This thesis focuses on the fast processing of visual information in natural scenes. It hinges on 2 chaptersboth containing a review of the literature and research papers describing experimental work completed during the thesis.Chapter 1 addresses first the degree of parallelism in the processing of natural scenes. In opposition withserial models postulating that objects are analyzed one after the other by the visual system, the detailedreview of the literature suggests a large part of parallelism is present in visual processing. Interference between object representations would occur mainly at the decisional level, probably within frontal areas. The first two papers of this thesis address the question of object categorization in natural scenes and present data in favor of this hypothesis. The second part of chapter 1 focuses on parallel processing which allows us to extract the meaning of the general context of a scene(back-ground). Paper 3 describes the efficiency ofthe visual system in extracting the global meaning of a scene in a rapid manner and suggests that it mightinteract in parallel with the categorization of objects. Paper 4 attempts to clarify the involvement of bottom-up and top-down visual factors in the analysis of natural scenes.Among all categories, human faces could be processed in a very specific way. Chapter 2 discusses some arguments in favor of the specificity of underlying mechanisms. Alternative explanations are suggested, allowing us to consider a unique model of visual processing for all object categories. Paper 5 shows that at the behavioral level human faces in natural scenes are not processed faster than other categories of familiar objects. Paper 6 tries to determine the processing time of these stimuli at the electrophysiological level. Several hypotheses are discussed. Paper 7 shows that the N170 is not as specific to human faces as commonly thought. What seems to be specific to human stimuli is the magnitude of the inversion effect at the behavioral and electrophysiological levels. All these results are discussed in the context of current models of visual processing.
