Brain Research, 157 (1978) 325-330 325 Elsevier/North-Holland Biomedical Press Input to primate motor cortex from posterior parietal cortex (area 5}. I. De- monstration by retrograde transport P. L. STRICK* and C. C. KIM** National Institute of Mental Health, Laboratory of Neurophysiology, Building 35, Room 2D-lO, Bethesda, Md. 20014 (U.S.A.) (Accepted July 27th, 1978) It has long been recognized that the motor cortex participates in the initiation and control of limb movements. Inputs to the motor cortex therefore take on a special importance because of their potential influence on voluntary activity. Prior studies have indicated that, of the cerebral cortical areas of the ipsilateral hemisphere, only premotor and somatic sensory areas have direct cortico-cortical connections with the arm area of the motor cortexg,Z0, z6. Other cortical areas have been thought to influence the motor cortex only indirectly, by way of pathways to premotor cortical areas or long loops through subcortical structures such as the cerebellum, basal ganglia and thalamusl,6,1°,~3, ~4. This report, however, demonstrates a substantial, direct input to the arm area of the primate motor cortex (area 4) from the posterior parietal association cortex (area 5). Similar anatomical observations have been made recently by Jones et al. s. The following paper 28 reports on an electrophysiological confirmation of these results. In that study intracortical microstimulation in area 4 was employed to antidromically activate area 5 neurons. These findings are of particular interest because of recent work suggesting that the posterior parietal association cortex contains a motor 'command apparatus' for the 'operation of the limbs, hands and eyes in immediate extrapersonal space '19. A connection between area 5 and the motor cortex would therefore link a source of 'motor commands' to a major origin of descending cortical output. Our methods have been described in detail elsewhereH,23, 24. Briefly, under sodium pentobarbital anesthesia (35 mg/kg), single injections of 30~ horseradish peroxidase (HRP, Sigma Type VI, dissolved in normal saline) were made at a depth of 1.0-1.5 mm into the arm area of the motor cortex 27 in 5 rhesus monkeys (Macaca mulatta). The animals were allowed to survive for 24-36 h, reanesthetized, and perfused with a solution containing 1 ~ paraformaldehyde and 1.25 ~ glutaraldehyde * Present address: Veterans Administration Hospital and Departments of Neurosurgery and Physiology, Upstate Medical Center, SUNY, Syracuse, N.Y. 13210, U.S.A. ** Present address: Department of Physiology, Catholic Medical College, Seoul, 110, Korea.