vision: intrinsic reasons of their functional attributes

Abstract Visual modules can be viewed as expressions of a marked analytic attitude in the study of vision. In vision psychology, this attitude is accompanied by hypotheses that characterize how visual modules are thought to operate in perceptual processes. Our thesis here is that there are what we call “intrinsic reasons” for the presence of such hypotheses in a vision theory, that is, reasons of a deductive kind, which are imposed by the partiality of the basic terms (input and output) in the definition of a module, and by peculiar characteristics of those terms. Specifically, we discuss three hypotheses of functional attributes: successive stages in the action of modules, residual indeterminacy of their effects, and the role of prior constraints. For each of the three, we indicate its occurrence in perceptual psychology, explain corresponding intrinsic reasons, and illustrate such reasons with examples.

Inflections of the Bayesian Paradigm in Perceptual Psychology.

Bayesian modeling has gained a conspicuous position in contemporary perceptual psychology. It can be examined from two viewpoints: a formal one, concerning the logical attributes of and the algebraic operations on the components of the models, and a substantive one, concerning the empirical meaning of those components. We maintain that, while there is homogeneity between Bayesian models of visual perception in their formal setup, remarkable differences can be found in their substantive aspect, that is, how the question “Where do probabilities come from?” is answered when designing the models. In particular, we focus on an inflection that we call “congenial” because it consistently embodies the inversion idea of the Bayes’ rule in terms of optical inversion and highlight delicate issues that face this inflection for a consistent realization of the scientific program it represents. We also suggest ideas concerning the organization of the Bayesian area within perceptual psychology, which appears variegated, with the congenial inflection in a central position, and a fringe of disputable classification along the border.

Olp: an R package for optimal linear partitions of finite sets of points on the plane.

Given a set of points on the plane and an assignment of values to them, an optimal linear partition is a division of the set into two subsets which are separated by a straight line and maximally contrast with each other in the values assigned to their points. We present a method for inspecting and rating all linear partitions of a finite set, and a package of three functions in the R language for executing the computations. One function is for finding the optimal linear partitions and corresponding separating lines, another for graphically representing the results, and a third for testing how well the data comply with the linear separability condition. We illustrate the method on possible data from a psychophysical experiment (concerning the size-weight illusion) and compare its performance with that of linear discriminant analysis and multiple logistic regression, adapted to dividing linearly a set of points on the plane.