The medial fronto-parietal network of the macaque: Different specializations for spatial parameters of reaching?

Neuronal populations in parietal and frontal cortices orchestrate a series of visuomotor transformations crucial for executing successful reaching movements. Two key nodes in this network are the dorsal premotor area F2 and the medial posterior parietal area V6A, which are strongly reciprocally connected (Gamberini et al., 2021). Extensive research in premotor areas primarily focused on center-out reaches, which did not account for reach depth. Therefore, it is unknown whether tuning by spatial and motor variables, such as reach direction and depth, is similar in frontal and parietal cortices and whether there are differences in the temporal evolution of activity that reflects functional specializations. To fill this gap, we recorded single-neuron activity from two regions of the medial fronto-parietal circuit in macaques performing an instructed delay reaching task toward targets varying in both direction and depth. We then compared how various spatial and temporal movement parameters are represented both at the neural and population levels across these two network nodes. Our results reveal that neurons in the medial posterior parietal cortex (mPPC) exhibit sharper spatial tuning, with a particularly robust representation of target depth, compared to the broader, more gradual activation patterns observed in the dorsal premotor cortex (PMd). Moreover, greater decoding accuracy for direction and depth has been found in mPPC. These findings suggest that mPPC is specialized for accurate and reliable spatial encoding that is essential for dynamic sensorimotor transformations, whereas PMd may primarily support the planning and initiation of motor actions directed toward a continuum of spatial locations.