Resumo

The execution of fine hand movement is a landmark in evolution, allowing certain primates to manipulate the external environment according to their needs. The posterior parietal cortex plays a crucial role in the implementation of such movements working as an essential cortical node for the integration of visual, somatosensory, and motor information. The Cebus Apella is the only New World monkey capable of performing precision grips by the opposition of the thumb with the index finger, making it a very interesting model for anatomical studies of the cortical connections subserving primate manual habilities. After a session of electrophysiological mapping of the parietal cortex, we injected the fluorescent neurotracer fast-blue in the hand/forearm representation of area 5, located in the anterior bank of the intraparietal sulcus (IPS). Additional neurotracer injections of fluoro-emerald and diamidino-yellow were respectively made in area 7b-IPS (located in the posterior bank of IPS, immediately caudal to area 5), and in the lateral portion of area 7b near the lateral sulcus (area 7b-L). After a survival of 14 days, the animal was perfused, and sagittal brain slices were subjected to histological processing. Retrogradely labeled neurons, blood vessels, and cortical section contours were plotted using a fluorescent microscope equipped with the Neurolucida system (MBF, Inc). Architectonic identification of cortical areas containing labeled cells was performed on adjacent Nissl-stained sections. Area 7b-L receives projections from a number of cortical areas, including areas of anterior (area 3a) and posterior (areas 5, MIP, VIP, 7b-IPS, PFG and PF) parietal cortex, somatosensory areas of lateral parietal cortex (areas S2 and PV), and areas of insular, motor, and pre-motor cortices. Additionally, area 7b-L receives projections from high-order visual areas (MST and FST) of the superior temporal lobule and from prefrontal fields located lateral to the principal sulcus. Therefore, area 7b-L satisfies the necessary criteria for a cortical node of somatosensory, visual, and motor integration. The region we called 7b-IPS is located at the anterior part of the posterior bank of IPS. This corresponds to the region in which the anterior intraparietal area (AIP) was described in macaque monkeys. In cebus, this cortical field receives projections from other posterior parietal cortical areas (MIP, VIP, PIP, LIP, PFG, PF, and 7b-L), from somatosensory areas S2 and PV, from premotor areas, and from prefrontal fields. We did not detect any connections from higher order visual areas or from areas located in the anterior bank of IPS to area 7b-IPS. In macaque monkeys, this region (AIP) is known to play a crucial role in visuomotor transformations for grasping. However in cebus monkeys, the connections described for area 7b-L seems more suitable to support this function. Finally, the representation of the hand in area 5 receives projections from areas of the somatosensory cortex (3a, 3b, 1 and 2), and from the posterior parietal cortex (MIP, VIP, PF, PFG and 7b-L). It also receives information from motor and premotor areas, from the cingulate cortex, and from the claustrum. Such connectional pattern give support to the notion that area 5 has an important role in the integration of somatosensory and motor information.