Spatial task instructions and global activation trends influence functional modularity in the cortical reach network

Humans can be instructed to ignore visual cues or use them as landmarks for aiming movements (Musa et al. 2024), but it is not known how such allocentric cues interact with egocentric target codes and general planning activity to influence cortical network properties. To answer these questions, we applied graph theory analysis (GTA) to a previously described fMRI dataset (Chen et al. 2014). Participants were instructed to reach toward targets defined in egocentric or landmark-centered (allocentric) coordinates. During Egocentric pointing, cortical nodes clustered into four bilateral modules with correlated BOLD signals: a superior occipital-parietal / somatomotor module, an inferior parietal / lateral frontal module, a superior temporal / inferior frontal module, and an inferior occipital-temporal / prefrontal module. The Allocentric task showed only three modules, in part because inferior occipital nodes were incorporated into the superior occipital-parietal / somatomotor module. Both tasks engaged local (within module) and global (between module) cortical hubs, but the Allocentric task recruited additional hubs associated with allocentric visual codes and ego-allocentric integration. Removing reach-related activation trends reduced global synchrony and increased clustering, specifically diminishing dorsoventral coupling in the allocentric task. Cross-validated decoding confirmed that modularity provided the best predicter of task type and suggest that temporal / parietal modules spanning prefrontal cortex play an important role in task instruction. These results demonstrate that activation trends related to motor plans influence global network integration, whereas task instructions influence intermediate / local network properties, such as the modular integration and hub recruitment observed in our Allocentric task.