Hee Kyung Yoon¹, Yaelan Jung¹, Daniel Dilks¹; ¹Emory University

The parietal cortex is widely thought to support visually guided actions, but whether it contains distinct regions specialized for different action classes – 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, yet whether these regions are truly dissociable is unclear because each has never been tested for the other function. Here, we addressed this question using fMRI in human adults. Participants viewed four types of stimuli: Dynamic Scenes (video clips of first-person motion through scenes), Static Scenes (static images taken from these same movies, rearranged such that first-person motion could not be inferred), Contextual Reaching (video clips of first-person reaching motion on a scene background), and Isolated Reaching (video clips of the same actions on a black background). A clear 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 condition. By contrast, SPOC responded significantly more to both reaching conditions than to either scene condition. Resting-state functional connectivity further supported this double dissociation: SPL showed stronger connectivity with “leg-motor cortex” than with “arm-motor cortex”, whereas SPOC showed the opposite pattern. Together, these findings reveal two distinct parietal systems: SPL for visually-guided navigation and SPOC for visually-guided reaching – clarifying how the parietal cortex organizes visually guided action.

Acknowledgements: R01 EY29724