The brain location and topographical organization of the cortical visual area V6 was studied in five hemispheres of four awake macaque monkeys. Area V6 is located in the caudal aspect of the superior parietal lobule (SPL). It occupies a 'C'-shaped belt of cortex whose upper branch is in the depth of the parieto-occipital sulcus (POS) and lower one is in the depth of the medial parieto-occipital sulcus (POM), with the medial surface of the brain as a zone of junction between the two branches. Area V6 contains a topographically organized representation of the contralateral visual field up to an eccentricity of at least 80 degrees. The lower visual field representation is located dorsally, in the ventral part of POS, and the upper field ventrally, in the dorsal wall of POM. The representation of the horizontal meridian forms the posterior border of V6. It is adjacent to area V3 in POS as well as in the caudal part of POM, on the ventral convexity of the brain. The lower vertical meridian forms the anterior border of V6, adjacent to area V6A. The upper vertical meridian is in the depth of POM. The representation of the central visual field is not magnified relative to that of the periphery. The central visual field (below 20-30 degrees of eccentricity) is represented in the medial-most aspect of the annectant gyrus, in the lateral part of the posterior bank of POS. The visuotopic organization of area V6 suggests a role in the analysis of the flow field resulting from self-motion, in selecting targets during visual searching as well as in the control of arm-reaching movements towards non-foveated targets.

The brain location, extent and functional organization of the cortical visual area V6A was investigated in macaque monkeys by using single cell recording techniques in awake, behaving animals. Six hemispheres of four animals were studied. Area V6A occupies a horseshoe-like region of cortex in the caudalmost part of the superior parietal lobule. It extends from the medial surface of the brain, through the anterior bank of the parieto-occipital sulcus, up to the most lateral part of the fundus of the same sulcus. Area V6A borders on areas V6 ventrally, PEc dorsally, PGm medially and MIP laterally. Of 1348 neurons recorded in V6A, 61% were visual and 39% non-visual in nature. The visual neurons were particularly sensitive to orientation and direction of movement of visual stimuli. The inferior contralateral quadrant was the most represented one. Visual receptive fields were also found in the inferior ipsilateral quadrant and in the upper visual field. Receptive fields were on average smaller in the lower visual field than in the upper one. Both central and peripheral parts of the visual field were represented. Large parts of the visual field were represented in small regions of area V6A, and the same regions of the visual field were re-represented many times in different parts of this area, without any apparent topographical order. The only reliable sign of retinotopic organization was the predominance of central representation dorsally and far periphery ventrally. The functional organization of area V6A is discussed in the view that this area could be involved in the control of reaching out and grasping objects.

The medial parieto-occipital cortex is a central node in the dorsomedial visual stream. Recent physiological studies in the macaque monkey have demonstrated that the medial parieto-occipital cortex contains two areas, the visual area V6 and the visuomotor area V6A. Area V6 is a retinotopically organized visual area that receives form and motion information directly from V1 and is heavily connected with the other areas of the dorsal visual stream, including V6A. Area V6A is a bimodal visual/somatosensory area that elaborates visual information such as form, motion and space suitable for the control of both reaching and grasping movements. Somatosensory and skeletomotor activities in V6A affect the upper limbs and involve both the transport phase of reaching and grasping movements. Finally, V6A is strongly and reciprocally connected with the dorsal premotor cortex controlling arm movements. The picture emerging from these data is that the medial parieto-occipital cortex is well equipped to control both proximal and distal movements in the online visuomotor guidance of prehension. In agreement with this view, selective V6A lesions in monkey produce misreaching and misgrasping with the arm contralateral to the lesion in visually guided movements. These deficits are similar to those observed in optic ataxia patients and suggest that human and monkey superior parietal lobules are homologous structures, and that optic ataxia syndrome is the result of the lesion of a 'human' area V6A.

The aim of this work was to study the cortical connections of area V6 by injecting neuronal tracers into different retinotopic representations of this area. To this purpose, we ®rst functionally recognized V6 by recording from neurons of the parietooccipital cortex in awake macaque monkeys. Penetrations with recording syringes were performed in the behaving animals in order to inject tracers exactly at the recording sites. The tracers were injected into the central or peripheral ®eld representation of V6 in different hemispheres. Irrespective of whether injections were made in the centre or periphery, area V6 showed reciprocal connections with areas V1, V2, V3, V3A, V4T, the middle temporal area/V5 (MT/V5), the medial superior temporal area (MST), the medial intraparietal area (MIP), the ventral intraparietal area (VIP), the ventral part of the lateral intraparietal area (LIP V ) and the ventral part of area V6A (V6A V ). No labelled cells or terminals were found in the inferior temporal, mesial and frontal cortices. The connections of V6 with V1, and with all the retinotopically organized prestriate areas, were organized retinotopically. The connection of V6 with MIP suggests a visuotopic organization for this latter. Labelling in V6A and VIP after either central or peripheral V6 injections was very similar in location and extent, as expected on the basis of the nonretinotopic organization of these areas. We suggest that V6 plays a pivotal role in the dorsal visual stream, by distributing the visual information coming from the occipital lobe to the sensorimotor areas of the parietal cortex. Given the functional characteristics of the cells of this network, we suggest that it could perform the fast form and motion analyses needed for the visual guiding of arm movements as well as their coordination with the eyes and the head.

Superior area 6 of the macaque monkey frontal cortex is formed by two cytoarchitectonic areas: F2 and F7. In the present experiment, we studied the input from the superior parietal lobule (SPL) to these areas by injecting retrograde neural tracers into restricted parts of F2 and F7. Additional injections of retrograde tracers were made into the spinal cord to define the origin of corticospinal projections from the SPL. The results are as follows: 1) The part of F2 located around the superior precentral dimple (F2 dimple region) receives its main input from areas PEc and PEip (PE intraparietal, the rostral part of area PEa of Pandya and Seltzer, [1982] J. Comp. Neurol. 204:196-210). Area PEip was defined as that part of area PEa that is the source of corticospinal projections. 2) The ventrorostral part of F2 is the target of strong projections from the medial intraparietal area (area MIP) and from the dorsal part of the anterior wall of the parietooccipital sulcus (area V6A). 3) The ventral and caudal parts of F7 receive their main parietal input from the cytoarchitectonic area PGm of the SPL and from the posterior cingulate cortex. 4) The dorsorostral part of F7, which is also known as the supplementary eye field, is not a target of the SPL, but it receives mostly afferents from the inferior parietal lobule and from the temporal cortex. It is concluded that at least three separate parietofrontal circuits link the superior parietal lobule with the superior area 6. Considering the functional properties of the areas that form these circuits, it is proposed that the PEc/PEip-F2 dimple region circuit is involved in controlling movements on the basis of somatosensory information, which is the traditional role proposed for the whole dorsal premotor cortex. The two remaining circuits appear to be involved in different aspects of visuomotor transformations.