Input to primate motor cortex from posterior parietal cortex (area 5). II. Identification by antidromic activation

In the preceding report 17 anatomical experiments are described which demonstrated a projection to the motor cortex (area 4) from the posterior parietal cortex (area 5). Precautions were taken in that study to reduce the possibility that neurons of area 5 were labeled from locations other than the gray matter of the motor cortex. However, since previous anatomical studies10,11, la did not indicate a projection of the density which Strick and Kim 17 observed, we wished to further substantiate its existence with electrophysiological techniques. In addition, this approach provides the opportunity to investigate the peripheral input reaching neurons of area 5 which project to the motor cortex. This is particularly important because of the long-standing difficulty in determining the origin of somatic sensory afferent input to the motor cortex. The anatomical demonstration of a projection from area 5 to the motor cortex raised the possibility that this pathway might be a source of afferent input to the motor cortex. Possible relations between parietal and motor cortices are of interest also because of early findings of perceptual and motor deficits following parietal cortex damage (cf. ref. 4), as well as recent findings that the activity of neurons of the posterior parietal cortex is related to motor operations of the limbs 14. The results were obtained from 3 M. irus monkeys. Surgical preparation was performed under Nembutal anesthesia (35 mg/kg). After opening the dura a double closed chamber 18 was attached to the skull surrounding a craniotomy which exposed the arcuate, central and intraparietal sulci from near the midline to beyond the lateral extent of the intraparietal sulcus. A head fixation device was attached to the skull with screws and cement. The animals were then placed in a primate chair and recovered from general anesthesia overnight. No additional anesthesia was administered. Experiments were carried out during the following two or 3 days. The motor cortex was first explored by intracortical microstimulation (ICMS) to delineate the forearm and hand areas. Tungsten in glass microelectrodes with tips of approximately 15 × 20