Role des axes structuraux (symetrie, elongation) dans la perception des formes
Institution:
Université Pierre Mendès France (Grenoble)Disciplines:
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Abstract EN:
This thesis explores the proposal that the human visual system processes the global spatial structure of two-dimensional (2-d) shapes relative to salient axes of symmetry and elongation. Four experimental studies were conducted, which fall in two parts. Part 1 focuses on whether implicit axes of symmetry and elongation determine perceived shape orientation (chapter 1), and representations underlying shape recognition (chapter 2). . The same set of simple geometrical shapes was used in two experimental paradigms involving visual search and categorisation. Part 2 (chapters 3 and 4) presents a neuropsychological case study of a simultanagnosic patient who is impaired at perceiving multiple objects. The role of axis-based descriptions in perceptual grouping between shapes was examined in this patient. In chapter 1, four experiments investigated the role of implicit axes of symmetry and elongation on visual search for shape orientation. The results suggest that the perception of shape orientation is mediated by descriptions in terms of perceptually salient axes of symmetry and elongation. Chapter 2 examined the of structural (axis-based) descriptions in shape recognition. Shortterm priming effects between 2-d shapes were studied in a categorization paradigm. Reliable priming (in terms of reductions in reaction time) between shapes on consecutive trials was observed a function of their structural correspondence on the dimensions, and as a function of the category structure manipulated across three experiments. Overall, the results provide evidence that perceptual priming between shapes is determined by their structural correspondence on symmetry and orientation, and that these attributes are encoded in representations mediating shape perception. In chapters 3 and 4, the effects of grouping by the alignment of implicit axes on the perception of multiple shapes was examined in a patient (gk) with simultanagnosia. Gk made various perceptual judgments about configurations of shapes aligned by collinearity of edges or internal axes. Differential effects of grouping conditions were observed, suggesting that grouping by axes or edges can support the allocation of visual attention to multiple objects. Taken together, the studies suggest that axis-based information can be used in the perception of shapes.
Abstract FR:
Un des objectifs majeurs des theories sur la reconnaissance d'objet est de decrire et modeliser comment le systeme perceptif construit une representation stable des formes et des objets qui nous environnent, malgre la variation de leurs projections retinales d'une perception a l'autre (entre deux perceptions, les objets peuvent changer de position, d'orientation, n'etre plus a la meme distance de l'observateur, etc). La these examine l'hypothese selon laquelle les formes seraient percues et representees en memoire dans un format structural base sur leurs axes de symetrie et/ou d'elongation (eg, marr, 1982). Cette proposition theorique est testee dans le cas des formes polygonales bidimensionnelles. La these est organisee en deux parties; la premiere (etude chez le sujet 'normal') montre que les axes de symetrie et/ou d'elongation des formes determinent la perception de l'orientation frontoparallele de ces dernieres (chapitre 1; quatre experiences de recherche visuelle) et leur representation en memoire (chapitre 2; trois experiences d'amorcage perceptif). La seconde partie (etude du patient gk, simultagnosique, i. E. Ayant de la difficulte a percevoir plusieurs objets a la fois) montre gk est, d'une part, sensible au groupement perceptif de formes bi-dimensionnelles base sur l'alignement de leurs axes de symetrie (vs de leurs contours) (chapitre 4; cinq experiences) et, d'autre part, capable de construire et d'utiliser dans l'hemichamp visuel souffrant d'extinction des representations axiales (structurales) basees sur les contours et les axes de symetrie des formes locales (chapitre 4; deux experiences). En resume, les donnees experimentales obtenues dans ce travail de these avec differents paradigmes experimentaux et differentes approches (psychologie, neuropsychologie) sont en faveur d'un systeme visuel elaborant une representation structurale des formes utilisable dans differentes taches perceptives, et peut-etre a differents niveaux de traitement.