No / BACKGROUND: Tripping is a common factor in falls and a typical safety strategy to avoid tripping on steps or stairs is to increase foot clearance over the step edge. In the present study we asked whether the perceived height of a step could be increased using a visual illusion and whether this would lead to the adoption of a safer stepping strategy, in terms of greater foot clearance over the step edge. The study also addressed the controversial question of whether motor actions are dissociated from visual perception. METHODOLOGY/PRINCIPAL FINDINGS: 21 young, healthy subjects perceived the step to be higher in a configuration of the horizontal-vertical illusion compared to a reverse configuration (p = 0.01). During a simple stepping task, maximum toe elevation changed by an amount corresponding to the size of the visual illusion (p<0.001). Linear regression analyses showed highly significant associations between perceived step height and maximum toe elevation for all conditions. CONCLUSIONS/SIGNIFICANCE: The perceived height of a step can be manipulated using a simple visual illusion, leading to the adoption of a safer stepping strategy in terms of greater foot clearance over a step edge. In addition, the strong link found between perception of a visual illusion and visuomotor action provides additional support to the view that the original, controversial proposal by Goodale and Milner (1992) of two separate and distinct visual streams for perception and visuomotor action should be re-evaluated.

Acompanha: Praxinoscópio e zootrópio: brinquedos ópticos na relação arte-ciência / O desenvolvimento científico foi responsável por consolidar o início da ciência moderna. A presente pesquisa procurou problematizar a necessidade de aproximar Arte e Ciência reelaborando os aparelhos ópticos Zootrópio e Praxinoscópio, desenvolvendo, assim, o conhecimento artístico e o cientifico por meio de uma atividade lúdica e interdisciplinar entre Arte e Física. Para tanto foi fundamental o princípio da percepção visual e o da ilusão óptica, utilizando-se da movimentação de quadros estáticos (frame by frame). A reconstrução destes instrumentos ancorou-se nas oficinas de um Projeto de Extensão intitulado “Técnicas de Representação da Imagem: Do Renascimento à Fotografia”, dividida em duas oficinas especificas “Zootrópio: uma arte vista entre frestas luminosas” e “Praxinoscópio: o reflexo fragmentado da animação”. Os dados obtidos na pesquisa foram analisados em suas modalidades qualitativa e descritiva, tendo como fonte direta as informações mais próximas do ambiente escolar natural. Na fundamentação da pesquisa destacaram-se alguns teóricos, que abordaram fenômenos físicos presentes na vida cotidiana, como Perelman, Arnheim, Crary, Santaella, Fogliano e Lucena envolvendo as diferentes análises sobre a percepção visual e sua influência para cada observador na identificação de diferentes objetos. A narração histórica e cronológica dos primeiros inventos ópticos foram baseados em Briggs e Burgue, abordando as questões que envolvem a imagem enquanto forma de comunicação, especialmente com o advento da fotografia. A pesquisa finaliza-se com os resultados das oficinas e as observações dos alunos do curso de Licenciatura em Ciências Naturais, da Universidade Tecnológica Federal do Paraná (UTFPR) - campus Ponta Grossa. / Scientific development was responsible for consolidating the beginning of modern science. The present research sought to problematize the need to approach Art and Science by reworking the optical devices Zootrope and Praxinoscope, thus developing artistic and scientific knowledge through a playful and interdisciplinary activity between Art and Physics. For this, the principle of visual perception and optical illusion was fundamental, using frameby-frame motion. The reconstruction of these instruments was anchored in the workshops of an Extension Project entitled "Techniques of Image Representation: From the Renaissance to Photography", divided into two specific workshops "Zootrope: an art seen between luminous cracks" and "Praxinoscope: the fragmented reflection of animation ". The data obtained in the research were analyzed in their qualitative and descriptive modalities, having as direct source the information closest to the natural school environment. Some of the theorists focused on physical phenomena present in everyday life, such as Perelman, Arnheim, Crary, Santaella, Fogliano and Lucena, involving the different analyzes on visual perception and their influence for each observer in the identification of different objects. The historical and chronological narration of early optical inventions were based on Briggs and Burgue, addressing issues involving the image as a form of communication, especially with the advent of photography. The research is finished with the results of the workshops and the observations of the students of the undergraduate degree in Natural Sciences, of the Federal Technological University of Paraná (UTFPR) - Ponta Grossa campus.

Cette thèse présente des modèles mathématiques pour la perception visuelle et s'occupe des phénomènes où on reconnait une brèche entre ce qui est représenté et ce qui est perçu. La complétion amodale consiste en percevoir un complètement d'un object qui est partiellement occlus, en opposition avec la complétion modale, dans laquelle on perçoit un object même si ses contours ne sont pas présents dans l'image [Gestalt, 99]. Ces contours, appelés illusoires, sont reconstruits par notre système visuelle et ils sont traités par les cortex visuels primaires (V1/V2) [93]. Des modèles géométriques de l'architecture fonctionnelle de V1 on le retrouve dans le travail de Hoffman [86]. Dans [139] Petitot propose un modèle pour le complètement de contours, équivalent neurale du modèle proposé par Mumford [125]. Dans cet environnement Citti et Sarti introduisent un modèle basé sur l'architecture fonctionnelle de la cortex visuel [28], qui justifie les illusions à un niveau neurale et envisage un modèle neuro-géometrique pour V1. Une autre classe sont les illusions d'optique géométriques (GOI), découvertes dans le XIX siècle [83, 190], qui apparaissent en présence d'une incompatibilité entre ce qui est présent dans l'espace object et le percept. L'idée fondamentale développée ici est que les GOIs se produisent suite à une polarisation de la connectivité de V1/V2, responsable de l'illusion. A partir de [28], où la connectivité qui construit les contours en V1 est modelée avec une métrique sub-Riemannian, on étend cela en disant que pour le GOIs la réponse corticale du stimule initial module la connectivité, en devenant un coefficient pour la métrique. GOIs seront testés avec ce modèle. / This thesis presents mathematical models for visual perception and deals with such phenomena in which there is a visible gap between what is represented and what we perceive. A phenomenon which drew the interest most is amodal completion, consisting in perceiving a completion of a partially occluded object, in contrast with the modal completion, where we perceive an object even though its boundaries are not present [Gestalt theory, 99]. Such boundaries reconstructed by our visual system are called illusory contours, and their neural processing is performed by the primary visual cortices (V1/V2), [93]. Geometric models of the functional architecture of primary visual areas date back to Hoffman [86]. In [139] Petitot proposed a model of single boundaries completion through constraint minimization, neural counterpart of the model of Mumford [125]. In this setting Citti and Sarti introduced a cortical based model [28], which justifies the illusions at a neural level and provides a neurogeometrical model for V1. Another class of phenomena are Geometric optical illusions (GOIs), discovered in the XIX century [83, 190], arising in presence of a mismatch of geometrical properties between an item in object space and its associated percept. The fundamental idea developed here is these phenomena arise due to a polarization of the connectivity of V1/V2, responsible for the misperception. Starting from [28] in which the connectivity building contours in V1 is modeled as a sub-Riemannian metric, we extend it claiming that in GOIs the cortical response to the stimulus modulates the connectivity of the cortex, becoming a coefficient for the metric. GOIs will be tested through this model.

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