Allocentric Versus Egocentric Representations for Visual Memory and Action in Human Cortex

In daily life, people frequently perform various aiming movements, such as reaching or making a saccade toward a cellphone. The early stage for executing such movements is to localize the target location precisely. A visual target can be represented and maintained in memory in two main reference frames: egocentric (body-fixed) or allocentric (world-fixed). However, the neural mechanisms for the allocentric spatial processing are poorly understood and for the Allo-Ego conversion are still unknown in humans. This thesis investigated the allocentric and egocentric mechanisms with a focus on target memory coding for reaching (study 1) and saccades (study 2) in healthy humans using event-related functional magnetic resonance imaging (fMRI) designs where the phase of memorized target representation was separated from the phase of motor planning and execution. I further examined neural substrates for Allo-Ego conversion of targets for reach in study 3 using different types of cues to specify reach target direction for two reach tasks before delay or response phases. I observed widely overlapping cortical areas in the egocentric and allocentric reach tasks as compared to the control, but higher activation in parietofrontal areas for the former, and higher activation in early visual areas for the latter. Further, directional selectivity in egocentric coordinates (target relative to gaze/midline) was observed in superior occipital and inferior occipital gyrus; on the other hand, directional selectivity in allocentric coordinates (target relative to a landmark) was revealed in inferior temporal gyrus and inferior occipital gyrus. These results indicate that different cortical mechanisms are involved in the representations of remembered reach targets. I found similar pattern of task-relevant activation and egocentric directional selectivity in the saccade study. However, different areas from those observed in the reach study showed allocentric directional selectivity of remembered saccade targets including precuneus and midposterior intraparietal sulcus, suggesting effector-specific (eye vs. hand) neural mechanisms. In study 3, I identified four areas in parietal and frontal cortex, i.e., posterior precuneus, angular gyrus, supramarginal gyrus and medial frontal gyrus that are specifically involved in converting allocentric target coding to egocentric representation as soon as the final target location for reach is specified.