| Goal-directed action involves transformations from visual inputs to motor commands. In most everyday situations, we use vision to guide movements of the hand when we perform various tasks, such as reaching to and grasping a cup of coffee, driving a car, using a tool, or working on a computer. In other words, eye-hand coordination is central to basic life-skills, work, and recreational activities of all people. And yet, the transformations from sensory inputs to motor action are poorly understood in humans.;First, I showed that human PPC is critical for both saccade and reach planning. Second, I determined that human PPC has functionally distinct regions for saccade and reach: a partially overlapping anterior-lateral cluster (in particular, regions over the medial intraparietal sulcus and angular gyrus) for both saccades and reach planning and a more medial-posterior region (in the superior parieto-occipital cortex) for reach planning only. Finally, I causally demonstrate that human PPC has two reach-related regions: the anterior-lateral cluster encodes hand position or movement vector, or both, whereas the more posterior-medial region encodes the movement goal.;This thesis provides novel causal evidence that PPC plays a fundamental role in the planning and transformation processes involved in visually guided movements in healthy humans.;Converging evidence spanning nonhuman primate neurophysiology and human neuropsychology suggests that posterior parietal cortex (PPC) is critical in spatial processing and visually guided action. Single-unit recordings in monkey have identified effector-specific regions in PPC, but functional neuroimaging in the human have yielded controversial results. This thesis investigated the early stages of movement planning by examining saccade and reach behaviour of healthy humans while transiently disrupting brain activity in selective parietal regions using a technique called transcranial magnetic stimulation (TMS). TMS interferes with the function of a particular cortical region, at a specific stage of processing in a given task, thereby inducing a temporary "virtual lesion", allowing for the investigation of the relations between cortical areas and behaviour. |
