Dissociable systems for visually guided navigation versus reaching and grasping in human parietal cortex.

Parietal cortex is thought to support visually guided actions, but whether it contains distinct regions specialized for different actions-such as navigation, reaching, and grasping-remains unknown. Prior work implicates the superior parietal lobule (SPL) in navigation and the superior parietal occipital cortex (SPOC) in reaching and grasping, yet whether these are truly dissociable is unclear. We addressed this using fMRI in adults. Participants viewed four stimulus types: Dynamic Scenes (first-person motion through scenes), Static Scenes (static images from these movies), Contextual Reaching and Grasping (first-person reaching and grasping on a scene background), and Isolated Reaching and Grasping (reaching and grasping actions on a black background). A double dissociation emerged: SPL responded significantly more to Dynamic than Static Scenes-consistent with its role in visually-guided navigation-and, critically, more to Dynamic Scenes than either reaching and grasping condition. By contrast, SPOC responded significantly more to both reaching and grasping conditions than to either scene condition. Resting-state functional connectivity further supported this double dissociation: SPL showed stronger connectivity with "leg-motor" than "arm-motor" cortex, whereas SPOC showed the opposite pattern. These findings reveal two distinct parietal systems: SPL for visually-guided navigation and SPOC for visually-guided reaching and grasping-clarifying how the parietal cortex organizes visually guided action.