Thesis defence : Rémi LAFITTE

Various coordinate systems are elaborated by the brain to assess the position and orientation of objects in relation to oneself and the environment. In this context, the straight-ahead and gravitational vertical are very important spatial reference frames. It is now accepted that the brain codes the direction of these reference frames from sensory integration mechanisms involving vision, somaesthesia, and the vestibular system. The disruption of these mechanisms after a right hemispheric stroke induces perceptual and representational deficits in several dimensions: on the one hand, a bias of the subjective straight-ahead to the right, strongly associated with spatial neglect, and on the other hand, a bias of the subjective vertical to the left (counter-clockwise), strongly associated with postural orientation disorders (lateropulsion). Intriguingly, recent clinical data have shown a very strong association in these same patients between spatial neglect, impaired verticality perception, and lateropulsion. These clinical evidences suggest the hypothesis of a spatial reference frame common for the perception of the straight-ahead and the gravitational vertical. These clinical associations need to be supported on an experimental basis. The general aim of this thesis was therefore to test this "common reference frame" hypothesis. In a first series of clinical studies, we further investigated the link between spatial neglect and impaired verticality perception, using direct (Study 1: neglect tests, verticality perception tests) and indirect (Study 2: writing and drawing tests) clinical measures of these deficits in individuals with right hemispheric stroke. We observed that patients with transmodal vertical tilts (visual and postural) systematically showed signs of spatial neglect, and that patients with abnormal writing and drawing tilts showed particularly severe spatial neglect and impaired verticality perception. In a second series of experimental studies, we tested our hypothesis in healthy individuals by investigating whether sensory/sensory-motor stimulations (Study 3) biased both straight-ahead and verticality perception. We also checked whether verticality perception shared common processes with the lateralized orientation of attention, known to interact with straight-ahead perception (Study 4). The results of Study 3 suggested a modest effect of prism adaptation on visual vertical, observed in two independent experiments. Study 4 showed that the lateral shift of visual attention, induced via a Posner-like paradigm, biased left-right perception but not visual vertical perception. Overall, these mixed results suggest that the brain elaborates representations of the straight-ahead and gravitational vertical via neural bases that are anatomically very close, but underlying sensory integration mechanisms that are probably distinct. Because of this strong anatomical proximity, the modulatory effect of prism adaptation on ego-centered perception could in turn modulate, to a limited extent, gravity-centered perception in healthy subject. In this perspective, it would be interesting to investigate the therapeutic effect of prism adaptation on impaired verticality perception after hemispheric stroke. Our results also encourage clinicians to screen for pathological writing and drawing tilts after right hemispheric stroke, to help in the early diagnosis of spatial neglect and lateropulsion.

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Encadrants :
- Directeur de thèse : Dominic PERENNOU - DPerennouchu-grenoble.fr (DPerennou[at]chu-grenoble[dot]fr)
- Co-encadrant : Eve DUPIERRIX - eve.dupierrixuniv-grenoble-alpes.fr (eve[dot]dupierrix[at]univ-grenoble-alpes[dot]fr)