Prefrontal connections of medial motor areas in the rhesus monkey

Abstract

Several areas on the medial surface of the frontal lobe in both monkeys and humans, including the supplementary motor area and specific areas within the ventral bank of the cingulate sulcus called the cingulate motor areas, have been implicated in the initiation and execution of skilled movements. These areas project directly to the motor cortex and spinal cord, and, on this basis alone, can be considered premotor areas. The present study investigated whether these premotor areas are specific targets of prefrontal cortical projections in the rhesus monkey and thereby provide links between this association cortex and motor effector pathways. Circumscribed injections of wheat germ agglutinin-conjugated horseradish peroxidase were placed into different cytoarchitectonic subdivisions of prefrontal cortex, and resultant retrograde and anterograde labeling examined with respect to designated premotor targets. Conversely, injections were also made in the supplementary and cingulate motor areas and labeled cells and terminals charted in the prefrontal cortex. A principal finding in this study is the identification of multiple prefrontal regions that project to the supplementary motor area, the cingulate motor areas, or both. Areas 46, 8a, 9, 11, and 12 are reciprocally connected with an area of the superior frontal gyrus in or near the supplementary motor area at its rostral margin. A smaller constellation of prefrontal areas, areas 46, 8a, and 11, is reciprocally connected with portions of cingulate cortex that have been classified as premotor arm and/or leg representations (Hutchins et al., Exp Brain Res 71:667-672, 1988). In accordance with numerous previous reports, prefrontal areas 46, 8a, 9, 10, 11, and 12 are reciprocally connected with "nonmotor" subdivisions of cingulate cortex. The results presented here specify the corticocortical connections by which prefrontal cortex may influence motor output.