`Object constancy' is the name given to the brain's ability to overcome the myriad environmental obstacles to visual perception and produce a stable, consistent internal representation of object shape. Changes in object orientation represent one such confound. It can be inferred from the time taken to recognise misoriented objects that we encode specific object views based on our experience of those objects and their typical orientations ('viewpoint-dependent recognition'). Such studies also suggest that we may recognise certain objects in a manner that is not dependent on their orientation ('viewpoint-invariant recognition'). Further studies indicate that the time to resolve two angularly disparate shapes (`mental rotation') increases as a function of their angular disparity. It is hypothesised, based on these findings, that viewpoint-dependent recognition and mental rotation share a common mechanism for transforming the global stimulus percept into alignment, but that viewpointinvariant recognition is achieved by some other, non-transformational means. This thesis presents studies that examine the cortical correlates of viewpoint-dependent and viewpointinvariant object recognition using novel objects to eliminate the confounding effects of prior experience. It also presents a study that directly compares the cortical correlates of mental rotation, viewpoint-dependent and viewpoint-invariant recognition. Further comparison of these object constancy processes is then made using electrophysiological markers of visuospatial transformation. The findings of these studies indicate that viewpoint-dependent recognition and mental rotation recruit a bilateral parietal-premotor network for the manipulation of global stimulus percepts, hypothesised to be the same mechanism as that used for physical object manipulation and prehension. Viewpoint-invariant recognition does not appear to recruit such a mechanism, and this process appears to be less expensive in terms of cognitive resources than transformational object constancy mechanisms. Thus, implementation of a viewpoint-invariant mechanism to recognise misoriented objects is preferable, but may not be possible where stimulus features are few or ambiguous. In recognising misoriented objects, viewpoint-dependent and viewpoint-invariant mechanisms initially proceed in parallel, but successful recognition of object invariant features may be sufficient to terminate the viewpoint-dependent mechanism.

En pleine extension, la population sans domicile fixe (SDF) est devenue une problématique sociétale majeure qui confronte le dispositif de veille sociale à son impuissance. L'objectif de cette recherche est de comprendre comment une frange de la population SDF parvient à entrer dans une dynamique d'insertion quand une autre se chronicise dans sa situation. Pour ce faire, trois groupes sont comparés: un groupe de 24 SDF fréquentant l'hébergement d'urgence, un groupe de 25 SDF inscrits dans une structure d'insertion et un groupe de 25 SDF, "chronicisés", accueillis dans une structure de stabilisation. Un entretien semi-directif est mené auprès de chaque participant, et sont administrées l'échelle d'anxiété et de dépression (Zigmond et Snaith, 1983) et l'échelle d'estime de soi de Rosenberg (1965). une adaptation a été nécessaire pour l'échelle de lieu de contrôle (IPAH-Jutras, 1987) et pour le Questionnaire de Soutien Social Perçu (Bruchon-Schweitzer et Quintard, 2001), également administrés. de nombreuses variables différencient significativement les groupes SDF. Ces résultats permettent d'établir des liens entre le type de prise en charge d'une part, la souffrance psychique, la qualité du lien social et la perturbation identitaire d'autre part. Totalement déstructurante pour les SDF, la situation d'hébergement d'urgence est source d'une intense souffrance psychique. Ayant préservé ce qui fonde leur identité, certains SDF parviennent à s'appuyer sur le lien social pour s'inscrire dans une dynamique d'insertion. A l'inverse, d'autres mettent en place des défenses psychiques et des stratégies de survie qui ont l'effet paradoxal de favoriser leur chronicisation tout en diminuant drastiquement leur souffrance. C'est en considérant la santé mentale des SDF et leurs stratégies d'adaptation que nous pourrons améliorer le dispositif afin que la prise en charge soit adaptée à cette population. / The growing population of homeless has become a major social issue which forces the Social Watch to face its own inefficiency. The purpose of this study is to understand how a part of the homeless manages to achieve a process of social reintegration while another stays in a chronic state of homelessness. To do so, three groups have been compared: a group of 24 homeless people using the emergency center, a group of 25 homeless registered in a social reintegration center and a group of 25 homeless, in a "chronic state of homelessness", sheltered in a center of stabilization. A semi-structured interview is carried out with each participant. their are assessed on the Hospital Anxiety and Depression Scale (Zigmond and Snaith, 1983) and the Rosenberg Self-Esteem Scale (1965). An adaptation is necessary for the Locus of Control Scale (IPAH - Jutras, 1987) and for the Perceived Social Support Questionnaire (Bruchon-Schweitzer et Quintard, 2001), which are also used. Numerous variables clearly dissociate these groups of homeless people. The results show the link between the type of social care, mental suffering, the strength of the social link and the disruption of one'x identuty. Deeply destructive for homeless people, the emergency center's environment is a source of intense psychological sufferinf. Having preserved what shapes their identity, some homeless manage to rely on this social link to start a pocess of integration. On the contrary, others build psychological fences and strategies to survive that paradoxically reinforce their chronic homelessness while drastically reducing their sufferings. By taking homeless people's mental health into account and their strategies to adapt, we will be abl to improve our plan of action so that this population can be taken care of properly.