Modular Integration of Auditory Instructions and Visual Cues into the Cortical Reach Network

Humans often need to integrate bottom-up stimulus attributes (e.g., object shape) with top-down instructions (e.g., how to pick it up) for adaptive behavior. To investigate how such cues are integrated into the cortical reach network, we employed an fMRI paradigm where participants viewed a cube to their left or right (visual cue) and were verbally instructed to use a horizontal or vertical grip (auditory cue), in varying order. Univariate voxel-wise and region-of-interest analysis confirmed the expected order-dependent activation of sensory regions, followed by robust parietofrontal activation. Graph-theory analysis revealed two significant subnetworks (modules): an Occipital-Parietal module (likely integrating the visual cue into the reach plan) and a Temporal-Frontal module (likely integrating the auditory cue into the grasp plan), both converging in somatomotor cortex. Cue-order influenced modularity, i.e., the module corresponding to the first cue tended to dominate contralateral motor cortex, and conversely a classification model decoded cue-order based on modularity. Local network hubs were order-independent and occupied traditional reach areas, whereas global (between module) hubs were order-dependent and more distributed. Overall, these results suggest that multisensory integration of top-down and bottom-up cues for action is a network phenomenon that relies on both lateral and serial communication between parallel sensorimotor modules.