Projections to the superior colliculus from inferior parietal, ventral premotor, and ventrolateral prefrontal areas involved in controlling goal-directed hand actions in the macaque.

We found that the macaque inferior parietal (PFG and anterior intraparietal [AIP]), ventral premotor (F5p and F5a), and ventrolateral prefrontal (rostral 46vc and intermediate 12r) areas forming a network involved in controlling purposeful hand actions ("lateral grasping network") are a source of corticotectal projections. Based on injections of anterograde tracers at the cortical level, the results showed that all these areas displayed relatively dense projections to the intermediate and deep gray layers of the ipsilateral superior colliculus (SC) and to the ventrally adjacent mesencephalic reticular formation. In the SC, the labeling tended to be richer in the lateral part along almost the entire rostro-caudal extent, that is, in regions controlling microsaccades and downward gaze shifts and hosting arm-related neurons and neurons modulated by the contact of the hand with the target. These projections could represent a descending motor pathway for controlling proximo-distal arm synergies. Furthermore, they could broadcast to the SC information related to hand action goals and object affordances extraction and selection. This information could be used in the SC for controlling orienting behavior (gaze and reaching movements) to the targets of object-oriented actions and for the eye-hand coordination necessary for appropriate hand-object interactions.