This study attempts to resolve to what extent individuals with Down syndrome benefit from advance information provided visually, versus the same information provided verbally when performing a manual aiming task. Adults with Down syndrome and undifferentiated developmental handicaps, as well as control subjects without a developmental handicap, performed manual aiming movements to targets 10.5 cm. away. On a particular trial, subjects were cued about the specific movement either visually or verbally. The cue provided either 50% or 80% certainty. Nonhandicapped control subjects initiated and completed their manual aiming movements more quickly than subjects with mental handicaps. As well, individuals with Down syndrome were found to be slower and more variable in reaction time than individuals in the other mentally handicapped group when valid information was provided verbally but not when the cue was provide visually. These results provide support for the Elliott and Weeks (1990) model of biological dissociation. Specifically, the atypical hemispheric lateralization for speech perception exhibited by individuals with Down syndrome results in a disruption in communication between functional systems responsible for the processing of verbal langauge, and the organization of complex movement. / Thesis / Master of Science (MS)

When exploring movement production, motor control researchers have been interested in investigating the relative contributions to different types of movement. In a research setting, a startling acoustic stimulus (SAS) can be used as a tool to explore the neural processes that are occurring when preparing and initiating a movement. Additionally, suprathreshold TMS is another tool which can induce a suppression of the cortical region of the brain, resulting in RT delays which provides us with the ability to assess the corticospinal contributions to a particular movement. The aim of the current study was to investigate potential differences in the planning and execution of bimanual versus unimanual wrist extension movements. It was of particular interest as to whether bimanual coupling occurs at the cortical level or in lower parts of the output pathway (reticulospinal). Participants (N=6) were instructed to complete a unimanual or bimanual wrist extension following a control go-signal or a SAS. For subset of trials, in order to explore the level of corticospinal excitability of the movement, suprathreshold TMS was applied over the left M1 during the task to induce a cortical silent period (CSP). Results revealed that theimpact of TMS on response initiation was not significantly different for unimanual task versus a bimanual task. Furthermore, the SP (silent period) only had an impact on the right limb and not the left during the bilateral task. Lastly, SAS did lead to shorter RTs for both the unimanual and bimanual wrist extension task, but the RT delay induced by TMS in the right limb was not shorter in SAS trials compared to control. The findings of the present study suggest that bimanual coupling may be occurring at the cortical level and in lower parts of the output pathway as there may be correlated neural activity in the two hemispheres occurring during bimanual wrist extension movements.

Le contexte temporel influence profondément la façon dont nous nous comportons. De manière similaire, il donne forme à l'activité du cortex moteur (LFP et potentiels d'action), pendant la préparation motrice, mais aussi en absence de préparation d'un mouvement. / The temporal context deeply shapes the motor cortical activity (spikes and LFPs), during movement preparation but also outside movement preparation.

Pour un mouvement de saisie, le système moteur doit contrôler un certain nombre de paramètres pour produire une commande motrice adaptée aux propriétés de l'objet. La compréhension des mécanismes mis en jeu dans l’élaboration de cette commande motrice repose ainsi sur plusieurs questions. Quelle est la nature des paramètres traités par le système nerveux ? Quelles sont les structures corticales impliquées ? Quand et comment ces paramètres sont-ils traités ? Durant l’exécution du mouvement ou durant la phase de préparation qui précède son initiation ? Ces questions sont autant de problématiques abordées dans ce travail de thèse dont l'objectif général est d'apporter une meilleure compréhension, d'une part, de l'organisation fonctionnelle des processus mentaux qui lient la perception à l'action, et d'autre part, de la façon dont ces processus se traduisent au niveau neurophysiologique. Nos recherches reposent notamment sur la comparaison entre l'homme et le singe étudiés dans un contexte expérimental similaire et réalisant une tâche de saisie manuelle identique. L'ensemble de nos travaux ont orienté notre réflexion vers 3 axes principaux. Premièrement, ils nous ont permis de préciser les principes fonctionnels qui régissent la préparation des mouvements de saisie manuelle. Deuxièmement, ils nous ont conduits à identifier plusieurs composantes qui caractérisent les modulations du rythme bêta (13 -35 Hz) et à dégager les principaux facteurs régissant leur présence ou leur absence. En ce sens, nous avons proposé une hypothèse qui permet d’interpréter dans un cadre théorique unifié la majorité des études proposant des interprétations souvent contradictoires du rythme bêta. / For grasping, the motor system has to control several movement parameters to produce a motor command adapted to the object properties. The understanding of the mechanisms involved in the development of this motor command relies on several questions. What kinds of parameters are processed by the nervous systems? What are the cortical structures involved? When and how these parameters are processed? During the execution or during the preparation phase preceding movement initiation? All these questions are addressed in this thesis which general objective is to provide a better understanding of the mental processes linking perception to action and to clarify how the functional organization of these processes is reflected in the neurophysiological activity. Our research is based in particular on the comparison between humans and monkeys studied in a similar experimental setting and performing an identical reach-to-grasp task. The results of this work led us to focus our discussion on three main axes. First, they allowed to specify the functional principles underlying the preparation of grasping movements. Second, we identified several components that characterize the modulations of the beta rhythm (15-35 Hz) and pinpointed the main factors governing their presence or absence. In this sense, we propose a hypothesis for interpreting in a unified theoretical framework a large number of studies that often provide conflicting interpretations of this sensorimotor rhythm.