Eye-hand coordination during reaching. I. Anatomical relationships between parietal and frontal cortex.

The anatomical and physiological substrata of eye-hand coordination during reaching were studied through combined anatomical and physiological techniques. The association connections of parietal areas V6A and PEc, and those of dorso-rostral (F7) and dorso-caudal (F2) premotor cortex were studied in monkeys, after physiological characterization of the parietal regions where retrograde tracers were injected. The results show that parieto-occipital area V6A is reciprocally connected with F7, and receives a smaller projection from F2. Local parietal projections to V6A arise from areas MIP and, to a lesser extent, 7m, PEa and PEC: On the contrary, parietal area PEc is strongly and reciprocally connected with the part of F2 located close to the pre-central dimple (pre-CD). Local parietal projections to PEc come from a distributed network, including PEa, MIP, PEci and, to a lesser extent, 7m, V6A, 7a and MST. Premotor area F7 receives parietal projections mainly from 7m and V6A, and local frontal projections mainly from F2. On the contrary, premotor area F2 in the pre-CD zone receives parietal inputs from PEc and, to a lesser extent, PEci, while in the peri-arcuate zone F2 receives parietal projections from PEa and MIP. Local frontal projections to F2 pre-CD mostly stem from F4, and, to a lesser extent, from F7 and F3, and CMAd; those addressed to peri-arcuate zone of F2 arise mainly from F5 and, to a lesser extent, from F7, F4, dorsal (CMAd) and ventral (CMAv) cingulate motor areas, pre-supplementary (F6) and supplementary (F3) motor areas. The distribution of association cells in both frontal and parietal cortex was characterized through a spectral analysis that revealed an arrangement of these cells in the form of bands, composed of cell clusters, or 'columns'. The reciprocal connections linking parietal and frontal cortex might explain the presence of visually related and eye-position signals in premotor cortex, as well as the influence of information about arm position and movement direction in V6A and PEC: The association connections identified in this study might carry sensory as well motor information that presumably provides a basis for a re-entrant signaling. This might be necessary to match retinal-, eye- and hand-related information underlying eye-hand coordination during reaching.