We traced the cortical connections of the anterior intraparietal (AIP) area, which is known to play a crucial role in visuomotor transformations for grasping. AIP displayed major connections with 1) areas of the inferior parietal lobule convexity, the rostral part of the lateral intraparietal area and the SII region; 2) ventral visual stream areas of the lower bank of the superior temporal sulcus and the middle temporal gyrus; and 3) the premotor area F5 and prefrontal areas 46 and 12. Additional connections were observed with the caudal intraparietal area and the ventral part of the frontal eye field. This study suggests that visuomotor transformations for object-oriented actions, processed in AIP, rely not only on dorsal visual stream information related to the object's physical properties but also on ventral visual stream information related to object identity. The identification of direct anatomical connections with the inferotemporal cortex suggests that AIP also has a unique role in linking the parietofrontal network of areas involved in sensorimotor transformations for grasping with areas involved in object recognition. Thus, AIP could represent a crucial node in a cortical circuit in which hand-related sensory and motor signals gain access to representations of object identity for tactile object recognition.

We have found that the 2 architectonic subdivisions of the prefrontal area 45, 45A and 45B, display connectivity patterns that clearly distinguish them from one another and from their neighboring architectonic areas. Area 45A is primarily connected to the frontal areas 45B, 12l, caudal 12r, 12o, 10, rostrodorsal 46, 9/8B, 44, 8/FEF (frontal eye field), and the SEF (supplementary eye field), temporal area IPa, and unique among all the studied areas, to the superior temporal polysensory (STP) area and auditory parabelt areas. Area 45B displayed much stronger frontal connections with the oculomotor areas 8/FEF, 8r, and the SEF than those of area 45A, primary connections with areas 12l, caudal 12r, 12o, and 8B, and unlike area 45A, with areas ventrorostral 46, rostral 12r, 12m, and 13m. Temporal connections were all virtually confined to areas IPa, intermediate TEa/m, and TE. Additional labeling was found in lateral intraparietal area. Our data suggest that 45A and 45B are 2 distinct areas, possibly playing a differential role in nonspatial information processing: area 45A corresponds to the prefrontal sector for which a role in communication behavior and homology with the human area 45 was proposed, whereas area 45B is a distinct prearcuate area, possibly affiliated to the oculomotor frontal system.

In both monkeys and humans the observation of actions performed by others activates cortical motor areas. An unresolved question concerns the pathways through which motor areas receive visual information describing motor acts. Using functional Magnetic Resonance Imaging (fMRI), we mapped the macaque brain regions activated during the observation of grasping actions, focusing on the superior temporal sulcus region (STS) and the posterior parietal lobe. Monkeys viewed either videos with only the grasping hand visible or videos with the whole actor visible. Observation of both types of grasping videos activated elongated regions in the depths of both lower and upper banks of STS, as well as parietal areas PFG and anterior intraparietal (AIP). The correlation of fMRI data with connectional data showed that visual action information, encoded in the STS, is forwarded to ventral premotor cortex (F5) along two distinct functional routes. One route connects the upper bank of the STS with area PFG, which projects, in turn, to the premotor area F5c. The other connects the anterior part of the lower bank of the STS with premotor areas F5a/p via AIP. While the first functional route emphasizes the agent and may relay visual information to the parieto-frontal mirror circuit involved in understanding the agent's intentions, the second route emphasizes the object of the action and may aid in understanding motor acts with respect to their immediate goal.

We traced the cortical connections of the anterior sector (F5a) of the macaque ventral premotor (PMv) area F5 and compared them with those of the adjacent F5 sectors, F5c and F5p. F5a displays a very dense "intrinsic" connectivity with F5c and F5p, premotor connections limited to F4 and F6/pre-SMA, relatively robust prefrontal connections with areas 46v and 12, and dense connections with rostral opercular frontal areas. Outside the frontal cortex, connections of F5a are dense with the SII region, relatively robust with inferior parietal areas PFG and AIP, weak with the inferior parietal area PF, and moderate with area 24. The comparison with data from injections in F5c and F5p showed that F5a, though sharing some common parietal connections with the other F5 sectors, displays several characterizing features providing robust evidence for its connectional distinctiveness. The present study provides evidence for a general organization of the PMv similar to that of the medial and dorsal premotor cortex, with F5a representing a pre-PMv area. Specifically, the present data suggest that F5a is a privileged site of integration, in the PMv, of parietal sensory-motor signals with higher-order information originating from prefrontal, rostral frontal opercular areas, and F6/pre-SMA. The results of this integration can be then broadcasted to the adjacent F5 sectors for the generation and control of hand actions and cognitive motor functions.