Cortical Networks for Visual Reaching: Physiological and Anatomical Organization of Frontal and Parietal Lobe Arm Regions

The functional and structural properties of the dorsolateral frontal lobe and posterior parietal proximal arm representations were studied in macaque monkeys. Physiological mapping of primary motor (Ml), dorsal premotor (PMd), and posterior parietal (area 5) cortices was performed in behaving monkeys trained in an instructed-delay reaching task. The parietofrontal corticocortical connectivities of these same areas were subsequently examined anatomically by means of retrograde tracing techniques. Signal-, set-, movement-, and position-related directional neuronal activities were distributed nonuniformly within the task-related areas in both frontal and parietal cortices. Within the frontal lobe, moving caudally from PMd to the Ml, the activity that signals for the visuospatial events leading to target localization decreased, while the activity more directly linked to movement generation increased. Physiological recordings in the superior parietal lobule revealed a gradient-like distribution of functional properties similar to that observed in the frontal lobe. Signal- and set-related activities were encountered more frequently in the intermediate and ventral part of the medial bank of the intraparietal sulcus (IPS), in area MIP. Movementand position-related activities were distributed more uniformly within the superior parietal lobule (SPL), in both dorsal area 5 and in MIP. Frontal and parietal regions sharing similar functional properties were preferentially connected through their association pathways. As a result of this study, area MIP, and possibly areas MDP and 7m as well, emerge as the parietal nodes by which visual information may be relayed to the frontal lobe arm region. These parietal and frontal areas, along with their association connections, represent a potential cortical network for visual reaching. The architecture of this network is ideal for coding reaching as the result of a combination between visual and somatic information. The act of reaching to visual targets requires the combination of information regarding the locations of external objects with information concerning the configuration of our own body segments with respect to those objects. The present studies were undertaken to investigate, within the cortical areas involved in reaching, the relationships between the representations of sensorimotor information and the anatomical substrates by which this information is combined and transformed in selected cortical areas that are directly involved in the control of movement: the primary motor cortex (MI), the dorsal premotor cortex (PMd), and parts of area 5 in the superior parietal lobule (SPL). The relationship between the physiological properties and the extrinsic cortical connectivity of these cortical regions was studied by combining physiological and anatomical approaches, to obtain data on both the representation and the flow of information to, and through, the cortical frontal and parietal areas involved in the control of reaching. The proximal arm representations of MI and PMd have