Judgments of spatial relationships are often made when the object or observer are moving. Behaviourally, there is evidence that these ‘dynamic’ judgments of spatial extent differ from static judgments. Here I used three separate techniques for exploring dynamic judgments: first, a line bisection paradigm was employed to study ocular and pointing judgments of spatial extent while manipulating line length, position, speed, acceleration, and direction of scanning (Experiments 1-4); second, functional MRI (fMRI) was used to examine whether distinct brain regions were involved in dynamic versus static judgments of spatial extent (Exp 5); and finally, a mathematical and computational model of dynamic judgments was developed to provide a framework for interpreting the experimental results. In the behavioural experiments, substantial differences were seen between static and dynamic bisection, suggesting the two invoke different neural processes for computing spatial extent. Surprisingly, ocular and pointing judgments produced distinct bisection patterns that were uncorrelated, with pointing somewhat more impervious to manipulations such as scan direction and position than ocular bisections. However, a new experimental task for probing dynamic judgments (the ‘no line’ Experiment 4) found that scan direction can influence both hand behaviour. Functional MRI demonstrated that dynamic relative to static judgments produced activations in the cuneus and precuneus bilaterally, left cerebellum, and medial frontal gyrus, with reduced activation relative to static judgments observed in the supramarginal gyrus bilaterally. Dynamic bisections relative to a control condition produced activations in the right precuneus and left cerebellum, as well as in left superior parietal lobule, left middle temporal gyrus, and right precentral gyrus. It may be the case that velocity processing and temporal estimates are integrated primarily in the cuneus and precuneus bilaterally to produce estimates of spatial extent under dynamic scanning conditions. These results highlight the fact that dynamic judgments of spatial extent engage brain regions distinct from those employed to make static judgments, supporting the behavioural results that these are separate and distinct. Finally, a mathematical model was proposed for dynamic judgments of spatial extent, based on the idea that, rather than using an ‘all-or-none’ approach, spatial working memory actually takes about 100 ms to reach full representational strength for any given point in space. The model successfully explains many of the effects seen in the behavioural experiments including the effects of scan direction, velocity, line length, and position. In conjunction with the neuroimaging data, it also suggests why neglect patients may fail to show rightward bisection biases when making dynamic judgments of spatial extent. Overall, this work provides novel insights into how the brain executes dynamic judgments of spatial extent.

Judgments of spatial relationships are often made when the object or observer are moving. Behaviourally, there is evidence that these ‘dynamic’ judgments of spatial extent differ from static judgments. Here I used three separate techniques for exploring dynamic judgments: first, a line bisection paradigm was employed to study ocular and pointing judgments of spatial extent while manipulating line length, position, speed, acceleration, and direction of scanning (Experiments 1-4); second, functional MRI (fMRI) was used to examine whether distinct brain regions were involved in dynamic versus static judgments of spatial extent (Exp 5); and finally, a mathematical and computational model of dynamic judgments was developed to provide a framework for interpreting the experimental results. In the behavioural experiments, substantial differences were seen between static and dynamic bisection, suggesting the two invoke different neural processes for computing spatial extent. Surprisingly, ocular and pointing judgments produced distinct bisection patterns that were uncorrelated, with pointing somewhat more impervious to manipulations such as scan direction and position than ocular bisections. However, a new experimental task for probing dynamic judgments (the ‘no line’ Experiment 4) found that scan direction can influence both hand behaviour. Functional MRI demonstrated that dynamic relative to static judgments produced activations in the cuneus and precuneus bilaterally, left cerebellum, and medial frontal gyrus, with reduced activation relative to static judgments observed in the supramarginal gyrus bilaterally. Dynamic bisections relative to a control condition produced activations in the right precuneus and left cerebellum, as well as in left superior parietal lobule, left middle temporal gyrus, and right precentral gyrus. It may be the case that velocity processing and temporal estimates are integrated primarily in the cuneus and precuneus bilaterally to produce estimates of spatial extent under dynamic scanning conditions. These results highlight the fact that dynamic judgments of spatial extent engage brain regions distinct from those employed to make static judgments, supporting the behavioural results that these are separate and distinct. Finally, a mathematical model was proposed for dynamic judgments of spatial extent, based on the idea that, rather than using an ‘all-or-none’ approach, spatial working memory actually takes about 100 ms to reach full representational strength for any given point in space. The model successfully explains many of the effects seen in the behavioural experiments including the effects of scan direction, velocity, line length, and position. In conjunction with the neuroimaging data, it also suggests why neglect patients may fail to show rightward bisection biases when making dynamic judgments of spatial extent. Overall, this work provides novel insights into how the brain executes dynamic judgments of spatial extent.

Peripersonal (reachable) and extrapersonal (beyond reach) space is linked to hand perception. Using a tool to reach farther than normal recalibrates previously unreachable space as peripersonal, evidenced by Intraparietal Sulcus (IPS) activity related to hand perception and lateral biases during line bisection. The current study looked at the role of a visual connection between the hand and body in the ability to manipulate objects within the extended area of reach. In an immersive virtual environment, participants bisected lines using a connected hand (via arm), a disconnected hand, or a floating dot. A rightward shift in bisection was seen only for the dot condition for far lines, indicating that it was the only "tool" incapable of extending peripersonal space.

Peripersonal (reachable) and extrapersonal (beyond reach) space is linked to hand perception. Using a tool to reach farther than normal recalibrates previously unreachable space as peripersonal, evidenced by Intraparietal Sulcus (IPS) activity related to hand perception and lateral biases during line bisection. The current study looked at the role of a visual connection between the hand and body in the ability to manipulate objects within the extended area of reach. In an immersive virtual environment, participants bisected lines using a connected hand (via arm), a disconnected hand, or a floating dot. A rightward shift in bisection was seen only for the dot condition for far lines, indicating that it was the only "tool" incapable of extending peripersonal space.

Nous sommes fonctionnellement et structurellement asymétriques. La symétrie parfaite que nous expérimentons subjectivement en observant l'espace qui nous entoure est, dans une certaine mesure, une illusion. La cognition visuospatiale, comme indiqué par les tâches de bissection de lignes, est généralement biaisée à gauche chez les sujets sains et à droite suite à des lésions de l'hémisphère droit causant la Négligence Spatiale Unilatérale (NSU). Ces biais peuvent être modulés et l'adaptation prismatique (AP) a démontré sa capacité à réduire les symptômes de la NSU et à induire des comportements similaires à la NSU chez les individus sains. La question de savoir comment ce type d'adaptation sensorimotrice module la cognition spatiale est encore débattue. L'objectif de cette thèse était d'utiliser des approches comportementales et physiologiques, pour examiner les mécanismes sous-jacents des effets de l'AP sur la cognition visuospatiale d'individus sains. Dans une 1ère étude comportementale, nous avons observé la présence d'une dynamique temporelle des effets survenant après l'AP. Suite à ce premier résultat, nous avons testé sur une période de temps plus longue les effets faisant suite à l'AP déviant la vision vers la droite ou la gauche, et nous avons dévoilé, dans une 2ème étude, des dynamiques temporelles différentes en fonction de la direction de l'AP. Dans une 3ème étude, nous avons utilisé la stimulation magnétique transcrânienne pour étudier la physiologie sous-tendant la modulation visuospatiale efficacement induite par l'AP. Les résultats de cette thèse appellent à un raffinement des modèles actuels de l'action de l'AP sur la cognition visuospatiale / We are functionally and structurally asymmetric. The perfect symmetry we subjectively experience through vision of the space around us is, to some extent, an illusion. Visuospatial cognition, as indexed by performance on line bisection tasks, is generally biased leftward in healthy individuals and pathologically rightward after right brain damage causing unilateral spatial neglect (USN). These biases can be modulated and prism adaptation (PA) is capable of both alleviating USN symptoms and inducing a rightward shift (the so-called “neglect-like behavior”) in healthy individuals. How this type of sensorimotor adaptation modulates spatial cognition is still debated. The goal of this thesis was to use both behavioral and physiological approaches to investigate the underlying mechanisms of PA’s effects on visuospatial cognition in healthy individuals. In a first behavioral study we found the presence of a temporal dynamic in PA after-effects. Based on this first finding we tested, over a longer period of time the PA after-effects following both right and leftward PA and unveiled, with the second study, different temporal dynamics depending on PA direction. In a third study we used transcranial magnetic stimulation to investigate the physiology underlying the effective visuospatial modulation induced by PA. The results of this thesis call for a refinement of the current models of PA action on visuospatial cognition